Absence of small lattice polarons above the Curie temperature in magnetoresistive manganites

Abstract

It is generally believed that the electrical transport in magnetoresistive manganites above T C is governed by small polarons associated with large lattice distortions. We present evidence which confirms the existence of magnetic clusters ͑polarons͒ associated with minimal lattice distortions above the Curie temperature. We take advantage of the fact that the Mössbauer spectra of the relatively large and slowly fluctuating magnetic polarons consist of incompletely collapsed magnetic component with extensive wings.

Anomalous magnetic and dynamic behavior in magnetoresistive compounds: origin of bulk colossal magnetoresistivity

“…Now the question arises whether these magnetic (clusters)polarons are dressed with lattice distortions. The Mössbauer spectra above T C for all studied manganites show very small quadrupole splitting (about 0.18 mm/s) [22,29] (and becomes even smaller below T C ) which indicates that the distortions imposed on the octahedron incorporating the Mössbauer probe by the surrounding Mn-O network is quite small. The probe senses distortions in a 10 -7 s period.…”

“…Experiment shows that as the temperature is lowered toward T C , the abundance (normalized area) instead of increasing, as expected according to the theory, goes down -the closer to the T C , more is the observed loss. This suggests that by approaching T C from above and keeping a narrow energy window, we lose gradually increasing amounts of spectral area associated with the fraction of clusters whose fluctuating rate becomes less than about 10 9 s -1 [29].…”

“…Taking into consideration the superparamagnetic-like behavior of mixed valence manganites at different temperatures [22][23][24][25][26][27][28][29]39] and the interaction energy between a modest H ext and the nano spin clusters (Fig. 3), we estimate approximately the average size of the magnetic clusters as roughly 20 Å in diameter (if approximated by a sphere) containing couple of hundred Mn 3+ /Mn 4+ atoms.…”

“…Now we have to address the question as to how the long range ferromagnetic order breaks down with formation of nano size magnetic clusters below T C and the survival of the short range order in the spin clusters up to a temperature close to 2T C [26,29,30,35]. This behavior contrasts sharply with that of regular ferromagnets like Fe and Ni, where magnetic correlations are observed only up to 1.1T C , and there is no evidence of formation of spin clusters below T C .…”

“…This means that as we approach T C from below, an increasing number of nano spin clusters are Anomalous magnetic and dynamic behavior in magnetoresistive compounds formed from the bulk, there is always some fraction of the material exhibiting bulk-like magnetism even up to 0.99T C (Fig. 2) [28,29]. The sextet lines are rather broad even at T/T C~0 .3 and become broader with increasing temperature (Fig.…”