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

– 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 easyplane and large Pt easy-axis 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 ) 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. Introduction. – Since the phenomenon of super-paramagnetism has been identified as one of the major limits for the conventional magnetic recording [1], significant research effort has been invested in the development of materials with large magnetic-anisotropy energy (MAE). Recent progress in the fabrication and characterization of granular and nano-particulate FePt films [2] puts even more emphasis on the understanding of the giant MAE of FePt and its temperature dependence. The latter property appears to be of critical importance for the development of future high-density magnetic-recording systems in particular for heat-assisted magnetic recording [1]. A systematic understanding of the temperature dependence of the MAE in itinerant magnets remains a challenge and one of the long-standing problems in the theory of magnetism. The proposed model deals with mixed localized and itinerant magnetic moments and thus bears general importance as large anisotropy is achieved by combining strongly magnetic elements with non-magnetic ones, where the latter have large spin-orbit coupling. The chemically ordered L10 phase of FePt has large uniaxial MAE with the first-order anisotropy constant K1 ≈ 10 erg/cc [1] based on the simple angular variation of MAE E ∼ K1 sin θ. In the L10 phase the cubic symmetry is broken due to the stacking of alternate planes of the 3d element (Fe) and the 5d element (Pt) along the [001] direction. It is well