Anomalous magnetic and Electrical properties of Fe2V2-xAlx (x=0-1) alloys.

Intriguing correlated electronic structure is revealed when vanadium is substituted in place of iron in ferromagnetic DO3-Fe3 Al intermetallic alloy and transforms it into a Fe2 VAl Heusler alloy with L21 crystal structure. This particular composition exhibits nonmagnetic semi-metallic character [1], [2]. Both DFT calculations and experiments revealed the existence of pseudogap or gap ranging up to 0.5 eV. The origin of the gap and the role of hybridization in these alloys is still debated [3]. These studies reveal that the vanadium reduces the moment, yet the role of aluminium in altering the electronic properties is still need to be investigated. Therefore, in the present investigation, we have studied the structural and magnetic properties of Fe2 V2-x Alx alloys with x varying from 0 to1 to unravel the role of Al in Fe2 VAl which is known to be exhibiting the features similar to that of equiatomic FeV. Alloy ingots were synthesized by arc melting the elemental constituents and subsequent annealing at 1273 K for 3 days. From X-Ray Diffraction studies it is observed that x=0 composition (FeV) stabilized in $\sigma -$phase, while a mixed phase (${\sigma }$ + cubic) is obtained for x=0.2. It is observed that as the Al-composition varies from 0.4 to 1 it exhibits a cubic phase. DC magnetic measurements were performed for all samples, which revealed that the magnetic character decreases with Al concentration. The extreme compositions (well ordered) of this series namely, FeV (with $\sigma -$phase) and Fe2 VAl (L21-phase) are found to be paramagnetic down to lowest temperature. The temperature dependence of magnetization curves shown in Fig.1 for composition x=0.2 suggest ferromagnetic character with Curie temperature (Tc) close to 240 K and for x=0.4 with Tc~130K, while the compositions x=0.8, 1 exhibit nonmagnetic behavior down to 5K (Fig.1). The magnetic isotherms of these compositions exhibit anomalous magnetic character i.e., exhibit higher coercivity at 300K compared to 5K (except for Fe2 VAl), with lower saturation magnetization. The existence of hysteresis behavior above Tc indicates the possibility of the presence of ferromagnetic clusters in intermediate compositions. Analysis of magnetic data suggests that disorder plays a significant role in addition to the composition. To see whether the magnetic anomalies affect the electrical transport, we carried out electrical resistivity measurements in the low temperature range, shown in Fig.2. The electrical resistivity shows a positive temperature coefficient of resistivity (TCR) for x=0 to x=0.8 and the resistivity at temperatures below 250K decreases with Al concentration which indicates the presence of Al improves the metallic nature of these samples. But the sample with highest Al concentration i.e Fe2 VAl shows negative TCR which also exhibits high values of resistivity compared to other compositions of the series. This kind of behavior can be attributed to band structure formed due to structural disorder in samples, x=0.2, 0.4, 0.8, and pseudo band gap formed in Fe2 VAl near Fermi level [4], [5]. From above studies, it is quite evident that the material going from metallic state to semi-metallic state with a small variation in Al concentration. The resistivity increased by two orders of magnitude from Fe2 V1.2 Al0.8 Fe2 VAl, whereas both the samples were paramagnetic down to 5K. An inflection point in resistivity curve near the magnetic transition temperature Tc is observed, which suggests a correlation between transport and magnetic properties. These magnetic and electrical transport anomalies show even non-magnetic aluminum plays a significant role in electronic properties and will be discussed in detail.