Spontaneous Hall effect in an antiferromagnet Mn₁₁Ge₈ with broken time-reversal symmetry

In conventional ferromagnets, the magnitude of anomalous Hall effect is proportional to net magnetization. On the other hand, in recent times, it was theoretically proposed that some antiferromagnets with broken time-reversal symmetry (TRS) can induce large spontaneous Hall effect despite vanishingly small net magnetization. In this study, we investigated the properties of Mn₁₁Ge₈ as a candidate of TRS-broken antiferromagnet. This material was reported to show weak spontaneous magnetization along b-axis, which implies the realization of antiferromagnetic order with broken TRS. By performing detailed magnetization measurement, we identified four distinctive magnetic phases as a function of temperature and magnetic field. We also found that the Hall resistivity profile does not simply scale with net magnetization, implying the additional Hall contribution originating from TRS-broken antiferromagnetic order.