Abstract
Two-sublattice general spin-s exchange antiferromagnets, assumed to have a stable configuration in applied uniform external magnetic fields parallel to their spontaneous spin-ordering direction in the absence of a field, are treated in the molecular- or mean-field approximation. The magnetic phase-boundary line of this antiferromagnetic configuration is shown to have simple closed parametric expressions. In the present approximate formalism the peculiar nonmonotonic phase-boundary line becomes of monotonic variation in the classical limit ofs → ∞. The possibility of strongly anisotropic two-sublattice parallel-field-stable antiferromagnets to function as efficient cooling systems is explored. This arises from their magnetic entropy increase on isothermal magnetization as a consequence of the large asymmetry developed through the respective cooperation and competition of the exchange-coupled spins with the external applied field in determining the effective fields acting on the spins of the two sublattices. The competing or negative sublattice has anomalous behavior in that its component molar spin entropy exhibits its maximum limit of1/2 R ln(2s+1) at low temperatures compared to the zero-field transition temperature, as long as the applied magnetic field strength is a substantial fraction of the critical field strength, above which the system remains paramagnetic down to the absolute zero. The transformation into the antiferromagnetic state is then prevented. The enormous magnetic or spin entropy of the system, persisting down to very low temperatures, according to the mean-field formalism, makes it possible to reach quite low temperatures on adiabatic magnetization from easily accessible initial states in the presence of an external magnetic field. Within the limitations of the model and of its treatment experimental investigations into the preparation of antiferromagnets with the required anisotropy may be of particular interest in connection with their possible use for the production of low temperatures.
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Goldstein, L. Mean-field theory of some magnetized antiferromagnets. J Low Temp Phys 14, 471–499 (1974). https://doi.org/10.1007/BF00658875
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DOI: https://doi.org/10.1007/BF00658875