P. A. Algarabel

A systematic study of the effect of oxygen content on the structural, magnetotransport, and magnetic properties has been undertaken on a series of ${\mathrm{LaMnO}}_{3+\mathrm{\ensuremath{\delta}}}$ samples, with $\ensuremath{\delta}=0,$ 0.025, 0.07, 0.1, and 0.15. Measurements of the ac initial magnetic susceptibility, magnetization, magnetoresistance, and neutron diffraction, including small-angle neutron scattering (SANS), were performed in the temperature range 1--320 K using high magnetic fields up to 12 T. The antiferromagnetic order found in ${\mathrm{LaMnO}}_{3}$ evolves towards a ferromagnetic order as \ensuremath{\delta} increases. This behavior is accompanied by a drastic reduction of the static Jahn-Teller distortion of the ${\mathrm{MnO}}_{6}$ octahedra. The ferromagnetic coupling weakens for $\ensuremath{\delta}>~0.1.$ The magnetic behavior is interpreted by taking into account two effects caused by the increase in \ensuremath{\delta}: cation vacancies and ${\mathrm{Mn}}^{4+}{/\mathrm{M}\mathrm{n}}^{3+}$ ratio enhancement. The orthorhombic crystallographic structure becomes unstable at room temperature for $\ensuremath{\delta}>~0.1.$ The sample $\ensuremath{\delta}=0.1$ shows a structural transition from rhombohedral to orthorhombic below ${T}_{S}\ensuremath{\approx}300\mathrm{K}$ with a huge change in the cell volume. All the studied compounds were found to be insulating at low temperatures with no appreciable magnetoresistance, except for $\ensuremath{\delta}=0.15,$ in which we observed a large value for the magnetoresistance. The SANS results indicate that magnetic clustering effects are important below ${T}_{C}$ for $\ensuremath{\delta}>~0.07,$ which could explain the intriguing ferromagnetic insulator state. In the $\ensuremath{\delta}=0.07$ and $\ensuremath{\delta}=0.10$ samples we found at temperatures below ${T}_{C}$ magnetic and structural anomalies that are characteristic of charge ordering.

We present direct evidence that the giant magnetocaloric effect recently discovered in the ${\mathrm{Gd}}_{5}{(\mathrm{S}\mathrm{i}}_{1.8}{\mathrm{Ge}}_{2.2})$ alloy is associated with a field-induced first-order structural transition from a ${P112}_{1}/a$ monoclinic (paramagnetic) to a Pnma orthorhombic (ferromagnetic) structure. A large volume contraction of $\ensuremath{\Delta}V/V\ensuremath{\cong}0.4%$ takes place spontaneously at the transition temperature, ${T}_{C}\ensuremath{\cong}240\mathrm{K}.$ The reported structural transition can be induced reversibly by application of an external magnetic field, producing strong magnetoelastic effects.

ac initial magnetic susceptibility, thermal expansion, and magnetostriction measurements were performed on a polycrystal of ${\mathrm{La}}_{0.60}{\mathrm{Y}}_{0.07}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ manganese oxide. We observed a spontaneous volume magnetic contribution ( $\ensuremath{\cong}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ at ${T}_{C}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}160\ifmmode\pm\else\textpm\fi{}5\mathrm{K}$). This extra contribution sharply decreases at ${T}_{C}$ as a consequence of the metal ( $M$)--insulator ( $I$) transition, and is suppressed under an applied magnetic field. This shows that the transition to the $M$ state is induced by the field in the $I$ regime, producing strong magnetoelastic effects. The present results demonstrate the close relationship between magnetoelastic and magnetotransport properties in this compound.

The effects of magnetic field and pressure on the unusual spontaneous behavior of ${\mathrm{La}}_{2/3}$${\mathrm{Ca}}_{1/3}$${\mathrm{MnO}}_{3}$ have been thoroughly investigated. Resistivity and volume thermal expansion, both under magnetic field and pressure, ac susceptibility under pressure, magnetostriction, magnetoresistance, and neutron diffraction measurements, have allowed us to determine the relevant underlying mechanisms in this system. Above ${\mathit{T}}_{\mathit{c}}$ the neutron measurements reveal short-range ferromagnetic correlations and the anomalous volume thermal expansion indicates that local distortions are present. Both experiments support the formation of magnetic polarons above ${\mathit{T}}_{\mathit{c}}$. At ${\mathit{T}}_{\mathit{c}}$ the compound undergoes a paramagnetic-ferromagnetic transition accompanied by an insulator-metal-like transition with anomalies in the electrical and volume properties. Above ${\mathit{T}}_{\mathit{c}}$ the magnetic field and the pressure favor electrical conduction by enhancing the double-exchange interaction. Below ${\mathit{T}}_{\mathit{c}}$ the metallic state is favored by the magnetic field and the pressure in a different way. \textcopyright{} 1996 The American Physical Society.