The Magnetic,transport And Ferroelectric Properties Of Transition Metal Oxides

Transition metal oxide materials in strongly correlated electron systems have been receiving attention over the past twenty years.The coupling of spin,orbital,charge and lattice degrees of freedom in these materials induces rich physical phenomena,such as superconductivity,Hall effect,colossal magnetoresistance effect,multiferroicity,and so on.This is also very important for some potential technology applications in spintronics,such as data storage,logical operation,and magnetoelectric controlling.The most typical representatives include the colossal magnetoresistance effect of manganites and multiferroic properties of BiFeO3 and TbMnO3.In recent twenty years,great progress has been made.However,due to the complex physical mechanisms,it is far from full understanding of the physics of these materials.Therefore,it is necessary to explore some new materials and study their physical properties.Based on this consideration,we focus our work on the following several aspects,and the dissertation includes several chapters as described in the following:The first chapter consists of two parts.The relevant physical mechanisms of 3d-4d manganites and ruthenate oxides will be introduced in the first part.We first outline the researches on manganites,including the crystal lattice,magnetic structure,phase separation,and colossal magnetoresistance.Then we describe the crystal structure,electronic structure,and magnetic properties of 4d ruthenate oxides ARuO3(A=Ca,Sr,Ba).In the second part,we give a review on the research progress of multiferroics and clarify some typical physical mechanisms.In the second chapter,the structure,ionic valences,magnetism,and magneto-transport behaviors of mixed valence oxides La1-xCaxMn1-xRuxO3(LCMRO)are systematically investigated.The simultaneous substitutions of La3+ and Mn3+ ions by Ca2+ and Ru4+respectively are confirmed by the structural and ionic valence characterizations,excluding the presence of Mn4+ and Ru3+ions.The enhanced ferromagnetism,induced metal-insulator transition,and remarkable magnetoresistance effect are demonstrated when the substitution level x is lower than-0.6,in spite of the absence of the Mn3+-Ru4+eg-orbital double-exchange.These anomalous magnetotransport effects are discussed based on the competing multifold interactions associated with the Mn3+-Ru4+super-exchange and strong Ru4+-Ru4+hopping,while the origins for the metal-insulator transition and magnetoresistance effect remain to be clarified.In the third chapter,we study the eg-orbital double-exchange mechanism as the core of physics of colossal magnetoresistance(CMR)manganites,which usually covers up the role of super-exchange at th et2g-orbitals.The role of the double-exchange mechanism is maximized in La0.7Ca0.3MnO3,leading to the concurrent metal-insulator transition and ferromagnetic transition as well as CMR effect.In this work,by a set of synchronous Ru-substitution and Casubstitution experiments on La0.7-yCa0.3+yMn1-yRuyO3,we demonstrate that the optimal ferromagnetism in La0.7Ca0.3MnO3 can be further enhanced.It is also found that the metalinsulator transition and magnetic transition can be separately modulated.By well-designed experimental schemes with which the Mn3+-Mn4+double-exchange is damaged as weakly as possible,it is revealed that this ferromagnetism enhancement is attributed to the Mn-Ru t2g ferromagnetic super-exchange.The present work allows a platform on which the electrotransport and magnetism of rare-earth manganites can be controlled by means of the t2g-orbital physics of strongly correlated transition metal oxides.In the fourth chapter,we report the structural,ferroelectric,and magnetoelectric coupling behaviors of the layered Ruddleson-Popper perovskite Ca3Mn2O7.It is suggested that the ferroelectric polarization and magnetoelectric coupling coincided with the coupled structural transition by the oxygen octahedron tilts and rotations.Here,we provide the first experimental demonstration of the ferroelectric transition temperature between 250K and 350K and the switchable polarization at low temperature.We also investigate the electric control of magnetization which induced by the ferroelectric domain reverse.More importantly,these results open a new avenue to search for improper ferroelctricity and electric control of magnetization in the Ruddleson-Popper family.In the fifth chapter,we report the low-temperature characterizations on structural,specific heat,magnetic,and ferroelectric behaviors of transition metal oxide compound Sr3NiTa2O9.It is suggested that Sr3NiTa2O9 is a spin-1 triangular lattice Heisenberg quantum antiferromagnet which may have weak easy-axis anisotropy.At zero magnetic field,a two-step transition sequence at TN1=3.35 K and TN2=2.74 K respectively is observed,corresponding to the upup-down(uud)spin ordering and 120° spin ordering respectively.The two transition points shift gradually with increasing magnetic field towards the low temperature,in accompanying with an evolution from the 120° spin structure(phase)to the normal oblique phases.Ferroelectricity in the 120° phase is clearly identified.The first-principles calculations confirm the 120° phase as the ground state whose ferroelectricity originates mainly from the electronic polarization.The sixth chapter is the summary and prospect of this thesis.