Long-range chemical order effects upon the magnetic anisotropy of FePt alloys from anab initioelectronic structure theory

Abstract

The magnetocrystalline anisotropy (MCA) of bulk and thick films of FePt is calculated from a 'first-principles' theory. The starting point is a description from electronic density functional theory for systems of interacting electrons moving in lattices of ions. Relativistic effects such as spin-orbit coupling are included. FePt readily transforms into a CuAu-type (L1 0) ordered phase and this coincides with the material's high anisotropy. Here we describe how to calculate the MCA of a partially ordered alloy and to extract its dependence on the long range chemical order parameter η. We present calculations of the MCA of FePt as a function of η and find excellent agreement with the experimental data of Okamoto et al (2002 Phys. Rev. B 66 024413) and others with respect to the magnetic easy axis, the magnitude of the MCA and its trend with decreasing η. We also study the paramagnetic phase of the ordered alloy using the 'disordered local moment' picture of metallic magnetism at finite temperatures. We calculate a Curie temperature of 935 K in reasonable agreement with experiment (710 K) and find the easy axis for the onset of ferromagnetic order to coincide with the magnetic easy axis found at low temperatures.

Magnetocrystalline anisotropy of FePt: A detailed view

“…Daalderop et al [18] and Ravindran et al [12] studied the influence of the band-filling on E MCA of FePt. Many groups studied the influence of the temperature on the MCA of FePt [31,32]. The dependence of the Curie temperature on the FePt grain size was investigated via model Hamiltonian calculations [33].…”

Discovery of metastable tetragonal disordered phase upon phase transitions in the equiatomic nanostructured FePd alloy

“…According to the modern first-principles theory of magnetic anisotropy [4][5][6], the magnetocrystalline anisotropy of L1 0 alloys is mainly related to the process of atomic ordering and depends on the degree of long order. It is experimentally shown that variation of temperature and duration of the ordering annealing and application of magnetic field or stresses in the course of this annealing, as well as of preliminary plastic deformation, are factors that exert a significant effect on the integral degree of long ordering, morphology and sizes of the meso-and microstructure elements of L1 0 alloys [7][8][9][10][11][12][13][14].…”

“…In the majority of theoretical and experimental works (e.g. [4][5][6][7][8][9][10][11][12][13][14][15]) the tetragonal distortion of the crystal lattice, which arises in the alloys upon the phase transformation A1 ! L1 0 , is considered to result exclusively from layer-by-layer distribution of atoms of different kinds in the directions h100i (Fig.…”

Observation of L1-like chemical ordering in a decahedral FePt nanoparticle by Cs-corrected high resolution transmission electron microscopy

“…3 The magnetic properties depend greatly on the degree of chemical order in the L1 0 phase. [4][5][6][7] However, FePt nanoparticles produced from room temperature sputtering and chemical synthesis process normally are in the disordered fcc phase and postsynthesis treatments are necessary to induce the phase transformation to the ordered L1 0 phase. Most postsynthesis treatments, such as thermal annealing, also lead to aggregation of the nanoparticles.…”