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Abstract

For a certain antiferromagnet, the magnetization does not increase gradually with increasing magnetic field but exhibits field region(s) typically at an integer fraction of its saturation magnetization. This phenomenon is understood by the supposition that such an antiferromagnet undergoes field-induced partitioning of its spin lattice into ferrimagnetic fragments. We searched for a theoretical basis for this supposition by investigating how external magnetic fields affect the magnetic entropy of such an antiferromagnet, to find that the field region of the magnetization plateau has a single magnetic phase, but a nonzero slope region of the magnetization curve has two magnetic phases of different magnetic entropy, and that the magnetic entropy of a single-phase region does not depend on magnetic field but that of a two-phase region does. We tested these predictions by carrying out magnetization and specific heat measurements for g-Mn3(PO4)2. It was found that the magnetic entropy of the two-phase region increases with field, indicating that field-induced breaking of magnetic bonds and hence field-induced partitioning of an antiferromagnetic spin lattice are time-averaged results of all allowed spin arrangements that occur repeatedly during static magnetization measurements. The temperature-dependent magnetic specific heats of g-Mn3(PO4)2 between 2 - 6 K shows a larger excitation gap when measured at 9 T than at 0 T, suggesting that these energy gaps reflect the two successive local excitations of linear Mn2+-Mn2+-Mn2+ ferrimagnetic trimers embedded in the antiferromagnetic spin lattice of g-Mn3(PO4)2 and arise from the Boltzmann factor associated with these excitations