Striping of orbital-order with charge-disorder in optimally doped manganites

The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the $${\frac{3}{8}}^{th}$$ 3 8 t h doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn$${}_{3}^{A^{\prime} }$$ 3 A ′ Mn$${}_{4}^{B}$$ 4 B O12), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn3+ and charge disordered (CD) Mn3.5+ layers in a 1:3 ratio. This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. This striping of CD Mn3.5+ with Mn3+ provides a natural mechanism though which long range OO can melt, giving way to a conducting state.

Localized electronic vacancy level and its effect on the properties of doped manganites

Oxygen vacancies are common to most metal oxides and usually play a crucial role in determining the properties of the host material. In this work, we perform ab initio calculations to study the influence of vacancies in doped manganites $$\text {La}_{(1-\text {x})} \text {Sr}_{\text {x}} \text {MnO}_{3}$$ La ( 1 - x ) Sr x MnO 3 , varying both the vacancy concentration and the chemical composition within the ferromagnetic-metallic range ($$0.2\,<\,\text {x}\,<\,0.5$$ 0.2 < x < 0.5 ). We find that oxygen vacancies give rise to a localized electronic level and analyse the effects that the possible occupation of this defect state can have on the physical properties of the host. In particular, we observe a substantial reduction of the exchange energy that favors spin-flipped configurations (local antiferromagnetism), which correlate with the weakening of the double-exchange interaction, the deterioration of the metallicity, and the degradation of ferromagnetism in reduced samples. In agreement with previous studies, vacancies give rise to a lattice expansion when the defect level is unoccupied. However, our calculations suggest that under low Sr concentrations the defect level can be populated, which conversely results in a local reduction of the lattice parameter. Although the exact energy position of this defect level is sensitive to the details of the electronic interactions, we argue that it is not far from the Fermi energy for optimally doped manganites ($$\text {x}\,\sim \,1/3$$ x ∼ 1 / 3 ), and thus its occupation could be tuned by controlling the number of available electrons, either with chemical doping or gating. Our results could have important implications for engineering the electronic properties of thin films in oxide compounds.

Chemical disorder induced positive magnetoimpedance in La0.7Pb0.3Mn0.35Fe0.65O3- and La0.7Pb0.3Mn0.3Fe0.7O3- manganites

We report structural, magnetic and magnetoimpedance properties of La0.7Pb0.3Mn0.35Fe0.65O3- and La0.7Pb0.3Mn0.3Fe0.7O3- manganites. Bulk samples were prepared by solid state method. Rietveld refinement of the X-ray diffraction pattern shows the crystallization of these samples in trigonal crystal system. Fe doping at Mn site in La0.7Pb0.3MnO3 increases the lattice parameters and induces oxygen non stoichiometry in the lattice of La0.7Pb0.3Mn0.35Fe0.65O3-and La0.7Pb0.3Mn0.3Fe0.7O3-. La0.7Pb0.3Mn0.3Fe0.7O3-composition shows ~180% positive magnetoimpedance at 1Tesla magnetic field while La0.7Pb0.3Mn0.35Fe0.65O3- shows ~75% positive magnetoimpedance at 320K. Magnetization versus applied magnetic field measurement curves show the magnetic moment of La0.7Pb0.3Mn0.35Fe0.65O3-and La0.7Pb0.3Mn0.3Fe0.7O3-do not saturate up to 2 tesla magnetic field at 300K. Fe doping at Mn site in these manganites created chemically modified systems where the origin of positive magnetoimpedance is found due to the presence of magnetic region inside of the nonmagnetic regions. Huge positive magnetoimpedance in 65% and 70% Fe doped manganites originated by maxwell wagner effect due to the chemical disorder induced by Fe in manganite lattice.

Low Energy Excitations in La1.2Sr1.8Mn2O7 investigated by ellipsometry

The low-energy excitations of the bilayered manganite La1.2 Sr1.8 Mn2 O7 have been explored by spectral ellipsometry from two faces of a single crystal over the range from 0.006 to 0.6 eV. This compound is a paramagnetic insulator at ambient temperature, with a transition to a ferromagnetic metal below a Curie temperature (Tc) of 125 K. Both the ab -plane and c -axis temperature-dependent conductivities have been determined. Essentially no temperature-dependent behavior is observed above Tc although below Tc both the phonon and electronic contributions are strongly temperature sensitive. The highest-frequency phonons, especially those involving Mn-O bond stretching, split and show frequency changes consistent with structural results in the literature, and furthermore there is clear evidence of an increase in electron-phonon coupling at and below Tc. We interpret the temperature-dependent electronic spectral contribution in the light of recent calculations that indicate that a mixed phase exists in the doped manganites below Tc, with coexisting regions of an itinerant large-polaron phase and a localized small-polaron phase. © 2005 The American Physical Society.

Low Energy Excitations in La1.2Sr1.8Mn2O7 investigated by ellipsometry

The low-energy excitations of the bilayered manganite La1.2 Sr1.8 Mn2 O7 have been explored by spectral ellipsometry from two faces of a single crystal over the range from 0.006 to 0.6 eV. This compound is a paramagnetic insulator at ambient temperature, with a transition to a ferromagnetic metal below a Curie temperature (Tc) of 125 K. Both the ab -plane and c -axis temperature-dependent conductivities have been determined. Essentially no temperature-dependent behavior is observed above Tc although below Tc both the phonon and electronic contributions are strongly temperature sensitive. The highest-frequency phonons, especially those involving Mn-O bond stretching, split and show frequency changes consistent with structural results in the literature, and furthermore there is clear evidence of an increase in electron-phonon coupling at and below Tc. We interpret the temperature-dependent electronic spectral contribution in the light of recent calculations that indicate that a mixed phase exists in the doped manganites below Tc, with coexisting regions of an itinerant large-polaron phase and a localized small-polaron phase. © 2005 The American Physical Society.