Mössbauer spectroscopy study of the randomly mixed two-dimensional antiferromagnet with competing spin anisotropies ${\mathrm{K}}_2$${\mathrm{Co}}_{\mathrm{x}}$${\mathrm{Fe}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{F}}_4$

We have studied the randomly mixed two-dimensional antiferromagnet ${\mathrm{K}}_2$ ${\mathrm{Co}}_{\mathrm{x}}$ ${\mathrm{Fe}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{F}}_4$ (x=0.27, 0.55, and 0.70) with Mössbauer 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±1 and 97±1 K, respectively. In the x=0.27 compound below $T_N$=64±1 K, the Fe moments acquire a static component along the c axis. Below $T_L$=27±2 K the transverse components also become ordered, at least on the Mössbauer 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°, depending on the local surroundings. The Mössbauer 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 $T_L$.