Half-metallic Co-based quaternary Heusler alloys for spintronics: Defect- and pressure-induced transitions and properties

Abstract

Heusler compounds offer potential as spintronic devices due to their spin-polarization and halfmetallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin-polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin-polarization. Using densityfunctional theory, we analyze the electronic and magnetic properties of equiatomic Heusler (L21) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (thermal antisites, binary swaps, and vacancies). Under pressure, CoMnCrAl undergoes a metallic transition, while half-metallicity in CoFeCrGe is retained for a limited range. Antisite disorder between Cr-Al pair in CoMnCrAl alloy is energetically the most favorable, and retain half-metallic character in Crexcess regime. However, Co-deficient samples in both alloys undergo a transition from half-metallic to metallic, with a discontinuity in the saturation magnetization. For binary swaps, configurations that compete with the ground state are identified and show no loss of half-metallicity; however, the minority-spin bandgap and magnetic moments vary depending on the atoms swapped. For single binary swaps, there is a significant energy cost in CoMnCrAl but with no loss of half metallicity. Although a few configurations in CoFeCrGe energetically compete with the ground state, however the minority-spin bandgap and magnetic moments vary depending on the atoms swapped. These informations should help in controlling these potential spintronic materials.PACS numbers: 31.15. 75.50.Cc,

Thermodynamic and thermoelectric properties of CoFeYGe (Y = Ti, Cr) quaternary Heusler alloys: first principle calculations

“…The Ytype-I structure with a space group F43m (#216) is found to be the most stable geometrical configuration of CoFeYGe (Y = Ti, Cr) compounds. The calculated lattice parameters are found to be in good agreement with previous theoretical [10,13] and experimental [11] results as shown in Table 2.…”

“…[9] Enamullah et al investigated the effect of defects and change of pressure on the electronic and magnetic properties of CoFeCrGe and CoMnCrAl, where CoFeCrGe alloy retained its half-metallicity for a limited range of pressure. [10] A theoretical and experimental study by Enamullah et al shows that CoFeCrGe alloy crystallizes in a Y-type structure and exhibits a half-metallic ferromagnetic behavior with high Curie temperature (866 K). [11] Density functional theory predicted that XX0YZ (X = Co, Ni, X0 = Fe; Y = Ti; Z = Si, Ge, As) quaternary Heusler alloys are halfmetallic.…”

Ordering tendencies and electronic properties in quaternary Heusler derivatives

“…Since the first quaternary Heusler derivatives adopting the Y structure have been proposed to possess halfmetallic properties 72 , the interest in these materials has developed rapidly with numerous publications dealing with the topic. [10][11][12]70,71,[73][74][75][76][77][78][79][80][81][82][83][84][85][86][87] Density-functional theory calculations have been used to identify promising systems among the NiCo-, 10,11,70,71,[73][74][75] NiFe- 11,75,76 and CoFebased 12,72,77-87 compounds. However, in most cases the phase stability of the Y structure is tested, if at all, only against stacking order variations of this Y structure (see, for instance, Refs.…”

“…However, in most cases the phase stability of the Y structure is tested, if at all, only against stacking order variations of this Y structure (see, for instance, Refs. 72 and 77) but rarely against disorder 86 or other states of order. Simultaneously, experimental investigations as a rule either point towards disordered structures 10,76,77,80,82 or, specifically for the case of X-ray diffraction on ordering between transition metal elements, cannot decide these issues.…”

Structural disorder and short-range order in full Heusler alloys Fe2VAl and Co2CrAl from first principles calculations

“…Real samples of the disordered phases apparently have a more complex structure compared to the model we used. For instance, the presence of vacancies may also have an effect on the magnetization and electronic structure [25]. In addition to the antisite defects between X and Y sublattices, X and Z, as well as Y and Z sublattices can also exchange with atoms to form B 2 and bcc (A 2 ) types of structure [20].…”