Spin-Lattice Relaxation in Cerium Magnesium Nitrate at Liquid Helium Temperature: A New Process

Abstract

The spin-lattice relaxation time of Ce³⁺ in cerium magnesium nitrate has been measured as a function of temperature and of magnetic fields up to 1000 gauss. In the temperature range between 3° and 1.9°K, the relaxation time is found to vary exponentially with inverse temperature, changing by a factor of 350. A theoretical treatment of spin-lattice relaxation appropriate to rare earth salts leads to predictions which agree both with the experimentally observed temperature dependence and with the magnitude of the relaxation time. The dominant relaxation process is one in which a phonon of high energy (» kT) is absorbed by the spin system resulting in a transition from one of the ground state doublet levels to the nearest excited level. Then, in a separate step, another high energy phonon is emitted and a transition is effected to the other level of the ground state doublet. The net result of this `two-step direct process' can be interpreted as spin-lattice relaxation; i.e. there results a net change in population between the ground state doublet levels which brings the spin system towards thermal equilibrium with the lattice. Such a mechanism, which is quite different from those which have been envisaged up to now for spin-lattice relaxation, should be appropriate for many of the rare earth salts.