Magnetoelectric Effects in Ferromagnetic Oxides with the Double Perovskite Structure

本文研究了La_&lt；1-x&gt；Bi_xMn_&lt；0.5&gt；Ni_&lt；0.5&gt；O_3的晶体结构和磁介电性能，(2)铁磁性和铁电性的竞争，(3)用X射线衍射、磁电测量和核磁共振研究了其磁电效应：(1)多晶La_t；采用常压软化学合成法成功地合成了x=0~0.9时具有双钙钛矿结构的0.5&gt；O_3。介电常数随x的增加而增大，铁磁居里温度(T&lt；Cm&gt；)随x的增加而降低。(2)通过对REMn_&lt；0.5&gt；(Ni_&lt；1-x&gt；Mg_x)_&lt；0.5&gt；Ni_&lt；0.5&gt；O_3(RE=稀土元素)中^&lt；55&gt；Mn的核磁共振研究表明，其共价性参数f(RE)为：双钙钛矿中的-Ni^&lt；2+&gt；键是由超精细磁场H_&lt；Hf&gt；在^&lt；55&gt；Mn。基于该知识，La_&lt；1-x&gt；Bi_xMn_&lt；0.5&gt；Ni_&lt；0.5&gt；Ni_&lt；0.5&gt；O_3中键的共价性参数f(X)为…用^&lt；55&gt；Mn对氧化物的核磁共振研究。F(X)的减小是由于Bi^&lt；3+&gt；-O^&lt；2-&gt；键与La^&lt；3+&gt；-O^&lt；-2-&gt；键相比，Bi^&lt；3+&gt；-O^&lt；-O^&lt；2-&gt；键的共价性较强；O^&lt；2-&gt；离子的电子更多地被吸引和填充在Bi^&lt；3+&gt；-O^&lt；2-&gt；键上，从而导致了Mn^&lt；中电子的不足。4+&gt；-O^&lt；2-&gt；-Ni^&lt；2+&gt；键。由Bi^&lt；3+&gt；-O^&lt；2-&gt；键的共价性引起的铁电性与由Mn^&lt；4+&gt；-O^&lt；2-&gt；-Ni^&lt；2+&gt；键的共价性引起的铁磁性相互竞争。(3)对于x≧0.5时，在接近T&lt；CM&gt；的磁场中，介电常数增大。当x=0.7时，介电常数在9T时的增量达7%，介电常数与温度和磁场的关系，包括频率色散，用麦克斯韦-瓦格纳模型进行了解释，该模型假定样品中含有较高介电常数的磁区分散在具有负磁阻的材料中。较少

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