Specific Heat of Some Rare Earth Iron Garnets and YIG at Low Temperatures

Heat capacity measurements of the iron garnets of Y, Gd, Er, Ho, and Yb between 1.4° and 20°K are presented. Below 5°K, the specific heat of YIG can be represented by the sum of a lattice term proportional to $T^3$ and the spin-wave contribution ${\text{2.15\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−3}}{T}^{\frac{3}{2}}\hspace{0.17em}\mathrm{joules/mole-deg}$ . This last term agrees satisfactorily with that calculated from a spin-wave analysis, in which the exchange interaction coefficients were those derived from Pauthenet's magnetization data. The results of the magnetic specific heat of the rare earth ions could be interpreted in terms of a Weiss molecular field acting on these ions. For ${\mathrm{Gd}}^{\text{3+}}$ and ${\mathrm{Yb}}^{\text{3+}}$ , this field was found to be, respectively, about ${\text{3.0\hspace{0.17em}×\hspace{0.17em}10}}^5$ and ${\text{1.5\hspace{0.17em}×\hspace{0.17em}10}}^5\hspace{0.17em}\mathrm{oe}$ below 20°K, in satisfactory agreement with that derived from Pauthenet's data.