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

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 ₉H ₂₄N 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 ₅H ₁₄N ₂) ₂N ₃(PF ₆).