A method is given for investigating the temperature dependence of the positron (e+) diffusivity that is analogous to the study of the diffusion of positive muons in solids through the longitudinal relaxation of the muon spin polarization. It makes use of the facts that e+ emitted from radioactive sources with velocity v are longitudinally polarized with helicity v/c and that the e+ spin polarization is essentially preserved during the implantation and slowing‐down process. The residual polarization of the annihilating e+ is determined by their “magnetic history”, i.e., by the magnetic fields “seen” by the e+ magnetic moments during their movement through the solid. Information on the residual polarization may be obtained by measuring the annihilation characteristics (in the present set‐up the Doppler broadening of the 2γ annihilation line) in magnetically saturated ferromagnetic substances. In this case the “polarimeter” serves also as the sample in which the e+ diffusivity is studied. The theory relating the polarization to the e+ diffusivity is developed for the specific case of α‐iron. Experiments on α‐iron single crystals between 77 and 770 K indicate that the mechanism of e+ diffusion is rather complicated and cannot be adequately described by propagation in band states or hopping alone.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics