The microscopic magnetic properties of the Cu02 planes in YBa2Cu306 63 (T, =62 K) have been investigated in Cu and 0 NMR experiments. Unlike the fully oxygenated Y-Ba-Cu-07 (T, =90 K), the various components of the Cu and 0 Knight-shift tensors show strong but identical temperature dependences in the normal state. This supports the picture that there is only one spin component in the Cu02 planes. The spin susceptibility deduced from Knight-shift results shows significant reduction with decreasing temperature in the normal state. The temperature dependences of the nuclear-spin-relaxation rates (1/T& ) are very different for the Cu and the 0 sites. 1/( T& T) at the O sites is nearly proportional to the spin susceptibility. 1/(T& T) at the Cu sites shows a broad peak around 150 K. We discuss these relaxation behaviors based on a model of the dynamical spin susceptibility proposed by Millis, Monien, and Pines.

The mean-square relative displacements (MSRD) of atomic pair motions in crystals are studied as a function of pair distance and temperature using the atomic pair distribution function (PDF). The effects of the lattice vibrations on the PDF peak widths are modelled using both a multi-parameter Born von-Karman (BvK) force model and a single-parameter Debye model. These results are compared to experimentally determined PDFs. We find that the near-neighbor atomic motions are strongly correlated, and that the extent of this correlation depends both on the interatomic interactions and crystal structure. These results suggest that proper account of the lattice vibrational effects on the PDF peak width is important in extracting information on static disorder in a disordered system such as an alloy. Good agreement is obtained between the BvK model calculations of PDF peak widths and the experimentally determined peak widths. The Debye model successfully explains the average, though not detailed, natures of the MSRD of atomic pair motion with just one parameter. Also the temperature dependence of the Debye model largely agrees with the BvK model predictions. Therefore, the Debye model provides a simple description of the effects of lattice vibrations on the PDF peak widths.

We present zero-field muon spin relaxation ͑SR͒ measurements of La 1.6Ϫx Nd 0.4 Sr x CuO 4 with xϭ0.125,0.15,0.2; La 1.475 Nd 0.4 Ba 0.125 CuO 4 , La 1.875 Ba 0.125 CuO 4 , and La 1.875 Ba 0.125Ϫy Sr y CuO 4 with y ϭ0.025,0.065. All of the samples with dopant concentrations xϩyр0.15 show similar static magnetic order with coherent precession of the muon spins below T N Ϸ30 K, with a T→0 ordered Cu moment Ϸ0.3 B . The samples with xϭ0.20 show no coherent precession but manifest two distinct relaxation regimes, typical of quasistatic magnetism. We then present transverse-field SR hysteresis measurements of the La 1.45 Nd 0.4 Sr 0.15 CuO 4 and La 1.4 Nd 0.4 Sr 0.2 CuO 4 systems that show a large superconducting response below approximately 7 K and 12 K, respectively. We argue that superconductivity and magnetic order coexist in the xϭ0.15 system.

The inhuence of hydrostatic pressure on the electrical resistivity p and ferromagnetic transition temperature T, of La& Ca"Mn03 is presented for x=0.21, 0.33, and 0.40. Pressure strongly decreases p and drives T, up at rates as large as +37 K/GPa for the x=0.21 specimen; this is significantly larger than dT, /dP values observed in conventional ferromagnets and about eight times larger than d Tz /dP observed in Ca& Sr Mn03 antiferromagnets. The transport mechanism in these oxides is influenced in a qualitatively similar way by both pressure and magnetic field. The results are interpreted in the framework of the doubleexchange interaction involving conduction via magnetic polarons.