Successive phase transitions to antiferromagnetic and weak-ferromagnetic long-range order in the quasi-one-dimensional antiferromagnet Cu3Mo2O9

Investigation of the magnetism of Cu3Mo2O9 single crystal, which has antiferromagnetic (AF) linear chains interacting with AF dimers, reveals an AF second-order phase transition at T-N=7.9 K. Although weak-ferromagnetic-like behavior appears at lower temperatures in low magnetic fields, complete remanent magnetization cannot be detected down to 0.5 K. However, a jump is observed in the magnetization below weak-ferromagnetic (WF) phase transition at T-c similar or equal to 2.5 K when a tiny magnetic field along the a axis is reversed, suggesting that the coercive force is very weak. A component of magnetic moment parallel to the chain forms AF long-range order (LRO) below T-N, while a perpendicular component is disordered above T-c at zero magnetic field and forms WF-LRO below T-c. Moreover, the WF-LRO is also realized by applying magnetic fields even between T-c and T-N. These results are explainable by both magnetic frustration among symmetric exchange interactions and competition between symmetric and asymmetric Dzyaloshinskii-Moriya exchange interactions.