| 研究実績の概要 |
We discovered a very large magnetoresistive effect in the family of BaMn2Pn2 antiferromagnetic insulators, where Pn stands for As, Sb, and Bi. In this effect, the conductivities of the materials are strongly enhanced when the antiferromagnetic moments are tilted by an external magnetic field. In the case of BaMn2Bi2, the effect is strong enough to suppress the insulating ground states and a metallic ground state is induced. Interestingly, the Neel temperatures of the BaMn2Pn2 are high, and no magnetic transition was observed. This makes our findings stand out from other well-known colossal magnetoresistive effects, in which the magnetic field induces a transition from an antiferromagnetic insulator to a ferromagnetic metals.
From the viewpoint of symmetry, the magnetoresistance in BaMn2Pn2 can be explained as an effect of the breaking the parity-time symmetry inherited from the antiferromagnetic order. In BaMn2Pn2, the spin up and spin down Mn ions can exchange their crystallographic positions via an inversion center; and hence the time- and space-inversion operations are intertwined. A space inversion simultaneously invert the spins of the Mn ions, and this is called parity-time symmetry. Under certain conditions, the parity-time symmetry allows or even facilitates the localization of carriers, being similar the effect of time reversal symmetry in more conventional materials. The parity-time symmetry is destroyed by a magnetic fields that tilts the antiferromagnetic moments, and in turn the localization can be lifted, producing a large negative magnetoresistance.
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