Sponsors:
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Organized by
the Institute of Experimental Physics, University of Bialystok
Andrzej Szewczyk
Institute of Physics, Polish Academy of Sciences, Warsaw, Poland
Manganites - phase diagram, specific heat and colossal magnetoresisitance
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The term "manganites" denotes a wide class of materials that are formed from the parent LaMnO3 compound via partial substitution of the trivalent lanthanum ions with other trivalent rare earth ions or with divalent ions such as Ca, Sr, or Ba. Along with the substitution of the divalent ions, Mn3+ ions are replaced with Mn4+ ones (to conserve electrical neutrality). This process influences dramatically transport (from insulating to metallic) and magnetic (from antiferromagnetic to ferromagnetic) properties, as well as the crystalline structure of the material. As the result, these compounds show very interesting physical phenomena, among them strong electron-electron correlations, so called, phase separation, strong spin-charge-lattice coupling, charge and orbital ordering, as well as colossal magnetoresistance and large magnetocaloric effect. The latter two effects are thought to be promising for application in computer memory elements and in magnetic refrigerators. All these factors made manganites attractive materials, studied very intensively over the last ten years (in fact, they became the second group of most frequently studied compounds after high-TC superconductors). In spite of the intensive investigation, many physical properties of manganites, including colossal magnetoresistance, are far from being satisfactorily understood.
After short overview of the problems mentioned above, the competing interactions occurring in manganites (Coulomb interactions, Hund's coupling, crystalline electric field, double exchange, Jahn-Teller effect, and superexchange) will be described. Next, the properties-composition phase diagram of the La1-xSrxMnO3 series for the full range of x (0 < x < 1), being the typical example of the phase diagrams of different manganite series, will be considered in detail. Particular emphasis will be given to the influence of structural, magnetic, magneto-structural, and charge/orbital ordering phase transitions on temperature dependences of specific heat. In particular:
- Differences in specific heat anomalies accompanying phase transitions from the paramagnetic to the antiferromagnetic phases of the type A, C, and G will be discussed.
- The unconventional first order magneto-structural phase transition from the A-type antiferromagnetic to the ferromagnetic phase, strongly influenced by magnetic field (the field of 9 T shifts the transition point by 33K) will be presented. Also the supplementary magnetization studies revealing the effect of coexistence of both phases will be outlined.
- The studies of the magnetocaloric effect accompanying the second order phase transition from the ferromagnetic to the paramagnetic phase will be presented. At this point, also parameters characterizing the magnetocaloric effect (isothermal change in entropy and adiabatic change in temperature induced by magnetic field), as well as advantages and shortcomings of the three basic methods used for studying this effect (magnetic, direct, and specific heat based method) will be described.
In the last part of the lecture, multiferroic (i.e., simultaneously magnetic and ferroelectric) properties occurring in some manganites, both in those that crystallize in the typical perovskite structure and in those that crystallize in the hexagonal structure, will be discussed.
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