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.
Optoelectronic applications with
transparent conducting oxides
have been made possible by modulating the carrier density of wide
band gap oxides with doping. We demonstrate the modulation of the
density of states (DOS) at the Fermi level in nanocrystalline CuAlO
2
particles synthesized using a sol–gel technique, as
a function of doping with a magnetic impurity (Ni). This behavior
is directly correlated with structural studies using X-ray diffraction
and magnetic properties which show a similar trend. Our results can
be understood in a picture where charge hopping occurs through surface
or defect states, rather than by direct hopping between the quantum-confined
states of the nanocrystal, and an increase in the DOS at the Fermi
level caused by the substitution of Ni atoms at the Al site.
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