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A Variation of the F‐Test for Determining Statistical Relevance of Particular Parameters in EXAFS Fits

A general problem when fitting EXAFS data is determining whether particular parameters are statistically significant. The F‐test is an excellent way of determining relevancy in EXAFS because it only relies on the ratio of the fit residual of two possible models, and therefore the data errors approximately cancel. Although this test is widely used in crystallography (there, it is often called a “Hamilton test”) and has been properly applied to EXAFS data in the past, it is very rarely applied in EXAFS analysis. We have implemented a variation of the F‐test adapted for EXAFS data analysis in the RSXAP analysis package, and demonstrate its applicability with a few examples, including determining whether a particular scattering shell is warranted, and differentiating between two possible species or two possible structures in a given shell.

What Can We Learn from a Detailed Study of the Temperature Dependence of σ, the Width of the Pair Distribution Function?

In many systems there is a significant coupling between the local structure and other properties of the system such as magnetism, electrical and thermal transport, metal/insulator transitions etc. In such materials, a detailed temperature‐dependent study of the width of the Pair Distribution Function (PDF), σ, can separate different contributions and provide a connection between the observed macroscopic observations and the underlying atomic interactions that produce them. The usual model for simple systems is that the T‐dependence of σ2 is described by an Einstein or Correlated Debye model, with one characteristic temperature for the system; in such models σ2(T) increases smoothly with T and has a slowly increasing slope. However that is not always the case: in structures with large unit cells containing several types of atoms, some atoms in the crystal can have a low Einstein temperature while others have a very high correlated Debye temperature as observed in a number of thermoelectric systems (skutter...

EVIDENCE FOR A UNIVERSAL RELATIONSHIP BETWEEN MAGNETIZATION AND CHANGES IN THE LOCAL STRUCTURE

X-ray Absorption Fine Structure (XAFS) measurements of the colossal magnetoresistance (CMR) sample La0.79Ca0.21MnO3 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 Tc. The change in σ2 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.

UNIVERSAL RELATIONSHIP BETWEEN MAGNETIZATION AND CHANGES IN THE LOCAL STRUCTURE IN QUASICUBIC AND BILAYER MANGANITES

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 extend our previous EXAFS investigations on the relationship between magnetization and local distortions to the bilayer materials La2-2xSr1+2xMn2O7 and to higher magnetic fields, for quasicubic La0.7Ca0.3MnO3. For each, there is a significant change in the broadening parameter σ of the Mn-O pair distribution function (PDF) associated with polaron formation as T is increased through Tc. Applying a magnetic field reduces the local distortion of the Ca sample for temperatures near Tc. For samples with sharp transitions, there is still a significant change of σ2 with T below Tc when the sample is fully magnetized, i.e. distortions are still present and continuing to be removed as T is lowered well below Tc. A simple model is presented for the magnetization process.