Semimetallic behavior inFe2VAl:NMR evidence

Abstract

We report the results of a 27 Al and 51 V nuclear magnetic resonance study of Fe 2 VAl at temperatures between 4 and 550 K. This material has been a subject of current interest due to indications of possible heavy fermion behavior. The low-temperature NMR relaxation rate follows a Korringa law, indicating a small density of carriers at the Fermi level. At elevated temperatures, the shifts and relaxation rates go over to a thermally activated response, a semiconductorlike behavior, consistent with separate low-lying bands removed from the Fermi-level. These results are consistent with recent electronic structure calculations, and can explain both the reported activated resistivity as well as the Fermi cutoff exhibited in photoemission studies. While we observe nonstoichiometric samples of (Fe 1Ϫx V x ) 3 Al to be magnetic, the xϭ0.33 composition is nonmagnetic, with narrow NMR linewidths. ͓S0163-1829͑98͒07739-X͔

Structure and magnetic order in Fe2+xV1−xAl

“…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.…”

“…From our Mössbauer data we can estimate for sc-Fe 2 VAl the required number of site exchanged Fe II/V pairs: to account for the broadening of the Mössbauer-line below T C it would require at least 6% Fe II on V sites. This value, which is more than one order of magnitude larger than the estimated level of Fe/V site exchanges for q-Fe 2 VAl of 0.3-0.4% [10,11], is inconsistent with the result of our neutron diffraction experiments, setting an upper limit of 3% Fe II on V sites. Therefore, we conclude that neither model proposed so far properly accounts for the Mössbauer spectra of Fe 2+x V 1−x Al, x≈0.…”

“…Our structural investigation proves that slowly-cooled Fe 2 VAl, which we established on a microscopic scale to be ferromagnetically ordered below T C = 13K, is structurally better ordered than quenched material, for which we found evidence for substantial Al off-stoichiometry. For the quenched material the bulk properties [13] resemble the behaviour of "Kondo-insulating like" or "pseudogap" Fe 2 VAl [1,2,5,[9][10][11]16]; this suggests that the materials investigated in those works are Al deficient.As pointed out in Ref.[5], because of the electron count Al deficiency might have a strong effect on the actual position of the Fermi level E F in a pseudogap system. Specifically, it was argued that Al deficiency moves E F out of the centre of the pseudogap into the slopes, implying that Al deficient material should be more metallic than non-deficient one.…”

Effect of onsite Coulomb repulsion on thermoelectric properties of full-Heusler compounds with pseudogaps

“…The actual g(E F ) should be somewhat larger. Okamura et al 3 using photoconductivity measurements also reported similar results for Fe 2 VAl with a band gap of 0.1-0.2 eV and a finite DOS at the Fermi level. The experimental results of the band gap and the temperature range in which they were measured are summarized in Table I.…”

“…These experimental results were quite intriguing. This dual nature was also seen in NMR experiments by Lue et al 2 By measuring the T -dependence of Knight shift (K s ) and the nuclear spin relaxation time (T 1 ) and fitting these two quantities to functions of E g /T , they found a band gap of 0.21-0.22 eV for K s and 0.27 eV for T 1 . Surprisingly, in addition to the activated behaviour, they also found metallic characteristics through Korringa law.…”