报告摘要：

Exploring the unusual Magnetic behavior of Intermetallic Alloys: Investigations on (i). Fe2VAl1-xMx (M=Sn, B), and (ii). RFe5Al7 (R=Gd, Dy)

by Venkatesh Chandragiri

The presentation is divided into two parts in which, initially recent results on the Fe-based Heusler alloys would be presented, followed by the discussion on the magnetic and transport behavior of rare earth based magnetically competitive sub-lattice systems.

It is well-known that Heusler-type compounds display remarkable ‘site-selectivity’ properties in the sense that substituted metal atoms show a preference for entering the lattice in specific crystallographic positions. In connection to this, the compound Fe2VAl has attracted special attention because of the intriguing behaviour of its electrical resistivity and magnetic properties [1-3]. In the present study, the ab-initio calculations on Fe2VAl alloy have been carried out by introducing B2, DO3, A2' and  XY-XZ type disorders in order to understand the role of site-disorder on the ground state of Fe2VAl. These studies showed an enhancement of individual spin moments of anti-site Fe atoms in DO3, A2' and XY-XZ type anti-site disorders, making the Fe2VAl alloy magnetically active [4]. A comprehensive study of magnetic and electrical transport properties of Fe2VAl1-xMx (M=Sn, Si, x=0-0.1) is also presented. The temperature dependence of transport and magnetization data on Fe2VAl1-xSnx (x=0-0.1) alloys showed a high temperature non-metallic (non-magnetic) to low temperature metallic (magnetic) transition. On the other side, no clear magnetic transition was seen in the Fe2VAl1-xSix (x=0-0.1) alloys with the metallic like transport behavior until the room temperature. A sharp decrease in the electrical resistivity and a substantial enhancement in thermoelectric power by doping Sn & Si elements were explained on the basis of electronic band structure of the corresponding material in which the Fermi level shifts slightly from the middle of their pseudo gap. Further, the unusual physical properties of Fe2VAl1-xBx (x=0-0.1) will be presented.

In the second part, the magnetic, magnetocaloric and magnetoresistive properties of Gd and Dy members of the rare-earth (R) series RFe5Al7, crystallizing in ThMn12 structure [5], known to order antiferromagnetically are explored.  Particularly, I would like to emphasize on the following novel findings: (i) There are multiple sign-crossovers in the temperature (T) dependence of isothermal entropy change (ΔS) in the case of Dy compound; in addition to nil ΔS at the magnetic compensation point known for two magnetic sub-lattice systems, there is an additional sign-crossover at low temperatures, indicative of ‘re-entrant inverse magnetocaloric’ phenomenon.  Corresponding sign reversals could also be observed in the magnetoresistance data. (ii). The plots of magnetoresistance versus magnetic field are found to be highly asymmetric with the reversal of the direction of magnetic-field (H) well below TN for both compounds, similar to that known for an antiferromagnetic tunnel junctions [6]. Finally, these to subtle changes are attributed to the competitive orientations of R and Fe moments induced by T and H.

Reference:

[1] Y. Nishino, M. Kato, S. Asano, K. Soda, M. Hayasaki, U. Mizutani, Semiconductorlike Behavior of Electrical Resistivity in Heusler-type Fe2VAl Compound, Phys. Rev. Lett., 79 (1997) 1909-1912.

[2] C.S. Lue, H.R. Joseph, Jr., K.D.D. Rathnayaka, D.G. Naugle, S.Y. Wu, W.H. Li, Superparamagnetism and magnetic defects in Fe2VAl and Fe2VGa, J. Phys. C: Condensed Matter, 13 (2001) 1585.

[3] C.S. Lue, J.H. Ross, Jr., C.F. Chang, H.D. Yang, Field-dependent specific heat in Fe2VAl and the question of possible 3d heavy fermion behavior, Phys. Rev. B, 60 (1999) R13941-R13945.

[4] C. Venkatesh, V. Srinivas, V.V. Rao, S.K. Srivastava, P.S. Babu, Effect of site disorder on the electronic properties of Fe2VAl Heusler alloy, J. Alloys Comp., 577 (2013) 417-425.

[5] I. Felner, I. Nowik, M. Seh, Ferrimagnetism and hyperfine interactions in RFe5Al7(R = rare earth), J. Magn. Magn. Mater., 38 (1983) 172-182.

[6] V. Chandragiri, K.K. Iyer, E.V. Sampathkumaran, Magnetic and magnetotransport behavior of RFe5A7 (R=Gd and Dy): Observation of reentrant inverse-magnetocaloric phenomenon and asymmetric magnetoresistance behavior, Phys. Rev. B, 92 (2015) 014407.