Role of longitudinal fluctuations in L10 FePt

Abstract

L1 0 FePt is a technologically important material for a range of novel data storage applications. In the ordered FePt structure the normally non-magnetic Pt ion acquires a magnetic moment, which depends on the local field originating from the neighboring Fe atoms. In this work a model of FePt is constructed, where the induced Pt moment is simulated by using combined longitudinal and rotational spin dynamics. The model is parameterized to include a linear variation of the moment with the exchange field, so that at the Pt site the magnetic moment depends on the Fe ordering. The Curie temperature of FePt is calculated and agrees well with similar models that incorporate the Pt dynamics through an effective Fe-only Hamiltonian. By computing the dynamic correlation function the anisotropy field and the Gilbert damping are extracted over a range of temperatures. The anisotropy exhibits a power-law dependence with temperature with exponent n ≈ 2.1. This agrees well with what observed experimentally and it is obtained without including a two-ion anisotropy term as in other approaches. Our work shows that incorporating longitudinal fluctuations into spin dynamics calculations is crucial for understanding the properties of materials with induced moments. arXiv:2001.03074v1 [cond-mat.mtrl-sci] 9 Jan 2020

The Magnetic Genome of Two-Dimensional van der Waals Materials

“…For elevated temperatures, the current atomistic approach used within the 2D Heisenberg model has some basic approximations assuming a fully classical heat bath and fully localized magnetic moments. While the latter approximation seems to be reasonable for a broad class of current 2D magnets, it may be necessary to introduce longitudinal spin fluctuations − to better describe the itinerant characteristics of local moments. An outstanding problem common to both 3D and 2D magnetic materials is the nature of the heat bath that drives thermal spin fluctuations and allows for energy dissipation.…”

Temperature scaling of two-ion anisotropy in pure and mixed anisotropy systems

“…It allows for the proper interpretation of unusual scaling exponents found in experimental data for permanent magnets [4][5][6]. Another theoretical explanation of two-ion anisotropy was put forward recently by Ellis et al [29], where no explicit two-ion anisotropy was included, yet the exponent l ∼ 2.1 was recovered for FePt by accounting for the longitudinal dynamics of the non-magnetic Pt moments mediating the exchange interactions. Metallic ferromagnets tend to retain some itinerant character, which may explain a predominant two-ion contribution in rare-earth-transition-metal compounds.…”

“…Rare-earthtransition-metal permanent magnets often exhibit even more complicated behavior with an increase of the anisotropy with temperature [10][11][12][13], corresponding to a negative scaling exponent. Theoretical work attributed the unusual scaling exponent in FePt either to the longitudinal dynamics of the induced Pt moments [14] or to two-ion anisotropy. Within a mean-field calculation, this term was shown to possess a scaling exponent of l = 2 [15], which in combination with the single-ion term (l = 3) successfully reproduced the exponent found experimentally in refined numerical calculations [16,17].…”

The role of interfacial intermixing on HAMR dynamics in bilayer media

“…In such a case in fact, the description of FePt where the Pt contribution is included via an effective augmented atomic spin moment starts losing its validity. There are reports of models that, by taking into consideration longitudinal spin fluctuations, can include Pt explicitly and allow for Pt magnetic moments to vary with the surrounding Fe environment [37]. Such an approach requires a detailed parameterisation of both Fe and Pt magnetic properties and efforts are on the way to develop an approach similar to that proposed by Ellis et al Moreover, the approximation of a weak exchange coupling between effective FePt atoms and those of the soft material when these are displaced in the opposite layer can be seen as a representation of the exchange coupling on FIG.…”