Magnetic and Structural Properties of LANTHANUM(2-X) Strontium(x) Copper OXYGEN(4+DELTA) Studied with Time - Perturbed Angular Correlations

Structural and magnetic properties of the high temperature superconductor La_{2-x}Sr_{x} CuO_{4-y} have been studied with ^{111} In/^{111}Cd perturbed angular gammagamma-correlations (PAC). In these measurements ppm of the radioactive probe, ^{111}In, is diffused into small samples by heat treatment. During the 2.8 day half-life of ^{111}In, the hyperfine spectrum of the I = 5/2, 85 ns. excited state of ^{111}Cd can be studied. A number of complex combined magnetic-dipole -electric-quadrupole interactions are observed. These depend simply on the sample preparation which controls the types and concentration of oxygen defects which exist in the sample. In particular, we have found that vacuum annealing results in a unique PAC spectrum. In this case, which only occurs when the samples are nearly exactly oxygen stoichiometric, the ^{111}Cd probe sits at the La site with no oxygen defects nearby. The principle axis of the electric field gradient ( omega_{o} = 240 Mrad/s) is found to be nearly parallel to the tetragonal c-axis and perpendicular to the hyperfine field ( omega_{L} = 10 Mrad/s). The temperature dependence and orientation of the hyperfine field, the electric field gradient and its asymmetry parameter (eta~ 0.1) have been studied for lightly doped La_{2-x }Sr_{x}CuO _4. We found first that hyperfine field is proportional to bulk magnetization, and second that eta is proportional orthorhombic distortion (To = 530 K). This unique sensitivity of our probe to both the magnetic and structural phase transitions in La _2CuO_{4+delta }, combined with the microscopic information on oxygen stoichiometry it can provide, have made ^{111}In/^{111 }Cd PAC ideal for studies of complex phase diagram of La_{2-x}Sr _{x}CuO _4. Results from these investigations include the first microscopic determination of the oxygen stoichiometry of La_{2-x}Sr _{x}CuO _4 and observation of the maximum Neel temperature; measurement of the magnetic critical exponent, beta = 0.50(4), and the precise determination of the antiferromagnetic phase boundary in La_ {2-x}Sr_{ x}CuO_4 for x < 0.02. However, as valuable as ^{111 }In/^{111}Cd PAC has been as a probe of long range properties of La _{2-x}Sr _{x}CuO_4 , its greatest potential still lies in its ability, as a local probe, to observe short range order. Already, a number of complex microscopic phenomena have been observed. The most interesting of which was the discovery that due its interaction with the incipient structural instability in La_{2-x}Sr _{x}CuO_4 , the ^{111}In/ ^{111}Cd ion itself can influence the local dynamics of the host sufficiently to induce a "local structural phase transition" at 150 K. The resulting change in symmetry is similar to the low-temperature -tetragonal phase which is found in La_{ 2-x}Ba_{x}CuO_4 for x = 1/8.