Structure and magnetic order in Fe_2+xV_1−xAl

Abstract

Abstract: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. ) IntroductionRecently, 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, Heuslertype 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][7][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, the anomalous low temperature specific heat has been attributed to ferromagnetic clusters with a density of 0.003-0.004/unit cell, consistent with the results from NMR experiments. These works, as well, are in broad agreement with the results from band structure calculations, which predict that via Fe/V site exchange or crystallographic superstructure formation ferromagnetic clusters or long-range order might be generated in Fe 2 VAl [6][7][8]. Recently, it has been claimed that s...

NMR and Mössbauer study of spin dynamics and electronic structure ofFe2+xV1−xLue1, Li2, Ross3 et al. 2003Phys. Rev. B200110View full textAdd to dashboardCite

“…The relaxation peaks can be attributed to critical slowing down of the local moment dynamics at these transition temperatures. Maxima in the µSR longitudinal relaxation rate were also observed by Maksimov, et al 10 attributed to the same mechanism.…”

“…We measured the spin-echo T 2 and observed a weak temperature increase at low temperatures, to a value of 210 µs at 4 K for x=0, consistent with the freezing-in of local fields as the system goes through its spin freezing at low temperatures. 7 Maksimov et al 10 have similarly seen a suppression of dynamic relaxation processes at the spin freezing point, in a µSR study.…”

“…7,19 Furthermore in Fe 2 VAl and Fe 2 VGa localized magnetic defects have been observed and associated with Fe antisites, 6,12,15 while there is also evidence for larger magnetic clusters in the materials. 7,19 No magnetic splitting was observed in Mössbauer studies near the Fe 2 VAl composition, 10,22,23 while the dominance of defects and clusters makes a percolative behavior seem likely.…”

“…Fe 3 Al is a metallic ferromagnet in which both Fe sites carry a magnetic moment. 20 In Fe 2+x V 1−x Al T c goes monotonically to zero as x goes to zero, 9 although Fe 2 VAl exhibits a sample-dependent low-temperature transition which may be residual ferromagnetism 10,21 or a superparamagnetic freezing temperature. 7,19 Furthermore in Fe 2 VAl and Fe 2 VGa localized magnetic defects have been observed and associated with Fe antisites, 6,12,15 while there is also evidence for larger magnetic clusters in the materials.…”

“…6,7 There has been considerable interest in understanding the role of these clusters for the magnetic transitions, and the unusual electronic behavior associated with these materials. 8,9,10 Both Fe 2 VAl and Fe 2 VGa have been shown to be semimetallic and intrinsically non-magnetic, via electronic structure calculations 11,12,13,14,15 NMR measurements, 7,16,17 and optical measurements.…”