m+SR studies on pure MnF2 and site-diluted (Mn/sub 0.5/Zn/sub 0.5/)F2

Positive muon spin rotation and relaxation measurements have been carried out on the antiferromagnets, pure MnF2 and site-diluted (Mn/sub 0.5/Zn/sub 0.5/)F2, above and below the Neel temperature T/sub N/ using single-crystal specimens. Two different muon signals have been found in the pure MnF2; with the precession frequency nu/sub A/ = 1.3 GHz for the site A and nu/sub B/ = 152 MHz for the site B measured in zero external magnetic field at T = 5 K. We propose a picture that the signal from the A site represents the ''muonium'' state, and discuss the characteristic features of muonium in magnetic materials. The spin relaxation rate 1/T1, measured in zero external field, decreases rapidly with decreasing temperature below T/sub N/. The mechanism of the spin relaxation above T/sub N/ is explained by the exchange fluctuations of the Mn moments, while below T/sub N/ by the Raman scattering of spin waves. At the same normalized temperature T/T/sub N/, 1/T1 observed in the diluted (Mn/sub 0.5/Zn/sub 0.5/)F2 is significantly larger than that in the pure MnF2 below T/sub N/. The difference between the pure and diluted systems is related to the large spectral weight of low-energy magnons in (Mn/sub 0.5/Zn/sub 0.5/)F2 found by neutron scattering