Specific Heat of Ferrites at Liquid Helium Temperatures

The specific heat of lithium, cobalt, magnesium, and nickel ferrite has been measured from 1.8°K to 5°K using a calorimetric technique similar to that used by Clement. A "heat switch" was used to cool the samples rather than a helium exchange gas, thereby avoiding helium desorption effects. In all cases the specific heat $C$ could be described by the relation $C={\alpha }_M{T}^{\frac{3}{2}}+\beta T^3$, where $T$ is the temperature and ${\alpha }_M$ and $\beta$ are constants. The Debye temperatures were computed from $\beta$ and were in good agreement with values obtained at liquid nitrogen temperatures. The $T^{\frac{3}{2}}$ temperature dependence verifies the spin-wave theory for these compounds although the values of ${\alpha }_M$ were consistently larger than anticipated from observed values of the Curie temperatures. Possible explanations for this are discussed. An extra term proportional to $T^{-2\mathrm{}}$ was observed for cobalt ferrite and this was identified as the nuclear contribution to the specific heat. From the magnitude of this term the magnetic field at the nucleus of the cobalt ion was evaluated and found to be approximately 410 koe.