Creation of room-temperature-multiferroic oxides and electric-field-controlled magnetization

研究了由于铁磁性和铁电性共存而在室温下表现出较大磁电效应的新材料。1.电场诱导磁化还原及其在氧化物中同时表现铁磁性和铁电性的机理：(1)La，Mn：BaTiO_3和BaTiO_3-LaMnO_3复合材料中的铁磁性和铁电性共存是由于少量的铁磁性(La，BA)MnO_3析出到铁电晶界或铁电颗粒内部。(2)BaTiO_3-LaMnO_3复合材料在室温下同时具有铁电性和相对铁磁磁化强度。随着LaMnO_3颗粒尺寸的减小，复合材料的密度和介电性能明显提高。当施加750V/mm的电场时，…的磁化强度降低了25%(3)在BaTiO_3/BaTiO_3-LaMnO_3复合材料/BaTiO_3结构中，复合材料夹在铁电绝缘体BaTiO_3层之间，观察到与复合材料相似的电场诱导磁化强度降低。这表明，观察到的磁化强度降低可能不是由于热效应，而是由于内禀的场致效应。介电极化引起的空穴浓度变化可能是引起磁化强度降低的原因。(4)在BaTiO_3/(La，BA)MnO_3/BaTiO_3层状结构中，在4V的外加电压下，电场诱导磁化强度降低了2.2%。这是磁化强度变化源于诱导极化的直接证据。2.新型多铁材料的研究(1)与BaTiO_3系相比，Pb(Zr，Ti)O_3-LaMnO_3复合材料具有更高的铁电居里温度和更大的极化。首次发现，在室温下，当外加电场为0.4kV/mm时，磁化强度降低了30%。较少

New materials exhibiting a large magnetoelectric effect due to coexistence of ferromagnetism and ferroelectricity at room temperature have been investigated. The following research results are obtained ;1.Electric-field-induced magnetization reduction and the mechanism in oxides exhibiting simultaneously ferromagnetism and ferroelectrics.(1)Coexistence of ferromagnetism and ferroelectricity observed in La, Mn : BaTiO_3 and BaTiO_3-LaMnO_3 composites are due to a small amount of ferromagnetic (La, Ba)MnO_3 phase precipitated into the grain boundaries or the inside of the ferroelectric grains.(2)The composites BaTiO_3-LaMnO_3 are materials simultaneously exhibiting ferroelectricity and a relatively ferromagnetic magnetization at room temperature. The density and dielectric properties of the composites markedly improved with the grain size of LaMnO_3 decreasing. When an electric field of 750 V/mm was applied to the composites, magnetization reduction as large as 25 % was observed for the … More first time.(3)An electric-field-induced magnetization reduction similar to that in the composites was observed for BaTiO_3/BaTiO_3-LaMnO_3 composite / BaTiO_3 structures where a composite was sandwiched between ferroelectric insulator BaTiO_3 layers. This suggests that the observed magnetization reduction is probably not due to thermal effect but intrinsic field-induced effect. Change of hole concentration in (La, Ba)MnO_3 induced by the dielectric polarization may be responsible for the magnetization reduction.(4)An electric-field-induced magnetization reduction of 2.2 % for the applied voltage of 4V was observed in a BaTiO_3/(La, Ba)MnO_3/BaTiO_3 layer structure. This is direct evidence showing the magnetization change to originate from the induced polarization.2.Search for new multiferroic materials(1)Pb(Zr, Ti)O_3-LaMnO_3 composites are new multiferroics with higher ferroelectric Curie temperature and larger polarization than BaTiO_3 system. It has been found for the first time that they exhibits a magnetization reduction as large as 30 % for a low applied electric field of 0.4 kV/mm at room temperature. Less