Low Temperature Thermal and Magnetic Properties of Haldane-Gap Antiferromagnet TMNIN

Heat capacity and susceptibility measurements are performed down to 70 mK under magnetic fields in the Haldane-gap antiferromagnet (CH3)4NNi(NO2)3 (TMNIN). We have observed no magnetic long range order within our experimental range of the temperature down to 70 mK and the magnetic field up to 7 T. The heat capacity reflects the field dependence of the energy gap and show a broad anomaly above a critical field HC, which has been also observed in NENP. In the adiabatic magnetization measurement, the spin temperature shows a minimum around Hc.These results are interpreted as increasing of the energy gap above Hc which may suppress the expected long-range-ordering.