Structure and Magnetic Order in Fe 2+x V 1−x Al

We present a detailed structural investigation via neutron diffraction of differently heat treated samples Fe 2 VAl and Fe 2+x V 1−x Al. Moreover, the magnetic behaviour of these materials is studied by means of µSR and Mössbauer-experiments. Our structural investigation indicates that quenched Fe 2 VAl, exhibiting the previously reported " Kondo insulating like " behaviour, is off-stoichiometric (6%) in itsAl content. Slowly cooled Fe 2 VAl is structurally better ordered and stoichiometric, and the microscopic magnetic probes establish long range ferromagnetic order below T C = 13K, consistent with results from bulk experiments. The magnetic state can be modelled as being generated by diluted magnetic ions in a non-magnetic matrix. Quantitatively , the required number of magnetic ions is too large as to be explained by a model of Fe/V site exchange. We discuss the implications of our findings for the ground state properties of Fe 2 VAl, in particular with respect to the role of crystallographic disorder. 1.) Introduction Recently, the magnetic phase diagram of the alloying series (Fe 1−x V x) 3 Al has been the focus of various detailed studies [1,2]. In particular, Heusler-type Fe 2 VAl has been reported to exhibit a very unusual behaviour for an intermetallic compound, namely a semiconductor-like resistivity close to a magnetic instability [1]. This was interpreted in terms of Kondo-insulating behaviour, analogous to the system FeSi [3,4]. In contrast, optical conductivity studies provided evidence for a pseudogap in the density of states of 1 Fe 2 VAl of 0.1-0.2eV [5], a view supported by various band structure calculations [6-8]. Notably, no temperature dependence of the gap features has been detected in these studies, apparently contradicting a Kondo insulator scenario for Fe 2 VAl. However, the pseudogap scenario itself does not account for the unusual resistivity of Fe 2 VAl, as in the absence of magnetic correlations it should predict no or a positive metallic magnetoresistance, in conflict with experimental observations [2,9]. Therefore, in Ref. 5 it has been speculated that the (magneto)resistivity of Fe 2 VAl reflects a mixture of electron excitation processes over the pseudogap and spin dependent scattering from impurities. Independently, on basis of specific heat and NMR-experiments it has been demonstrated that in Fe 2 VAl crystallographic disorder, assumed to be present in form of atomic site exchange between Fe and V atoms, substantially affects the ground state properties of this compound [10,11]. In particular, …