Successive phase transitions to antiferromagnetic and weak-ferromagnetic long-range orders in quasi-one-dimensional antiferromagnet Cu$_3$Mo$_2$O$_9$

TL;DR

This study reveals successive phase transitions in Cu$_3$Mo$_2$O$_9$, showing antiferromagnetic and weak-ferromagnetic long-range orders driven by magnetic frustration and Dzyaloshinskii-Moriya interactions, with detailed magnetic behavior observed at low temperatures.

Contribution

It provides the first detailed analysis of the successive magnetic phase transitions and the interplay of exchange interactions in Cu$_3$Mo$_2$O$_9$.

Findings

Antiferromagnetic transition at T_N=7.9 K.

Weak ferromagnetic behavior appears below T_c≈2.5 K.

Magnetic frustration and Dzyaloshinskii-Moriya interactions explain the observed phenomena.

Abstract

Investigation of the magnetism of Cu$_3$Mo$_2$O$_9$ single crystal, which has antiferromagnetic (AF) linear chains interacting with AF dimers, reveals an AF second-order phase transition at $T_{\rm N} = 7.9$ 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 $T_{\rm c} \simeq 2.5$ 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 $T_{\rm N}$, while a perpendicular component is disordered above $T_{\rm c}$ at zero magnetic field and forms WF-LRO below $T_{\rm c}$. Moreover, the WF-LRO is also realized with applying magnetic fields even between $T_{\rm c}$ and $T_{\rm N}$. These results are explainable by both magnetic frustration among symmetric exchange interactions and competition between symmetric and asymmetric Dzyaloshinskii-Moriya exchange interactions.