Characteristics of a granular electronic system in Heusler-type Fe_2+xV_1−xAl

Abstract

We report comprehensive measurements of the magnetic, transport, and thermal properties of the Heusler-type compound Fe2+xV1-xAl, with x values near the ferromagnetic quantum critical point, xc ∼ 0.05. At T ∼ 60 K, a prominent Schottky-like anomaly appeared in the specific heat; this anomaly was correlated with a smaller pseudo-gap formation in magnetic susceptibility, magnetoresistance, and thermoelectric power. Furthermore, a magnetic anomaly observed in the magnetic susceptibility and resistivity at T ∼ 4 K was suppressed significantly by applying a magnetic field. A magnetically inhomogeneous phase arose below T ∼ 60 K, which appeared to consist of ferromagnetic and paramagnetic clusters.

Composition induced metal–insulator quantum phase transition in the Heusler type Fe2VAl

“…Likewise, as we previously claimed for Fe2+xV1−xAl near the paramagneticferromagnetic QCP at x  0.05 [12,21], the low-temperature enhancement in C/T that we observed at x  −0.05 and the singular temperature dependence, approximated by T −1 , can be reminiscent of a Griffiths-McCoy singularity near the QCP (Fig. 4(a)) [26].…”

“…Note that  was also reduced near the paramagneticferromagnetic QCP at xc m . It is plausible that at x > 0, the field-induced Schottky anomaly could be attributed to the ferromagnetic Griffiths singularity/phase, as we previously speculated [21].…”

“…where CHT was estimated to fit an experimental value of C(x = 0) by means of T + T 2 + T 4 above T = 8 K and was estimated numerically by means of a polynomial series with respect to temperature for T > 30 K [18,12]. As indicated previously, the low-temperature upturn in C/T [9,11,12,[18][19][20][21] and the Schottky-like peak at T  50 K [12,21] are ubiquitous in the range −0.10  x  0.20. The low-temperature upturn in C/T observed at x  0.02 approximately followed a power law with respect to temperature, T −1 , down to T = 2 K, with a nonuniversal exponent which varied continuously as a function of x, specifying divergence of thermodynamic quantities ( Fig.…”

“…The characteristic divergences are reminiscent of a quantum Griffiths phase, which occurs in many disordered correlated systems. The existence of a quantum Griffiths phase in the vicinity of the paramagnetic-ferromagnetic QCP has been confirmed by comprehensive measurements of thermodynamic quantities of transition metallic compounds such as Fe2+xV1−xAl [21], FeV1−xMxGa (M = Ti and Rh) [22], Ni1−xVx [23], and Fe1−xCoxS2 [24] and in the rare earth intermetallic compound Ce1−xLaxRhIn5 [25].…”

“…5(a)). These inconsistencies with the J = 3/2 cluster model can be explained reasonably well by the fact that the antisite Fe occupying the V site facilitates the establishment of itinerant (ferromagnetic) features in Fe2+xV1−xAl at x > 0 [12] and the fact that an inhomogeneous state (Griffiths phase) that appears to consist of ferromagnetic and paramagnetic clusters emerges at 0 < x < xc m [21]. In a theoretical study, Neto et al found that a Schottky anomaly in the specific heat, due to a magnetic field, appears close to the QCP for magnetic ordering in magnetic metallic systems [26].…”