Magnetic anisotropy energy of disordered tetragonal Fe-Co systems fromab initioalloy theory

Abstract

We present results of systematic fully relativistic first-principles calculations of the uniaxial magnetic anisotropy energy (MAE) of a disordered and partially ordered tetragonal Fe-Co alloy using the coherent potential approximation (CPA). This alloy has recently become a promising system for thin ferromagnetic films with a perpendicular magnetic anisotropy. We find that existing theoretical approaches to homogeneous random bulk Fe-Co alloys, based on a simple virtual crystal approximation (VCA), overestimate the maximum MAE values obtained in the CPA by a factor of four. This pronounced difference is ascribed to the strong disorder in the minority spin channel of real alloys, which is neglected in the VCA and which leads to a broadening of the d-like eigenstates at the Fermi energy and to the reduction of the MAE. The ordered Fe-Co alloys with a maximum L10-like atomic long-range order can exhibit high values of the MAE, which, however, get dramatically reduced by small perturbations of the perfect order.

Magnetic properties of(Fe1−xCox)2Balloys and the effect of doping byEdström1, Werwiński2, Iuşan3 et al. 2015Phys. Rev. B

“…The qualitative shape of the MAE curve as function of Co concentration agrees with experiment for fullpotential calculations in the VCA. However, the quantitative discrepancy is severe as commonly observed when studying the MAE in the VCA [16][17][18]. Calculations in the atomic sphere approximation, treating disorder in the CPA, solve the problem of the quantitative disagreement with experiment on the Fe-rich side of the alloy but instead yield a qualitatively incorrect behavior on the Co-rich side.…”

“…The highest uniaxial anisotropy, with MAE = 1.5 MJ/m 3 , occurs for the (Fe 0.6 Co 0.4 ) 2 B alloy. Quantitative overestimation of the MAE for the whole concentration range brings reminiscence to the case of tetragonally strained Fe/Co alloys, where VCA treatment of substitutional disorder [11] also led to an overestimation of MAE compared to experiments [13] or more sophisticated treatments of disorder [16,17]. Therefore, in the next subsection we will also present results based on a more advanced description of disorder, the coherent potential approximation (CPA).…”

“…Recently, however, the availability (and thus price) of the rare-earth elements became rather volatile, calling for development of replacement materials which would use less or none of * Corresponding author: jan.rusz@physics.uu.se the rare-earth elements. Intense research efforts have started worldwide, revisiting previously known materials, such as Fe 2 P [5][6][7], FeNi [8], or Fe 16 N 2 [9], doing computational data mining among the large family of Heusler alloys [10], exploring the effects of strain [11][12][13][14][15][16][17] and doping by interstitial elements [18,19], multilayers such as Fe/W-Re [20] or, as a limiting case of multilayers, the L1 0 family of compounds [21], or promising Mn-based systems [22][23][24][25][26][27], among others.…”

UH3-based ferromagnets: New look at an old material

“…The calculation employed the tight-binding linear muffin-tin orbital (TB-LMTO) method in the atomic-sphere approximation (ASA) combined with the coherent-potential approximation (CPA) [14][15][16] for the treatment of random binary U 1-x Zr x alloys with Zr concentrations x < 0.5 [8]. The alloy structure kept as bcc (no H atoms for simplicity) with the lattice parameter equal to that of bcc UH 3 (a = 416 pm).…”

Increased magnetocrystalline anisotropy in epitaxial Fe-Co-C thin films with spontaneous strain

“…However, competitive alternatives are still missing. The alloy Fe-Co has often been considered a promising candidate for rare earth free permanent magnets [2][3][4] . Its very high intrinsic magnetic moment 5,6 is one prerequisite.…”