A review of studies of magnetism and magnetic materials by means of the μSR technique

The μSR technique with positive muons has been used to probe interstitial magnetic fields in the elemental ferromagnets Fe, Co, Ni, Gd, and Dy, in antiferromagnetic Cr and rare earth metals, in insulating compounds MnO, Fe2O3, manganese aluminosilicate glasses, the coupound MnSi and the alloys Fe-Al, Pd-Fe, Cu-Mn and AuFe. The results for the elemental ferromagnets have provoked substantial theoretical effort to explain the measured hyperfine fields. These fields turn out to be negative, i.e., directed opositely to the bulk magnetization, as has been found for other low-valence impurity probes. Local field enhancement factors have been calculated by non-linear electronic screening theory using the jellium approximation. The problem is complicated by the finite extent of the muon wave-function and by the distortions of the lattice and electronic wave-functions due to the presence of the muon. The results obtained for a few alloys provide additional information on the magnetic structure surrounding impurities and on the muon-impurity interaction. The results for Fe and Cr have yielded muon diffusion parameters for those metals. Muon experiments on spin glasses such as Cu-Mn were helpful in illucidating the nature of the freezing transition displayed by impurity spins. The hyperfine field in MnSi has been identified as an itinerant d-electron effect. (Auth.)