Magnetoelastic properties in field-induced magnetic ordered phases are studied theoretically based on a Ginzburg-Landau theory. A critical field for the field-induced ordered phase is obtained as a function of temperature and pressure, which determine the phase diagram. It is found that magnetic field dependence of elastic constant decreases discontinuously at the critical field, Hc, and that it decreases linearly with field in the ordered phase (H>Hc). We found an Ehrenfest relation between the field dependence of the elastic constant and the pressure dependence of critical field. Our theory provides the theoretical form for magnetoelastic properties in field- and pressure-induced ordered phases.
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