Relationship between macroscopic physical properties and local distortions of low-dopingLa1−xCaxMnO3: An EXAFS study

Abstract

A temperature-dependent EXAFS investigation of La 1−x Ca x MnO 3 is presented for the concentration range that spans the ferromagnetic-insulator (FMI) to ferromagnetic-metal (FMM) transition region, x = 0.16, 0.18, 0.20, and 0.22; the titrated hole concentrations are slightly higher y = 0.2, 0.22, 0.24, and 0.25 respectively. For this range of Ca concentrations 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. This raises a question as to the nature of the ferromagnetic (FM) coupling mechanism in such insulating samples. 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 (associated with the hole charge carriers) have a small distortion/site and are magnetized first. The large Mn-O distortions that remain at low T are attributed to a small fraction (< 30%) of fully Jahn-Teller-distorted Mn sites that are either unmagnetized or antiferromagnetically ordered. 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 (that involve holes) and that such sites do not percolate at these concentrations.

Evidence of active role played by the nonmagnetic element Sr in magnetostructural coupling inSrRuO3

“…Thus, a deviation from correlated Debye model suggests important contributions such as Jahn-Teller ͑JT͒ distortion, polaron formation, etc., which are beyond the consideration of this model. A similar deviation found in manganites in an earlier study 18 was attributed to the Jahn-Teller type of distortions. While Jahn-Teller type of distortions may be applicable to parameters related to Ru-O bonds, it may be ruled out for Sr-Sr bond.…”

“…This is in contrast to the previous observations of magnetoelastic effects and distortion-mediated magnetic transitions in various other compounds, all involving the magnetic ions. 17,18 Interestingly, such thermal evolution nucleates at a temperature, T Ã higher than the Curie temperature revealing a possible precursor effect.…”

Identification of Mn Site in BaTiO3by Synchrotron X-Ray Absorption Spectroscopy Measurements

“…In addition, the EXAFS measurement allows the identification of the nearest neighboring atoms to the absorbing atom to determine the coordination environment for the metal of interest [26]. Recently, both techniques have been successfully applied to various classes of materials to obtain their local structures [27][28][29][30][31][32][33][34][35]. Our previous experimental and theoretical works on applying the XAS techniques to determine the local structure of PZT-based materials have already been established [36][37][38].…”

Detailed relationship between local structure, polarons, and magnetization forLa1−xCaxMnO3Bridges1, Downward2, Neumeier3 et al. 2010Phys. Rev. BSelf Cite274201View full textAdd to dashboardCite

“…Over the last decade, many authors have observed significant local structure changes in the Mn-O pair distribution as a function of temperature using both extended x-ray absorption fine structure ͑EXAFS͒ spectroscopy [16][17][18][19][20][21][22][23][24][25] and neutron or x-ray pair-distribution function ͑PDF͒ analysis. [26][27][28][29][30] These investigations show that within the CMR regime, the average JT distortions at low T are very small ͑ϳ0 for some samples͒, but grow rapidly as T approaches T c .…”