Anomalous Magnetoresistance in Antiferromagnetic Semiconductor NaMnBi*

The robustness of antiferromagnetic order to perturbations from external magnetic fields and the lack of a net moment give antiferromagnetic (AFM) materials advantages over ferromagnetic materials for potential spintronic device applications. The development of AFM systems with interesting properties is vital to the realization of AFM spintronics. The semiconductor NaMnBi (T_N ~340 K) exhibits anomalous magnetoresistance depending on growth conditions, with after growth quenching increasing the positive MR from 25% at 2 K and 9 T in the unquenched sample to 10000% after quenching, which also introduces hysteresis in the magnetic susceptibility.

Composition analysis indicates a stoichiometry of Na_0.92MnBi and Na_0.81MnBi_0.86 for the as-grown and quenched crystals, respectively. Neutron scattering experiments were carried out to characterize the structures of the samples. Both samples have the same tetragonal symmetry, however, the magnetic moment undergoes canting to the (011) direction from the collinear c-axis alignment in the quenched sample. Moreover, an increase in the diffuse scattering is also observed. Local structure analysis of the disorder of the crystal structure gives insight into the mechanism of the MR behavior.