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Phase Separation Vs. Homogeneous State In Complex Magnetic Oxides (Anatoly M.Balagurov, Russia)

Topics

  • There exist several different types of phase separation (PS) in magnetic oxides, which are important for understanding of their physical properties, including superconductivity (SC) and colossal magnetoresistance (CMR). Among them: microscopic (or electronic) PS with length-scale of about < 102 Å and meso- (or macro-)scopic chemical or "structural" PS with length-scale of about ~ 103 Å. Microscopic PS, which is driven by charge (electrons or holes) segregation, was theoretically substantiated for magnetic oxides already in 70-s [1] and can explain the existence of small magnetic droplets found in CMR compounds. On the other hand, the observed coexistence of FM and AFM structures in manganites found in diffraction experiments can be naturally explained as spatially separated two phase state: AFM-dielectric and FM-metallic with regions of mesoscopic dimensions (see, for instance, [2, 3]). The similar behaviour - the presence of micro- and macroscopic phase separation - was found recently in HTSC superconductors [4, 5]. It is revealed as successive transitions into chemically inhomogeneous state followed by charge segregation and appearing of superconductivity. One may conclude that this tendency could be an intrinsic property of systems with strong competition between different types of ordering. There are many theoretical attempts of explanation these effects (see, for instance, [6, 7]) but no common view still exists.
    The paper updates present-day results concerning PS in HTSC and CMR compounds, which have been mainly obtained by neutron diffraction and mSR technique. As an example, a series of experiments which has been performed with CMR manganite (La1-yPry)0.7Ca0.3MnO3 , 0 ≤ y ≤ 1, is analysed. The experiments have been carried out with isotopicaly enriched samples, at several neutron diffractometers and mSR-spectrometers, in wide range of temperature (1.5 - 300 K) and magnetic field (0 - 40 kOe). As an example of PS in HTSC's, the results of the comprehensive study of La2CuO4+x, 0.02 ≤ x ≤ 0.04, single crystals with low oxygen mobility are presented.

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