Magnetization-step spectra of(CH3−NH3)2Mn

Abstract

The compounds ͑CH 3 -NH 3 ͒ 2 Mn x Cd 1−x Cl 4 are among the better physical realizations of a diluted Heisenberg antiferromagnet on the square lattice. The magnetization of three powder samples, with x = 0.063, 0.067, and 0.157, was measured at temperatures T Х 20 mK in magnetic fields B up to 17 T. Magnetization-step ͑MST͒ spectra were obtained with a much higher resolution than in the earlier MST study at T = 0.6 K. The earlier study uncovered only two spectral lines, near 6.6 and 13 T. These lines were attributed to nearest-neighbor ͑NN͒ pairs. The higher-resolution richer spectrum observed at T Х 20 mK is interpreted using a theory which includes two exchange constants: the largest exchange constant, J ͑1͒ , and the second-largest, J ͑2͒ . A summary of the relevant results of this theory is given. The main result of the present work is the determination of J ͑2͒ by two independent methods. The first method used a series of MSTs that was observed at T Х 20 mK in magnetic fields below 2 T. These MSTs, interpreted as the MSTs from J ͑2͒ pairs, gave J ͑2͒ / k B = −0.227± 0.010 K. The second method used the fine-structure ͑FS͒ splitting of the spectral line near 6.6 T. It gave J ͑2͒ / k B = −0.208± 0.006 K. The FS splitting of the spectral line near 13 T is smaller, as expected, and is consistent with the same value of J ͑2͒ . Attempts to identify the neighbor ͑in the cation lattice͒ that is associated with J ͑2͒ were unsuccessful. This failure is attributed to previously observed deviations from a random Mn distribution. The field separation between the strongest spectral line in the FS near 13 T and the strongest line near 6.6 T gave J ͑1͒ / k B = −4.39± 0.05 K, in excellent agreement with the earlier MST study. Much earlier measurements of the spin-wave dispersion curves in ͑CH 3 -NH 3 ͒ 2 MnCl 4 identified J ͑1͒ as the NN exchange constant J 1 . Comments about the theoretical interpretation of the present data are relegated to two appendixes.

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