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

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Abstract
Heat capacity and susceptibility measurements are performed down to 70 mK under magnetic fields in the Haldane-gap antiferromagnet (CH 3 ) 4 NNi(NO 2 ) 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 H C , which has been also observed in NENP. In the adiabatic magnetization measurement, the spin temperature shows a minimum around H C . These results are interpreted as increasing of the energy gap above H C which may suppress the expected long-range-ordering.