Paramagnetic Relaxation at Very Low Temperatures

A series of experiments using paramagnetic resonance at microwave frequencies and in the 1-4°K temperature range on ${\mathrm{Gd}}_2$ ${\mathrm{Mg}}_3$ ${\left({\mathrm{NO}}_3\right)}_{12}$·24${\mathrm{H}}_2$O, ${\mathrm{K}}_3$Cr${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_6$, and Cu${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2$ ${\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$·6${\mathrm{H}}_2$O indicate the following characteristics for their relaxation: (1) It is the lattice-bath relaxation which limits the total rate of relaxation. (2) The spin-lattice relaxation time is several orders of magnitude smaller than the normally observed values of $T_1$. (3) Breadth of the lattice modes is very much larger than the width of the resonances in diluted crystals, and in particular, for 1% paramagnetic concentration of the Cu salt, the breadth is several hundreds of megacycles/second. (4) Breadth of the lattice modes increases with increasing concentration of paramagnetic centers. (5) Two nearby resonances are rapidly brought to the same effective temperature by exchange of energy through the lattice modes. (6) The relaxation time $T_1$ is dependent on crystal size. Some consequences of these results are also discussed.