μ⁺ SR studies on pure MnF₂ and site-diluted (Mn_0.5Zn_0.5)F₂SR studies on pure MnF₂ and site-diluted (Mn_0.5Zn_0.5)F₂

Abstract

Positive muon spin rotation and relaxation measurements have been carried out on the antiferromagnets, pure MnF₂ and site-diluted (Mn_0.5Zn_0.5)F₂, above and below the Néel temperature T_N using single-crystal specimens. Two different muon signals have been found in the pure MnF₂; with the precession frequency υ_A for the site A and υ_B 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_N. The mechanism of the spin relaxation above T_N is explained by the exchange fluctuations of the Mn moments, while below T_N by the Raman scattering of spin waves. At the same normalized temperature T/T_N, 1/T₁ observed in the diluted (Mn_0.5Zn_0.5)F₂ is significantly larger than that in the pure MnF₂ below T_N. The difference between the pure and diluted systems is related to the large spectral weight of low-energy magnons in (Mn_0.5Zn_0.5)F₂ found by neutron scattering.