Enhancement of magnetic frustration caused by Zn doping in quasi-one-dimensional quantum antiferromagnet Cu3Mo2O9

We studied the effects of Zn doped into Cu sites on the magnetism of Cu3Mo2O9. The spin system consists of spin-1/2 antiferromagnetic (AF) uniform chains and AF dimers. Each chain is coupled to neighboring dimers by other interactions, giving rise to magnetic frustration. A component parallel to the chain of magnetic moment in the chain forms AF long-range order (LRO) below the transition temperature T-N, whereas a perpendicular component is disordered in zero magnetic field and above 2.0K. The perpendicular component forms weak ferromagnetic LRO (WF-LRO) only in finite magnetic fields H's. We have found that WF-LRO in H disappears with a very small amount of Zn doping, although the decrease of TN is negligible at corresponding Zn concentrations. These results are explained by the enhancement of magnetic frustration among symmetric exchange interactions caused by Zn doping.