Structural, magnetic and electronic properties of stoichiometric Co2 YZ Heusler alloys (Y = Cr, Fe, Mn and Z = Al, Si, Ge) have been studied by means of ab initio calculations and Monte Carlo simulations. The investigations were performed in dependence on different levels of approximations in DFT (FP and ASA modes, as well as GGA and GGA + U schemes) and external pressure. It is shown that in the case of the GGA scheme the half-metallic behavior is clearly observed for compounds containing Cr and Mn transition metals, while Co2FeZ alloys demonstrate the pseudo half-metallic behavior. It is demonstrated that an applied pressure and an account of Coulomb repulsion (U) lead to the stabilization of the half-metallic nature for Co2 YZ alloys. The strongest ferromagnetic inter-sublattice (Co–Y) interactions together with intra-sublattice (Co–Co and Y–Y) interactions explain the high values of the Curie temperature obtained by Monte Carlo simulations using the Heisenberg model. It is observed that a decrease in valence electrons of Y atoms (i.e. Fe substitution by Mn and Cr) leads to the weakening of the exchange interactions and to the reduction of the Curie temperature. Besides, in the case of the FP mode Curie temperatures were found in a good agreement with available experimental and theoretical data, where the latter were obtained by applying the empirical relation between the Curie temperature and the total magnetic moment.

https://iopscience.iop.org/article/10.1088/0022-3727/49/35/355004/pdf

Electronic and magnetic properties of the Co2-based Heusler compounds under pressure: first-principles and Monte Carlo studies

A ${\mathrm{Co}}_{2\ensuremath{-}x}{\mathrm{Ti}}_{x}\mathrm{FeGe}\phantom{\rule{4pt}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}1)$ Heusler alloy series has been synthesized and investigated in steps of $x=0.125$. While the parent ${\mathrm{Co}}_{2}\mathrm{FeGe}$ composition was previously reported to exhibit multi-phase structures, the novel substitution of Ti for Co is successful in stabilizing single-phase microstructures for compositions from $x=0.125$ to 0.625. The single-phase compositions crystallized in a face-centered cubic crystal structure. Soft-ferromagnetic behavior and a very high Curie temperature, as high as 881 K, is obtained for alloys with the lowest Ti concentration. Magnetic and electrical properties of these alloys suggest the possibility of half-metallic behavior in them. Both experimental findings and theoretical calculations support that the substitution of Ti atoms displaces Fe atoms towards vacated Co sites. The theoretical calculations also predict half-metallic character in the alloys after Ti substitution. The alloys are found to be unusually hard, with Vicker's hardness approaching 12 GPa for the most Ti-rich material.

Tunable properties and potential half-metallicity in (Co 2 -x Ti x )FeGe Heusler alloys: An experimental and theoretical investigation

Magnetism and half-metallicity in bulk and (100), (111)-surfaces of Co2ScSb full Heusler alloy for spintronic applications

Abstract In this work, first-principles calculations of the structural, electronic and magnetic properties of Heusler alloys CoMnYAl, CoMnYGa and CoMnYIn are presented. The full potential linearized augmented plane waves (FP-LAPW) method based on the density functional theory (DFT) has been applied. The structural results showed that CoMnYZ (Z = Al, Ga, In) compounds in the stable structure of type 1+FM were true half-metallic (HM) ferromagnets. The minority (half-metallic) band gaps were found to be 0.51 (0.158), 0.59 (0.294), and 0.54 (0.195) eV for Z = Al, Ga, and In, respectively. The characteristics of energy bands and origin of minority band gaps were also studied. In addition, the effect of volumetric and tetragonal strain on HM character was studied. We also investigated the structural, electronic and magnetic properties of the doped Heusler alloys CoMnYGa1−xAlx, CoMnYAl1−xInx and CoMnYGa1−xInx (x = 0, 0.25, 0.5, 0.75, 1). The composition dependence of the lattice parameters obeys Vegard’s law. All alloy compositions exhibit HM ferromagnetic behavior with a high Curie temperature (TC).

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The effect of pressure and alloying on half-metallicity of quaternary Heusler compounds CoMnYZ (Z = Al, Ga, and In)

In this work, we report a systemetic study of Co substitution for Ru in Ru 2 FeSi. Our previous studies showed that Ru 2 FeSi and Co 2 FeSi are antiferromagnetic and ferromagnetic, respectively. In order to understand the influence of Co substitution for Ru, (Ru 1− x Co x ) 2 FeSi alloys were prepared by arc melting, followed by annealing at 1273 K for 3 days. Structural and magnetic studies were carried out on the quaternary alloys by powder X-ray diffraction and vibrating sample magnetometry, respectively. At room temperature, Ru rich compositions i.e., x =0 and x =0.25 exhibited disordered B2 structure, but with increase in Co concentration L2 1 ordering appeared in the alloys. From magnetization measurements, it is seen that with increasing x , the ferromagnetic state becomes dominant with the appearance of spontaneous magnetization and increase in the value of magnetization of the alloys. Saturation magnetization measured at 15 kOe increased from 1.02 emu/g to 149.96 emu/g as x was increased from 0 to 1.

Magnetic and structural properties of (Ru1−xCox)2FeSi alloys

Magnetic and crystallographic properties of bulk Co2Fe(Ga1−xSix) alloys with 0≤x≤1 are reported in this work. The alloys with x=0.75 and 1.00 exhibit L21 structure whereas the alloys with x=0, 0.25 and 0.50 crystallized in the disordered A2 phase. Unit cell volume of this series of alloys decreased from 189.1 to 178.5 A3 as x was increased from 0 to 1.00. All alloy compositions exhibit ferromagnetic behavior with a high Curie temperature (TC) which showed a systematic variation with x (1089 K, 1075 K, 1059 K, 1019 K and 1015 K for x=0, 0.25, 0.5, 0.75 and 1.00, respectively). The saturation magnetization moment Ms for the alloys with x=0, 0.25 and 0.50 are 5.05μB, 5.23μB, 5.49μB, respectively, in accordance with the Slater–Pauling rule, but alloys with x=0.75 and 1.00 deviated from this rule. The effective moment per magnetic atom (pc) of the alloys was estimated from the inverse DC magnetic susceptibility data above TC. A comparison of Ms with pc reveals the half-metallic character of the alloys.

Dibyashree Chakraborty

Papers

Magnetic properties of Co2Fe(Ga1−xSix) alloys