Polaron framework to account for transport properties in metallic epitaxial manganite films

Abstract

We propose a model for the consistent interpretation of the transport behavior of manganese perovskites in both the metallic and insulating regimes. The concept of polarons as charge carriers in the metallic ferromagnetic phase of manganites also solves the conflict between transport models, which usually neglects polaron effects in the metallic phase, and, on the other hand, optical conductivity, angle-resolved spectroscopy, and neutron scattering measurements, which identify polarons in the metallic phase of manganites down to 6 K. Transport characterizations of epitaxial La 0.7 Sr 0.3 MnO 3 thin films in the thickness range of 5-40 nm and temperature interval of 25-410 K have been accurately collected. We show that taking into account polaron effects allows us to achieve an excellent fit of the transport curves in the whole temperature range. The current carriers density collapse picture accurately accounts for the properties variation across the metal-insulator transitions. The electron-phonon coupling parameter γ estimations are in a good agreement with theoretical predictions. The results promote a clear and straightforward quantitative description of the manganite films involved in charge transport device applications and promises to describe other oxide systems involving a metal-insulator transition. Ferromagnetic manganites are a prototypical example of the so-called half-metallic materials, i.e., materials with 100% spin polarization at 0 K. Although their possible application in commercial spintronics is prevented by a relatively low Curie temperature (T C 370 K), manganites represent an ideal laboratory tool to test spin transport in various materials and to search for pioneering device paradigms [1,2]. Thus, the use of manganites have significantly contributed to the field of organic spintronics, where almost half of the reported devices have La 0.7 Sr 0.3 MnO 3 (LSMO) as an injector [3]. In this context, revealing in a most exhaustive and comprehensive way, the transport properties of these materials looks both captivating and thought provoking.The transport properties of manganites are strongly linked to their ferromagnetism and are generally described in the framework of the so-called double-exchange mechanism: below T C the exchange interaction between Mn cations through oxygen anions favors ferromagnetism and electron delocalization along the Mn-O-Mn bonds, while above T C , thermal disorder disrupts this delocalization leading to a metal-insulator transition (MIT) [4]. Nevertheless, this basic picture appears to be conceptually insufficient to describe the physics of manganites, in general, and LSMO, in particular [5]: the strong role of polaron effects in manganites has been pointed out [6], and good experimental evidence has been provided by a variety of methods [6][7][8][9][10][11][12][13]. Moreover, manganites were proposed as an example of a polaron Fermi liquid [7].Although the presence of polarons in the metallic phase of LSMO (down to 6 K) has been demonstrated by optical * patrizio.gr...

Surface Nanostructures in Manganite Films

“…High quality colossal magnetoresistance manganite LSMO films were grown on cubic ( a = 3.905Å) SrTiO 3 (STO) substrates using the channel spark ablation (CSA) technique262728. Bulk LSMO is rhombohedral at RT with a pseudocubic parameter a = 3.873Å and a distorted pseudo-cubic angle α = 90.26°29.…”

Structural and transport properties of La1−xAgxMnO3 (x=0.075, 0.1, 0.125 and 0.15) manganites

“…Recently, a careful and complete study of La 0.7 Sr 0.3 MnO 3 (LSMO) thin films and related compounds with magnetic, transport, and structural properties are reported that are essentially required for technological applications. It is shown that that the Zener double exchange model, a strong electron-phonon interaction and polaron transport is successful in explaining the electrical resistivity in insulating and metallic phase of these manganites [58]. In passing, we note that in thin films of the low bandwidth manganite Pr 0.68 Ca 0.32 MnO 3 , it is showed that the structural change and the modification of the Coulomb interactions are substantial for magnetoresistive materials.…”

Electrical transport in the ferromagnetic state of silver substituted manganites La1−xAgxMnO3 (x = 0.05 and 0.1)

“…A careful study reveals that the Zener double exchange model, a strong electron-phonon interaction and polaron transport, is successful in explaining the electrical resistivity in the insulating and metallic phases of these manganites. 63 A minimum in the resistivity under the application of magnetic field is also important in manganites. The low temperature resistivity minimum is seen in noble-metal alloys with magnetic impurity and is explained in terms of Kondo effect (logarithmic dependence of resistivity on temperature).…”