Hydrothermal Synthesis And Magnetic Properties Of Several ABO₃-type Rare Earth Manganates And Ferrites

ABO3-type composite oxides show great tolerance for A-and B-site ions in theaspects of ionic radius and valence. It provides probability and convenience fordoping modification of existing materials and synthesis of new materials. Thereinto,ABO3-type rare-earth manganates and ferrites have become a research focus in thefields of materials and condensed matter physics because of their richmagnetic/electric properties and especially the excellent physical effects such asgiant magnetoresistance, multiferroics, etc. At present, the reaserch therein is mostlybased on high-temperature synthesis. But many outstanding candidates aremetastable phases, which are hard to form or form into the pure phase in hightemperatatures, thus hindering the reaserch progress. Hydrothermal synthesisprovides probability for synthesizing varieties of metastable phases. In this work,four series ABO3-type rare-earth manganates and ferrites were chosen as researchobject, hydrothermal technique was used to synthesize pure-phased samples, and thecrystal structures, chemical compositions, valence states, morphologies, magneticproperties, etc. were studied.Four pure-phased orthorhombic Y1-xCaxMnO3(x=0,0.07,0.55and0.65)samples with fine crystallization and morphologies were successfully synthesizedvia the low-temperature hydrothermal technique. Y1-xCaxMnO3(x=0) andY1-xCaxMnO3(x=0.07) show the behavior of spin glass. Y1-xCaxMnO3(x=0.55)shows obvious charge-ordering, antiferromagnetic transition and the behavior ofspin glass. Y1-xCaxMnO3(x=0.65) shows strong signals of charge ordering andantiferromagnetic transition, but no behavior of spin glass. Drastic increase ofmagnetization at low temperatures was observed in Y1-xCaxMnO3(x=0.65),suggesting the existence of Dzialoshinsky-Moriya anisotropic superexchange.Obvious magnetic hysteresis was observed in Y1-xCaxMnO3(x=0.65) at4K,suggesting the existence of strong local short-range ferromagnetic interaction.Y1-xCaxMnO3(x=0.55) and Y1-xCaxMnO3(x=0.65) show the charge transportbehavior of semiconductors and a small magnetoresistance, and the electricconduction mechanism accords with the variable range hopping mode of thehighly-correlated electrons.Pure-phased metastable rare-earth orthoferrites RFeO3(R=Pr, Nd, Sm, Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb and Lu) with fine crystallization and morphologies were successfully synthesized via the low-temperature hydrothermal technique. Thefactors that affect the formation of target products such as media alkalinity, reactiontemperature and time etc. were fully studied, and optimum routes were concluded.The crystal structures, morphologies were characterized, and the crystal nucleationand growth mechanism were analyzed. The magnetic properties, including Fe3+antiferromagnetic transition, spin reorientation, spin glass transition, magnetizationreversal, Re3+antiferromagnetic transition, etc., were characterized and discussed.Compared with samples synthesized by solid state sintering in literatures, thesamples in this work show some difference in the magnetic properties, for example,spin reorientation was observed in EuFeO3and LuFeO3.Pure-phased orthorhombic RMn0.5Fe0.5O3(R=Tb, Dy and Ho) weresuccessfully synthesized via the low-temperature hydrothermal technique. Thecrystal structures, morphologies, chemical compositions and valences, magneticproperties etc. were analyzed. Magnetization reversal was observed inTbFe0.5Mn0.5O3. A high magnetic field can reverse the magnetization direction, andthe spin reorientation temperature has a linear relation with the magnetic fieldintensity. Re3+antiferromagnetic transition was observed in TbFe0.5Mn0.5O3andDyFe0.5Mn0.5O3at low temperatures. The three samples show residual magnetism at2K and300K.Hexagonal RMn1-xFexO3(R=Er, Tm, Yb and Lu; x=0,0.1,0.3and0.5) weresuccessfully synthesized via the low-temperature hydrothermal technique. Structuralanalysis shows that the samples of ErMnO3and RMn1-xFexO3(R=Tm, Yb and Lu;x=0,0.1,0.3and0.5) are all pure hexagonal phase, but the samples of ErMn1-xFexO3(x=0.1,0.3and0.5) comprise hexagonal phase and orthorhombic phase. Fe dopingstretches the Mn-O bond along the c axis and shortens the Mn-O bond in the a-bplane, thus leading increase of c and decrease of a. The ferromagnetictransformation TNwas elevated by Fe doping and a linear relation was observedbetween ΔTNand Fe doping x.