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

Abstract

Measurements of the spin precession frequencies ωμ and transvers relaxation rates Γ2 of positive muons (μ+) magnetic phase of several iron-cobalt alloys and coabalt, and in amorphous Feo.91Zr0.09 alloys are discussed with particular reference to the influence of critical phenomena on ωμ and Γ2. Although the temperature dependence ωμ(T) is well described by scaling law ωμ ∝ (Tc−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-Tc)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 ωμ and that of M below Tc the frequency shifts observed in the paramagnetic regime are probably not directly proportional to the magnetic susceptibility. In amorphous Fe0.91Zr0.09 allosy frequency shifts and transverse relaxation rates are found up to nearly 2 Tc. Due to the shape of the Fe0.91Zr0.09 sample, demagnetization inhomogeneities certainly play an important role.