Magnetoresistance (MR), thermo power, magnetization and Hall effect measurements have been performed on Co-doped Bi2Se3 topological insulators. The undoped sample shows that the maximum MR as a destructive interference due to a π-Berry phase leads to a decrease of MR. As the Co is doped, the linearity in MR is increased. The observed MR of Bi2Se3 can be explained with the classical model. The low temperature MR behavior of Co doped samples cannot be explained with the same model, but can be explained with the quantum linear MR model. Magnetization behavior indicates the establishment of ferromagnetic ordering with Co doping. Hall effect data also supports the establishment of ferromagnetic ordering in Co-doped Bi2Se3 samples by showing the anomalous Hall effect. Furthermore, when spectral weight suppression is insignificant, Bi2Se3 behaves as a dilute magnetic semiconductor. Moreover, the maximum power factor is observed when time reversal symmetry (TRS) is maintained. As the TRS is broken the power factor value is decreased, which indicates that with the rise of Dirac cone above the Fermi level the anomalous Hall effect and linearity in MR increase and the power factor decreases.

No abstract

Abstract. We report the experimental observation of spin reorientation in the double perovskite Ho 2 FeCoO 6 . The magnetic phase transitions in this compound are characterized and studied through magnetization and specific heat, and the magnetic structures are elucidated through neutron powder diffraction. Two magnetic phase transitions are observed in this compound -one at T N1 ≈ 250 K, from paramagnetic to antiferromagnetic, and the other at T N2 ≈ 45 K, from a phase with mixed magnetic structures to a single phase through a spin reorientation process. The magnetic structure in the temperature range 200 K -45 K is a mixed phase of the irreducible representations Γ 1 and Γ 3 , both of which are antiferromagnetic. The phase with mixed magnetic structures that exists in Ho 2 FeCoO 6 gives rise to a large thermal hysteresis in magnetization that extends from 200 K down to the spin reorientation temperature. At T N2 , the magnetic structure transforms to Γ 1 . Though long-range magnetic order is established in the transition metal lattice, it is seen that only short-range magnetic order prevails in Ho 3+ -lattice. Our results should motivate further detailed studies on single crystals in order to explore spin reorientation process, spin switching and the possibility of anisotropic magnetic interactions giving rise to electric polarization in Ho 2 FeCoO 6 .

We report the theoretical prediction of a class of spintronic materials, namely, bipolar magnetic semiconductors (BMSs), also supported by our experimental data. BMSs possess a unique band structure with unequal band gaps for spin-up and spin-down channels and thus are useful for tunable spin-transport-based applications such as spin filters. The valence band and conduction band in BMSs approach the Fermi level through opposite spin channels and hence facilitate reversible spin polarization that is controllable via applied gate voltage. We report the quaternary Heusler alloy VNbRuAl to exactly possess the band structure of a BMS. A rigorous normal x-ray diffraction (XRD) fitting along with synchrotron XRD data confirms that this alloy crystallizes in the LiMgPdSn structure with partial B2 disorder. Transport measurements show a two-channel semiconducting behavior and a quasilinear dependence of negative magnetoresistance, indicating the possible semiconducting nature. The thermoelectric power data not only confirm the semiconducting nature but also give a strong indication of the BMS nature. Interestingly, VNbRuAl also appears to show features of a fully compensated ferrimagnetic (FCF) behavior with vanishing magnetization and significantly high ordering temperature (>900 K). Theoretical simulations of the special quasirandom structure predict partial B2 disorder to be mainly responsible for the coexistent BMS and FCF-like behavior. This study opens up the possibility of finding another class of materials for antiferromagnetic spintronics, with great significance for both fundamental and applied fronts.

No abstract