EPR study of the phase transition in Mn2+-doped ammonium hydrogen oxalate hemihydrate single crystals.

Abstract

The EPR spectra of ${\mathrm{Mn}}^{2+}$-doped ${\mathrm{NH}}_{4}$${\mathrm{HC}}_{2}$${\mathrm{O}}_{4}$\ensuremath{\cdot}1/${2\mathrm{H}}_{2}$O single crystals have been studied at the X band between 123 and 290 K. ${\mathrm{Mn}}^{2+}$ ions substitute for ${\mathrm{NH}}_{4}^{+}$ ions. Below the phase-transition temperature ${\mathit{T}}_{\mathit{c}}$ of the crystal, each hyperfine line of the spectrum is split into two components. The critical exponent \ensuremath{\beta} has been determined to be 0.41(3) from the line splitting below ${\mathit{T}}_{\mathit{c}}$.