Magnetoelectric Effects in Ferromagnetic Oxides with the Double Perovskite Structure

通过x射线衍射、磁学和电学测量以及^<55>Mn NMR研究了La_<1-x>Bi_xMn_<0.5>Ni_<0.5>O_3的晶体结构和磁介电性能，研究了La_<1-x>Bi_xMn_<0.5>Ni_<0.5>O_3的铁磁性和铁电性的竞争，研究了La_<1-x>Bi_xMn_<0.5> O_3的磁电效应。(1)采用常温软化学合成方法，在x=0 ~ 0.9的条件下成功合成了具有双钙钛矿结构的La_<1-x>Bi_xMn_<0.5>Ni_<0.5>O_3。(2)通过对rem_ <0.5>(Ni_<1-x>Mg_x)_<0.5>Ni_<0.5>O_3 （RE =稀土元素）中Mn <4+>- o <2->- ni <2+>键的核磁共振，从超细磁场H_<HF>计算了双钙钛矿中Mn <4+>- o <2->- ni <2+>键的共价参数f（RE）。在此基础上，对La_<1-x>Bi_xMn_<0.5>Ni_<0.5>Ni_<0.5>O_3中键的共价参数f (x)进行了^<55>Mn核磁共振研究，发现其共价参数f (x)随x的增大而减小。T_<CM>随x的增大而减小，可以用f(x)的减小来解释。f (x)的减小是由于Bi^<3+>- o ^<2->键的共价较La^<3+>- o ^<2->键强；O^<2->离子较多的电子被吸引并填充在Bi^<3+>-O^<2->键中，导致Mn^<4+>-O^<2->- ni ^<2+>键中电子缺乏。由Bi^<3+>- o ^<2->键的共价产生的铁电性和由Mn^<4+>- o ^<2->- ni ^<2+>键产生的铁磁性通过两个键的共价竞争而相互竞争。(3)当x≧0.5时，在T_<CM>附近的磁场中介电常数增大。当x=0.7时，介电常数在9 t的T_<CM>处的增量达到7%，介电常数的温度和磁场依赖性，包括频率色散，基于Maxwell-Wagner模型进行了解释，假设样品的非均匀性，其中包含分散在负磁阻材料中的相对高介电常数域。少

We have investigated (1) the crystal structure and the magnetic-dielectric properties, (2) competition of the ferromagnetism and ferroelectricity, and (3) the magnetoelectric effects of La_<1-x>Bi_xMn_<0.5>Ni_<0.5>O_3 by means of the x-ray diffraction, magnetic and electric measurements, and ^<55>Mn NMR.(1) Polycrystalline La_<1-x>Bi_xMn_<0.5>Ni_<0.5>O_3 with the double perovskite structure has been successfully synthesized for x=0〜0.9 by soft chemical synthesis at ambient pressure. The dielectric constant increases and the ferromagnetic Curie temperature (T_<CM>) decreases with increasing x.(2) It has been clarified through NMR of ^<55>Mn in REMn_<0.5>(Ni_<1-x>Mg_x)_<0.5>Ni_<0.5>O_3 (RE = rare earth elements) that the covalency parameter, f(RE), for Mn^<4+>-O^<2->-Ni^<2+> bond in the double perovskite is evaluated from the hyperfine magnetic field, H_<HF>, at ^<55>Mn. Based on the knowledge, the covalency parameter, f (x), for the bond in La_<1-x>Bi_xMn_<0.5>Ni_<0.5>Ni_<0.5>O_3 is fou … More nd to decrease with increasing x by ^<55>Mn NMR study of the oxides. The decrease of T_<CM> with increasing xis naturally explained with the decrease of f(x).The decrease of f (x) is attributed to the strong covalency of Bi^<3+>-O^<2-> bond compared with La^<3+>-O^<2-> bond; more electrons of O^<2-> ions are attracted to and populate in Bi^<3+>-O^<2-> bond resulting in deficiency of electrons in Mn^<4+>-O^<2->-Ni^<2+> bond. The ferroelectricity arising from the covalency of Bi^<3+>-O^<2-> bond and the ferromagnetism arising from that of Mn^<4+>-O^<2->-Ni^<2+> bond compete with each other through the competition of the covalencies of both bonds.(3) For x≧0.5, the dielectric constant increases in a magnetic field near T_<CM>. For x=0.7, the increment of the dielectric constant amounts to 7% at T_<CM> in 9 T. The temperature and the magnetic field dependence of the dielectric constant, including the frequency dispersion, has interpreted based on the Maxwell-Wagner model assuming an inhomogeneity of the sample which contains domains with relatively high dielectric constant dispersed in the material with negative magneto-resistance. Less