High-Pressure And High-Temperature Synthesis Of Cubic Quadruple Perovskite Oxides With Magnetoelectric Multiferroicity

The magnetoelectric multiferroics with spin-induced ferroelectricity always show stronger magnetoelectric coupling,which not only have rich physical significance in fundamental physics but also are promising for potential applications in developing spintronic devices and multiple-state memory devices.In this work,based on La Mn₃Cr₄O₁₂ system as the first one to be found magnetoelectric multiferroicity in a cubic perovskite lattice,we have carried out a series of experiments on designing new materials and investigating the spin-driven ferroelectricity.The main results are shown as follows:1.An A-site ordered perovskite Sm Mn₃Cr₄O₁₂ with a cubic Im-3 space group was prepared at high pressure and temperature conditions.Two antiferromagnetic phase transitions are found to occur with decreasing temperature to T_N1≈145 K and T_N2≈41K,due to the spin orderings of the B-site Cr³⁺and A’-site Mn³⁺magnetic sublattices,respectively.The total spin structure composed of these two magnetic sublattices can break the space inversion symmetry and therefore induce a spontaneous ferroelectric phase transition at T_N2.More interestingly,the magnetoelectric multiferroicity of the polycrystalline Sm Mn₃Cr₄O₁₂ exhibits remarkable magnetic and electric field dependent anisotropic quadratic behaviors upon a proper magnetoelectric poling procedure.When the applied magnetic field is parallel to the electric field,the magnetic field can significantly enhance the electric polarization.In contrast,the polarization is sharply suppressed if the magnetic field is perpendicular to the electric field,irrespective of the sign of the magnetic field.2.An A-site ordered perovskite Tb Mn₃Cr₄O₁₂ with a cubic Im-3 space group was prepared at high pressure and temperature conditions.Calculation results show that the set of spin-induced ferroelectric polarizations in cubic perovskite oxides AMn₃Cr₄O₁₂（A=La and Tb）resides on the topological Roman surface,a non-orientable two-dimensional manifold formed by sewing a M（?）bius strip edge to that of a disc.The induced polarization may travel in a loop along the non-orientable M（?）bius strip or orientable disc depending on how the spin evolves as controlled by external magnetic field.Experimentally,the periodicity of polarization can be the same or the twice of the rotating magnetic field,being well consistent with the orientability of disc and M（?）bius strip,respectively.This path dependent topological magnetoelectric effect presents a way to detect the global geometry of the surface and deepens our understanding of topology in both mathematics and physics.3.We studied the B-site Mn⁴⁺-doping effects on La Mn₃Cr₄O₁₂ to raise the spin-induced ferroelectric transition temperature.An A-site ordered but B-site disordered quadruple perovskite oxide Pb Mn₃Cr₃Mn O₁₂ was synthesized by high-pressure and high-temperature methods.The compound crystallizes in space group Im-3 with the charge distribution of Pb²⁺Mn₃³⁺Cr₃³⁺Mn⁴⁺O₁₂.Three antiferromagnetic phase transitions are found to occur at T_N1≈155 K,T_N2≈81 K,and T_N3≈74 K,respectively,due to the complex B-site and A’-B intersite spin interactions.Compared with the isostructural La Mn₃Cr₄O₁₂ with negligible A’-B intersite spin coupling,the substitution of Mn⁴⁺,which has identical electronic configuration with that of Cr³⁺(t³_2g),into the B site can introduce A’-B intersite spin interactions.As a result,the A’-site spin-related ordering temperature increases significantly while the B-site one remains little changed in the current Pb Mn₃Cr₃Mn O₁₂.This work opens up a way to enhance the A’-site spin ordering temperature in quadruple perovskite oxides.4.We studied the effects of B-site Fe³⁺-doping on the magnetoelectric multiferroic La Mn₃Cr₄O₁₂ and successfully synthesized La Mn₃Cr_4-xFe_xO₁₂（x=0,1,2）.From XRD measurements,a stable crystal symmetry of space group Im-3 can be found despite B-site doping.With increasing x,the B-site spin ordering temperature increases accompanied with more short-range spin order.In addition,the B-site Fe³⁺-doping can sharply suppress the A’-site spin ordering with a decrease of magnetic transition temperature.The dielectric constant and polarization measurements indicate that the magnetoelectric coupling and spin-induced ferroelectricity are also significantly suppressed compared to La Mn₃Cr₄O₁₂ due to Fe³⁺substitution.