Spin dynamics in exchange-biased F/AF bilayers

The spin dynamics of the ferromagnetic pinned layer of ferro-antiferromagnetic coupled NiFe/MnNi bilayers is investigated in a broad frequency range (30 MHz-6 GHz). A phenomenological model based on the Landau-Lifshitz equation for the complex permeability of the F/AF bilayer is proposed. The experimental results are compared to theoretical predictions.Comment: 12 pages, 3 figures, 1 tabl

Magnetization reversal and spin dynamics exchange in biased F/AF bilayers probed with complex permeability spectra

The spin dynamics of the ferromagnetic pinned layer of ferro-antiferromagnetic coupled NiFe/MnNi bilayers is investigated in a broad frequency range (30 MHz-6 GHz). A phenomenological model based on the Landau-Lifshitz equation for the complex permeability of the F/AF bilayer is proposed. The experimental results are compared to theoretical predictions. We show that the resonance frequencies, measured during the magnetization, are likewise hysteretic.Comment: 4 pages, 4 figure

Asymmetric magnetization reversal in exchange biased polycrystalline F/AF bilayers

This paper describes a model for magnetization reversal in polycrystalline Ferromagnetic/Antiferromagnetic exchange biased bilayers. We assume that the exchange energy can be expanded into cosine power series. We show that it is possible to fit experimental asymmetric shape of hysteresis loops in exchange biased bilayer for any direction of the applied field. The hysteresis asymmetry is discussed in terms of energy considerations. An angle beta is introduced to quantify the easy axis dispersion of AF grains.Comment: 15 pages, 4 figure

Thermal simulation of magnetization reversals for size-distributed assemblies of core-shell exchange biased nanoparticles

A temperature dependent coherent magnetization reversal model is proposed for size-distributed assemblies of ferromagnetic nanoparticles and ferromagnetic-antiferromagnetic core-shell nanoparticles. The nanoparticles are assumed to be of uniaxial anisotropy and all aligned along their easy axis. The thermal dependence is included by considering thermal fluctuations, implemented via the N\'eel-Arrhenius theory. Thermal and angular dependence of magnetization reversal loops, coercive field and exchange-bias field are obtained, showing that F-AF size-distributed exchange-coupled nanoparticles exhibit temperature-dependent asymmetric magnetization reversal. Also, non-monotonic evolutions of He and Hc with T are demonstrated. The angular dependence of Hc with T exhibits a complex behavior, with the presence of an apex, whose position and amplitude are strongly T dependent. The angular dependence of He with T exhibits complex behaviors, which depends on the AF anisotropy and exchange coupling. The resulting angular behavior demonstrates the key role of the size distribution and temperature in the magnetic response of nanoparticles.Comment: Revised arguments in Introduction and last sectio

FMR studies of exchange-coupled multiferroic polycrystalline Pt/BiFeO3_3/Ni81_{81}Fe19_{19}/Pt heterostructures

An experimental study of the in-plane azimuthal behaviour and frequency dependence of the ferromagnetic resonance field and the resonance linewidth as a function of BiFeO$_3$ thickness is carried out in a polycrystalline exchange-biased BiFeO$_3$/Ni$_{81}$Fe$_{19}$ system. The magnetization decrease of the Pt/BiFeO$_3$/Ni$_{81}$Fe$_{19}$/Pt heterostructures with BiFeO$_3$ thickness deduced from static measurements has been confirmed by dynamic investigations. Ferromagnetic resonance measurements have shown lower gyromagnetic ratio in a perpendicular geometry compared with that of a parallel geometry. The monotonous decrease of gyromagnetic ratio in a perpendicular geometry as a function of the BiFeO$_3$ film thickness seems to be related to the spin-orbit interactions due to the neighbouring Pt film at its interface with Ni$_{81}$Fe$_{19}$ film. The in-plane azimuthal shape of the total linewidth of the uniform mode shows isotropic behaviour that increases with BiFeO$_3$ thickness. The study of the frequency dependence of the resonance linewidth in a broad band of 3 to 35 GHz has allowed the determination of intrinsic and extrinsic contributions to the relaxation as function of BiFeO$_3$ thickness in perpendicular geometries. In our system the magnetic relaxation is dominated by the spin-pumping mechanism due to the presence of Pt. The insertion of BiFeO$_3$ between Pt and Ni$_{81}$Fe$_{19}$ attenuates the spin-pumping damping at one interface.Comment: paper accepted for publication in the Journal of Physics D: Applied Physic

Nano-structures ferromagnétique - anti-ferromagnétique pour applications en hyperfréquences

L'axe de recherche développé au cours de la thèse a particulièrement été focalisé sur l utilisation de matériaux nouveaux pour des applications en haut de bande HP. Afin de satisfaire de telles exigences, il convient de concevoir des matériaux ayant la particularité de posséder des valeurs de permittivité et de perméabilité élevées dans les fréquences micro-ondes comprises entre 1 et 20 gigahertz. Cette dernière propriété nécessite d élaborer des matériaux à anisotropie magnétique uniaxiale bien définie, à forte aimantation à saturation et à coercitivité faible, les deux premières propriétés étant difficilement compatibles. La motivation initiale de ce travail porte ainsi sur l élaboration, l optimisation et l étude de la dynamique de spin à haute fréquence de structures multicouches d épaisseur nanométrique de type ferromagnétique/isolant, avec les ferromagnétiques choisis NiFe et CoFeZr et les isolants de type diamagnétique Al203 et anti-ferromagnétique NiO. Le couplage avec la couche de NiO permet d induire une anisotropie unidirectionnelle. De plus, nous nous sommes particulièrement intéressés aux effets de dimensionnalité réduite sur les propriétés magnétiques dynamiques. Afin d élaborer ces matériaux, nous avons mené une étude systématique des paramètres magnétiques statiques et structuraux en fonction des conditions de dépôt. Cela nous a permis de sélectionner les conditions de croissance appropriées et de mettre en évidence la corrélation entre les propriétés structurales et magnétiques. Les performances dynamiques de ces matériaux (CoFeZr) les placent comme de bons candidats pour certaines applications RF nécessitant à la fois une perméabilité et des fréquences de résonance élevées. Les mesures systématiques conduites sur les deux structures à base de Py et de CoFeZr, nous ont permis de dégager, d une part, le rôle clé de l interface pour ces matériaux aux épaisseurs concernées, mais aussi d évaluer les différents termes d anisotropie présents et notamment un comportement remarquable de l anisotropie uniaxiale pour les systèmes Py/Al2O3 avec le basculement de l axe d anisotropie à une épaisseur critique de 5 nm. De plus, un traitement par analyse REM et l utilisation du modèle de Meiklejohn et Bean nous permettent de déterminer le désalignement entres les axes d anisotropie des systèmes couplés par échange, ce qui peut se révéler complexe selon d autre méthode. Dans l étude de la relaxation magnétique, nous avons utilisé les larges possibilités de la résonance ferromagnétique large bande afin de discriminer et de quantifier les différentes contributions à la relaxation des différents systèmes étudiés. Un point fort de cette étude de la relaxation magnétique est l observation et la quantification de l anisotropie de l amortissement intrinsèque (aG) induite et surtout proportionnelle au couplage d échange.The aim of the research developed during the thesis has been particularly focused on new materials for applications in high-band RF. To meet such requirements, it should be developed materials with hight permittivity and hight permeability (in the high microwave frequencies between 1 and 20 gigahertz). This last property requires to develop materials to well-defined uniaxial magnetic anisotropy, high saturation magnetization and low coercivity, the first two properties are hardly compatible. The initial motivation of this work involves the development, optimisation and the study of spin dynamics at high frequency multilayer structures with nanometer-thick type ferromagnet / insulator with ferromagnetic NiFe and selected CoFeZr and insulators diamagnetic type Al203 and antiferromagnetic NiO. Coupling with the NiO layer can induce a unidirectional anisotropy. In addition, we focused on the effects of reduced dimensionality on the dynamic magnetic properties. To elaborate these materials, we conducted a systematic study of static magnetic and structural parameters based on the deposition conditions. This allowed us to select the appropriate growth conditions and to highlight the correlation between the structural and magnetic properties. The dynamic performance of these materials (CoFeZr) are interesting for RF applications requiring both permeability and higher resonance frequencies. Routine measurements conducted on the two structures based on Py and CoFeZr allowed us to identify the one hand, the key role of the interface for these materials with thicknesses involved, but also to evaluate the different terms anisotropy including a noteworthy behavior of the uniaxial anisotropy for systems with Py/A1203, the tilting of the anisotropy axis at critical thickness around 5 nm. Additionally treatment by RFM analysis and use of Meiklejohn and Beau model we can determine the misalignment between the axes of anisotropy in exchange coupled systems, this may be complicated by other method. In the study of magnetic relaxation, we used appropriate method to broadband RFM to discriminate and quantify relaxation s contributions of different systems. A important point of this study of relaxation magnetic is observation and quantification of the anisotropy of the intrinsic damping (aG), mainly proportional to the exchange coupling.BREST-BU Droit-Sciences-Sports (290192103) / SudocSudocFranceF

Spin dynamics in exchange-biased F/AF bilayers

12 pages, 3 figures, 1 tableThe spin dynamics of the ferromagnetic pinned layer of ferro-antiferromagnetic coupled NiFe/MnNi bilayers is investigated in a broad frequency range (30 MHz-6 GHz). A phenomenological model based on the Landau-Lifshitz equation for the complex permeability of the F/AF bilayer is proposed. The experimental results are compared to theoretical predictions