Magnetic Resonance in ${\mathrm{K}}_2$Cu${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O

We have observed the microwave electron spin resonance of ${\mathrm{Cu}}^{2+}$ and the proton nuclear magnetic resonance in the temperature range $0.17\leqq T\leqq 4.2°$K and in fields in the range 3 to 6 kOe for single crystals of ${\mathrm{K}}_2$Cu${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O, an ideal Heisenberg insulating ferromagnet with a Curie temperature $T_c=1.1\mathrm{}°$K. The temperature dependence of the magnetization, as measured by the proton-magnetic-resonance field, is in close agreement with the molecular-field model. The dependence of the microwave ferromagnetic resonance on temperature and sample geometry is in agreement with Kittel's equation. Numerous sharp magnetostatic modes are observed and compared with the theory of Walker. The ferromagnetic resonance line width has a temperature-independent component of a few Oe, and a temperature-dependent component which narrows from 65 to 1 Oe as the temperature varies from 4.2 to 0.7°K. From the observed shift in the ferromagnetic resonance below 0.6°K due to the hyperfine (hfs) field of the copper nuclei, we find that the hfs constant $A=1.5\mathrm{}\times {10}^{-2\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. We observe a new type of ferromagnetic resonance induced by a microwave field parallel to the dc field, which we interpret as a "forbidden" hyperfine resonance corresponding to the simultaneous flip of an electron spin and a nuclear spin.