Universal Relationship between Magnetization and Changes in the Local Structure ofLa1−xCaxMnO3: Evidence for Magnetic Dimers

Abstract

We present extensive x-ray absorption fine structure measurements on La(1-x)Ca(x)MnO(3) as a function of the B field (to 11 T) and Ca concentration, chi(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 form at temperature T(*) above T(c).

Magnetization and electronic structure of polycrystalline La1‐xCax MnO3 (x =0.19, 0.17)

“…The prepared samples have been investigated [5] that Griffiths phase exists above Curie temperature with the downturn characteristic of temperature dependent inverse susceptibility, which may be due to the disorder caused by structure and competition between ferromagnetic phase and paramagnetic phase. The Griffiths phase temperature region of x=0.19 sample is larger than that of x=0.17 sample, and both samples have large effective spin, indicating the formation of magnetic dimers [10,11]. In order to further understand the magnetization of samples, measurement of hysteresis loops of magnetization shown in Fig.…”

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

“…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.…”

“…In the most recent EXAFS work, Downward et al 17 proposed a model where M develops via the aggregation of two-site polarons ͑Downward et al call them "dimerons"͒ or multiples of such pairs; we will use the term dimeron to refer to a two-site polaron here. The dimerons consist of pairs of coupled Mn sites with aligned spins; initially one Mn site would correspond to an e g electron site while the other is a hole site.…”

“…The dimerons consist of pairs of coupled Mn sites with aligned spins; initially one Mn site would correspond to an e g electron site while the other is a hole site. 17 The two Mn spins are coupled via double exchange ͑DE͒, mediated by an electron ͑or hole͒ hopping rapidly back and forth between the two Mn sites. In addition, because the two sites share a JT distortion between them, the net distortion per site for the pair is reduced since the electron ͑or hole͒ is partially delocalized.…”

Lattice Strain Accompanying the Colossal Magnetoresistance Effect inEuB6

“…Despite considerable efforts to understand the inter-play between spin, charge and lattice degrees of freedom in the CMR effect for the various materials, see e.g. [7][8][9][10][11][12], no general picture has evolved yet. For the manganites, in particular, the reason for that may be related to their complexity due to the simultaneous action of strong crystal-electric field (CEF) and Jahn-Teller (JT) effects.…”