X-ray Absorption Fine Structure (XAFS) measurements of the colossal magnetoresistance (CMR) sample La 0.79 Ca 0.21 MnO 3 at high fields indicate a decrease in the width parameter of the pair distribution function, σ, as the applied magnetic field is increased for T near T c . The change in σ 2 from the disordered polaron state varies approximately exponentially with magnetization irrespective of whether the sample magnetization was achieved through a change in temperature or the application of an external magnetic field. This suggests a more universal relationship between local structure and the sample magnetization than was previously indicated.
A temperature-dependent EXAFS investigation of ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$, with x rays polarized parallel to the $c$ axis and in the $ab$ plane, has shown an extremely sharp and anisotropic jump in the broadening $\ensuremath{\sigma}$ of the $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{O}$ pair-distribution function (PDF) near the ferromagnetic transition temperature, ${T}_{c}$. This jump is associated with an increase in Jahn-Teller (JT) distortions as the temperature increases through ${T}_{c}$. The data show that changes in ${\ensuremath{\sigma}}^{2}$ as $T$ is lowered below ${T}_{c}$ is linearly correlated with the sample magnetization, and that there is a break in the slope of this correlation when the sample is $\ensuremath{\sim}80%$ magnetized, consistent with the recently proposed dimeron model. A JT distortion clearly exists well above ${T}_{c}$ and some distortions still remain for a range of temperatures below ${T}_{c}$. In agreement with recent work on the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ (LCMO) system, the data indicate that two types of distorted sites must occur; one is associated with the hole charge carriers (two-site polaron called a dimeron) and has a very small distortion/site; the second is associated with unpaired electron sites and has a similar distortion/site as observed for corresponding sites in LCMO. Furthermore, the broadening of the $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{O}$ PDF shows a second and more subtle increase above ${T}_{c}$ near ${T}^{*}\ensuremath{\approx}250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a change in structure that correlates well with features in several other experiments.
We present extensive X-ray Absorption Fine Structure (XAFS) measurements on La_{1-x}Ca_xMnO_3 as a function of B-field (to 11T) and Ca concentration, x (21-45%). These results reveal local structure changes (associated with polaron formation) that depend only on the magnetization for a given sample, irrespective of whether the magnetization is achieved through a decrease in temperature or an applied magnetic field. Furthermore, the relationship between local structure and magnetization depends on the hole doping. A model is proposed in which a filamentary magnetization initially develops via the aggregation of pairs of Mn atoms involving a hole and an electron site. These pairs have little distortion and it is likely that they pre-form at temperatures above T_c.
We present a detailed extended x-ray absorption fine structure (EXAFS) analysis of the thermoelectric clathrates ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$, both of which have an unusually low thermal conductivity attributed to a ``rattler'' motion of the filler atoms Eu and Sr. The EXAFS results show that the $\mathrm{Ga}∕\mathrm{Ge}$ lattice is quite stiff with a high correlated Debye temperature $\ensuremath{\sim}400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Eu is on-center in the site 1 cage but off-center $(0.445\ifmmode\pm\else\textpm\fi{}0.020\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$ in the large cage called the Eu2 site. The results for Sr are similar, but $\ensuremath{\sim}75%$ are off-center $0.40\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $\ensuremath{\sim}25%$ are on-center in the Sr2 site. Both results are in reasonable agreement with diffraction results. The temperature dependence of the nearest neighbor pair distribution widths yield low Einstein temperatures ($80\ifmmode\pm\else\textpm\fi{}10$ and $100\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for Eu1 and Sr1, respectively, and $95\ifmmode\pm\else\textpm\fi{}10$ and $125\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the shortest $\mathrm{Eu}2\ensuremath{-}\mathrm{Ga}∕\mathrm{Ge}$ and $\mathrm{Sr}2\ensuremath{-}\mathrm{Ga}∕\mathrm{Ge}$ pairs). In contrast, the more distant $\mathrm{Eu}2∕\mathrm{Sr}2\ensuremath{-}\mathrm{Ga}∕\mathrm{Ge}$ pair distributions within the $\mathrm{Eu}2∕\mathrm{Sr}2$ cage are strongly disordered even at low $T$, indicating considerable local disorder. This indicates that the off-center Eu or Sr atom is bonded to the side of the site 2 cage. This has two important implications for the thermal conductivity: it increases the coupling between the ``rattler'' vibrations and the lattice phonons and it introduces a symmetry-breaking large mass defect.
A new empirical model of the plasmapause location has been developed using density data from the plasma wave receiver onboard the CRRES spacecraft for nearly 1000 orbits. The “plasmapause” is identified here as the innermost sharp gradient in density (change of a factor of 5 in <0.5 L). Such a sharp gradient was observed on 73% of the CRRES inbound and outbound orbits that returned data. The plasmapause location is expressed as a linear function of Kp (previous 12 hour maximum) and local time. The model gives the linear best fit location of the plasmapause as well as the standard deviations of the model parameters. We found a slight noon‐midnight asymmetry with the plasmapause located on average an L shell farther from the Earth at midnight than in the noon sector. This is in the opposite sense to the noon‐midnight asymmetry found previously. Significant variability (with standard deviations up to +/− 1 L shell) in the plasmapause location is seen and suggests that though the mean plasmapause is roughly circular, the instantaneous plasmapause has significant time variable localized structure at all local times but most especially in the duskside sector.
We report X-ray Absorption Fine Structure (XAFS) measurements of the colossal magneto-resistance (CMR) samples La0.79Ca0.21MnO3 and La0.7Ca0.3MnO3 at high magnetic fields. For T near Tc, the width parameter of the pair distribution function, σ, decreases significantly as the applied magnetic field is increased. For a given magnetization, M, the decrease in σ2 from the disordered polaron state is the same, irrespective of whether M was achieved through a change in temperature or the application of an external magnetic field. This universal behavior can be modeled as a quadratic function of magnetization, and suggests a new model for the magnetization process.
