Relationship between macroscopic physical properties and local distortions of low doping La{1-x}Ca{x}MnO3: an EXAFS study

TL;DR

This study uses temperature-dependent EXAFS to explore how local Mn-O distortions relate to magnetic and electronic properties in La{1-x}Ca{x}MnO3 across the ferromagnetic-insulator to ferromagnetic-metal transition, revealing distinct distortion types and magnetization behaviors.

Contribution

It provides new insights into the local distortions and magnetization mechanisms in low-doping La{1-x}Ca{x}MnO3, especially in the ferromagnetic-insulator phase, extending understanding from colossal magnetoresistance materials.

Findings

FMI samples show similar distortion-magnetization correlations as CMR samples.

Approximately 50% of Mn sites have small distortions and magnetize first.

Large distortions persist at low T, linked to unmagnetized or antiferromagnetic Mn sites.

Abstract

A temperature-dependent EXAFS investigation of La{1-x}Ca{x}MnO3 is presented for the concentration range that spans the ferromagnetic-insulator (FMI) to ferromagnetic-metal (FMM) transition region, x = 0.16-0.22. The samples are insulating for x = 0.16-0.2 and show a metal/insulator transition for x = 0.22. All samples are ferromagnetic although the saturation magnetization for the 16% Ca sample is only ~ 70% of the expected value at 0.4T. We find that the FMI samples have similar correlations between changes in the local Mn-O distortions and the magnetization as observed previously for the colossal magnetoresistance (CMR) samples (0.2 < x < 0.5) - except that the FMI samples never become fully magnetized. The data show that there are at least two distinct types of distortions. The initial distortions removed as the insulating sample becomes magnetized are small and provides direct evidence that roughly 50% of the Mn sites have a small distortion/site and are magnetized first. The large remaining Mn-O distortions at low T are attributed to a small fraction of Jahn-Teller-distorted Mn sites that are either antiferromagnetically ordered or unmagnetized. Thus the insulating samples are very similar to the behavior of the CMR samples up to the point at which the M/I transition occurs for the CMR materials. The lack of metallic conductivity for x <= 0.2, when 50% or more of the sample is magnetic, implies that there must be preferred magnetized Mn sites and that such sites do not percolate at these concentrations.