Field-induced magnetic long-range order in the ferromagnetic-antiferromagnetic alternating Heisenberg chain system (CH3)2CHNH3CuCl3 observed by specific heat measurements

The compound (CH3)(2)CHNH3CuCl3 has recently been found to be an ideal ferromagnetic-antiferromagnetic alternating Heisenberg chain with the energy gap Delta/k = 17 similar to 18K [H. Manaka et al.: J. Phys. Sec. Jpn. 66 (1997) 564]. When an external field H is applied to this compound, the energy gap should vanish at the critical field H-c1 = Delta/g mu(B). Just above H-c1, the three-dimensional magnetic long-range order is expected, if the interchain coupling exists. Motivated by this expectation, we performed specific heat measurements over a temperature range from 0.5 to 5 K applying H up to 12 T. As a result. we found a lambda-type anomaly of specific heat over 10 similar to 12T, which indicates a transition to the three-dimensional long range order. The temperature at which the lambda-type anomaly appears increases with increasing H. The critical field obtained by extrapolating the curve of the lambda peak position versus H is 9.0 T; which is close to that obtained from a magnetization measurement.