What can we learn about critical magnetic phenomena from muon spin rotation experiments?

Measurements of the spin precession frequencies ω_μ and transvers relaxation rates Γ₂ of positive muons (μ⁺) magnetic phase of several iron-cobalt alloys and coabalt, and in amorphous Fe_o.91Zr_0.09 alloys are discussed with particular reference to the influence of critical phenomena on ω_μ and Γ₂. Although the temperature dependence ω_μ(T) is well described by scaling law ω_μ ∝ (T_c?T)^β′ where α′ varies strongly with the co concentration in the FeCo alloys, it is shown that the exponent α′ is not identical to the static scalling parameter β describing the critical behaviour of the spontaneous magnetization M. In paramagnetic iron we find Γ₂∝(T-T_c)^0.72, which is attributed to the effect of dynamical spin fluctuations on the μ⁺. Due to the apparent difference between the temperature dependence of ω_μ and that of M below T_c the frequency shifts observed in the paramagnetic regime are probably not directly proportional to the magnetic susceptibility. In amorphous Fe_0.91Zr_0.09 allosy frequency shifts and transverse relaxation rates are found up to nearly 2 T_c. Due to the shape of the Fe_0.91Zr_0.09 sample, demagnetization inhomogeneities certainly play an important rôle.