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

We present the results of our extensive Mössbauer effect studies carried out on a wide variety of mixed valence manganites as well as other types of magnetoresistive materials, including pyrochlore Tl₂Mn₂O₇ and the chalcospinels Fe₀.₅Cu₀.₅Cr₂S₄ and FeCr₂S₄ with absolutely different natures of the magnetism, in a search for similarities linked to their magnetoresistive behavior. The double exchange electron transfer and coupling between the electrons and Jahn–Teller lattice distortions invoked by most theories to explain the colossal magnetoresistivity and associated metal–insulator transition in manganites are not applicable to pyrochlore nor to chalcospinels. Nevertheless, we find intriguing similarities in the anomalous magnetic and dynamic behavior among these widely different systems at, above, and below the Curie temperature Tc, which shed light on the origin of bulk magnetoresistivity in general. All these compounds share the following features. The long-range ferromagnetic order breaks down even below the Curie temperature, with the formation of nano-size spin clusters. Softening of the lattice was observed near Tc. The short-range interactions in these spin clusters survive well above Tc. When an external magnetic field is applied, the spin clusters coalesce to form large clusters, with considerable lowering of the resistivity. There is a strong evidence that the existence of nano-size spin clusters with superparamagnetic-like behavior near Tc is a prerequisite for the occurrence of bulk magnetoresistivity

Competition of Zener and polaron phases in doped CMR manganites

Inspired by the strong experimental evidence for the coexistence of localized and itinerant charge carriers close to the metal-insulator transition in the ferromagnetic phase of colossal magnetoresistive manganese perovskites, for a theoretical description of the CMR transition we propose a two-phase scenario with percolative characteristics between equal-density polaron and Zener band-electron states. We find that the subtle balance between these two states with distinctly different electronic properties can be readily influenced by varying physical parameters, producing various ``colossal'' effects, such as the large magnetization and conductivity changes in the vicinity of the transition temperature.Comment: 8 pages, 5 figure

Mixed-phase description of colossal magnetoresistive manganites

In view of recent experiments, indicating the spatial coexistence of conducting and insulating regions in the ferromagnetic metallic phase of doped manganites, we propose a refined mixed-phase description. The model is based on the competition of a double-exchange driven metallic component and a polaronic insulating component, whose volume fractions and carrier concentrations are determined self-consistently by requiring equal pressure and chemical potential. The resulting phase diagram as well as the order of the phase transition are in very good agreement with measured data. In addition, modelling the resistivity of the mixed, percolative phase by a random resistor network, we obtain a pronounced negative magnetoresistance in the vicinity of the Curie temperature $T_C$.Comment: 7 pages, 6 figure

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

We present the results of our extensive Mössbauer effect studies carried out on a wide variety of mixed valence manganites as well as other types of magnetoresistive materials, including pyrochlore Tl₂Mn₂O₇ and the chalcospinels Fe₀.₅Cu₀.₅Cr₂S₄ and FeCr₂S₄ with absolutely different natures of the magnetism, in a search for similarities linked to their magnetoresistive behavior. The double exchange electron transfer and coupling between the electrons and Jahn–Teller lattice distortions invoked by most theories to explain the colossal magnetoresistivity and associated metal–insulator transition in manganites are not applicable to pyrochlore nor to chalcospinels. Nevertheless, we find intriguing similarities in the anomalous magnetic and dynamic behavior among these widely different systems at, above, and below the Curie temperature Tc, which shed light on the origin of bulk magnetoresistivity in general. All these compounds share the following features. The long-range ferromagnetic order breaks down even below the Curie temperature, with the formation of nano-size spin clusters. Softening of the lattice was observed near Tc. The short-range interactions in these spin clusters survive well above Tc. When an external magnetic field is applied, the spin clusters coalesce to form large clusters, with considerable lowering of the resistivity. There is a strong evidence that the existence of nano-size spin clusters with superparamagnetic-like behavior near Tc is a prerequisite for the occurrence of bulk magnetoresistivity

Emission Mössbauer study of CMR manganite La₀.₈Ca₀.₂MnO₃. I. Anomalous ferromagnetism

Using ⁵⁷Co emission Mössbauer technique, we present clear evidence that in Ca-doped manganite, the magnetic and paramagnetic phases coexist below Tc, with the abundance of the latter increasing with temperature. In contrast with the regular ferromagnetic materials, the variation of the hyperfine internal magnetic field Hint with temperature deviates considerably from the Brillouin relation, and exhibits an abrupt drop at Tc. These features characterize the magnetic transition as a first-order transition. The non-Brillouin behavior of Hint(T) and the temperature dependence of the shape of the magnetically split sextet indicate the presence of spin fluctuations in this material well below Tc

Emission Mössbauer study of CMR manganite La₀.₈Ca₀.₂MnO₃. II. Step-by-step snapshots of the metal-insulator transition

We follow the step-by-step progression of events while approaching the Curie temperature Tc from below using ⁵⁷Co substituent as a microprobe in an emission Mössbauer study combined with resistivity measurements. In the temperature range 0.33<T/Tc ≤ 1, the material consists of a mixture of ferromagnetic and paramagnetic regions within the same matrix. An increase of the amount of paramagnetic fraction is accompanied by a decrease in electron delocalization in the ferromagnetic regions. At T≥Tc, the electrons are localized to neighboring Mn⁴⁺/Mn³⁺ pairs only, in about 46% of the paramagnetic species. The strength with which Mn atoms are bound to the neighbors also decreases progressively and rather steeply in the range 0.65<T/Tc=1. Zero field resistivity, ρ0, follows linearly with the amount of the paramagnetic phase in the range 0.65<T/Tc<1 and still shows metal-like behavior up to T/Tc≈1.03. Remove selecte

Mössbauer study of 1% 57 Fe doped ferromagnetic insulator La 0.825 Ca 0.175 MnO 3

We have employed magnetization measurements, Mössbauer and ESR spectroscopic techniques, in order to study the ferromagnetic insulating (FMI) compound La 1-x Ca xMnO 3 (x=0.175) doped with 1% 57 Fe. We have used two samples; one prepared in air which has cation vacancies and a second in inert atmosphere, which is stoichiometric. An abrupt change of the experimental results is obtained, by all techniques, in the ferromagnetic insulating regime, in the temperature region of T O/O// ≈60 K, where an orbital rearrangement is suggested to occur. An analysis of these findings points to an orbital rearrangement transformation. Ferromagnetic resonance reveals considerable differences between stoichiometric and cation deficient samples, indicating anisotropy of the exchange interactions in the former sample with significant temperature dependence, most pronounced in the vicinity of T O/O// Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2005