Nuclear Magnetic Relaxation in Antiferromagnetics

The mechanism of the thermal relaxation of nuclear spins related with the nuclear magnetic resonance in antiferromagnetics is studied theoretically. The predominant mechanism is provided by the fluctuating local magnetic field at the nucleus coming from the electron spins which flip from time to time due to exchange interactions among them. At low temperatures this mechanism can be described in terms of the inelastic scatterings of spin waves by nuclear magnetic moments, and the thermal transition probability can be calculated from this point of view. At high temperatures the relaxation time is obtained from the local field spectra which are calculated with the use of the high temperature approximation assuming a Gaussian distribution. The calculated relaxation time of protons in CuCl2·2H2O agrees well with the experiment both in order of magnitude and in the qualitative nature of its temperature dependence. However, for MnF2, whose nuclear resonnce has not yet been observed in the range 300∼1.5 °K, the calculated relaxation time is always longer than 10−5 sec and it becomes rapidly longer as temperature is lowered. This contradicts with the interpretation of Bloembergen and Poulis who accounted for the absence of the resonance by assuming that T1 is shorter than 10−6 sec. As a possible origin of the absence of the resonance in the frequency range of their observation, effect of the hyperfine interaction is suggested.