Field-dependent specific heat inFe2VAland the question of possible3dheavy fermion behavior

Abstract

Specific heat measurements on Fe 2 VAl show the previously reported upturn in electronic specific heat coefficient (␥) to be sample dependent, and related to magnetic defects. These measurements, in temperatures as low as 0.6 K and magnetic fields up to 8 T, indicate the presence of Schottky anomalies arising from magnetic clusters having a moment 3.7 B. This result is in good agreement with theoretical estimates for Fe antisite defects in the material. The inherent ␥ϭ1.5Ϯ0.3 mJ/mol K 2 deduced from this work is considerably less than previously reported, and the behavior does not appear consistent with heavy fermion behavior. However, the mass enhancement is significant when compared to nuclear magnetic resonance and band calculations, and we propose a spin-fluctuation mechanism. ͓S0163-1829͑99͒51444-6͔

Structure and magnetic order in Fe2+xV1−xAl

“…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.…”

“…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.…”

“…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.…”

Weak itinerant ferromagnetism in Heusler-typeFe2VAl0.95

“…7,[10][11][12][13][14] Consequently, the samples at x Ϸ 0 and ␦ Ϸ 0 are located at the brink of ferromagnetic order, close to the ferromagnetic quantum critical point, i.e., T C → 0. 6,8 Stimulated by these findings, theoretical work claimed Fe 2 VAl is a nonmagnetic semimetal with the pseudogap at the Fermi level. [15][16][17] The considerable mass enhancement of the conduction carriers was attributed to excitonic correlations 16 or spin fluctuations.…”

“…17 However, subsequent specific heat, C͑T͒, measurements in applied magnetic fields showed that the upturn in C / T at low temperatures results from a Schottky contribution of magnetic clusters in Fe 2 VAl. 8 The effects of off-stoichiometry have not been investigated quantitatively in the framework of nearly and weak ferromagnetism at x Ϸ 0 and ␦ Ϸ 0 yet. Another interesting aspect of Fe 2 VAl is its potential for application in thermoelectric devices, since off-stoichiometry and heattreatment procedures may results in an enhancement of the Seebeck coefficient.…”