Ab initio electronic and magnetic properties of half-metallic NiCrSi and NiMnSi Heusler alloys: The role of defects and interfaces

Abstract

Using state-of-the-art first-principles calculations we study the properties of the ferromagnetic Heusler compounds NiYSi where Y stands for V, Cr or Mn. NiCrSi and NiMnSi contrary to NiVSi are half-metallic at their equilibrium lattice constant exhibiting integer values of the total spin magnetic moment and thus we concentrate on these two alloys. The minority-spin gap has the same characteristics as for the well-known NiMnSb alloy being around ∼1 eV. Upon tetragonalization the gap is present in the density of states even for expansion or contraction of the out-of-plane lattice parameter by 5%. The Cr-Cr and Mn-Mn interactions make ferromagnetism extremely stable and the Curie temperature exceeds 1000 K for NiMnSi. Surface and interfaces with GaP, ZnS and Si semiconductors are not half-metallic but in the case of NiCrSi the Ni-based contacts present spinpolarization at the Fermi level over 90%. Finally, we show that there are two cases of defects and atomic-swaps. The first-ones which involve the Cr(Mn) and Si atoms induce states at the edges of the gap which persists for a moderate-concentration of defects. Defects involving Ni atoms induce states localized within the gap completely destroying the half-metallicity. Based on single-impurity calculations we associate these states to the symmetry of the crystal.

Effect of disorder on the magnetic properties of cubic Mn2RuxGa compounds: A first-principles study

“…9 The very small tetragonalization of the lattice observed in the experiments only marginally affects our results and thus has been neglected. 12 Our results suggest that actually it is the disorder caused by the distribution of the Mn and Ru atoms at the various sites which is responsible for the observed behavior of the total spin magnetic moment in the samples and Mn 2 Ru 0.5 Ga is not a half-metal contradicting the assumption in Ref. First, we consider the case of Mn 2 Ga. As stated also in Ref.…”

Effects of structural change on half-metallicity of CoMnZ (Z=P, As, Sb): A first-principle study

“…It can be seen that the Fm-3m CoMnSb presents no half-metallicity, which is consistent with the results reported by Ksenofontov [13]. Two possible reasons are responsible for it: (a) in MnSb sub-cell, Mn atoms lost the four nearest Co atoms, which severely decreases the hybridization between Mn and Co atoms and results in the anti-bonding states moving to lower energy, (b) Co atoms shifting to the vacancy sites promotes a shift of the minority anti-bonding e g bands towards the lower energies [19]. The effects of two kinds of distortions (volume and tetragonal distortions) on the stability of half-metallicity are investigated.…”

The minority spin surface bands of CoS2(001)

“…Surface composition and order are seen to be strongly dependent upon surface preparation [37], but the samples appear to be single crystals with no evidence of twinning or grain boundaries in the LEED or x-ray diffraction. The data reported here are for the stoichiometric surface, with the surface termination in the dense Co-S plane [36], illustrated schematically in figure 1.…”

“…Interface and surface effects then become critical. Minority spin surface states in the nominally half-metallic systems are well known [21,[26][27][28][29][30][31][32][33][34][35][36][37] and can 'evolve' into minority spin interface states in the presence of overlayers [21,[32][33][34][35][36][37]. The existence of such spin-polarized surface-localized electronic states may have profound consequences; the introduction of interfaces can increase the minority spin population at the Fermi level, with the consequent loss of half-metallic character, even near T = 0.…”