Exchange interactions and Curie temperatures in Cr-based alloys in the zinc blende structure: Volume- and composition-dependence from first-principles calculations

Abstract

We present calculations of the exchange interactions and Curie temperatures in Cr-based pnictides and chalcogenides of the form CrX with X=As, Sb, S, Se and Te, and the mixed alloys CrAs50X50 with X=Sb, S, Se, and Te. The calculations are performed for Zinc Blende (ZB) structure for 12 values of the lattice parameter between 5.44 and 6.62Å, appropriate for some typical II-VI and III-V semiconducting substrates. Electronic structure is calculated via the linear muffin-tin-orbitals (LMTO) method in the atomic sphere approximation (ASA), using empty spheres to optimize ASArelated errors. Whenever necessary, the results have been verified using the full-potential version of the method, FP-LMTO. The disorder effect in the As-sublattice for CrAs50X50 (X=Sb, S, Se, Te) alloys is taken into account via the coherent potential approximation (CPA). Exchange interactions are calculated using the linear response method for the ferromagnetic (FM) reference states of the alloys, as well as the disordered local moments (DLM) states. These results are then used to estimate the Curie temperature from the low and high temperature side of the ferromagnetic/paramagnetic transition. Estimates of the Curie temperature are provided, based on the mean field and the more accurate random phase approximations. Dominant antiferromagnetic exchange interactions for some low values of the lattice parameter for the FM reference states in CrS, CrSe and CrTe prompted us to look for antiferromagnetic (AFM) configurations for these systems with energies lower than the corresponding FM and DLM values. Results for a limited number of such AFM calculations are discussed, identifying the AFM[111] state as a likely candidate for the ground state for these cases.

Half-metallicity and magnetism of GeTe doped with transition metals V, Cr, and Mn: A theoretical study from the viewpoint of application in spintronics

“…(3) or (4) when, in addition to the robust moments on the magnetic atoms, there are induced moments on apparently non-magnetic atoms, interstitial spaces, or, in case of the LMTO method, empty spheres. This problem has been discussed in detail in our previous publication 35 and references cited therein. As shown by Sandratskii et al, 33 the calculation of T c using RPA is considerably more involved even for the case where only one secondary induced interaction needs to be considered, in addition to the principal interaction between the strong moments.…”

“…35,36 Here, i and j are site indices, e i is the unit vector pointing along the direction of the local magnetic moment at site i, and J ij is the exchange interaction between the moments at sites i and j. The calculations are based on a mapping procedure due to Liechtenstein et al [37][38][39] This method was later extended to random magnetic systems by Turek et al, using CPA and the TB-LMTO method.…”

Pressure dependence of Curie temperature and resistivity in complex Heusler alloys

“…In fact, it is just the band energy, not the total energy, which is considered in the mapping, by appealing to the so-called "magnetic force theorem" (see Ref. 25 and references therein). For finite temperature (T = 0) calculations, as would be appropriate for the DLM reference state, one needs to consider the mapping of the free energy F = U − T S, where U is the internal energy and S is the entropy.…”

“…Similar calculations have also been carried out for the intermetallic compound GdAl 2 24 and for Cr-based compounds in the zincblende structure. 25 A systematic theoretical study of the pressure-dependence of T c in Heusler alloys was carried out recently by Sasioglu et al, 26 where the experimentally observed increase of T c with pressure 9,11 in Ni 2 MnSn was reproduced correctly, albeit only qualitatively so. There exists some other studies of Heusler alloys in which the dependence of the exchange integrals on volume contraction and expansion 17,[27][28][29] was explored.…”

Stability and half-metallicity of the (001) and (111) surfaces of CrTe with rocksalt structure

“…Since Akinaga et al 1 predicted theoretically and confirmed experimentally in 2000 the half-metallic (HM) ferromagnetism in zinc-blende (ZB) CrAs, transition-metal pnictides and chalcogenides (TM-PCs) with ZB structure have attracted increasing research interest in the past ten years, because most of this class of compounds were predicted to HM ferromagnets, [2][3][4][5][6][7][8][9][10] i.e., compounds for which only one spin channel is metallic, resulting in 100% spin polarization at the Fermi level. The half-metallicity and the comparable lattice structure with conventional binary semiconductors make ZB TM-PCs promising candidates of spininjector materials in spintronic devices.…”