The sensitivity of ρ to changes in volume which occur through: (1) applied pressure, (2) variations in temperature, and (3) phase transitions, is evaluated for some selected CMR oxides. It is argued that the changes in volume associated with (2) and (3), which are referred to as self pressures, are equivalent in magnitude to changes in volume resulting from pressures in the range of 0.18 to 0.45 GPa. Through consideration of thermal expansion and electrical resistivity data, it is shown that these self pressures are responsible for large features in the electrical resistivity and are an important component for the occurrence of the metallicity below Tc. It is suggested that this is a manifestation of a strong volume dependence of the electron phonon coupling in the CMR oxides.

We report the results of a detailed polarization and temperature dependence study of the Raman scattering from La1.2Sr1.8Mn2O7. The Raman spectra reveal three general spectral features. First, there are sharp peaks due to long-wavelength optic phonons. Phonons, attributed to the distortion of the MnO6 octahedra, reveal an anomalous behavior which correlates with the transition from a paramagnetic-insulating (PI) to a ferromagnetic-metallic (FM) phase at a critical temperature Tc. Second, there is an electronic continuum that is suppressed for ω < 500 cm−1 at T ≪ Tc. Third, broad peaks between 400 cm1 to 800 cm−1 seen in the PI state, surprisingly, disappear in the FM state. The implications of these results are discussed.

The radio frequency (RF) response of La1-xSrxMnO3 single crystals reveal a variety of features associated with the structural, electronic and magnetic properties of the system. The resonance technique operating at ∼ 4 MHz employed in this study is sensitive to small changes in both the magnetic susceptibility and resistivity of the samples. Very sharp changes in frequency are observed at the ferromagnetic (FM) and structural phase transitions in both the metallic (x = 0.175) and insulating (0.125) crystals studied.

In addition to the known transitions identified as FM and orthorhombic distortions, our experiments show rich structures which are not observed in conventional DC magnetization and transport experiments. Our results demonstrate that RF experiments are ideally suited to investigate the complex phase diagram in the manganites.

The colossal frequency change that we observe at the FM transition in the La1-xSrxMnO3 crystals is indicative of the enormous potential for using these materials in high frequency switching applications.

We studied the magnetic and electronic transport properties of a single crystal sample of La0.64Pb0.36MnO3 in the temperature range 5 K to 350 K and magnetic field up to 5.5 T. A magnetic transition is found at 210 K. The single crystal sample is ferromagnetic below the transition temperature (Tc) and becomes paramagnetic at temperatures T > Tc. Magnetization measurements along three different orthorhombic crystal axes show no significant difference. The magnetoresistance approaches a maximum value of about -60% at Tc in 5 T magnetic field strength and has qualitatively different field dependence below and above Tc. The scaling behavior between resistivity and magnetic moment is examined for temperatures both below and above the transition. A low temperature (T<15 K) dp / dT < 0 effect is attributed to possible quantum tunneling of carriers between neighbouring distortions.

Thin films of colossal magnetoresistance material La0.7Ca0.3MnO3 were implanted with different fluence 200keV Cr ions. Resistivity measurements in zero and applied fields of up to 8T were made in order to determine the effects of the implanted magnetic ions on the magnetoresistance (MR). As the Cr fluence was increased, the resistivity increased and the metal-insulator transition (MI) temperature was suppressed to values below the experimentally accessible temperature range as a result of oxygen loss and the creation of defects. However, for the highest fluence of 5×1015 ions/cm2, a re-entrant metal-insulator type transition was observed. Furthermore a significant improvement in the low field MR was observed for fields less than 500mT. These results are interpreted in terms of substitution of Cr ions onto Mn sites and the creation of a magnetically inhomogeneous material and the influence of oxygen deficiency.

The study of doped and undoped La1-xMxMnO3 single crystals (x = 0 to x = 0.3) in the temperature range from 5 K to 423 K is reported. The activity of phonon vibrational modes in optical spectra is analyzed and a comparison to experimental Raman data is presented. Different contributions from optical phonons to the Raman scattering process are discussed. It is shown that both the value of doping and temperature have a significant influence on Raman features, and these are strongly affected by structural and magnetic phase transitions. Due to the symmetry present in distorted lanthanum manganese compounds their Raman spectra were found to contain comparable contributions from first- and second-order phonon scattering. The common features related to both the reduction of the Jahn-Teller (JT) distortion and the temperature-induced phase transition were found in the Raman spectra of La1-xMxMnO3 crystals. An anomalous behavior of the low-frequency Raman mode in doped systems was found and explained in terms of the temperature-dependent ionic radius of the La/Sr ion sites. It was found that the strong dependence of the Ag Raman mode on the value of doping may serve as a useful tool for optical characterization of La1-xMxMnO3 compounds, including the inhomogeneity of the surface.

Measurements of the temperature dependence of the resistivity ρ(T) and thermoelectric power α(T) under several hydrostatic pressures on l8O/l6O isotope-exchanged (La1-xNdxCa0.3MnO3 polycrystalline samples spanning the structural O'-O orthorhombic transition have demonstrated that the perovskite tolerance factor t increases with pressure, signaling an unusually compressible Mn-0 bond. They have also indicated a change at the O'-O transition from static to dynamic cooperative Jahn-Teller deformations, from a second-order to a first-order magnetic transition, from Mn(IV) to two-Mn polarons in the paramagnetic region, from Mn(IV) polarons to a vibronic state below Tc, and a phase segregation in the O phase above Tc that traps out mobile polarons into ferromagnetic Mn(IV)-rich clusters within a Mn(IV)-poor matrix. Specific heat data show a transfer of spin entropy to configurational entropy on cooling through Tcin the O phase, and a single-crystal study of La1-xSrxMnO3, x = 0.12 and 0.15, has demonstrated a transition from polaronic to itinerant e electrons below Tc within the O phase. Magnetic-susceptibility measurements in low fields confirm the phase segregation above Tc and the existence of a ferromagnetic glass below Tc in the O phase. The intrinsic “colossal” magnetoresistance (CMR) is attributed to the growth to their percolation threshold of the ferromagnetic clusters existing above Tc, and the dramatic rise in Tc with increasing tolerance factor t in the O-orthorhombic phase to an increase in the density, and also the mobility, of the untrapped polarons above Tc.

Anisotropie charge transport and magnetic properties have been investigated for single crystals of the layered manganite, La2–2xMn2O7 (0.3≤x≤0.5). Remarkable variations in the magnetic structure as well as in the charge-transport properties are observed with changing doping-level x. A crystal with x=0.3 behaves like a 2-dimensional ferromagnetic metal in the temperature region between ∼90 K and ∼270 K, and shows the interplane tunneling magnetoresistance at lower temperatures. These characteristic charge-transport properties are attributed to the interplane magnetic coupling between the adjacent MnO2 bilayers, and are strongly affected by the application of pressure as well as low magnetic fields through the change in magnetic structure. With increase of the carrier concentration toward x=0.5, the charge-ordered phase is stabilized and dominates the charge transport and magnetic properties.