Incommensurate phase in Mn2+ doped (NH4)2ZnCl4 studied by ESR

Ferroelectric (NH4)(2)ZnCl4 crystals doped with Mn2+ were studied by electron spin resonance (ESR) in the temperatures range from 80 degrees C to 160 degrees C. The crystal was found to undergo an incommensurate phase transition at 135"C and an incommensurate-commensurate transition at 92 degrees C. The temperature dependence of the ESR line shape while passing through these changes are in good agreement with the predictions of the phase-solition theory. The parameters h(1) and h(2) determining the variation of the resonant magnetic field show a power-law behavior as a function of temperature. The critical exponent beta of this crystal was determined two independent ways to be 0.35+/-0.02. The solition density remains fairly constant throughout the incommensurate phase and drops abruptly to zero at the incommensurate-commensurate transition, a behavior that is quite different from those of other incommensurate systems.