Substrate polarization effects in two-dimensional magnetic arrays

The magnetostatic energy of a two-dimensional (2D) periodic array of magnetic particles (or a thin film with periodic magnetization) is evaluated, including additional energy terms due to a polarizable substrate. The polarization of the substrate is solved self-consistently using surface charges. This requires describing the magnetic potential of the 2D array in terms of an equivalent surface charge distribution. Analytic expressions for the magnetostatic self-energy of the 2D array as well as the energy due to the interaction of the magnetic structure and polarizable substrate are presented. It is shown how substrates with large susceptibility significantly alter the stray-field energy and, hence, the magnetic properties of the array, even promoting a spin-reorientation transition. Our results suggest that system properties can be controlled in a simple way by exploiting substrates with tunable polarizabilityWe acknowledge funding by Consolider-Ingenio en Nanociencia Molecular Ref. No. CSD2007-00010, by the Comunidad de Madrid through Project No. S2009/MAT-1726, and Project No. FIS 2010-18847 from MICIN

Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films

We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e. (001), (110) and (1-18). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, the magnetic properties of the LSMO films can be interpreted with just the uniaxial term with the anisotropy axis given by the film morphology, whereas the strength of this anisotropy depends on both structure and film thickness. In particular, LSMO films grown on nominally flat (110)-oriented STO substrates presents a well defined uniaxial anisotropy originated from the existence of elongated in-plane [001]-oriented structures, whereas LSMO films grown on nominally flat (001)-oriented STO substrates show a weak uniaxial magnetic anisotropy with the easy axis direction aligned parallel to residual substrate step edges. Elongated structures are also found for LSMO films grown on vicinal STO(001) substrates. These films present a well-defined uniaxial magnetic anisotropy with the easy axis lying along the step edges and its strength increases with the LSMO thickness. It is remarkable that this step-induced uniaxial anisotropy has been found for LSMO films up to 120 nm thickness. Our results are promising for engineering novel half-metallic magnetic devices that exploit tailored magnetic anisotropy.Comment: 10 pages, 10 figures, 1 tabl

Role of anisotropy configuration in exchange-biased systems

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.We present a systematic study of the anisotropy configuration effects on the magnetic properties of exchange-biased ferromagnetic/antiferromagnetic (FM/AFM) Co/IrMn bilayers. The interfacial unidirectional anisotropy is set extrinsically via a field cooling procedure with the magnetic field misaligned by an angle bFC with respect to the intrinsic FM uniaxial anisotropy. High resolution angular dependence in-plane resolved Kerr magnetometry measurements have been performed for three different anisotropy arrangements, including collinear bFC =0º and two opposite noncollinear cases. The symmetry breaking of the induced noncollinear configurations results in a peculiar nonsymmetric magnetic behavior of the angular dependence of magnetization reversal, coercivity, and exchange bias. The experimental results are well reproduced without any fitting parameter by using a simple model including the induced anisotropy configuration. Our finding highlights the importance of the relative angle between anisotropies in order to properly account for the magnetic properties of exchange-biased FM/AFM systems

First-principles quantum transport modeling of spin-transfer and spin-orbit torques in magnetic multilayers

We review a unified approach for computing: (i) spin-transfer torque in magnetic trilayers like spin-valves and magnetic tunnel junction, where injected charge current flows perpendicularly to interfaces; and (ii) spin-orbit torque in magnetic bilayers of the type ferromagnet/spin-orbit-coupled-material, where injected charge current flows parallel to the interface. Our approach requires to construct the torque operator for a given Hamiltonian of the device and the steady-state nonequilibrium density matrix, where the latter is expressed in terms of the nonequilibrium Green's functions and split into three contributions. Tracing these contributions with the torque operator automatically yields field-like and damping-like components of spin-transfer torque or spin-orbit torque vector, which is particularly advantageous for spin-orbit torque where the direction of these components depends on the unknown-in-advance orientation of the current-driven nonequilibrium spin density in the presence of spin-orbit coupling. We provide illustrative examples by computing spin-transfer torque in a one-dimensional toy model of a magnetic tunnel junction and realistic Co/Cu/Co spin-valve, both of which are described by first-principles Hamiltonians obtained from noncollinear density functional theory calculations; as well as spin-orbit torque in a ferromagnetic layer described by a tight-binding Hamiltonian which includes spin-orbit proximity effect within ferromagnetic monolayers assumed to be generated by the adjacent monolayer transition metal dichalcogenide.Comment: 22 pages, 9 figures, PDFLaTeX; prepared for Springer Handbook of Materials Modeling, Volume 2 Applications: Current and Emerging Material

Accurate determination of the specific absorption rate in superparamagnetic nanoparticles under non-adiabatic conditions

We report on a general description of non-adiabatic calorimetry measurements for improving the accuracy on the determination of the specific absorption rate of superparamagnetic nanoparticles subjected to alternating magnetic fields. We perform experiments on reduced volumes of iron oxide nanoparticles dispersed in aqueous media under different thermal equilibrium conditions. We introduce a simple model, which considers linear thermal losses to precisely reproduce the complete time evolution of temperature. The control and the quantification of heat losses lead to higher accuracy for determining the specific absorption rate in superparamagnetic nanoparticles. © 2012 American Institute of Physics.This work has been partially supported by EU-FP7 MULTIFUN Project (No. 262943), by Spanish Ministry of Economy and Competitiveness (MAT2010-21822-C02-01 and CSD2007-00010) and NANOBIOMAGNET Project (S2009/MAT-1726) funded by Comunidad de Madrid. F.J.T and A.B. acknowledge financial support from Ramon y Cajal subprogram (RYC-2011-09617 and RYC-2007-01727, respectively).Peer Reviewe

Substrate-induced magnetic anisotropy in La0.7Sr0.3MnO3 epitaxial thin films grown onto (110) and (1 8) SrTiO3 substrates

International audienceWe show a detailed magneto-optical Kerr study at room temperature of well characterized epitaxial La0:7Sr0:3MnO3 (LSMO) thin lms grown onto (110) and (1 18) SrTiO3 substrates. The lms present a well-de ned uniaxial (two-fold) magnetic anisotropy ascribed to substrate-induced anisotropy. In particular, the in-plane uniaxial anisotropy in the(110)-oriented LSMO lms originates from the existence of elongated in-plane [001]-oriented structures. Similar elongated structures, parallel to the [110] crystallographic direction, are found for LSMO lms grown on (1 18) STO surfaces. In all lms, such a uniaxial magnetic anisotropy is characterized by an easy axis lying along the elongated structures. Furthermore, the vectorial-resolved hysteresis loops as a function of the in-plane applied eld direction are interpreted in terms of rotation and propagation and nucleation of magnetic domains processes. Our results demonstrate the tailoring of magnetic anisotropy by exploiting the substrate-induced anisotropy in epitaxial thin films