Mössbauer spectroscopy study of the randomly mixed two-dimensional antiferromagnet with competing spin anisotropies K2CoxFe1-xF4.

Abstract

We have studied the randomly mixed two-dimensional antiferromagnet ${\mathrm{K}}_{2}$${\mathrm{Co}}_{\mathrm{x}}$${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{F}}_{4}$ (x=0.27, 0.55, and 0.70) with M\"ossbauer absorption. In the latter two compounds a phase transition to a uniaxial antiferromagnetic phase with the Fe moments aligned along the Co easy axis (c axis) is observed, and ${T}_{N}$=90\ifmmode\pm\else\textpm\fi{}1 and 97\ifmmode\pm\else\textpm\fi{}1 K, respectively. In the x=0.27 compound below ${T}_{N}$=64\ifmmode\pm\else\textpm\fi{}1 K, the Fe moments acquire a static component along the c axis. Below ${T}_{L}$=27\ifmmode\pm\else\textpm\fi{}2 K the transverse components also become ordered, at least on the M\"ossbauer time scale, confirming previous neutron-diffraction experiments. A comparison with the results of this technique further shows the Co and Fe spins to differ in orientation by up to 30\ifmmode^\circ\else\textdegree\fi{}, depending on the local surroundings. The M\"ossbauer spectra at 4.2 K reveal a widening of the distribution of the hyperfine parameters when x approaches the multicritical value. In the case of x=0.27, the widening increases sharply towards ${T}_{L}$, and reflects substantial fluctuations of the transverse components persisting over an unusually large critical regime above ${T}_{L}$.