No abstract

The thermodynamics and kinetics of the magnetic states of the semiconductor-like compound La0.7Sr0.3Mn0.7Co0.3O3 were studied by detailed magnetic measurements. The spontaneous magnetic state was found to be a cluster-glass phase which can cross over to a ferromagnetic state under an external magnetic field H. The defreezing temperature Tf for the cluster-glass phase and the merging temperature Tm for zero field cooled and field cooled magnetizations are related to the field H by Hj=H0j(1−Tj/Tc)nj, where H0j and nj are constants and j=f,m. Codoping tends to destroy the double exchange in Mn4+–O–Mn3+ and broadens the coexistence region of the cluster-glass and ferromagnetic states. The magnetic relaxation was found to deviate from the usual logarithmic time dependence and follow a power law.

On the basis of the self-consistent Bogoliubov-de Gennes equations and a tight-binding lattice model, we investigate the quasiparticle spectrum of vortex-lattice state in pure d-, mixed dϩis, and d x 2 Ϫy 2ϩ id xy-wave superconductors. For a d-wave case, the local density of states ͑LDOS͒ at the vortex core shows a multipeak structure, and the positions of peaks as well as the width of splitting between peaks are sensitively dependent on both the magnetic-field strength and the orientation of the vortex lattice. For the mixed dϩis-and d x 2 Ϫy 2ϩ id xy-wave pairing states, we observe a double-peak structure of the local density of states at vortex center, where the two peaks are asymmetrically situated around the Fermi energy. By taking into account the matrix-element effect, the local density of states appears to be qualitatively consistent with the scanningtunneling-microscopy experimental data.

The longitudinal and transverse ultrasonic velocities and attenuations, the transport and magnetic properties, as well as the structural changes of polycrystalline La0.25Ca0.75Mn0.93Cr0.07O3 have been studied systematically. A velocity minimum and an attenuation peak for both longitudinal and transverse waves occur around the temperature Tco of charge ordering transition accompanied by the structural change from a cubic to a tetragonal phase. Near temperature TN (where the c-axis length shows a maximum), the sharp decrease of velocities for both waves and the appearance of a longitudinal attenuation peak are attributed to the antiferromagnetic transition together with the anomalous change in c-axis length of the tetragonal phase. The simultaneous occurrence of the ultrasonic, lattice, charge ordering, magnetic, and transport anomalous characteristics indicates the strong electron–phonon and spin–phonon interactions in the system.