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Recently, a lot of attention has been paid to a novel phase formation as the effect of competing interactions, for example, due to geometrical frustration, and quantum criticality. Here, we present our recent findings of unusual time-reversal-symmetry broken phase formed in a spin liquid state of the geometrically frustrated itinerant magnet Pr$_2$Ir$_2$O$_7$.
The metallic pyrochlore oxide Pr$_2$Ir$_2$O$_7$ forms a spin liquidlike state below 2 K despite the antiferromagnetic interaction of the energy scale of 20 K. Detailed measurements of the anomalous Hall effect in this temperature regime have found a zero field anomalous Hall effect with virtual absence of uniform magnetization. This spontaneous Hall effect indicates that the time-reversal-symmetry is broken in this spin liquidlike state, suggesting a formation of nontrivial spin order due to spin chirality. As the second topic, we would like to discuss the possible exotic phase transitions in two-dimensional triangular Heisenberg antiferromagnets. While vector chirality is expected to play an important role, phase transitions could be strongly affected by anisotropy of spins. Insights obtained through our recent experiments will be discussed. This is the work performed in collaboration with S. Onoda, T. Tayama, T. Sakakibara, M. Tokunaga, S. Tanaka, K. Onuma, and Y. Maeno. |
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