Electronic and Magnetic properties of Fe-based Heusler alloys: role of defects and disorder

Intermetallic Fe2TMAl (where TM=V, Cr, Ti, Mn, Nb) Heusler alloys have recently triggered an intense research activity to understand its unusual physical properties. The most intriguing characteristics of Fe2VAl alloy are high resistivity (ρ~103 μΩ-cm), negative temperature coefficient of resistance and nonmagnetic behavior down to 2K in spite of having 50% Fe as one of the constituents. Similarly, a negative temperature coefficient of resistivity in the whole temperature range was reported experimentally for Fe2CrAl, but was predicted to be a half metallic ferromagnet from band structure calculation. These physical properties are very sensitive to substitutions as well as anti-site disorder, resembling doped semiconductor-like behavior. However the results published on magnetic and electrical transport behavior gave a rather confusing picture of the situation. In this work we carried out a comprehensive study through simulations and experiments on Cr, Nb and Cu substituted Fe2VAl alloys and the role of disorder and defects on physical properties. When the V atoms of cluster glass type Fe2VAl alloy are replaced with Cr atoms it appears that the antiferromagnetic coupling reduces and ferromagnetic component increases to establish long-range magnetic order for Fe2CrAl. The role of anti-site disorder on electronics properties has been demonstrated in Fe2V1-xCrxAl alloys through a systematic analysis of structural, magnetic and magneto-transport data. From the present analysis, we have shown that Fe2CrAl exhibits a double magnetic transition, first at T= 208 K being long range ferromagnetic order and second at T= 313 K results from Cr clusters generated due to site disorder. We present a detailed magnetic critical behavior study of Fe2CrAl alloy, but the estimated critical exponents are in between the theoretically predicted values for three-dimensional Heisenberg and mean-field interaction models. From the experimental data and electronic structure simulations on Fe2V1-xYxAl (Y=Cr, Nb and Cu) alloys, we suggest that the perfectly ordered Fe2VAl composition is a nonmagnetic, semimetal. A magnetic metallic state can be realized due to site disorder/defects. We have also shown that not only electron density but also lattice constant plays a major role in generating unusual yet useful properties for various applications. From these investigations, it is clear that Fe2VAl alloy is in the vicinity of magnetic and electrical percolation threshold due to its peculiar band structure.