The temperature and pressure dependence of the Fermi contact field of positive muons in ferromagnetic Ni

The spin precession frequency omega mu/pi 2 of positive muons and the transverse mu(+) spin relaxation rate Gamma(2) have been measured on three Ni single crystals as a function of temperature and hydrostatic pressure between 7 and 660 K and up to pressures of 0.6 GPa. It is shown that above 260 K the Fermi contact field acting on the mu(+) magnetic moments is proportional to the saturation magnetization, whereas at lower temperatures it is temperature independent apart from a small anomaly at about 8 K. The volume dependence B-Fermi does not exhibit any structure that might be attributed to the occupation of metastable sites by the mu(+). The observed relaxation rates are compatible with the hypothesis that they are caused by internal strains associated with dislocations.