Magnetic exchange coupling and Curie temperature of Ni(1+x)MnSb(x=0,0.25,

Abstract

We study the dependence of magnetic interactions and Curie temperature in Ni (1+x) MnSb system on the Ni concentration within the framework of the density-functional theory. The calculation of the exchange parameters is based on the super-cell and frozen-magnon approaches. The Curie temperatures, TC , are calculated within the random-phase approximation. In agreement with experiment we obtain decrease of the Curie temperature with increasing Ni content. Recently rapid development of spin electronics intensified the researches on the ferromagnetic materials that are suitable for the spin injection into a semiconductor [1]. One of the promising classes of these materials is the Heusler alloys. Heusler alloys have been intensively studied theoretically [2,3] and experimentally [4,5] as possible sources of spin-polarized carriers for spintronics applications. Among the properties useful for the applications are high Curie temperature, high electron spin polarization at the Fermi level and very small lattice mismatch with widely employed semiconductors (e.g., Ni2MnIn and InAs) [6]. Some of the Heusler compounds were found to have half-metallic ground state [2] which is characterized by a 100% carrier spin-polarization. An interesting combination of physical properties makes Heusler alloys the subject of intensive experimental and theoretical investigations [7,8,9].In the present work we report the theoretical study of the exchange interactions and Curie temperature of Ni (1+x) MnSb system as a function of Ni concentration. In particular, we focus on the effect of local environment of Mn atoms on half-metallicity and exchange in- * ersoy@mpi-halle.de teractions. We show that, occupation of vacant sites in NiMnSb by Ni immediately leads to disappearance of the half-metallicity and to substantial decrease of both inter-sublattice (Mn-Ni) and intra-sublattice (Mn-Mn) exchange interactions.The calculations are carried out with the augmented spherical waves (ASW) method [10] within the generalized gradient approximation (GGA) [11] for the exchange-correlation potential. In all calculations the experimental values of the lattice parameters are used [12]. The radii of all atomic spheres are chosen equal [13]. We use cubic super cell for 0.25 ≤ x ≤ 0.75.We describe the interatomic exchange interactions in terms of the classical Heisenberg HamiltonianIn Eq.(1), the indices µ and ν number different sublattices and R and R ′ are the lattice vectors specifying the atoms within sublattices, s µ R is the unit vector pointing in the direction of the magnetic moment at site (µ, R).