High Pressure-high Temperature Synthesis And Propetries Of Multiferroic Materials With Perovskite Structure

Single-phase multiferroic material is a class of single-phase compounds which hasboth ferroelectric and magnetic properties. Firstly, multiferroic materials have bothferroelectric and magnetic properties; secondly, there may be coupling effects betweenferroelectric and magnetic properties, which can achieve the mutual regulation offerroelectric and magnetic materials.Normally, multiferroic materials are categorized into two different classesdepending on the origin of the ferroelectric properties. The first class called properferroelectrics is exemplified by BiFeO3, BiMnO3and Bi2NiMnO6, which are typicalmultiferroic materials universal with both relative high ferroelectric Curie temperatureand high magnetic Curie temperature. However, this class of materials generally showweak magnetoelectric coupling, because of the different origins of anti/ferromagneticand ferroelectric orderings that FM desires transition metals with unpaired3d electronsand unfilled3d orbital, whereas FE polarization requires transition metals with filled3dorbital. Members of the second class known as improper ferroelectrics have broughtbetter magnetoelectric coupling effect, exemplified by orthorhombic RMnO3(R=Tb-Luand Y) and GdFeO3.Orthorhombic RMnO3(o-RMnO3) has attracted great research interests owing tothe strong coupling between magnetic ordering and ferroelectric ordering. Themagnetoelectric is useful for storing information in a memory device. Usually the operating temperature is too low to meet the practical applications. Although manystudies have been carried out, the finding of new multiferroics with higher transitiontemperatures is still lacking. Previous studies have mostly focused on doping at therare-earth site to increase the magnetic transition temperature while protecting theferroelectric properties at the same time. Unfortunately, the multiferroic properties haveyet witnessed any significant improvement as desired. Recent reports suggest that asmall quantity of Cr3+doping at Mn3+sites significantly enhances the FM component ofthe samples although the AFM order is dominant. Furthermore, the newdouble-perovskite compound Tb0.5Ca0.5MnO3treated as Mn4+doping at Mn3+siteshowed a high ferroelectric transition temperature at about270K. These are excitingsignals that the carefully chosen atoms doping at the B site may effectively improve themultiferroic peoperties while avoide limination of the FE polarization.The most widely studied in RMnO3(R=Tb-Lu and Y) is o-TbMnO3, withantiferromagnetic (AFM) ordering at41K and ferroelectric ordering at27K. Anothertypical improper ferroelectric, albeit less studied because of the extraordinary conditionssuch as high-pressure sintering required during synthesis, othorhombic YMnO3(o-YMnO3) is an orthorhombic distorted perovskite (ABO3) in Pnma (No.62) spacegroup. And o-YMnO3shows antiferromagnetic ordering at about42K and ferroelectricordering at21K. Compared with TbMnO3, o-YMnO3is a less complex material to bestudied because the Y3+ion is nonmagnetic. The substitution of the3d5ion Fe3+and3d3ion Cr3+in o-YMnO3gives Fe3+-O-Mn3+(e2-O-1) and Cr3+-O-Mn3+(0gegeg-O-)interaction, thus improving antiferromagnetic transition temperature withoutsignificantly changing the structures. Furthermore, YCrO3is also a multiferroic materialwith antiferromagnetic order at140K and ferroelectric order at437K. Fascinatingproperties are thus expected through the introducing of Cr3+into o-YMnO3. But thedoping product of o-YMnO3, especially high-quality single phase product was barelyreported in the past years. This fact also suggests that it’s difficult to substitute Mn or Yions by other elements in o-YMnO3. The high-quality samples may lead to moreaccurate magnetism and dielectric test results. So the lacking of high-quality samples limited the research of this kind of materials.The Hexagonal YbFeO3thin-film shows good ferroelectric properties at Curietemperature of470K. The remanent polarizarion is as large as10μC/cm2. To understandthe reason why this material can exhibit ferroelectric polarization reported in theliterature, two very important factors can not be ignored—the absence of mirror-plane ofhexagonal structure of P63cm and the empty5dz2(Yb)-2pz(OA) interaction. The bulkmaterial of YbFeO3belongs to the Pnma space group, in general, while no hexagonalstructure YbFeO3bulk material have been reported. To obtain multiferroic properties inthe bulk material, we designed and synthesized a series compands of Yb ions dopedInFeO3. First, InFeO3has a hexagonal structure. Though InFeO3belongs to the P63/mmcspace group which has a mirror-plane, the random presence of Yb ions makes anonmirror symmetry for Yb ions. Second, this system also has an empty5dz2(Yb)-2pz(OA) interaction of Yb ions. Otherwise, indium-based perovskite have been reported asan multiferroic near room temperature. So good multiferroic is expected in theIn1-xYbxFeO3.The highly pure crystals of Y2FeMnO6(YFMO) and Y2CrMnO6(YCMO) havebeen firstly synthesized using flux method under high temperature of1573K and highpressure of6GPa. Both YFMO and YCMO have orthorhombic structures in space groupPnma. The temperature-dependent magnetization and the nonlinear M-H hysteresisloops of both materials indicate that an antiferromagnetic transition occurs at Néeltemperature of328K for YFMO, and a ferrimagnetic transition occurs at74K forYCMO. YFMO is a relaxor ferroelectric in which three dielectric relaxations wereobserved at245K,328K and358K, respectively. The first relaxation process is due toMaxwell-Wagner polarization at the grain boundary whereas the second and the thirdrelaxation behaviours arise from the beginning and the ending of antiferromagneticordering, respectively. The presence of the dielectric anomaly near TNindicates themagnetoelectric effect. Ferroelectric hysteresis loops and PUND (positive-up&negative-down) pulse data reveal weak ferroelectric behaviors of YFMO at77K.Otherwise one dielectric constant anomaly has been found for YCMO at about390K.But no ferroelectric polarization was observed in YCMO. A new series of multiferroic materials In1-xYbxFeO3(0.1≤x≤0.3) with giantdielectric constant were firstly prepared. The substitution of Yb ions for In induced theweak ferroelectric properties of this system. Dielectric measurement shows that thedielectric property is closely related to the crystal structures. The crystallize in spacegroup P63/mmc similarly to InFeO3. The increase in the Yb content leads to anincrement in a and reduction in c parameters. Magnetic measurements show thatIn0.8Yb0.2FeO3has antiferromagnetic transition temerature at about150K. And onedielectric constant change has been fund for In0.8Yb0.2FeO3at about553K.