Dependence of Spin-Lattice Relaxation Time Upon Magnetic Field for Two Salts of Neodymium

Measurements made below 1.5^∘{}K of the spin-lattice relaxation rate T₁^-1 of Nd³⁺ in lanthanum fluoride and lanthanum magnesium nitrate as a function of the separation δ of the levels of the ground doublet give clear evidence for a dependence T₁^-1∝δ⁵coth(δ2kT), as expected for the direct process. There is also a second contribution at lower fields in LaF₃, T₁^-1∝δ³coth(δ2kT), which we believe arises from "forbidden" relaxation transitions in the neodymium ions which have a hyperfine structure. The relaxation rate for the forbidden transition has been calculated, and it has been shown that under certain circumstances it can be more rapid than the relaxation rate for allowed transitions. There is a third contribution which depends upon the concentration of Nd³⁺ and may be due to cross relaxation to coupled pairs of Nd³⁺ ions. In lanthanum magnesium nitrate there is a phonon bottleneck as well as the direct process. Both salts show an Orbach process above 2^∘{}K. A modification of the usual pulse saturation technique has been used to obtain these measurements which gives some advantages over the standard method.