Critical Phonon Scattering near a Ferromagnetic Transition and Its Effect on the Thermal Conductivity of Cu${\mathrm{K}}_2$${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O

By comparing the thermal conductivity in a magnetic field large enough to align the spin system with the conductivity at zero field, it has been possible to measure the change in the thermal conductivity due to critical scattering near the transition in the cubic Heisenberg ferromagnet Cu${\mathrm{K}}_2$ ${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O. A phonon scattering law deduced from these results agrees well with ultrasonic attenuation measurements on the ferromagnet gadolinium. Some comparison has been possible between these results and dynamical scaling laws in their present form.