Pressure Induced Structural Transitions in Manganites with Orbital Ordering
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Pressure Induced Structural Transitions in Manganites with Orbital Ordering Michael D. Kaplan*,**, George O. Zimmerman** *Physics and Chemistry Departments, Simmons College, 300 The Fenway, Boston, 02115 **Physics Department, Boston University, 590 Commonwealth Avenue, Boston, MA 02215 Abstract In order to explain magneto-elastic and pressure measurements in doped colossal magneto-resistance manganites, we postulate a doublet-triplet model and use it to predict the occurrence of the various phase transitions.
Measurements of magnetic, magneto-elastic and transport properties of the low doped colossal magneto-resistance manganites of LaxSr1-x MnO3 crystals reveal a series of unusual anomalous properties [1-3]. These anomalies are related to the existence of three types of phase transitions: 1) a structural transition from insulating paramagnetic to insulating anti-ferromagnetic phase; 2) a phase transition from anti-ferromagnetic insulating phase to a ferromagnetic metallic phase; 3) a low temperature transition to a high symmetry (cubic?) phase. The high temperature structural transition (TJT= 269°K in Lax Sr1-x MnO3) is the result of the cooperative Jahn-Teller effect, and the low temperature phase transition is related to some orbital ordering (T00=147°K); the assumption that this phase transition has pure orbital, without any lattice participation, origin [1-3], is questionable, but this is not the subject of the present discussion. The most characteristic anomalies of the intermediate crystal phase in the temperature range between T00 and TJT are the discontinuities in the crystal lattice cell strains as a function of the external magnetic fields. In addition, the influence of hydrostatic pressure on the properties of manganites has attracted a lot of attention as a means to clarify the interplay between the electron and the lattice degrees of freedom and as a manifestation of the film-substrate interaction [4-10]. The analysis of these as well as some other experimental results has led us to the conclusion that they cannot be explained if only the ground electronic state of Mn3+ cations is taken into account. We suggest that a possible triplet excited state, close to the ground doublet, plays an important role in the phase transitions. Aiming to elucidate the possible triplet state contribution, we develop a doublet-triplet model of phase transitions in manganite type crystals based on the cooperative Jahn-Teller effect [11,12]. Here we are applying the doublet-triplet model for the analysis of the hydrostatic pressure influence on the magneto-elastic properties of crystals and comparing the results with the data for the uniaxial pressure influence discussed earlier [13 ]. In the framework of our approach we have obtained the self-consistent equations describing two sub-lattice order parameters, x and y, in the presence of the uniaxial inplane (Px ≠ 0 or Py ≠ 0) and out-of-plane (Pz ≠ 0) pressure and the hydrostatic pressure, Phydro. The last is taken into account by modulating the crystal field energ
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