Temperature-dependent magnetic properties of FePt: Effective spin Hamiltonian model

Abstract

A model of magnetic interactions in the ordered ferromagnetic FePt is proposed on the basis of first-principles calculations of non-collinear magnetic configurations and shown to be capable of explaining recent measurements of magnetic anisotropy energy (MAE). The site (Fe,Pt) resolved contributions to the MAE have been distinguished with small Fe easy-plane and large Pt easyaxis terms. This model has been tested against available experimental data on the temperature dependence of MAE showing scaling of uniaxial MAE (K1(T)) with magnetization (M(T)) K1(T ) ∼ M (T ) γ characterized by the unusual exponent of γ = 2.1. It is shown that this unusual behavior of the FePt can be quantitatively explained within the proposed model and originates from an effective anisotropic exchange mediated by the induced Pt moment. The latter is expected to be a common feature of 3d-5d(4d) alloys having 5d/4d elements with large spin-orbit coupling and exchange enhanced Stoner susceptibility.

Orientation and temperature dependence of domain wall properties in FePt

“…We have calculated the effective micromagnetic parameters A and K, including their directionality. K is found to be essentially independent of direction, and the range of values which we obtain for K is consistent with the large anisotropy energy of FePt (about 8 × 10 6 J/m 3 corresponding to a zero temperature Stoner-Wohlfarth coercive field of about 16T [2]). The value for A is also consistent with the Curie temperature of FePt (a Curie temperature of about 700K corresponds to an exchange constant of about 2 × 10 −11 J/m).…”

“…The fundamental interactions at the electronic level are strongly modified by this structure. In particular it has been shown [2] that the Fe moments can be considered as localized, while the Pt induced moments have to be treated as essentially delocalized. Nevertheless, it is possible to construct a classical spin Hamiltonian involving only the Fe degrees of freedom, with the introduction of a 2-ion anisotropy term and a modified exchange term [2].…”

“…This model [2] has been constructed on the basis of first-principles calculations of non-collinear configurations calculated using constrained local spin density functional theory [6] and site-resolved magneto-crystalline anisotropy [7]. It is used here to investigate the domain wall properties of FePt.…”

“…The full Hamiltonian, as described in Ref. [2], including the cou-pling to a magnetic field B and dipole-dipole coupling, is,…”

Thermally assisted current-induced magnetization reversal in SrRuO3

“…Assuming a nucleation volume of cylindrical shape with the film thickness (37.5 nm) as the cylinder height, the estimated radius of the cylinder is ∼ 1.5 nm, on the order of the domain wall width. We should, however, take into consideration that E 0 b is the barrier at T = 107 K, where the magnetization is ∼ 0.7 of its saturation value and K u may also be suppressed [30,31]. Therefore, the volume could be larger although still on the same order.…”