Nuclear relaxation in antiferromagnetic crystals

Abstract

Synopsis A general theory of the spin-lattice relaxation process of nuclear spins in antiferromagnetic crystals, resulting from the magnetic dipolar interaction between the nuclear spins and the antiferromagnetic spins is developed. A detailed calculation of the relaxation time t1 is carried out on the basis of the spin wave approximation for the antiferromagnetic spin system. At not too high temperatures the dominant relaxation processes are the Raman spin wave processes. For negligible anisotropy of the antiferromagnetic spin system, t1 varies as 1/T3 at low temperature (T ≪ TN, where TN is the Neel temperature), while for a finite anisotropy the temperature dependence of t1 is much stronger. The theory is compared with the experimental results of Hardeman, Poulis and v. d. Lugt on the protons in CuCl2.2H2O. The order of magnitude of t1 at the lowest temperature (1.2°K) agrees with the theoretical value, but the temperature dependence is stronger than predicted. Possible sources of this strong temperature dependence are indicated. The dependence of t1 on the orientation of the external field with respect to the crystal axes is discussed, and a phenomenological method of interpreting the angular dependende of t1 in terms of spin wave theory is suggested. Finally the non-observability of the nuclear fluorine resonance in MnF2 is disucssed.