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Investigations of the spin dynamics of the geometrically frustrated pyrochlore (Tb(p)Y(1-p))2Ti2O7, using muon spin relaxation and neutron spin echo, as a function of magnetic coverage p, have been carried out. Our major finding is that paramagnetic fluctuations prevail as T-->0 for all values of p, and that they are sensitive to dilution, indicating a cooperative spin motion. However, the percolation threshold pc is not a critical point for the fluctuations. We also find that the low temperature spectral density has a 1/f behavior, and that dilution slows down the spin fluctuations.

A study has been made of the internal fields acting on Fe 57 nuclei in some spinel ferrites, with particular reference to the low-temperature order-disorder transition in magnetite, using the techniques of Mossbauer absorption. For the Fe 3+ ions at both the octahedral and tetrahedral sites in nickel ferrite (NiFe 2 0 4 ) at 300°K, 7Fe20 3 at 85° and 300°K, and magnetite (Fe30 4 ) at 85 °K, the effective magnetic field at the Fe 67 nuclei is the same and equal to about 5.1 XlO 5 oe. In magnetite, the value of H e u in the Fe 2+ ions is about 4.5X10 5 oe at 85°K. Measurements on Fe 3 04 at room temperatures provide a microscopic confirmation of Verwey's hypothesis that above the transition temperature of magnetite there is a fast exchange between the ferrous and ferric ions in the octahedral sites.

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Results of magnetization measurements on carefully prepared single crystals of SmCo, are presented over the temperature range 4 -970 K. The contribution of the rare-earth sublattice magnetocrystalline anisotropy was evaluated using the data on YCos as "a blank. " These results are interpreted on the basis of a singleion model. The following Hamiltonians were employed to obtain eigenvalues: &(parallel to c axis) = 'AL ' S + XcF +2@&S, H, ", 3'. (perpendicular to c axis) ='A L'S+3'cF +2p~S H, ".H, "was regarded as arising mainly from the cobalt sublattice. Contributions arising from the multiplets H7~2 and H9~2 to the ground multiplet H5~2 were included. The crystal field was evaluated in terms of Racah's tensor-operator technique employing 3-j and 6-j symbols. With the two sets of eigenvalues obtained from the above expressions, the anisotropy energy to rotate the magnetization direction from the axis to the plane was calculated and compared with experimental results on single crystals of SmCo, . It is shown that the sign of the crystalfield parameter predicts the correct easy direction of magnetization and that the free-energy values calculated from 4 to 970 K are in reasonable agreement with experiment.