Muon Korringa relaxation

Significant muon spin-lattice relaxation is found in a number of non-magnet ic semimetals and metals. Measured in longitudinal magnetic field, the rela xation rate is independent of field up to several kilogauss and generally i ncreases monotonically with temperature. This suggests a form of Korringa r elaxation, originating in the hyperfine interaction between the implanted m uons and the conduction electrons. Bearing in mind that NMR data on Korring a relaxation refers chiefly to the host nuclei, the muon offers a probe of conduction-electron encounter at an interstitial site, linking the topic to the nature of defect screening in metals, to relaxation by spin-density fl uctuations in magnetic materials and to spin- and charge-exchange on parama gnetic muonium centres in semiconductors. Data are presented for C (graphit e), Bi, Pb and Cd and compared with the Korringa predictions using known va lues of the muon Knight shift. Control experiments are described on Zn and Cu, both pure and deliberately doped with magnetic impurity. For graphite, an interpretation is given in terms of charge-exchange on a molecular radic al formed by the chemical reaction of interstitial muonium. (C) 2000 Elsevi er Science B.V. All rights reserved.