Magnetoelectric Effects in Ferromagnetic Oxides with the Double Perovskite Structure

本文用X射线衍射、磁、电测量和~ 1HNMR研究了La_ Bi_xMn_ Ni_ O_3的晶体结构和磁介电性质，铁磁性和铁电性的竞争，以及磁电效应<1-x><0.5><0.5><55>。(1)采用<1-x><0.5><0.5>软化学合成法在常压下成功地合成了具有双钙钛矿结构的La_ Bi_xMn_ Ni_ O_3多晶（x=0 ~ 0.9）。随着x的增加，介电常数增加，铁磁居里温度（T_<CM>）降低. (2)通过<55>REMn_<0.5>（Ni_<1-x>Mg_x）_<0.5>Ni_<0.5>O_3（RE =稀土元素）中~（40）Mn的NMR研究，阐明了在~（40）Mn处，<2->由超精细磁场H_估算出双钙钛矿中Mn^&lt;4+&gt;-O^ -Ni^&lt;2+&gt;键<HF>的共价参数f（RE）<55>。在此基础上，得到了La_ Bi_xMn_ Ni_ Ni_ O_3中键的共价参数f（x）<1-x><0.5><0.5><0.5>为 关于我们 通过<55>对氧化物的~ 1 Mn NMR研究发现，随着x的增加，氧化物的还原性降低。f<CM>（x）的降低是由于Bi^&lt;3+&gt;-O^^键的共价性比<2->La^&lt;3 +&gt;-O ^<2->键强，O^离子的电子更多<2->地被吸引到Bi^&lt;3+&gt;-O ^键中，从而<2->导致Mn^&lt;4+&gt;-O^<2->-Ni^&lt;2+&gt;键中的电子不足。Bi^&lt;3+&gt;-O^键的共价性所产生的铁电性<2->和Mn^&lt;4+&gt;-O^<2->-Ni^&lt;2+&gt;键的铁磁性通过两个键的共价性的竞争而相互竞争。(3)当x ≥ 0.5时，在磁场T_附近，介电常数增大<CM>.当x=0.7时，在9 T时，介电常数增加了7%<CM>.基于Maxwell-Wagner模型解释了介电常数的温度和磁场依赖性，包括频率色散，该模型假设样品的不均匀性，该样品包含分散在具有负磁阻的材料中的具有相对高介电常数的畴。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