We present a computational model of crystallographically amorphous ferrimagnetic alloys using a stochastic Landau-Lifshitz-Gilbert equation of motion for atomistic spins and an atomistic spin Hamiltonian with Heisenberg exchange. The spontaneous equilibrium magnetization is calculated and a comparison with a mean field model is made. The simulations show excellent agreement with experiments on GdFeCo using x-ray magnetic circular dichroism to determine the individual sublattice magnetizations. The calculated temperature dependence of the magnetization shows a polarization of the Gd sublattice leading to a common Curie temperature, in agreement with the experimental data. The intersublattice exchange is shown to be an important energy transfer channel for ultrafast dynamics.

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Crystallographically amorphous ferrimagnetic alloys: Comparing a localized atomistic spin model with experiments

Classical magnetic dipolar interactions can make significant contributions to magnetic anisotropy in amorphous ferrimagnetic alloys with slight structural anisotropies on either atomic or microstructural scales. Simple expressions have been derived for this magnetic anisotropy in terms of structural anisotropy parameters, p, V, and Δx. For an alloy with atomic‐scale structural anisotropy (pair ordering), p is a measure of alignment of unlike nearest‐neighbor pairs. For an alloy with aligned elongated composition inhomogeneities, V is the volume fraction of inhomogeneities and Δx is the composition difference (atom fraction) between the inhomogeneities and their surroundings. Accounting for magnetic anisotropy observed experimentally in bias‐sputtered GdCo‐based alloy films solely in terms of pair ordering and classical dipolar interactions requires p∼0.015. Accounting for the anisotropy solely in terms of composition inhomogeneities requires Δx∼0.1 for V=0.5. Although such atomic‐scale structural anisotro...

Magnetic anisotropy from dipolar interactions in amorphous ferrimagnetic alloys

To investigate the mechanism of perpendicular magnetic anisotropy K u in rare earth-transition metal amorphous films, K u was measured for (Gd 0.75 R 0.25 ) 19 Co 81 films with varying the rare earth element R. Results were analysed based on a single ion anisotropy model using a point charge approximation. The dependence of K u on rare earth element is well explained with the single ion anisotropy. A model was proposed in which anelastic deformation during deposition of the film is responsible for the induction of K u . Magnetostriction can also be explained with a point charge model with screening.

Single ion model for perpendicular magnetic anisotropy in RE-TM amorphous films

The influence of ion sputtering on the elemental analysis of solid surfaces

Ferrimagnetic rare earth - transition metal Tb-Fe alloy thin films exhibit a variety of different magnetic properties, which depends strongly on composition and temperature. In this study, first the influence of the film thickness (5 - 85 nm) on the sample magnetic properties was investigated in a wide composition range between 15 at.% and 38 at.% of Tb. From our results, we find that the compensation point, remanent magnetization, and magnetic anisotropy of the Tb-Fe films depend not only on the composition but also on the thickness of the magnetic film up to a critical thickness of about 20-30 nm. Beyond this critical thickness, only slight changes in magnetic properties are observed. This behavior can be attributed to a growth-induced modification of the microstructure of the amorphous films, which affects the short range order. As a result, a more collinear alignment of the distributed magnetic moments of Tb along the out-of-plane direction with film thickness is obtained. This increasing contribution of the Tb sublattice magnetization to the total sample magnetization is equivalent to a sample becoming richer in Tb and can be referred to as an “effective” composition. Furthermore, the possibility of all-optical switching, where the magnetization orientation of Tb-Fe can be reversed solely by circularly polarized laser pulses, was analyzed for a broad range of compositions and film thicknesses and correlated to the underlying magnetic properties.

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Ferrimagnetic Tb–Fe Alloy Thin Films: Composition and Thickness Dependence of Magnetic Properties and All-Optical Switching

A model will be presented to explain the bias voltage dependence of magnetic anisotropy in sputtered amorphous films of gadolinium cobalt alloys. It has been found that the magnetic anisotropy increases with bias voltage up to a critical bias voltage (∠150 V depending on the sputter gas) then decreases precipitously at higher voltages. The model used to explain this behavior is based on selective resputtering of atoms in different surface sites causing an anisotropic distribution of Gd and Co atoms with respect to the growth direction. For example, a Gd adatom in contact with three surface atom neighbors can have 0, 1, 2, or 3 cobalt neighbors. The sputtering threshold for each type of Gd adatom will be different so those with a low threshold will be selectively removed giving a structural anisotropy. This anisotropic atomic distributions can cause different kinds of anisotropy with respect to the growth direction depending on the type of amorphous film. For example, both easy‐axis (GdCo) and hard‐axis (G...

J. J. Cuomo

Papers

Selective resputtering‐induced anisotropy in amorphous films