Paramagnetic Resonance below 1°K: Spin-Lattice Relaxation ofCe3+andNd3+in Lanthanum Magnesium Nitrate

An apparatus is described for performing paramagnetic resonance experiments at $\nu \approx 9.6$ kMc/sec at temperatures down to $T\approx 0.2°$K, obtained by adiabatic demagnetization. It is used to extend the spin-lattice relaxation rate ${T_1}^{-1\mathrm{}}$ measurements of the previous paper on Nd in the double nitrate ${\mathrm{La}}_2$ ${\mathrm{Mg}}_3$ ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_{12}$.24${\mathrm{H}}_2$O down to 0.3°K, where the direct process is very well displayed. For $z\perp H$ the result is ${T_1}^{-1\mathrm{}}=0.3\mathrm{}coth\left(\frac{h\nu }{2kT}\right)$ ${\sec }^{-1\mathrm{}}$, to be compared to the theoretical estimate ${T_1}^{-1\mathrm{}}=0.6\mathrm{}coth\left(\frac{h\nu }{2kT}\right)$ ${\sec }^{-1\mathrm{}}$. At the lowest temperatures the relaxation rate tends toward the temperature-independent value of spontaneous phonon emission.