Electron paramagnetic resonance inK2Mn1−xMxF4withM=Fe, Co, and Ni

The EPR linewidth of the quadratic-layer antiferromagnets ${\mathrm{K}}_2{\mathrm{Mn}}_{1-x}{M}_x{\mathrm{F}}_4$, with $M={\mathrm{Fe}}^{2+}(x\le 0.015), {\mathrm{Co}}^{2+}\mathrm{}(x=0.008\mathrm{}), \mathrm{and} {\mathrm{Ni}}^{2+}(x\le 0.125)$, has been studied as a function of the temperature in the paramagnetic regime. In all cases, spin diffusion is only weakly affected by the impurities. The linewidth is, however, severely broadened by Fe and Co due to fast spin-lattice relaxation. The effects of Ni substitution are substantially smaller. With phenomenological equations of motion for the various magnetizations spin-lattice relaxation rates of the impurities are derived. When approaching the Néel temperature $T_N$, the critical enhancement of the linewidth is drastically modified in case of Fe and Co. Below a temperature depending on the concentration and the dopant, a universal reduction of the critical exponent $p$ by about unity is found. Above this point, which occurs at $\frac{(T-T_N)}{T_N}$ up to 0.8, the exponent $p\approx 2.5$ of the pure system is maintained. The results are consistent with an impurity-induced phonon-assisted damping additive to the critical slowing down. In case of Ni, the critical increase remains close to that of the pure system except in the proximity of $T_N$ at the highest concentration.