Magnetocaloric Effect And Critical Behavior Of Perovskite-type Rare-earth Titanate Crystals

Perovskite transition metal oxides system has been the forefront research fieldsof condensed matter physics and materials physics and other disciplines, becausethere are many strange and interesting physical phenomena, as well as many otherrich and complex physical connotations. The interplay and coupling between thespin, orbit, and lattice degree of freedom makes the perovskite rare earth titanatesRTiO₃（R=rare earth element or Y） system exhibiting many peculiar physicalphenomena. Sufficient research and understanding of the titanate system will helpresearchers to clarify the role of orbital degrees of freedom in strongly correlatedelectron system, reveal the mysteries of the coupling between magnetic and orbitaldegrees of freedom, and enhance the understanding of the complex interactions ofstrongly correlated electron system. With decreasing the size of R3+ions in theRTiO₃system, the magnetic ordering of Ti3+ions changes from antiferromagneticfor R=La, Pr, Nd, Sm to ferromagnetic for R=Gd,…, Lu, Y. The crossoverbehavior of the magnetic ordering is needed to be further studied. There are thesharp change of the magnetization and the orbital order-disorder transition at themagnetic ordering temperature. Therefore, the existence of large magnetocaloriceffect and magnetocaloric effect associated orbital ordering are very worthresearching in the system. The critical behavior study will contribute to theunderstanding of the ferromagnetic phase transition and the magnetic interactions inthe RTiO₃. Since the magnetization measurements are plagued by the contributionfrom magnetic rare earth, the critical behavior study of the ferromagnetic RTiO₃willmeets some problems by the conventional Arrott method. This requires us to explorea new method to determine the critical behavior of the ferromagnetic RTiO₃, anddeepen the understanding of the types of the ferromagnetic interactions in thesystem. Therefore, in this dissertation we studied the crossover behavior near theantiferromagnetic （AFM） to ferromagnetic （FM） phase transition boundary, themagnetocaloric effect and the critical behavior of the RTiO₃system. The mainresults were listed as following:Through the specific heat and magnetic data, the complex magnetic structureand the coupling between spin, orbital and lattice were revealed in SmTiO₃near theAFM-to-FM phase transition boundary in RTiO₃system. The special crystal fieldenvironment as well as the special spin-orbit structure leads to the two-dimensionalanisotropic characteristics. The abnormal specific heat peak under high magneticfield at c axis was originated from the weak ferromagnetism in SmTiO₃. The RTiO₃（R=Dy-Yb） system exhibits a large low magnetic fieldmagnetocaloric effect. For instance, the ΔSMand ΔTadreaches9.64J kg^-1K^-1and4.14K for DyTiO₃under a magnetic change of2T, respectively. The low-fieldmagnetic entropy change in RTiO₃system is quite impressive in the oxidemagnetocaloric materials. The origin and physical mechnism of the largemagnetocaloric effect was analyzed. The simultaneous anomalies of the latticeparameters due to the orbital order and the coupling of the magnetism and lattice canstrongly influence the magnetic entropy change. The universal behavior was alsofound in the temperature and magnetic field dependence of the magnetic entropychange curves, which was explained by the scaling relation theory.The magnetocaloric effect scaling law method is applied to the study of thecitical behavior of magnetic phase transition based on the relationship of themagnetocaloric effect and the critical behavior. Because of the influence frommagnetic rare earth R3+ions, the Arrott plot method leads to the incorrect criticalexponents. Here we report critical exponents for most ferromagnetic members in theRTiO₃family by measuring magnetocaloric effect and applying the correspondingscaling laws. Our results indicate that the ferromagnetic coupling in the RTiO₃canbe well-described by the3D Heisenberg model.The specific heat and the enhanced magnetic refrigeration capacity in GdTiO₃were studied. The λ-type anomaly occurs at TC=35K in zero-field specific heatcomes from the cooperation behavior of the magnetic sublattice Ti3+ions. TheSchottky specific heat anomaly appears at TS≈8K was derived from the groundstate doublet splitting of the Gd3+ions in the role of Gd-Ti exchange field. Thepresence of low-temperature Schottky-like anomaly arising from the splitting of theground-state doublet of Gd3+ion, which enlarges the temperature span of large MCE,and consequently resulting an enhanced RC.The metamagnetic transition phenomenon and the anomalous （inverse）magnetocaloric effect in TbTiO₃were studied. Below the Curie temperature TC,magnetic field induced metamagnetic transition is associated with the ground statemagnetic structures in TbTiO₃. The change of the magnetic entropy withtemperature as well as the external magnetic field and the performance for thecoexistence of normal and inverse magnetocaloric effect were originated from themetamagnetic transition. In addition, the anomalous hysteresis loop below the20Kwas associated with the magnetic anisotropy energies.