Добірка наукової літератури з теми "Magnetic materials with perpendicular magnetic anisotropy"

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (p. 34-35).<br>Co/Ni multilayers display perpendicular magnetic anisotropy and have applications in magnetic devices that could lead to a large increase in the density of magnetic storage. Co/Ni 10-(2 Å Co/ 8Å Ni) and 10-(2 Å Co/ 4 Å Ni) multilayers were deposited with ion beam sputtering on either ion beam sputtered copper or direct current magnetron sputtered gold buffer layers of various thicknesses. The effect of the the roughness and the degree of (1 1 1) texture of the buffer layers and the multilayers on the perpendicular magnetic anisotropy of the deposited multilayers was examined. In addition the effect of the deposition method used to fabricate the samples, ion beam sputtering, was analyzed. The magnetic behavior of the multilayers was examined with alternating gradient magnetometry and vibrating sample magnetometery, the structure of the buffer layers and the multilayers was characterized with X-ray diffraction, and the roughness of the surface of the multilayers was characterized with atomic force microscopy. None of the deposited films showed perpendicular magnetic anisotropy and instead showed parallel magnetic anisotropy which was found to have occurred for every sample due to either a low degree of (1 1 1) texture in the buffer layer and the Co/Ni multilayer, a too high degree of roughness in the buffer layer and the Co/Ni multilayer or a combination of these two factors. In addition it was hypothesized that as the samples were deposited with sputtering, diffusion and alloying at the multilayer interfaces may have contributed to the multilayers having parallel magnetic anisotropy instead of perpendicular magnetic anisotropy.<br>by Boris Rasin.<br>S.B.

Les effets de transfert de spin sont devenus un sujet de recherche majeur ces quinze dernières années. Cependant, un manque de vérifications expérimentales pour beaucoup de modèles décrivant les effets de transfert de spin peut être constaté. Ceci est surtout lié à un manque de systèmes magnétiques modèles permettant un contrôle précis des paramètres pertinents utilisés dans les modèles théoriques. Dans ce travail deux systèmes magnétiques à aimantation perpendiculaire ont été analysés : les alliages amorphes de Co1-xTbx élaborés par pulvérisation cathodique et les super-réseaux [Co/Ni](111) élaborés par épitaxie par jets moléculaires. L'anisotropie et l'aimantation, qui sont des paramètres pertinents dans beaucoup de modèles sur le transfert de spin, sont variables dans une large gamme. L'origine de cette anisotropie est discutée. La structure des domaines magnétiques est analysée et les résultats des mesures de transport sont interprétés. Pour les super-réseaux [Co/Ni](111) une forte polarisation en spin au niveau de Fermi est démontrée grâce à des expériences de photo émission résolue en spin et un coefficient d'amortissement intrinsèque [alpha] très faible est trouvé. Il est conclu que les alliages amorphes de Co1-xTbx et les super-réseaux [Co/Ni](111) sont des systèmes modèles pour le transfert de spin<br>Spin transfer torque effects have become a research subject of high interest during the last 15 years. However, in order to probe the fundamental physics of spin transfer torque model systems are needed. For a model system it must be as simple as possible to tune the significant parameters (magnetic and structural). In this work we analyze the suitability of two materials for this need. The studied materials are amorphous Co1-xTbx alloys elaborated by sputtering and MBE grown [Co/Ni](111) superlattices. Both systems show perpendicular magnetic anisotropy (PMA), which provides a uniaxial anisotropy to the system. This anisotropy and the magnetization, which are significant parameters for many models on spin transfer torque, can be tuned in a large range of values. The origin of this PMA is discussed. The domain structure is analyzed and transport measurements are interpreted. In addition we show a strong spin polarization of the electrons close to the Fermi level by doing photoemission experiments. A small intrinsic Gilbert damping parameter [alpha] is found by FMR spectroscopy. We conclude that both materials are good candidates to be used as model systems for spin transfer torque

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.<br>This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.<br>Cataloged from student-submitted PDF version of thesis.<br>Includes bibliographical references (pages 57-58).<br>CoCrPt has potential applications as a memory storage technology because of its perpendicular magnetic anisotropy (PMA) characteristics. An underlayer can be used to ensure the out-of-plane magnetization required for PMA functionalities. Ti, with a lattice constant of a = 2.95 Å can be used to encourage uniaxial c-axis growth in CoCrPt (lattice constant a ~/= 2.55 Å, dependent on exact composition). In this report, varying thicknesses of Ti (t = 0, 20, 40, 60, 70, 80, 90, 100nm) and CoCrPt (t = 50, 75, 90, 100, 125, 150nm) were sputtered onto naturally oxidized silicon substrates. Using various characterization methods, these films were investigated in order to better understand the system. The exact composition of the CoCrPt films was found to be approximately Co₆₀.₂Cr₁₆.₄Pt₂₃.₄, with a Curie temperature of about 600 °C. The addition of a Ti underlayer resulted in an increase in coercivity to approximately 1250 Oe for t > 60nm. However, switching field distribution and saturation magnetization appear to be independent of underlayer thickness. All samples show evidence of out-of-plane growth and the roughness of the films increases until it also plateaus at about t = 60nm. When CoCrPt thickness is varied on a constant Ti underlayer, the PMA properties of the materials decrease with increasing thickness due to increased disorder and potential relaxation of the lattice in thicker films. The switching field distribution shows a significant increase, implying that a thicker film has a more homogenous distribution of grain sizes. XRD peaks confirm out-of-plane growth and suggest a trend of increasing c lattice constant as the thickness of the film increases.<br>by Margaret Marie Kane.<br>S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (pages 155-168).<br>Magnetic based devices such as hard disk drives (HDDs) are widely used in the computer industry because of their high memory capacity, non-volatility and low cost compared to semiconductor-based solid state disk drives (SSDs). However, they also suffer from low energy efficiency and low speed, due to the requirement for mechanical motion in order to access the data. In my thesis, I will first give a brief introduction to the motivation and background in the study of magnetic domain walls (DWs), which have attracted great attention due to their ability to be moved by field and/or current and corresponding potential applications in high speed memory or logic devices. I will then discuss how to geometrically control the behaviors of DWs in a ferromagnetic nanowire. I will first discuss how natural geometry distortions such as edge tapering from sputtering on an undercut resist profile and wire width variation from the patterning process would affect DW behavior, including static configurations, stability and dynamics under current pulsing. I will then discuss how similar geometrical effects will affect the properties of materials with perpendicular magnetic anisotropy (PMA). The same geometry modulation will have different effects depending on the origin of the PMA. Such results are confirmed by observing the magnetic reversal process. Besides the study on 180DWs, we will then discuss the field and current effects on 360 degree DWs (360DWs), which have many unique properties compared to 180DWs and are an alternative candidate for DW based devices. I will then discuss control of 360DW behavior by designing a geometrical heterostructure. We have found that by utilizing the asymmetric Oersted field originated from the heterostructure, we are able to control the 360DWs depending on their chirality. The structure can function as a 360DW chirality filter, which provides extra freedom in DW-based applications. These studies were conducted by a combination of micromagnetic simulations and experimental implementations. Techniques being used including OOMMF micromagnetic simulations, Comsolfinite element simulations, electrical measurements, magnetic force microscopy and other characterization techniques.<br>by Jinshuo Zhang.<br>Ph. D.

Cette thèse est consacrée à l’étude des mécanismes de propagation de parois magnétiques dans des films et des pistes magnétiques basés sur des matériaux à anisotropie magnétique perpendiculaire qui sont très prometteurs pour les mémoires magnétiques non volatiles d’ultra haute densité. Je me suis principalement intéressé à l’influence des défauts structuraux sur les mécanismes de dépiegeage de parois en utilisant la technique de microscopie Kerr ainsi que des mesures de transport. Trois résultats importants ont été mis en évidence : (1) Dans des vannes de spin de type CoNi/Cu/CoNi, il existe une forte influence du champ dipolaire généré par la couche dure qui peut influencer la nucléation parasite de paroi magnétique dans la couche libre et créer une propagation asymétrique sous l’effet d’un courant polarisé. J’ai aussi montré que dans des pistes sub-50nm, le renversement de l’aimantation s’effectue par des événements multiples de nucléation à cause de la présence de centres de piégeage fort qui bloquent la propagation ; (2) En visualisant la géométrie des domaines magnétiques et en étudiant les lois de reptation, j’ai montré la présence d’une faible densité de défauts structuraux et de faibles champs de propagation dans les multicouches texturés/amorphe de CoNi-CoFeB et cristallisés de Ta-CoFeB-MgO ; (3) J’ai finalement mis en évidence un effet du transfert de spin à de faibles densités de courant (≈5x1011 A/cm2) dans les pistes de CoNi-CoFeB. J’ai aussi montré une forte influence du champ d’Oesterd sur la propagation de parois liée à la présence de faibles champs de propagation. Finalement, dans le cas des pistes basées sur des films cristallisés de Ta-CoFeB-MgO, j’ai pu mesurer la vitesse sur 10 ordres de grandeur et montrer que les parois se propagent à des champs de propagation ultra faibles (0,1mT)<br>This work is focused on the study of magnetic domain wall propagation mechanisms in the thin films and wires based on materials with perpendicular magnetic anisotropy which are promissing for the non-volatile magnetic memory of ultra high density. I’m interested in the influence of structural defects on the mechanisms of domain wall propagation by using the Kerr microscopy technique and the transport measurements. Three important results were obtained: (1) In the spin valve structure of CoNi/Cu/CoNi, a strong influence of the dipolar magnetic field induced by the hard layer can generate a parasitic nucleation in the soft layer and create an asymmetric domain wall propagation driven by a spin polarized current. I also demonstrated that in sub-50nm wires, the nature of magnetization reversal process is the multiple nucleation events because of strong pinning centers that hinder the domain wall motion; (2) By observing the magnetic domain geometry et studying the creep law, I have pointed out that in the CoNi-CoFeB multilayers and the crystallized Ta-CoFeB-MgO multilayers, the structural defect density is low and the propagation fields can be reduced; (3) I found a spin-transfer effect with low current density (≈5x1011 A/cm2) in CoNi-CoFeB wires. I also demonstrated that the Oersted field can strongly influence the domain wall motion, especially in the material with low propagation field. Finally, in the Ta-CoFeB-MgO wires, I could measure a wide range of domain wall velocity and I show that the domain wall can move at a very low propagation field (0.1mT)

La possibilité de synthétiser des hétérostructures formées de matériaux 2D offre des perspectives majeures pour l'amélioration des composants spintroniques actuels ou la réalisation de nouveaux dispositifs. Le contrôle et la bonne compréhension des propriétés physiques de ces systèmes constituent de fait un enjeu technologique majeur. Au cours de cette thèse, nous avons étudié, à l'aide de calculs ab initio basés sur la théorie de la fonctionnelle de la densité (DFT), des hétérostructures formées de monocouches de dichalcogénures de métaux de transition (TMDCs) et de cristaux ferromagnétiques présentant une anisotropie magnétique perpendiculaire. Trois objectifs principaux ont été définis : (i) comprendre comment utiliser la proximité magnétique pour lever la dégénérescence des vallées et quantifier l'effet Zeeman des vallées ; (ii) évaluer la possibilité d'injecter un gaz d'électrons polarisé en spin dans des vallées spécifiques du feuillet de TMDC ; (iii) examiner l'impact de la proximité sur le couplage spin-orbite dans le feuillet de TMDC et sur les phénomènes Rashba et Dresselhaus dans ces systèmes. Nous avons d'abord étudié des multicouches possédant une électrode constituée d'un métal et d'une barrière isolante non 2D. Dans le système Fe/MgO/MoS2, nous avons calculé qu'un transfert d'électrons spontané s'opère de la couche de Fe vers le monofeuillet de MoS2, donnant lieu à la formation d'un gaz d'électrons non polarisé en spin. Nous avons établi un modèle expliquant la compétition entre les effets spin-orbite de type Rashba et Dresselhaus et les effets de proximité magnétique sur les bandes de valence de MoS2 : Ce modèle nous a permis de montrer que les effets de proximité sont prédominants pour une faible épaisseur de MgO (&lt;0.42 nm), et tendent à disparaître au profit des effets spin-orbite pour à plus forte épaisseur (&gt; 1.06 nm). Nous avons prédit qu'il est possible d'obtenir des effets spin-orbites plus forts en remplaçant l'électrode de Fe par une électrode non-magnétique de V. Afin d'augmenter les effets de proximité magnétique, nous avons finalement décider d'étudier des hétérostructures [Co1Ni2]n/h-BN/WSe2, dans lesquelles [Co1Ni2]n est un super réseau à anisotropie magnétique perpendiculaire et h-BN un isolant bidimensionnel. Pour ce système, nous prédisons qu'il serait possible d'avoir une polarisation en spin des vallées aux points K et K'. Finalement, nous avons étudié les propriétés particulières de l'hétérostructure de van der Waals Graphène/CrI3/WSe2,dans laquelle l'électrode magnétique est également remplacée par des matériaux 2D<br>The ability to synthesize heterostructures made up of 2D materials provides significant opportunities for improving current spintronic components or developing new devices. Thus, the control and deep understanding of the physical properties of these systems become a critical technological challenge. During this thesis, we examined heterostructures composed of transition metal dichalcogenide (TMDC) monolayers and ferromagnetic crystals exhibiting perpendicular magnetic anisotropy, using ab initio calculations based on density functional theory (DFT). We focus on three main goals: (i) understanding how to use magnetic proximity to lift valley degeneracy and quantify the valley Zeeman effect; (ii) assessing the possibility of injecting spin-polarized electron gas into specific valleys of the TMDC sheet; (iii) investigating the impact of proximity on spin-orbit coupling in the TMDC sheet and on the Rashba and Dresselhaus phenomena in these systems. We first studied multilayers with an electrode made up of a metal and a non-2D insulating barrier. In the Fe/MgO/MoS2 system, we computed that a spontaneous electron transfer occurs from the Fe layer to the MoS2 monolayer, leading to the formation of a non-spin-polarized electron gas. We established a model explaining the competition between Rashba and Dresselhaus-type spin-orbit effects and magnetic proximity effect on the MoS2 valence bands: This model allowed us to show that proximity effect predominate for thin MgO (&lt;0.42 nm) and tend to disappear in favor of spin-orbit effects for thicker layers (&gt; 1.06 nm). We predicted that stronger spin-orbit effects can be achieved by replacing the Fe electrode with a non-magnetic V electrode. To boost the magnetic proximity effects, we finally decided to study [Co1Ni2]n/h-BN/WSe2 heterostructures, in which [Co1Ni2]n is a superlattice with perpendicular magnetic anisotropy, and h-BN is a two-dimensional insulator. For this system, we predict that it could be possible to have a spin polarization of the valleys at the K and K' points. Ultimately, we explored the unique properties of the van der Waals heterostructure Graphene/CrI3/WSe2, where the magnetic electrode is also replaced by 2D materials