Unusual magnetic ordering observed in nanosized S = 1/2 quantum spin system (CH3)2NH2CuCl3

Specific heat measurements were performed on nanometer-sized particles of an S = 1/2 quantum spin system, DMACuCl3 [DMA = (CH3)2NH2]. A tiny peak was observed at a low temperature of approximately 1.5 K at zero magnetic field, in addition to a well-defined and commonly observed peak with the bulk system at approximately 0.8 K. Under magnetic fields, it increased in intensity and coincided with the phase boundary responsible for the field-induced magnetic ordering phase above about 6 T. 1H-NMR studies confirmed that such extra peaks also indicated magnetic ordering. The resultant phase diagram is considerably different from that observed previously in the bulk system. Such a difference may be caused by the presence of extra spins found in a nanosized system by ESR measurements.