The thermal conductivity of paramagnetic crystals in a magnetic field at low temperatures.

The thermal conductivities of KMgF3 crystals doped with Ni2+ and Co2+ and crystals of cerium ethylsulphate have been measured in the temperature range 1 to 4.2 K and in magnetic fields up to 52 kilogauss. In all cases the conductivity changes which are observed may be qualitatively interpreted as being due to bands of phonons within a certain energy range being scattered by interaction with the spin system, the energy range being changed as the magnetic field is varied. For the Co2+ doping it is possible that interaction with pairs of Co ions is being observed, rather than with single ions. For the Ni2+ ions and the Co2+ pairs it would appear that in zero field the ground state doublet is separated from a higher singlet with a separation which has a spread of about 3 /cm. Cerium ethylsulphate has two low lying doublets whose g values are very different parallel and perpendicular to the hexagonal axis. The conductivity behaviour of crystals cut along these two axes is therefore quite different and this is briefly discussed