Evidence for a Universal Relationship Between Magnetization and Changes in the Local Structure

Abstract

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 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 ofLa1−xCaxMnO3: Evidence for Magnetic Dimers

“…Previously, we have shown for a few samples [2,12] that the decrease in average local distortion, 2 , as T is decreased below T c , is a simple function of the sample magnetization, M. Here is the width of the Mn-O PDF obtained from extended XAFS (EXAFS), and 2 is the T-or B-induced change in 2 . By extending those measurements to high fields (9-11 T) for some samples, and to a range of Ca concentrations, 0:21 x 0:45, we find that the plots of 2 versus M (each normalized to their value at low T) depend primarily on hole concentration.…”

Evidence for magnetic dimerons in the anisotropic bilayer systemLa1.2Sr1.8Mn2O7: An EXAFS study

“…16,17 The individual slopes, m, for the x = 0.4 bilayer sample are given in the legend ͑Fig. 7͒; their ratio is ϳ3.2.…”

“…12-16 and references therein͒ to probe distortions around the Mn site as a function of temperature, sample magnetization, and hole concentration ͑the nominal hole concentration is approximately the dopant concentration͒. A summary of these results, including a recent study, 17 are the following: ͑i͒ there is a rapid increase in the broadening, , of the Mn-O pair distribution function ͓attributed to the formation of Jahn-Teller ͑JT͒ polarons͔ as the temperature is increased up to T c over the entire concentration range that exhibits CMR behavior; above T c , the increase of with T is slow; ͑ii͒ upon cooling below T c , the distortion removed, D = ⌬͑ 2 ͒, initially is small and increases linearly with the sample's magnetization M; ͑iii͒ when the fractional magnetization, M M 0 ͑M 0 is the saturation magnetization at low T͒ reaches approximately twice the hole concentration, 2x, there is a rapid increase in the slope of D versus M M 0 ; ͑iv͒ measurements as a function of magnetic field 16,17 show that broadening associated with the formation of JT polarons is spread over a wider T range, yet the plot of D versus M M 0 is nearly universal; the distortion D is determined primarily by M and is independent of whether the value of M is achieved via a change in B or in T.…”