Magnetic phase diagram of the S = 1/2 antiferromagnetic ladder (CH3)2CHNH3CuCl3

(CH3)2CHNH3CuCl3 is one of the best laboratory models for the S = 1/2 spin ladder comprising ferromagnetic rungs and antiferromagnetic legs. We have determined the phase boundary between the low-field spin liquid and the field-induced ordered phase of this compound to 100 mK in temperature by means of specific-heat and magnetocaloric-effect measurements for two crystal orientations, with the magnetic field perpendicular to either the so-called B or C plane. The critical field of antiferromagnetic ordering shows power-law dependences on temperature, indicative of the Bose-Einstein condensation of spin triplets due to closing of the energy gap by the field.