On the interaction of nuclear spins in a crystalline lattice

Abstract

The exchange of energy between a system of nuclear spins, immersed in a strong magnetic field H o , and the lattice vibrations of insulating crystals has been investigated experimentally by the method of nuclear magnetic resonance absorption. The resonance in calciumfluoride, some hydrated sulphates and some alkalihalides, has been observed between 300°K and 1°K, at 30.5 and 9.5 Mc/sec. The measured spin-lattice relaxation times T 1 range from 10 −3 to 10 4 sec, and disagree violenty — in many cases by more than a factor 10 6 — with Waller's theory of paramagnetic relaxation. It is shown that the relaxation time T 1 is determined by paramagnetic impurities occurring in the lattice. An order of magnitude theory is developed taking these impurities into account. The quanta absorbed by the system of nuclear spins from the external radiofrequeney field diffuse toward the impurities. The energy is then transferred to the lattice vibrations via the fluctuating magnetic field from the impurity. The first process depends on the concentration of the impurities, the second on H o and on the relaxation time ϱ of the impurity. The experimental data receive at least qualitative explanation in terms of these quantities. In some cases a direct heat contact between the system of nuclear spins and the system of impurity spins proves to be important. Then T i is independent of the lattice temperature.