Dissertations / Theses on the topic 'Antiferromagnet'
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1
Rule, Kirrily. "Magnetic ordering in the two dimensional antiferromagnet, FePS₃." Monash University, School of Physics and Materials Engineering, 2004. http://arrow.monash.edu.au/hdl/1959.1/9725.
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Juhász, Junger Irén. "Green-function theory of anisotropic Heisenberg magnets with arbitrary spin." Doctoral thesis, Universitätsbibliothek Leipzig, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-70957.
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In this thesis, anisotropic Heisenberg magnets with arbitrary spin are investigated within the second-order Green-function theory. Three models are considered.
First, the second-order Green-fuction theory for one-dimensional and two-dimensional Heisenberg ferromagnets with arbitrary spin S in a
magnetic field is developed. For the determination of the introduced vertex parameters sum rules, higher-derivative sum rules, and regularity conditions are derived, and the equality of the isothermal and the longitudinal uniform static Kubo susceptibilities is required. Thermodynamic quantities, such as the specific heat, magnetic susceptibility, transverse and longitudinal correlation lengths are calculated. Empirical formulas describing the dependence of the position and height of the susceptibility maximum on the magnetic field are given. An anomal behavior of the longitudinal correlation length is observed. The appearance of two maxima in the temperature dependence of the specific heat is discussed.
Further, as an example of a system with an anisotropy in the spin space, the S=1 ferromagnetic chain with easy-axis single-ion anisotropy is studied. Justified by the up-down symmetry of the model with respect to $S_i^z -> -S_i^z$, $\\langle S_i^z \\rangle=0$ is set. Two different ways of the determination of the introduced vertex parameters are presented. The transverse nearest-neighbor correlation function, spin-wave spectrum and longitudinal correlation length are analyzed. The effects of the single-ion anisotropy on the transverse and longitudinal uniform static
susceptibilities as well as on the appearance of two maxima in the temperature dependence of the specific heat are examined.
Finally, as examples of spatial anisotropic spin systems,layered Heisenberg
ferromagnets and antiferromagnets with arbitrary spin are studied within the rotation-invariant Green-function theory. The long-range order is described by the condensation term, which is determined from the requirement that in the ordered state the static susceptibility has to diverge at the ordering wave vector. For determination of the introduced vertex parameters, the sum rule and the isotropy condition are used and also assumptions regarding the temperature dependence of some parameters are made. The main focus is put on the calculation of the specific heat, the Curie temperature, and the Néel temperature in dependence on the interlayer coupling and the spin-quantum number. Empirical formulas describing the dependence of the transition temperatures on the ratio of interlayer and intralayer couplings are given.
For all three models, the results of the Green-function theory are compared to available results of exact approaches (Quantum Monte Carlo, exact diagonalization, Bethe-ansatz method) and to available experimental data.
3
Jackson, Jerome. "Computer simulations of exchange bias : the Role of the antiferromagnet." Thesis, University of York, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.534908.
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Takatsu, Hiroshi. "Anomalous Hall effect in the metallic triangular-lattice antiferromagnet PdCrO2." 京都大学 (Kyoto University), 2010. http://hdl.handle.net/2433/120637.
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Nambu, Yusuke. "Novel Magnetic State of the Two-Dimensional Triangular Antiferromagnet NiGa2S4." 京都大学 (Kyoto University), 2009. http://hdl.handle.net/2433/124400.
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Chan, Chi Hung. "3-dimensional Heisenberg antiferromagnet in cubic lattice under time periodic magnetic field /." View abstract or full-text, 2009. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202009%20CHANC.
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Güttler, Monika, Alexander V. Generalov, M. M. Otrokov, K. Kummer, Kristin Kliemt, Alexander Fedorov, Alla Chikina, et al. "Robust and tunable itinerant ferromagnetism at the silicon surface of the antiferromagnet GdRh2Si2." Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2017. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-214847.
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Spin-polarized two-dimensional electron states (2DESs) at surfaces and interfaces of magnetically active materials attract immense interest because of the idea of exploiting fermion spins rather than charge in next generation electronics. Applying angle-resolved photoelectron spectroscopy, we show that the silicon surface of GdRh2Si2 bears two distinct 2DESs, one being a Shockley surface state, and the other a Dirac surface resonance. Both are subject to strong exchange interaction with the ordered 4f-moments lying underneath the Si-Rh-Si trilayer. The spin degeneracy of the Shockley state breaks down below ~90 K, and the splitting of the resulting subbands saturates upon cooling at values as high as ~185 meV. The spin splitting of the Dirac state becomes clearly visible around ~60 K, reaching a maximum of ~70 meV. An abrupt increase of surface magnetization at around the same temperature suggests that the Dirac state contributes significantly to the magnetic properties at the Si surface. We also show the possibility to tune the properties of 2DESs by depositing alkali metal atoms. The unique temperature-dependent ferromagnetic properties of the Si-terminated surface in GdRh2Si2 could be exploited when combined with functional adlayers deposited on top for which novel phenomena related to magnetism can be anticipated.
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Güttler, Monika, Alexander V. Generalov, M. M. Otrokov, K. Kummer, Kristin Kliemt, Alexander Fedorov, Alla Chikina, et al. "Robust and tunable itinerant ferromagnetism at the silicon surface of the antiferromagnet GdRh2Si2." Nature Publishing Group, 2016. https://tud.qucosa.de/id/qucosa%3A30007.
Full textAbstract:
Spin-polarized two-dimensional electron states (2DESs) at surfaces and interfaces of magnetically active materials attract immense interest because of the idea of exploiting fermion spins rather than charge in next generation electronics. Applying angle-resolved photoelectron spectroscopy, we show that the silicon surface of GdRh2Si2 bears two distinct 2DESs, one being a Shockley surface state, and the other a Dirac surface resonance. Both are subject to strong exchange interaction with the ordered 4f-moments lying underneath the Si-Rh-Si trilayer. The spin degeneracy of the Shockley state breaks down below ~90 K, and the splitting of the resulting subbands saturates upon cooling at values as high as ~185 meV. The spin splitting of the Dirac state becomes clearly visible around ~60 K, reaching a maximum of ~70 meV. An abrupt increase of surface magnetization at around the same temperature suggests that the Dirac state contributes significantly to the magnetic properties at the Si surface. We also show the possibility to tune the properties of 2DESs by depositing alkali metal atoms. The unique temperature-dependent ferromagnetic properties of the Si-terminated surface in GdRh2Si2 could be exploited when combined with functional adlayers deposited on top for which novel phenomena related to magnetism can be anticipated.
9
Collins, Alexander Rory Physics Faculty of Science UNSW. "Quantum lattice models." Publisher:University of New South Wales. Physics, 2008. http://handle.unsw.edu.au/1959.4/43408.
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This thesis presents studies of the low energy properties of nseveral frustrated spin-1/2 Heisenberg antiferromagnets using various analytic and computational methods. The models studied include the union jack model, the alternating Heisenberg chain, the Heisenberg bilayer model, and the spin-Peierls model. The union jack model is a Heisenberg antiferromagnetic spin model with frustration, and is analyzed using spin-wave theory. For small values of the frustrating coupling $\alpha$, the system is N{\' e}el ordered, while for large $\alpha$ the frustration is found to induce a canted phase. Spin wave theory with second order corrections finds the critical coupling at $\alpha \simeq 0.645$,which agrees quantitatively with series expansion results. No intermediate spin-liquid phase is found to exist between the two phases. The alternating Heisenberg chain is studied using an alternative triplet-wave expansion formalism for dimerized spin systems, modification of the ??bond operator?? formalism of Sachdev and Bhatt. Projection operators are used to confine the system to the physical subspace, rather than constraint equations. Comparisons are made with the results of dimer series expansions and exact diagonalization. The S=1/2 Heisenberg bilayer spin model at zero temperature is studied in the dimerized phase using analytic triplet-wave expansions and dimer series expansions. The occurrence of two-triplon bound states in the S=0 and S=1 channels, and antibound states in the S=2 channel, is predicted with triplet-wave theory and confirmed by series expansions. All bound states are found to vanish at or before the critical coupling separating the dimerized phase from the N{\' e}el phase. The critical behavior of the total and single-particle static transverse structure factors is also studied by series expansion methods and found to conform with theoretical expectations. The Heisenberg spin-Peierls model with dispersive, gapless phonons is studied with Density Matrix Renormalization Group methods. We investigate the zero temperature properties of the model using the crossover method. The calculations were found to converge poorly and no conclusive results could be found using this method. An analysis of the convergence problems and the discovery of an anomalous triplet ground state is presented in this chapter.
10
Hooley, Chris. "The Kagome antiferromagnet and related models : studies in low-dimensional geometrically frustrated quantum magnetism." Thesis, University of Oxford, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.301860.
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Attenborough, Michael. "A Moessbauer and #mu#SR study of the magnetic ordering of some antiferromagnet compounds." Thesis, University of Liverpool, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320505.
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Robinson, Matthew D. "An Information Theoretic Study of the Ising Antiferromagnet with Quenched Dilution on a Triangular Lattice." Fogler Library, University of Maine, 2003. http://www.library.umaine.edu/theses/pdf/RobinsonMD2003.pdf.
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de, Vries Mark Alexander. "Iron jarosite and zinc paratacamite as models of the classical and the quantum kagomé antiferromagnet." Thesis, University of Edinburgh, 2007. http://hdl.handle.net/1842/13603.
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In this thesis two experimental antiferromagnets with the particularly interesting lattice resembling the kagomé basket-weaving pattern, are studied. The kagomé topology frustrates the classical antiferromagnetic Néel state, the commonly encountered magnetic groundstate in transition-metal compounds. Extensive theoretical work has shown that in particular the S = 1/2 kagomé antiferromagnet should not show any symmetry-breaking transition to a classical long-range-ordered state. Hence, experimental realisations of this system should in theory allow a unique insight into properties of the symmetrical quantum-mechanical magnetic groundstate. Recently a viable experimental realisation of the S = 1/2 kagomé antiferromagnet has been discovered, the x = 1 phase of zinc-paratacamite ZnxCu(4-x)(OH)6Cl2. Here samples of stoichiometry 0.15 ≤ x ≤ 1 were synthesised and characterised. Using muon-spin relaxation spectroscopy on these samples, it was found that for x > 0.6 the spins do not freeze, even at 50 mK. From neutron powder diffraction for x = 1 and heat capacity measurements on zinc paratacamite with 0.5 ≤ x ≤ 1, it was found that ~6% antisite disorder is present in the x = 1 phase, and that samples with Zn stoichiometry 0.8 ≤ x ≤ 1 model the S = 1/2 kagomé antiferromagnet equally well. No quantum critical phase transition to a quantum spin liquid is found. Instead, the groundstate of this model system is magnetic even for x > 0,8. The field dependence of the heat capacity provides additional evidence that the total magnetic quantum number Stot is not a conserved quantity, despite the fact that no symmetry breaking transition occurs in the magnetic degrees of freedom. In a polarised X-ray spectroscopy experiment on synthesised single crystals of the classical (S = 5/2) kagomé antiferromagnet iron jarosite KFe3 (SO4)2(OH)6, it is shown that the Fe3+ ion with a 6S free-ion configuration can acquire a large orbital angular momentum in the solid state. The high-resolution spectra are reproduced in excellent detail.
14
Merdan, Mohammad Ghanim Merdan. "Study of the excited states of the quantum antiferromagnets." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/study-of-the-excited-states-of-the-quantum-antiferromagnets(dfd01ec6-806c-464a-9b68-910217679c3c).html.
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We investigate the quantum dynamics of the spins on different Heisenberg antiferromagnetic spin lattice systems. Firstly, we applied the coupled-cluster method to the spin-1/2 antiferromagnetic XXZ model on a square lattice by employing an approximation which contains two-body long-range correlations and high-order four-body local correlations. Improvement is found for the ground-state energy, sublattice magnetization, and the critical anisotropy when comparing with the approximation including the two-body correlations alone. We also obtain the full excitation spectrum which is in good agreement with the quantum Monte Carlo results and the high-order spin-wave theory. Secondly, we study the longitudinal excitations of quantum antiferromagnets on a triangular lattice by a recently proposed microscopic many-body approach based on magnon-density waves. We calculate the full longitudinal excitation spectra of the antiferromagnetic Heisenberg model for a general spin quantum number in the isotropic limit. Similar to the square lattice model, we find that, at the center of the first hexagonal Brillouin zone Γ(q=0) and at the magnetic ordering wavevectors ±[Q= (4π/3,0)], the excitation spectra become gapless in the thermodynamic limit, due to the slow, logarithmic divergence of the structure factor. However, these longitudinal modes on two-dimensional models may be considered as quasi-gapped, as any finite-size effect or small anisotropy will induce a large energy gap, when compared with the counterpart of the transverse spin-wave excitations. We have also investigated the excited states of the quasi-one-dimensional quantum antiferromagnets on hexagonal lattices, including the longitudinal modes based on the magnon-density waves. A model Hamiltonian with a uniaxial single-ion anisotropy is first studied by a spin-wave theory based on the one-boson method; the ground state thus obtained is employed for the study of the longitudinal modes. The full energy spectra of both the transverse modes (i.e., magnons) and the longitudinal modes are obtained as functions of the nearest-neighbor coupling and the anisotropy constants. We have found two longitudinal modes due to the non-collinear nature of the triangular antiferromagnetic order, similar to that of the phenomenological field theory approach by Affleck. The excitation energy gaps due to the anisotropy and the energy gaps of the longitudinal modes without anisotropy are then investigated. We then compares our results for the longitudinal energy gaps at the magnetic wavevectors with the experimental results for several antiferromagnetic compounds with both integer and non-integer spin quantum numbers, and we find good agreements after the higher-order contributions are included in our calculations.
15
Gustainis, Peter. "Field induced phase transition in one dimensional Heisenberg antiferromagnet model studied using density matrix renormalization group." Thesis, University of British Columbia, 2017. http://hdl.handle.net/2429/61214.
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This thesis examines the Heisenberg antiferromagnetic spin chain in one dimension (1D) with a crystal field splitting term and applied magnetic field term. We use theoretical techniques from quantum field theory and conformal field theory (CFT) to make predictions about the excitation spectrum for our model. We then use Density Matrix Renormalization Group (DMRG) numerical techniques to simulate our spin chain and extract the energy spectrum as we vary our crystal field splitting and magnetic field terms. These results are compared and we examine where theoretical calculations accurately describe our system. This work is motivated by recent experimental work done on SrNi₂Vi₂O₈ by Bera et al. [1] which is a quasi-1D material with weakly coupled spin chains in the bulk. These 1D chains are expected to be described by the Hamiltonian we study in this thesis, and we neglect interchain coupling. We first consider our system where the crystal field splitting term is set to zero, which can be described theoretically using a mapping to the non linear sigma model (NLSM). Near the critical field, it undergoes a Bose condensation transition whose excitation spectrum can be mapped to non-interacting fermions in 1D. We then consider large negative crystal field splitting, and find that near small applied magnetic field we can describe some excited states using Landau-Ginsburg theory. Near critical field, we show that the transition is in the Ising universality, and use results from CFT to predict the spectrum for finite size systems. This allows us to make predictions about where the transition field would be for very large or infinite system size. Finally, we examine our crystal field splitting tuned to the value obtained in Ref. 1, which is a small, negative value. We observe qualitative elements in this spectrum from the spectra obtained at zero and large negative crystal field splitting.Science, Faculty of
Physics and Astronomy, Department of
Graduate
16
Matan, Kittiwit. "Neutron scattering and magnetization studies of the spin correlations on the kagomé lattice antiferromagnet KFe₃(Oh)₆(SO₄)₂." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/45433.
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Includes bibliographical references (p. 203-222).Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, February 2008.
(cont.) The former represents a continuous planar rotational symmetry corresponding to the SO(2) symmetry, while the latter is a discrete symmetry associated with the Z2 symmetry. Depending on which measurements are performed, the critical behavior of the system can belong to either SO(2) or Z2 universality classes with two distinct critical temperatures; one is associated with the spontaneous breaking of the Z2 symmetry, and the other corresponds to a topological order (BKT transition) due to vortex- antivortex binding. The former occurs at a slightly higher temperature than the latter. Neutron scattering measurements show a signature of the BKT transition, while specific heat measurements show a feature of the 2D Ising transition. Above TN, the in-plane spin gap vanishes, and the system retains the SO(2) symmetry when measured with neutron scattering. On the other hand, specific heat measurements show a feature of the 2D Ising transition, since the underlying symmetry of the spin Hamiltonian is the time-reversal or Z2 symmetry.
The collective behavior of interacting magnetic moments can be strongly influenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting spin dynamics and chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. We report studies of the spin-wave excitations and spin chirality on a two-dimensional geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large single crystal samples of KFe3(OH)6 (SO4)2, an ideal kagom6 lattice antiferromagnet. The spin-wave excitations have been measured using high-resolution inelastic neutron scattering. We directly observe a flat mode which corresponds to a lifted "zero energy mode," verifying a fundamental prediction for the kagome lattice. A simple Heisenberg spin Hamiltonian provides an excellent fit to our spin-wave data. The antisymmetric Dzyloshinskii-Moriya interaction is the primary source of anisotropy and explains the low-temperature magnetization and spin structure. In addition, combined thermodynamic and neutron scattering measurements reveal that the phase transition to the ordered ground-state is unusual. At low temperatures, application of a magnetic field induces a transition between states with different non-trivial spin- textures. The transition indicated by the sudden increase in the magnetization arises as the spins on alternating layers, which are previously oppositely canted due to the ferromagnetic interplane coupling, rotate 1800 to align the canting moment along the c-axis. These observations are consistent with the ordering induced by the Dzyloshinskii-Moriya interaction. Elastic neutron scattering measurements in high field verify the 180' spin rotation at the transition. The critical behavior in jarosite cannot be categorized by any known universality classes. We propose a scenario where both 2D XY and 2D Ising symmetries are present.
by Kittiwit Matan.
Ph.D.
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Wong, Ming-Wai. "Studying the mutual interaction between ferromagnet and antiferromagnet planes by using the Schwinger-Boson mean field theory /." View Abstract or Full-Text, 2002. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202002%20WONG.
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Adams, Daniel J. "Ferromagnetic Resonance Studies of Coupled Magnetic Systems." ScholarWorks@UNO, 2016. http://scholarworks.uno.edu/td/2121.
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The high-frequency properties of coupled magnetic systems have been investigated using vector network analyzer ferromagnetic resonance (VNA-FMR) spectroscopy. SAF structures consist of two ferromagnetic layers separated by a non-magnetic spacer, coupled through the indirect exchange interaction. The ferromagnetic layers of our samples were composed of FeCoB separated by a layer of Ru. The thickness of Ru was varied in the range of 8 to 18 Å among the samples studied. Antiferromagnetic coupling can be quickly identified by the major hysteresis loop (MHL).
A new way of displaying FMR data for these trilayer samples is presented which completely preserves the anisotropy effects while fully characterizing the angular dependence of FMR. The advantage of our representation is that the high-frequency data can be easily compared to the static switching behavior at any angle obtained through susceptibility measurements.
Ferromagnetic resonance; Coupled; Synthetic antiferromagnet; Magnetization switching
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Yang, Rong. "ATOMIC-SCALE AND SPIN STRUCTURE INVESTIGATIONS OF MANGANESE NITRIDE AND RELATED MAGNETIC HYBRID STRUCTURES PREPARED BY MOLECULAR BEAM EPITAXY." Ohio University / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1152124059.
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Helton, Joel Strader. "The ground state of the spin-1/2 kagomé lattice antiferromagnet : neutron scattering studies of the zinc-paratacamite mineral family." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/52786.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student submitted PDF version of thesis.
Includes bibliographical references (p. 201-214).
The magnetic properties of the geometrically frustrated quantum magnets clinoatacamite, Cu2(OH)3Cl, and herbertsmithite, ZnCu3(OH)6Cl2, are studied by means of neutron scattering measurements as well as specific heat, susceptibility, and mag-netization measurements. These materials are studied to investigate the nature of the ground state of the spin-1 2 kagomé lattice antiferromagnet, as such a system is considered ideal for the emergence of spin liquid physics. Clinoatacamite, a distorted kagomé lattice antiferromagnet with weak inter-plane coupling, undergoes a Néel or- dering transition at TN ~/~ 6.2 K and shows evidence of a static local moment in the disordered phase below 18 K. Our experiments suggest two-dimensional Ising fluctuations at the Néel transition. A proposed spin ordering model is developed that suggests an order structure below TN and two-dimensional short range order of the kagomé plane spins up to 18 K. The inelastic spectrum is analyzed in terms of spin waves in an ordered kagomé lattice antiferromagnet with a Dzyaloshinskii-Moriya interaction. Herbertsmithite is the first structurally perfect spin- 1 2 kagomé lattice antiferromagnet. Susceptibility, specific heat, and neutron scattering measurements show no sign of any spin freezing or transition to a long range ordered state down to 50 mK. The data shows magnetic excitations extending adjacent to the ground state, suggesting the lack of any measurable spin gap. Several hypotheses are explored as possible explanations for the apparent lack of a spin gap.
(cont.) Dynamic susceptibility data display an unusual scaling relation, suggesting proximity to a quantum critical point. In sum, a wide range of data suggest that herbertsmithite displays a disordered gapless spin liquid ground state.
by Joel Strader Helton.
Ph.D.
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Han, Tianheng Ph D. Massachusetts Institute of Technology. "Thermodynamic and neutron scattering study of the spin- 1/2 kagome antiferromagnet ZnCu₃(OH)₆Cl₂ : a quantum spin liquid system." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/79516.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2012.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 183-200).
New physics, such as a quantum spin liquid, can emerge in systems where quantum fluctuations are enhanced due to reduced dimensionality and strong frustration. The realization of a quantum spin liquid in two-dimensions would represent a new state of matter. It is believed that spin liquid physics plays a role in the phenomenon of high-Tc superconductivity, and the topological properties of the spin liquid state may have applications in the field of quantum information. The Zn-paratacamite family, ZnCu₃(OH)₆Cl₂ for x > 0.33, is an ideal system to look for such an exotic state in the form of antiferromagnetic Cu2+ kagome planes. The x = 1 end member, named herbertsmithite, has shown promising spin liquid properties from prior studies on powder samples. Here we show a new synthesis by which high-quality centimeter-sized single crystals of Zn-paratacamite have been produced for the first time. Neutron and synchrotron x-ray diffraction experiments indicate no structural transition down to T = 2 K. The magnetic susceptibility both perpendicular and parallel to the kagome plane has been measured for the x = 1 sample. A small, temperature-dependent anisotropy has been observed, where Xz / Xp > 1 at high temperatures and Xz / Xp < 1 at low temperatures. Fits of the high-temperature data to a Curie-Weiss model also reveal anisotropies for [Theta]cw's and g-factors. By comparing with theoretical calculations, the presence of a small easy-axis exchange anisotropy can be deduced as a primary perturbation to the dominant Heisenberg nearest neighbor interaction. These results have great bearing on the interpretation of theoretical calculations based on the kagome Heisenberg antiferromagnet model to the experiments on ZnCu₃(OH)₆Cl₂. Specific heat measurements down to dilution temperatures and under strong applied magnetic fields show a super linear temperature dependence with a finite linear term. Most importantly, we present neutron scattering measurements of the spin excitations on a large deuterated single crystal sample of herbertsmithite. Our observation of a spinon continuum in a two-dimensional magnet is unprecedented. The results serve as a key fingerprint of the quantum spin liquid state in herbertsmithite.
by Tianheng Han.
Ph.D.
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Powalski, Michael [Verfasser], Götz S. [Akademischer Betreuer] Uhrig, and Kai Phillip [Gutachter] Schmidt. "Dynamics and interaction of magnons in the two dimensional Heisenberg antiferromagnet / Michael Powalski ; Gutachter: Kai Phillip Schmidt ; Betreuer: Götz S. Uhrig." Dortmund : Universitätsbibliothek Dortmund, 2017. http://d-nb.info/1137380446/34.
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Blouzon, Camille. "Photoelectric and magnetic properties of multiferroic domain walls in BiFeO3." Thesis, Paris 6, 2016. http://www.theses.fr/2016PA066006/document.
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De tous les matériaux multiferroïques, BiFeO3 est celui qui est le plus étudié. C’est un ferroélectrique, antiferromagnétique dont les températures de transition sont bien au-dessus de la température ambiante. De plus, le couplage magnétoélectrique entre ces deux paramètres d’ordre a été observé aussi bien dans les cristaux que dans les couches minces. BiFeO3 possède également la plus grande polarisation ferroélectrique jamais mesurée, 100µC/cm². De gros efforts sont fournis pour comprendre et exploiter les propriétés physiques de ce matériau. Dans ce but, il est important de pouvoir contrôler sa structure en domaines afin d’étudier les phénomènes émergeant aux parois de ces domaines. C’est l’objectif de cette thèse : étudier quelques une des propriétés de BiFeO3, comme la photoélectricité et le magnétisme, tout en prêtant en parallèle une attention particulière à la caractérisation de ces propriétés, dans un domaine et dans une paroi, avec des techniques originales telles que la microscopie de photocourants à balayage (MPB) et le rayonnement synchrotron ou les champs magnétiques intenses. Les images obtenues par MPB, révèlent qu’un champ dépolarisant proche d’une paroi de domaine à 180° peut améliorer de manière significative le rendement des effets photoélectriques : les parois de domaines peuvent être générées et positionnées dans le but de contrôler localement le rendement de l’effet photoélectrique. De plus, l’imagerie de la figure de diffraction de surface d’un réseau de parois de domaines dans des couches minces, par diffusion magnétique résonante de rayons X, permet de montrer que les parois de domaines entraînent la formation de structures magnétiques particulières qui pourraient donner lieu à une aimantationAmong all multiferroics, BiFeO3 is a material of choice because its two ordering temperatures are well above 300K. It is a ferroelectric antiferromagnet, and magnetoelectric coupling has been demonstrated in bulk and in thin films. Remarkably, BiFeO3 has the largest polarization of all known ferroelectrics (100µC/cm²). A huge research effort is carried out worldwide to understand and exploit the physical properties of this material which requires to design and tailor BiFeO3 on many scales. In this sense, developing methods and tools to control the domain structure is essential to explore new emergent phenomena arising at domain walls. This is the aim of the present PhD work. Some of the original properties of BiFeO3 have been investigated including its photoelectric and magnetic properties. A particular attention is given to characterize in a parallel fashion bulk properties and domain walls properties, using original techniques of characterization such as Scanning Photocurrent Microscopy (SPCM), scattering synchrotron facilities or high field pulses. SPCM mapping reveals that depolarizing fields in the vicinity of a 180° domain wall can significantly improve the photovoltaic efficiency. Thus domain walls can be generated and precisely positioned in order to tailor the local photovoltaic efficiency. Moreover, X-ray resonant magnetic scattering on thin films with periodic domain structure shows that domain walls generate specific magnetic structures with possible uncompensated magnetization
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Radu, Cosmin. "Study of Magnetization Switching in Coupled Magnetic Nanostructured Systems." ScholarWorks@UNO, 2008. http://scholarworks.uno.edu/td/894.
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A study of magnetization dynamics experiments in nanostructured materials using the rf susceptibility tunnel diode oscillator (TDO) method is presented along with a extensive theoretical analysis. An original, computer controlled experimental setup that measures the change in susceptibility with the variation in external magnetic field and sample temperature was constructed. The TDO-based experiment design and construction is explained in detail, showing all the elements of originality. This experimental technique has proven reliable for characterizing samples with uncoupled magnetic structure and various magnetic anisotropies like: CrO2 , FeCo/IrMn and Co/SiO2 thin films. The TDO was subsequently used to explore the magnetization switching in coupled magnetic systems, like synthetic antiferromagnet (SAF) structures. Magnetoresistive random access memory (MRAM) is an important example of devices where the use of SAF structure is essential. To support the understanding of the SAF magnetic behavior, its configuration and application are reviewed and more details are provided in an appendix. Current problems in increasing the scalability and decreasing the error rate of MRAM devices are closely connected to the switching properties of the SAF structures. Several theoretical studies that were devoted to the understanding of the concepts of SAF critical curve are reviewed. As one can notice, there was no experimental determination of SAF critical curve, due to the difficulties in characterizing a magnetic coupled structure. Depending of the coupling strength between the two ferromagnetic layers, on the SAF critical curve one distinguishes several new features, inexistent in the case of uncoupled systems. Knowing the configuration of the SAF critical curve is of great importance in order to control its switching characteristics. For the first time a method of experimentally recording the critical curve for SAF is proposed in this work. In order to overcome technological limitations, a new way of recording the critical curve by using an additional magnetic bias field was explored.
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Stepanov, Petr. "Spin and Charge Transport in Monolayer and Trilayer Graphene in the Quantum Hall Regime." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1532103607994675.
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Staats, Michael. "Numerische Untersuchungen von Gleichgewichts- und Nichtgleichgewichtseigenschaften verdünnter Antiferromagnete - Numerical investigations of equilibrium and non-equilibrium properties of diluted antiferromagnets." Gerhard-Mercator-Universitaet Duisburg, 2001. http://www.ub.uni-duisburg.de/ETD-db/theses/available/duett-09122001-125103/.
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Magnetic models with strong disorder are the topic of this work. Diluted antiferromagnets in an external field (DAFF) are of special interest, both from a theoretical, and from an experimental point of view. In this work critical exponents at various points in the temperature vs. magnetic field phase diagram are determined by the analysis of exact calculated ground states and of Monte Carlo simulations. The specific heat and its critical exponent deserve special attention. Simulations of irreversibilities of the specific heat are in agreement with latest measurements. Further emphasis is put on the investigation of the non-equilibrium dynamics of the DAFF. The relaxation dynamics of the DAFF is governed by thermal activation. Further it is shown that also the DAFF exhibits Aging effects, as they are known from many other disordered systems
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Geißler, David. "Plastizität, deformationsinduzierte Phänomene und Élinvareigenschaften in antiferromagnetischen austenitischen FeMnNiCr-Basislegierungen." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-89042.
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Hoch manganhaltige Eisenbasislegierungen sind bei Raumtemperatur austenitisch und antiferromagnetisch (afm). Dabei besteht die Besonderheit, dass sich durch Legierung die afm Übergangstemperatur (Néeltemperatur) so einstellen lässt, dass sie nahe Raumtemperatur liegt. FeMn-Basislegierungen zeigen in Abhängigkeit von der Zusammensetzung Transformation Induced Plasticity (TRIP) und/oder Twinning Induced Plasticity (TWIP), d.h. die niedrige Stapelfehlerenergie dieser Legierungen führt zu verformungsinduzierter, metastabiler Phasenbildung (TRIP) bzw. zur Bildung von Verformungszwillingen (TWIP) und dadurch zu außerordentlich hoher Duktilität bei gleichzeitig hoher Verfestigung. Darüber hinaus haben FeMn-Basislegierungen einen ausgeprägten Magnetovolumeneffekt und magnetoelastischen Effekt durch magnetische Ordnung. Daher sind die untersuchten FeMnNiCr-Basislegierungen auch prototypisch für afm Élinvarlegierungen. Da Élinvar jedoch für invariable Elastizität steht, bedingt eine Anwendung als temperaturkompensierte Konstantmodullegierungen die Glättung der ausgeprägten magnetischen Anomalien, die industriell noch in keiner Anwendung realisiert wurde. Der Vorteil dies für eine Anwendung zu erreichen, läge in der Unempfindlichkeit feinmechanischer Bauelemente, gegenüber magnetischen Feldern, die bei den industriell verfügbaren ferromagnetischen Élinvarlegierungen nicht gewährleistet ist. Mit Bezug zu feinmechanischen Schwingsystemen spielen dabei neben der Einstellung der magnetoelastischen Eigenschaften die Prozessierbarkeit, Kaltumformbarkeit und Festigkeit sowie deren wechselseitige Beeinflussung eine große Rolle. Die vorliegende Arbeit befasst sich daher mit der Anwendbarkeit der untersuchten FeMnNiCr-Legierungen. Dabei wurden grundlegende Untersuchungen zur Plastizität durchgeführt, die die mechanische Zwillingsbildung in diesen Legierungen charakterisiert und ein Modell der mechanischen Zwillingsbildung bei kleinen plastischen Dehnungen vorschlägt, das eine Abschätzung der Stapelfehlerenergie erlaubt. Die Untersuchung des Antiferromagnetismus umgeformter Proben zeigt das Auftreten thermoremanenter Magnetisierung (TRM), deren Größe mit dem Umformgrad der untersuchten Proben skaliert. Sie wird den durch Umformdefekte erzeugten unkompensierten Momenten in der afm Spinstruktur zugeschrieben. Diese werden durch eine magnetische Feldkühlung magnetisiert und koppeln durch Austauschwechselwirkung an die umgebende antiferromagnetische Matrix unterhalb der Néeltemperatur. Das komplexe thermomagnetische Verhalten der unkompensierten Momente wird experimentell beschrieben und phänomenologisch gedeutet. Die Weiterentwicklung und Bewertung technischer, ausscheidbarer FeMnNiCrBe- und FeMnNiCr(Ti, Al)-Legierungen wird mit Bezug zu den grundlegenden Untersuchungen dargestellt. Es wird gezeigt, dass die neu entwickelten ausscheidbaren FeMnNiCr(Ti, Al)-Legierungen eine vielversprechende Ausgangsbasis darstellen, afm Élinvarlegierungen technisch umzusetzenHigh manganese iron-base alloys are austenitic and antiferromagnetic (afm) at room temperature. By further alloying it is possible to tune the afm transition temperature (Néel temperature) near room temperature. FeMn-base alloys show extraordinary strain hardening as well as ductility because of Transformation Induced Plasticity (TRIP) and/or Twinning Induced Plasticty (TWIP), i.e. in dependence on composition the generally low stacking fault energy in these alloys allows for the mechanically induced formation of metastable phases (TRIP) or deformation twinning (TWIP). Furthermore, magnetic order causes distinct magnetovolume and magnetoelastic effects in these afm FeMn-base alloys. The investigated FeMnNiCr-base alloys are therefore prototypic for afm Élinvar alloys. However, as Élinvar is meant for invariant elasticity, an application as temperature compensated alloy with constant elastic modulus requires the smoothing of the pronounced magnetic anomalies, that is not industrially available yet. The advantage of afm Élinvar alloys in precision mechanics applications, would be their impassiveness with respect to magnetic fields that is not achievable by their ferromagnetic counterparts. For precision components like mechanic oscillators not only the tuning of the magnetoelastic properties but also the processing, cold formability and mechanical properties as well as their interplay have strong influence. Therefore this work addresses the applicability of the studied FeMnNiCr alloys. Elementary investigations on plasticity characterise the occurrence of TWIP in these alloys and propose a modell for deformation twinning at low plastic strains that allows for an estimation of the stacking fault energy. The investigations on the antiferromagnetism of deformed samples show the appearance of thermoremanent magnetisation (TRM). Its magnitude scales with the degree of deformation. The TRM is therefore attributed to uncompensated moments in the afm spin structure due to deformation induced defects. These are magnetised by a magnetic field cooling and couple to the afm matrix by exchange interaction below the Néel temperature. The complex thermomagnetic behaviour of the uncompensated moments is experimentally described and phenomenologically explained. The further development and assessment of engineering-grade pecipitable FeMnNiCrBe and FeMnNiCr(Ti, Al) alloys is presented in relation to the aforementioned elementary investigations. It is shown that the newly developped precipitable FeMnNiCr(Ti, Al) alloys are good candidates for afm Élinvar alloys in application
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Adams, Daniel J. "Magnetization Dynamics in Coupled Thin Film Systems." ScholarWorks@UNO, 2019. https://scholarworks.uno.edu/td/2578.
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A study is presented detailing experimental investigations of magnetization dynamics in nanostructured systems which are coupled magnetically. This work seeks to characterize the anisotropy of such systems through experimental techniques which probe microwave resonant absorption in the materials.
A custom-built experimental setup, designed and assembled in our labs, is explained in detail. This setup allows for angular-dependent ferromagnetic resonance (FMR) measurements in the sample plane through vector network analyzer spectroscopy and is adaptable to two different types of coplanar waveguides. This technique has proven effective for characterization of multiple types of magnetic systems, including multilayered structures as detailed here, with different types of anisotropies while allowing us to draw analogies with more common characterization techniques. The angular FMR setup has been used to study coupled systems, such as those coupled through the Ruderman–Kittel–Kasuya–Yosida interaction as well as exchange-biased structures. These types of coupled systems have technological impacts and are highly applied in the components of magnetoresistive random access memory. Using this new characterization technique, properties of synthetic antiferromagnets have been revealed which had not been observed before.
In addition to these experiments, magnetic susceptibility and FMR in exchange biased systems have been investigated at temperatures as low as 2 K. This investigation used a new FMR spectrometer and was one of the first studies to use this instrument.
For the first time a new method of identifying several types of coupling which can be present in layered nanostructures is presented and supported through comparison with known techniques, thus connecting a new characterization technique for layered structures with decades-old procedures. Many results within this work are also supported theoretically with computer simulations.
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Mai, Thuc T. "Optical spectroscopy of cooperative phenomena and their symmetries in solids." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555629359625425.
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Higgs, Thomas David Charles. "Magnetic structures and proximity effects in rare-earth/transition metal ferromagnetic and superconductor systems." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/280118.
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The antiferromagnetic coupling between a rare-earth (RE) and a tran- sition metal (TM) ferromagnet can be exploited to engineer normal state and superconducting functional devices. RE/TM ferromagnetic multi- layers were previously used as spin-mixers to generate spin-triplet su- percurrents. This was possible due to magnetic inhomogeneity present in the devices, however the precise nature of the inhomogeneity was not understood. Here we present a comprehensive study of the Ni/Gd/Ni system using a powerful element-specific measurement technique: x-ray magnetic circular dichroism. In order to analyse the experimental results we present a novel model based on the Stoner-Wohlfarth model, which shows that significant inhomogeneity exists at the Ni/Gd interfaces due to the competition between the exchange energies within the system and the Zeeman energy of the applied magnetic field. The experiment and model together provide a complete overview of the Ni/Gd/Ni system due to the breadth of temperatures and thicknesses studied. The knowledge gained from this work is then applied to designing and test- ing new spin valves based on the intrinsic inhomogeneity at the RE/TM interface, and both Ni/Gd- and Gd/Ho-based devices show reversible magnetic switching behaviour which alters the superconducting critical temperature.
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Fabianová, Kateřina. "Magnetotransportní vlastnosti FeRh nanodrátů." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2018. http://www.nusl.cz/ntk/nusl-382269.
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Železo-rhodium (FeRh) je látka procházející magnetickou fázovou přeměnou prvního druhu z antiferomagnetické (AF) do feromagnetické (FM) fáze, ke které dochází při zahřátí materiálu nad teplotu fázové přeměny nebo působením dostatečně velkého magnetického pole. Tato fázová přeměna je mimo jiné provázena výraznou změnou entropie, magnetizace a elektrického odporu, přičemž její tvar a poloha teploty přeměny je silně závislá na stechiometrii krystalu, na příměsích, tlaku a v případě tenkých vrstev na napjatosti vrstvy způsobené substrátem. Tato práce se zaměřuje na studium magnetotransportních vlastností drátů připravených z tenkých FeRh vrstev rostlých na substrátech indukujících různou napjatost vrstvy. Jedním z hlavních jevů studovaných v této práci je anizotropní magnetorezistance (AMR) projevující se změnou odporu pro různé natočení magnetických momentů v látce vůči směru elektrického proudu. AMR byla studována jak ve FM fázi, tak i v AF fázi FeRh. Byla změřena hodnota AMR ve vysokoteplotní FM fázi a objeveno neočekávané chování AMR ve zbytkové FM fázi v nízkoteplotním stavu. Dále byla pozorována výrazná závislost AMR na orientaci měřených segmentů vůči krystalografickým směrům FeRh.
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El, Kanj Aya. "Unraveling spin wave dynamics in ferro- and antiferro- magnetic materials : a step towards ultrafast magnonics." Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASP024.
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Cette thèse porte le sujet de la magnonique, un domaine de recherche étudiant le transport des ondes de spin (SWs) dans les matériaux magnétiques pour les technologies beyond-CMOS. L'objectif de mon travail repose sur la génération, la propagation et la détection des SWs dans les matériaux ferromagnétiques (FM) et antiferromagnétiques (AFM), principalement le Grenat de Fer d’Yttrium (Y₃Fe₅O₁₂, connu par YIG) et l'Hématite (α-Fe₂O₃). En conséquence, trois résultats principaux ont été obtenus : 1. Spectroscopie de temps de vol sur le YIG : Nous introduisons une méthode d'extraction d'informations temporelles sur les paquets de SWs propagatifs à l'aide d'un Analyseur de Réseau Vectoriel (VNA). L'approche implique la transformée de Fourier inverse des données de domaine fréquentiel, montrant son utilité dans les VNA disponibles commercialement. Nous réalisons donc des mesures sur un film de YIG, ayant un spectre complexe de mode. On arrive alors à identifier les modes ainsi que d'isoler leur contribution afin de pouvoir étudier les propriétés de chaue mode. À partir de ces résultats, nous avons pu mieux comprendre le comportement des SWs dans une ligne de retard de YIG et ainsi adapter la conception des antennes selon l'application souhaitée. 2. Ondes de spin non dégénérées, non réciproques et ultra-rapides dans l'hématite : Les ondes de spin dans les AFM sont également étudiés depuis des années, mais leurs observation en expérimentale font toujours défaut. Les AFM cantés tel que l'hématite avec la présence des interactions Dzyaloshinskii-Moriya et sous l'influence d’un champs magnétique appliqué, permettent un régime d'échange dipolaire de SWs. Nous avons réussi donc à détecter et démontrer la présence de tels SWs ayant un caractère non réciproques ultra-rapides et non dégénérés, en utilisant des moyens de détection inductifs sur l'hématite épais. En utilisant la spectroscopie de temps de vol, nous constatons que les paquets d'ondes de magnons peuvent se propager aussi rapidement que 20 km/s pour les modes de bulk réciproques et jusqu'à 6 km/s pour les ondes de spin de surface se propageant parallèlement au vecteur Néel. Ces résultats peuvent faire avancer le domaine de la magnonique antiferromagnétique et dévoiler la physique riche des SWs cohérents. 3. Magnon spintronics sur Hematite : Une technique alternative aux mesures inductives pour détecter les SWs peut être réalisée en utilisant une détection électrique. Cela est accompli en utilisant un transducteur métallique à base de platine via l'effet Hall de spin inverse (ISHE). Nous obtenons une tension de sortie sur l'hématite comparable à celle obtenue dans un FM comme le YIG, marquant une détection électrique efficace des SWs propagatifs à travers ISHE sensible à la surface. Un autre effet a également été obtenu dans cette thèse ; la réalisation de la rectification de diode de spin dans l'hématite. Grace à ces études on a pu démontrer que les effets de spin-pumping représentent un outil prometteur pour détecter la dynamique de SW dans les AFM. Avec ces études, nous visons à contribuer dans l’avancement des applications radiofréquences, ouvrant la voie au développement de la recherche magnonique et magnonique spintronique ultra-rapide pour les futures technologies de l'informationThis thesis holds the subject of magnonics, a research field investigating spin waves (SWs) transport in magnetic materials for potential future beyond CMOS technologies. The aim of my work relies on the generation, propagation, and detection of spin waves in ferromagnetic and antiferromagnetic materials, with a specific focus on Yttrium Iron Garnet (Y₃Fe₅O₁₂, known as YIG) and Hematite (α- Fe₂O₃). As a consequence, three main results have been achieved: 1. Time of flight spectroscopy on YIG: We introduce a method for extracting time-domain information on propagating spin-wave packets using a Vector Network Analyzer. The approach involves the inverse Fourier Transform of frequency domain data, showing its utility within the commercially available VNAs. We therefore conduct measurements on a 500 nanometer-thick YIG film, a well-known system in the aim to validate the technique's application. It results in isolating a single SW mode profile from a complex spectrum, enabling the identification of mode origins in addition to conducting individual mode study through time gating. From these results we were able to provide a better understanding of the SWs behavior in a YIG delay line, and therefore tailoring the antennas design according to the needed application. 2. Non-degenerated, non-reciprocal and ultra-fast spin-waves in Hematite:Spin waves in AFM have also been investigated for years, yet experimental observations are still lacking. Canted AFM, such as hematite with the presence of bulk Dzyaloshinskii-Moriya interaction and under applied magnetic fields, allow a dipole exchange regime of AFM spin waves. In this thesis, we were able to detect and demonstrate the presence of ultra-fast nonreciprocal SWs in this regime using inductive means of detection on a bulk Hematite. By using time-of-flight spectroscopy we find that the magnon wave packets can propagate as fast as 20 km/s for reciprocal bulk modes and up to 6 km/s for surface modes propagating parallel to the Néel vector. These findings can push forward the field of antiferromagnetic magnonics, and in unravelling the rich physics of coherent SWs. 3. Magnon spintronics on Hematite: An alternative technique to the inductive measurements to detect the spin waves can be done by using an electrical detection of antiferromagnetic spin-waves. This is achieved by using a platinum-based metallic transducer via the inverse spin hall effect. The response’s output voltage on Hematite is comparable to that obtained in ferromagnets like YIG, marking an efficient electrical detection of propagating spin-waves through the surface-sensitive inverse spin-Hall effect with a platinum-based metallic transducer. Another effect was also achieved in this thesis that is the realization of spin diode rectification in Hematite. In this work we evidence that as expected the Oersted field excitation dominates over spin-torque effects in the case of a single crystal. We also observe the presence of non-linear effects when pumping the system with high powers. To go further, we examine by simultaneously applying both AC and DC currents, the DC dependent characteristics of the system to validate our statements. These mentioned features further demonstrate that spin-pumping effects represent a promising tool to detect the spin-wave dynamics in antiferromagnets, and favorize their integration in magnonic devices. With these findings, we aim to unlock the potential for future advancements in radiofrequency applications, paving the way for the development of ultrafast magnonic and magnon spintronic based research for future ICT technologies
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Streib, Simon [Verfasser], Peter [Akademischer Betreuer] Kopietz, Roser [Akademischer Betreuer] Valenti, and Götz [Akademischer Betreuer] Uhrig. "Theoretical investigations of the spin-1/2 triangular lattice antiferromagnet Cs2CuCl4 in the spin-liquid and high-magnetic-field regimes / Simon Streib. Betreuer: Peter Kopietz. Gutachter: Peter Kopietz ; Roser Valentí ; Götz Uhrig." Frankfurt am Main : Univ.-Bibliothek Frankfurt am Main, 2016. http://d-nb.info/108130667X/34.
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Wollny, Alexander. "Fractional Moments and Singular Field Response." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2017. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-219916.
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In this PhD thesis, the physics of vacancies in two-dimensional ordered Heisenberg antiferromagnets is investigated. We use semi-classical methods to study the influence of a single vacancy in long-range ordered states, with a focus on non-collinear order. Here, on a classical level, a magnetic distortion is created as the spins readjust in response to the vacancy.
We use the non-collinear $120^\\circ$ state on the frustrated triangular lattice as an example, where we determine the impurity contributions to the magnetization and susceptibility. An important discovery is the vacancy moment not being quantized due to non-universal partial screening. The resulting effective moment $m_0 \\ll S$ can be observed as a fractional prefactor to an impurity-induced Curie response $m_0^2/(3k_BT)$ at finite temperature. This is in sharp contrast to collinearly ordered states. Here the moment is always quantized to the bulk spin value, $m_0=S$.
Furthermore, we present a detailed analysis of the vacancy-induced distortion cloud. Due to Goldstone modes, it decays algebraically as $r^{-3}$ with distance $r$ to the vacancy. Using leading-order $1/S$-expansion, we determine the quantum corrections to both size and direction of the distorted magnetic moments.
Secondly, we study the same problem in the presence of an external magnetic field $h$, both for the square and triangular lattice. For the triangular lattice we use a biquadratic exchange term $K$ to stabilize a unique ground state from a degenerate manifold. The finite-field vacancy moment $m(h)$ is generated by field-dependent screening clouds, as different non-collinear bulk states evolve with increasing field. These distortion clouds decay exponentially on a magnetic length scale $l_h\\propto 1/h$. Most importantly, we find that the magnetic-field linear-response limit $h \\rightarrow 0^+$ is generically singular for $SU(2)$ ordered local-moment antiferromagnets, as the vacancy moment in zero field differs fundamentally from even an infinitesimal but finite field, $m(h \\rightarrow 0^+)\\neq m_0$.
Moreover, a part of the screening cloud itself becomes universally singular. Particularly for spin-flop states, this leads to a semi-classical version of perfect screening. We present general arguments to support these claims, as well as microscopic calculations. Another remarkable result is an impurity-induced quantum phase transition for overcompensated vacancies in the $M=1/3$ plateau phase on the triangular lattice with $K<0$. We close our analysis with a discussion about important limits for finite vacancy concentrations, as well as a possible experimental verification of our predictions.
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Sinclair, John. "Novel antiferromagnetic materials." Thesis, University of York, 2018. http://etheses.whiterose.ac.uk/21962/.
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There is intense interest in new antiferromagnetic materials due to the development of antiferromagnetic spintronics. Currently, the material IrMn is used, however, Iridium is one of the scarcest and most expensive elements. In this work, a number of novel, thin film antiferromagnetic materials were produced using sputter deposition and then assessed using magnetic and structural characterisation techniques as well as a temperature dependent resistivity technique developed during the project.
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Huminiuc, Teodor. "Novel antiferromagnets for spintronic devices." Thesis, University of York, 2017. http://etheses.whiterose.ac.uk/18864/.
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Spin electronic or spintronic devices which are used in hard disk drive (HDD) read heads are expected to replace the current silicon based transistor technology used in volatile memories. A prime example for the net advantage of employing spin rather than electric charge manipulation is found in the newly developed magnetic random access memory (MRAM) which is proposed as a replacement for the dynamic random access memory (DRAM) based on three terminal metal-oxide-semiconductor (MOS) devices. Besides the decrease of energy consumption by a factor three arising from manipulating electron angular momentum, the magnetic memories are non-volatile hence they do not require constant power to store information. This allows for additional energy saving due to data stability when the storage device is powered off.
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Shirai, Masae. "Experimental investigations of frustrated antiferromagnets." Thesis, University College London (University of London), 2007. http://discovery.ucl.ac.uk/1446113/.
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This thesis describes the investigation of frustrated magnetic systems, focusing on the pyro-chlore antiferromagnets (AFM). The rare-earth pyrochlore oxides R2B2Oj {R+ = rare earth, = transition metal), crystallizes in the Fd3m space group, with the magnetic ions (R) and the non-magnetic transition metal (B) ions situated on the 16c and on the 6d sites, respectively. As a result, the magnetic ions form lattices of corner-sharing tetrahedra-a stage in which frustration is manifested. Erbium titanate Er2Ti2C > 7 and stannate ESO are both frustrated pyrochlore antiferromagnets which are expected to have very similar magnetic properties. However, Er2Ti207 magnetically orders at 7n 1.1 K but Er2Sn207 remains disordered down to F 150 mK. The main aim of this thesis is therefore to investigate the origin of this difference. The magnetic properties of the series Er2Ti2.jrSn.xO7 were studied as a function of jc (jc = 0 2). Firstly, from the powder neutron diffraction patterns obtained, it was shown that the detected order diminishes as jc * 2. However, in contrast to the previously reported result, a long-ranged ordering transition was detected for Er2Ti207 (at 80 mK) as well as for Er2Ti2 < 37 (at 1.2 K). The ordered magnetic structure was determined while Er2Ti207 orders into the Champion state (one of the ground state of the pyrochlore (lll)-XY AFM), Er2Sn207 orders into a ground state of a Heisenberg pyrochlore AFM with dipolar interaction (Palmer-Chalker state). Secondly, the existence of the quantum critical point (QCP) in the composition range 1.5 < x < 2 was investigated by means of low temperature magnetization measurements. The critical composition jccrjticai 1-55 was determined. Thirdly, the single ion anisotropy of the series ETiSnxO (x = 0 2) was studied by analyzing the crystal electric field (CEF) measured by the inelastic neutron scattering. As a result, the single-ion moments were shown to retain a constant size ( 3 Ub) throughout the series. In contrast, the nature of the single ion anisotropy changed from being XY-like for jc = 0 into being Ising-like for jc = 2 (with respect to the < 111)- axes). Finally, the temperature-field phase diagram of the experimental realization of the Heisenberg pyrochlore AFM -Gd2Ti207, was investigated by means of powder neutron diffraction.
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Ulloa, Osorio Camilo Edgardo. "Aspects of antiferromagnetic spintronics." Tesis, Universidad de Chile, 2016. http://repositorio.uchile.cl/handle/2250/140609.
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Magíster en Ciencias, Mención FísicaLa spintrónica se perfila como una de las corrientes mas atractivas y prometedoras dentro de la materia condensada gracias a la diversidad de fenómenos presentes, como el efecto Hall de spin, la magneto-resistencia gigante. En la spintrónica el estudio de materiales antiferromagnéticos es interesante pues dentro de sus propiedades se encuentran su abundancia natural y la posibilidad de disminuir las escalas temporal y espacial de los fenómenos presentes en ellos. Un ejemplo es la utilización de estos materiales en memorias magnéticas, pues gracias a la ausencia de magnetización neta en un material antiferromagnético es posible almacenar información en regiones de menor tamaño debido a la nula interacción dipolar entre dominios magnéticos. Esta tesis esté compuesta de tres trabajos teóricos orientados al desarrollo de la spintrónica antiferromagnética. En la primera parte se presenta la teoría efectiva de un sistema antiferromagnético no colineal. Para esto consideramos un sistema anisotrópico y con interacción de intercambio entre spines vecinos. A través de un parámetro de orden perteneciente al grupo de rotaciones estudiamos la dinámica de las excitaciones de baja energía del sistema obteniendo como resultado una familia de solitones topológicos que están descritos por la ecuación de sine-Gordon. Finalmente comparamos nuestros resultados con simulaciones numéricas de un sistema de momentos magnéticos obteniendo resultados completamente concordantes. La segunda parte corresponde al estudio de un cristal magnónico antiferromagnético. A partir de una teoría fenomenológica estudiamos la dinámica del campo de magnetización bajo el efecto de interacción de intercambio, y anisotropía uniaxial. A través de una modulación periódica de la anisotropía y del campo magnético caracterizamos el espectro de ondas de spín y las estructura de bandas del sistema. En la tercera y última parte se presenta el estudio de la generación de corrientes de spin mediante deformaciones de una red antiferromagnética gracias a efectos cuánticos. Este fenómeno, conocido como efecto piezospintrónico, es estudiado en dos modelos de interés: grafeno antiferromagnético y zinc-blende antiferromagnético. Este efecto, en conjunto con el efecto Hall de spín inverso pueden ser útiles para la detección de corrientes de spín puras.
Spintronics is one of the most attractive and promising areas in condensed matter due to the diversity of phenomena present in it as the spin Hall e ect and the giant magnetoresistance. In spintronics the study of antiferromagnetic materials is interesting due to their natural abundance and the possibility of decreasing the temporal and spatial scale of the phenomena in which they are involved. One example of this is the use of antiferromagnetic materials in magnetic memories, where due to the absence of net magnetization it is possible to store information in smaller regions because of the null dipolar interaction between domains. This thesis is made of three theoretical works focused in di erent aspects of antiferromagnetic spintronics. In the rst chapter we present the e ective theory of a non collinear antiferromagnet. For this we consider an anisotropic system with exchange interaction among neighbor spins. By making use of an order parameter in the rotation group we study the dynamics of low energy excitations of the system obtaining as result a family of topological solitons which are described by the sine-Gordon equation. Finally we compare our results with numerical simulations of a system of magnetic moments obtaining totally concordant results. The second chapter corresponds to the study of an antiferromagnetic magnonic crystal. From a phenomenological theory we study the dynamics of the magnetization eld under the e ect of exchange interaction and uniaxial anisotropy. Through a periodic modulation of the anisotropy and of the magnetic eld we characterize the spin wave spectra and the band structure of the system. In the third and last chapter we show the study of generation of spin currents by deformation of an antiferromagnetic lattice thanks to quantum mechanical e ects. This phenomenon, known as piezospintronic e ect, is studied in two interesting models: antiferromagnetic graphene and antiferromagnetic zinc-blende. This e ect together with the inverse spin Hall e ect could be useful for the detection of pure spin currents. v
Este trabajo ha sido parcialmente financiado por Proyecto Fondecyt N° 1150072, Proyecto Basal N° FB0807- CEDENNA, y Anillo de Ciencia y Tecnología N° ACT 1117
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Meschke, Matthias. "Untersuchung der magnetischen Eigenschaften kubischer Antiferromagnete." [S.l.] : [s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=961045450.
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Velkov, Hristo [Verfasser]. "Skyrmion dynamics in antiferromagnets / Hristo Velkov." Mainz : Universitätsbibliothek Mainz, 2018. http://d-nb.info/1160488932/34.
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Dupont, Maxime. "Dynamics and disorder in quantum antiferromagnets." Thesis, Toulouse 3, 2018. http://www.theses.fr/2018TOU30092/document.
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La physique de la matière condensée, et notamment les systèmes fortement corrélés, amènent à des problèmes parmi les plus stimulants et difficiles de la physique moderne. Dans ces systèmes, les interactions à plusieurs corps et les corrélations entre les particules quantiques ne peuvent être négligées, sinon, les modèles échoueraient simplement à capturer les mécanismes physiques en jeu et les phénomènes qui en découlent. En particulier, le travail présenté dans ce manuscrit traite du magnétisme quantique et aborde plusieurs questions distinctes à l'aide d'approches computationnelles et méthodes numériques à l'état de l'art. Les effets conjoints du désordre (i.e. impuretés) et des interactions sont étudiés concernant un matériau magnétique spécifique : plutôt qu'une phase de la matière dite localisée, attendue à fort champ magnétique, une phase ordonnée induite par le désordre lui-même est mise en lumière, avec une réapparition inattendue de la cohérence quantique dans ledit composé. Par ailleurs, la réponse dynamique d'aimants quantiques à une perturbation externe, comme celle mesurée dans des expériences de résonance magnétique nucléaire ou de diffusion inélastique de neutrons est étudiéeCondensed matter physics, and especially strongly correlated systems provide some of the most challenging problems of modern physics. In these systems, the many-body interactions and correlations between quantum particles cannot be neglected; otherwise, the models would simply fail to capture the relevant physics at play and phenomena ensuing. In particular, the work presented in this manuscript deals with quantum magnetism and addresses several distinct questions through computational approaches and state-of-the-art numerical methods. The interplay between disorder (i.e. impurities) and interactions is studied regarding a specific magnetic compound, where instead of the expected many-body localized phase at high magnetic fields, a novel disorder-induced ordered state of matter is found, with a resurgence of quantum coherence. Furthermore, the dynamical response of quantum magnets to an external perturbation, such as it is accessed and measured in nuclear magnetic resonance and inelastic neutron scattering experiments is investigated
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Saunders, Timothy E. "Phase transitions in geometrically frustrated antiferromagnets." Thesis, University of Oxford, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.670121.
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Deisz, John James. "Dynamical properties of one-dimensional antiferromagnets /." The Ohio State University, 1991. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487688507501641.
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Yu, Sisheng. "Spin Dynamics in Antiferromagnetic Heterostructures." The Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1586599000240225.
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Lake, Alysia C. I. "Neutron scattering studies of alternating chain antiferromagnets." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360395.
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Dinóla, Isabel Cristina Souza. "Super Antiferromagneto de Ising com campo uniforme." Universidade Federal do Amazonas, 2009. http://tede.ufam.edu.br/handle/tede/4543.
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The phase diagram of the two-dimensional super-antiferromagnetic (SAF) Ising model in the presence of a magnetic field is investigated within the framework of a real-space renormalization-group approximation. We consider nearest neighbor ferromagnetic interactions along the x(y) direction and antiferromagnetic interactions in the y(x) direction. The system presents a ordered phase at low temperatures and zero fields. The presence of a magnetic field induces a competition between the energy interactions of the SAF Hamiltonian. The resulting behavior has been a matter of controversy in the last years. We depicted the main results in the magnetic field versus temperature phase diagram. A second-order transition line separates a super-antiferromagnetic phase from a field induced ferromagnetic phase. Our study reveals that the magnetic field induces a phase transition at a single temperature value, thus, we did not find any evidence of reentrant behavior as claimed by some authors.
Utilizamos uma técnica de grupo de renormalização no espaço real para estudar o sistema super antiferromagneto (SAF) de Ising bidimensional sob a influência de um campo magnético externo. Neste modelo as interações de primeiros vizinhos na direção x são ferromagnéticas e na direção y são antiferromagnéticas. Este sistema apresenta uma fase ordenada, para baixas temperaturas e campos nulos, com uma estrutura de linhas ferromagnéticas e colunas antiferromagnéticas. A aplicação do campo magnético induz uma competição entre as energias de interação do modelo e o comportamento resultante desta competição tem sido objeto de estudo e gerado algumas controvérsias nos últimos anos. Na presença do campo magnético observa-se, além da fase SAF, a fase ferromagnética induzida pelo campo (FIC). Apresentamos neste trabalho o diagrama de fases completo do sistema SAF no plano temperatura versus campo magnético. O diagrama de fases obtido mostra uma linha de transição de segunda ordem separando a fase SAF da fase FIC. Nossos resultados contrariam resultados anteriores que preveêm um comportamento reentrante no diagrama de fases do sistema SAF.
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Panigrahy, Sujit. "Magnetic skyrmions in cobalt-based synthetic antiferromagnets." Electronic Thesis or Diss., université Paris-Saclay, 2023. http://www.theses.fr/2023UPASP187.
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Les skyrmions magnétiques sont des textures topologiquement protégées qui se comportent comme des particules et qui pourraient être utilisées dans de futurs dispositifs de spintronique. Dans les ferromagnétiques, leur vitesse induite par le courant électrique est limitée et ils souffrent d'une déviation liée à leur topologie non triviale. Dans ce travail, nous explorons leurs propriétés dans les antiferromagnétiques synthétiques (SAF), qui ont été proposés pour surmonter ces limites. A l'aide de simulations et de modèles, nous montrons que ces matériaux sont en effet prometteurs et confèrent aux skyrmions de nouvelles propriétés, comme une masse. Ensuite, dans deux chapitres expérimentaux, nous montrons comment concevoir des échantillons appropriés. Nous développons d'abord une méthodologie pour surmonter l'application d'un champ magnétique externe qui est généralement nécessaire dans les films ferromagnétiques, mais dont l'application serait sans effet dans les antiferromagnétiques. Enfin, nous démontrons le mouvement de skyrmions induit par le courant dans des antiferromagnétiques synthétiques, et nous montrons des vitesses cinq fois plus grandes que dans les ferromagnétiques ainsi qu'un mouvement sans déviation. Les résultats obtenus sont à la fois fondamentalement et technologiquement pertinents pour les dispositifs skyrmioniques basés sur les SAF au-delà des systèmes ferromagnétiques conventionnelsMagnetic skyrmions are topologically protected textures behaving as particles, and that could be used in future spintronic devices. In ferromagnets, their velocity induced by electrical current is limited and they suffer from a deflection related to their non-trivial topology. In this work, we explore their properties in synthetic antiferromagnets (SAF), that have been proposed to overcome these limits. Using simulations and models, we show that such materials are indeed promising and give to skyrmions new properties, like a mass. Then, in two experimental chapters, we show how to design suitable samples. We first develope a methodology to overcome the application of an external magnetic field that is usually required in ferromagnetic films, but whose application would not be efficient in antiferromagnets. Then, we finally demonstrate the current induced skyrmion motion in synthetic antiferromagnets, and show velocities five time larger than in ferromagnets and deflectionless motion. The results obtained hence are both fundamentally and technologically relevant for SAF based skyrmionics devices beyond the conventional ferromagnetic systems
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Ritchey, Ian. "Highly correlated spin systems in two dimensions." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239597.
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Brehmer, Sven. "Niederenergetische Anregungen in eindimensionalen Quanten-Antiferromagneten." [S.l. : s.n.], 1998. http://deposit.ddb.de/cgi-bin/dokserv?idn=955900182.
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Trzeciecki, Mikołaj. "Second harmonic generation from antiferromagnetic interfaces." [S.l. : s.n.], 2000. http://deposit.ddb.de/cgi-bin/dokserv?idn=96147792X.
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