Magnetismo

Magnetism of mixed quaternary Heusler alloys: (Ni,T)2 MnSn (T=Cu,Pd) as a case study
S.K. Bose
Department of Physics, Brock University, St. Catharines, Ontario, Canada, L2S 3A1

J. Kudrnovsk´ and V. Drchal y
Institute of Physics, Academy of Sciences of the Czech Republic, CZ-182 21 Praha 8, Czech Republic

arXiv:1008.4060v1 [cond-mat.mtrl-sci] 24 Aug 2010

I. Turek
Charles University,Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, CZ-12116 Prague 2, Czech Republic (Dated: August 25, 2010) The electronic properties, exchange interactions, finite-temperature magnetism, and transport properties of random quaternary Heusler Ni2 MnSn alloys doped with Cu- and Pd-atoms are studied theoretically by means of ab initio calculations over theentire range of dopant concentrations. While the magnetic moments are only weakly dependent on the alloy composition, the Curie temperatures exhibit strongly non-linear behavior with respect to Cu-doping in contrast with an almost linear concentration dependence in the case of Pd-doping. The present parameter-free theory agrees qualitatively and also reasonably well quantitatively with the availableexperimental results. An analysis of exchange interactions is provided for a deeper understanding of the problem. The dopant atoms perturb electronic structure close to the Fermi energy only weakly and the residual resistivity thus obeys a simple Nordheim rule. The dominating contribution to the temperature-dependent resistivity is due to thermodynamical fluctuations originating from thespin-disorder, which, according to our calculations, can be described successfully via the disordered local moments model. Results based on this model agree fairly well with the measured values of spin-disorder induced resistivity.
PACS numbers: 71.23.-k,72.25.Ba,75.10.Hk,75.30.Et

I.

INTRODUCTION

Heusler alloys were first studied by the German chemist Friedrich Heusler in 1903, starting with theordered alloy Cu2 MnSn. Because of their interesting physical properties they have been studied intensively in the past as well as more recently.1 The most widely studied Heusler alloys are those with the formula Ni2 MnZ (Z=Sn,Ga,In) and Co2 XY (X=Mn, Fe; Y=Al, Si). The former group is of interest because of potential technological applications based on their magnetic shape memory effect,2 themagnetocaloric effect,3 and the recently observed giant (negative) magnetocaloric effect.4 The latter group holds the promise of application in spintronic devices, thanks to their halfmetallicity at room temperature and above, lattice constant matching with the III-V semiconductors, and large bandgaps. Large tunneling magnetoresistance was measured recently in Co2 MnSi/AlO/Co2 MnSi magnetic tunnelingjunctions.5 Structurally, most Heusler alloys crystallize in two different cubic phases, having either the L21 (X2 YZ) or the C1b (XYZ) symmetry. They can be best visualized as being composed of four interpenetrating fccsublattices, shifted along the body diagonal in the order X-Y-X-Z or X-Y-E-Z, where E denotes the empty, i.e.unoccupied, sublattice.1 An important feature of Heusler alloys is thepresence of chemical or substitutional disorder. It is often the non-stoichiometric composition with respect to the ideal systems such

as Ni2 MnSn or Ni2 MnSb which interpolates between the L21 Heusler and the C1b semi-Heusler alloys.6 Examples are the magnetic shape memory alloys of the type Ni2 Mn1+x Sn1−x (Mn-nonstoichiometry), or the Ni2−x MnSb (Ni-nonstoichiometry) alloys. In addition to thechemical disorder due to nonstoichiometry, a native chemical disorder exists even in ’ideal’ ordered alloys X2 YZ and XYZ. In particular, halfmetallic Heusler alloys are very susceptible to such native disorder, a typical example being the Co2 MnSi alloy which exists in the B2-like structure due to Mn-Si disorder. Finally, there are complex quaternary alloys like the semi-Heusler (Ni,Cu)MnSb...