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Register a new userStrong charge ordering above room temperature in B-site disordered electron-doped manganite SrMn0.875Mo0.125O3-{delta}
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Added by
Niranjan Lalla Prasad
Publication date
2015
fields
Physics
and research's language is
English
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Low as well as high-temperature electron and x-ray diffraction studies have been carried out on a rare-earth free B-site disordered electron-doped manganite SrMn0.875.Mo0.125O3-{delta} in the temperature range of 83K to 637K. These studies reveal the occurrence of strong charge ordering (CO) at room temperature in a pseudo tetragonally distorted perovskite phase with space-group Pmmm. Non integral modulation vector of 8.95 times along [-110] indicates a charge density wave type modulation. The CO phase with basic perovskite structure Pmmm transforms to a charge disorder cubic phase through a first order phase transition at 355K. Supporting temperature dependent measurements of resistance and magnetization show a metal-insulator and antiferromagnetic transitions across 355K with a wide hysterisis ranging from 150K to 365K. The occurrence of pseudo tetragonality of the basic perovskite lattice with c/a < 1 together with charge-ordered regions with 2-dimensional modulation have been analyzed as the coexistence of two CO phases with 3dx2/3dy2 type and 3dx2-y2 type orbital ordering.
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Room-temperature ferrimagnetism was discovered for the anti-site-disordered perovskite Ca2MnOsO6 with Tc = 305 K. Ca2MnOsO6 crystallizes into an orthorhombic structure with a space group of Pnma, in which Mn and Os share the oxygen-coordinated-octahedral site at an equal ratio without a noticeable ordered arrangement. The material is electrically semiconducting with variable-range-hopping behavior. X-ray absorption spectroscopy confirmed the trivalent state of the Mn and the pentavalent state of the Os. X-ray magnetic circular dichroism spectroscopy reveals that the Mn and Os magnetic moments are aligned antiferromagnetically, thereby classifying the material as a ferrimagnet which is in accordance with band structure calculations. It is intriguing that the magnetic signal of the Os is very weak, and that the observed total magnetic moment is primarily due to the Mn. The Tc = 305 K is the second highest in the material category of so-called disordered ferromagnets such as CaRu1-xMnxO3, SrRu1-xCrxO3, and CaIr1-xMnxO3, and hence, may support the development of spintronic oxides with relaxed requirements concerning the anti-site disorder of the magnetic ions.
The two-electron doped rare earth mangnites Ca_1-x Ce_x MnO_3 (x = 0.1,0.2) are probed using resistivity, ac susceptibility and electron paramagnetic resonance (EPR) measurements across their respective charge ordering (CO) temperatures T_CO = 173 K and 250 K. The EPR g factor and intensity as well as the transport and magnetic behaviours of the two compositions are qualitatively similar and are as expected for CO systems. However, the EPR linewidth, reflective of the spin dynamics, for x = 0.1, shows a strongly anomalous temperature dependence, decreasing with decreasing temperature below T_CO in contrast with the sample with x = 0.2 and other CO systems. Keeping in view the evidence for magnetic frustration in the system, we propose that the anomalous temperature dependence of the linewidth is the signature of the occurrence of a disorder driven spin liquid phase, present along with charge ordering.
We have investigated possible spin and charge ordered states in 3d transition-metal oxides with small or negative charge-transfer energy, which can be regarded as self-doped Mott insulators, using Hartree-Fock calculations on d-p-type lattice models. It was found that an antiferromagnetic state with charge ordering in oxygen 2p orbitals is favored for relatively large charge-transfer energy and may be relevant for PrNiO$_3$ and NdNiO$_3$. On the other hand, an antiferromagnetic state with charge ordering in transition-metal 3$d$ orbitals tends to be stable for highly negative charge-transfer energy and can be stabilized by the breathing-type lattice distortion; this is probably realized in YNiO$_3$.
We show for the system La1-xCexCoO3 (0.1 <= x <= 0.4) that it is possible to synthesize electron-doped cobaltites by the growth of epitaxial thin films. For La1-xCexCoO3, ferromagnetic order is observed within the entire doping range (with the maximum of the Curie temperature, Tc, at x ca. 0.3), resulting in a magnetic phase diagram similar to that of hole-doped lanthanum cobaltites. The measured spin values strongly suggest an intermediate-spin state of the Co ions which has been also found in the hole-doped system. In contrast to the hole-doped material, however, where Tc is well above 200 K, we observe a strong suppression of the maximum Tc to about 22 K. This is likely to be caused by a considerable decrease of the Co3d - O2p hybridization. The observed intriguing magnetic properties are in agreement with previously reported theoretical results.
We present an explanation for the puzzling spectral and transport properties of layered cobaltates close to the band-insulator limit, which relies on the key effect of charge ordering. Blocking a significant fraction of the lattice sites deeply modifies the electronic structure in a way that is shown to be quantitatively consistent with photoemission experiments. It also makes the system highly sensitive to interactions (especially to intersite ones), hence accounting for the strong correlations effects observed in this regime, such as the high effective mass and quasiparticle scattering rate. These conclusions are supported by a theoretical study of an extended Hubbard model with a realistic band structure on an effective kagom`e lattice.
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