Specific heat of an S=1 quasi-1D bond alternating antiferromagnet in a magnetic field

Abstract We report the results of specific heat measurements in a magnetic field on single crystal samples of the S =1 quasi-1D bond alternating antiferromagnet Ni(C 9 H 24 N 4 )NO 2 (ClO 4 ). The compound has the singlet ground state with an excitation energy gap and the gap closes by application of magnetic field ( H c ). We observed a sharp peak corresponding to the long-range ordering above H c both parallel and perpendicular to the chain at low temperatures. The temperature-magnetic field phase diagram is discussed in comparison with those of the Haldane system Ni(C 5 H 14 N 2 ) 2 N 3 (PF 6 ).