Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India.
Nanotechnology. 2013 Dec 20;24(50):505711. doi: 10.1088/0957-4484/24/50/505711. Epub 2013 Nov 27.
Ferromagnetic BiFeO3 nanocrystals of average size 9 nm were used to form a composite with antiferromagnetic CuO nanosheets, with the composition (x)BiFeO3/(100-x)CuO, x = 0, 20, 40, 50, 60, 80 and 100. The dispersion of BiFeO3 nanocrystals into the CuO matrix was confirmed by x-ray diffraction and transmission electron microscopy. The ferromagnetic ordering as observed in pure BiFeO3 occurs mainly due to the reduction in the particle size as compared to the wavelength (62 nm) of the spiral modulated spin structure of the bulk BiFeO3. Surface spin disorder of BiFeO3 nanocrystals gives rise to an exponential behavior of magnetization with temperature. Strong magnetic exchange coupling between the BiFeO3 nanocrystal and the CuO matrix induces an interfacial superparamagnetic phase with a blocking temperature of about 80 K. Zero field and field cooled magnetizations are analyzed by a ferromagnetic core and disordered spin shell model. The temperature dependence of the calculated saturation magnetization exhibits three magnetic contributions in three temperature regimes. The BiFeO3/CuO nanocomposites reveal an exchange bias effect below 170 K. The maximum exchange bias field HEB is 1841 Oe for x = 50 at 5 K under field cooling of 50 kOe. The exchange bias coupling results in an increase of coercivity of 1934 Oe at 5 K. Blocked spins within an interfacial region give rise to a remarkable exchange bias effect in the nanocomposite due to strong magnetic exchange coupling between the BiFeO3 nanocrystals and the CuO nanosheets.
平均粒径为 9nm 的铁磁 BiFeO3 纳米晶与反铁磁 CuO 纳米片形成复合材料,组成(x)BiFeO3/(100-x)CuO,x=0、20、40、50、60、80 和 100。X 射线衍射和透射电子显微镜证实了 BiFeO3 纳米晶在 CuO 基体中的分散。纯 BiFeO3 中观察到的铁磁有序主要是由于与体相 BiFeO3 螺旋调制自旋结构的波长(62nm)相比,粒径减小所致。BiFeO3 纳米晶的表面自旋无序导致磁化强度随温度呈指数变化。BiFeO3 纳米晶与 CuO 基体之间的强磁交换耦合诱导出具有约 80K 阻塞温度的界面超顺磁相。零场和磁场冷却磁化通过铁磁芯和无序自旋壳模型进行分析。计算得到的饱和磁化强度随温度的变化在三个温度范围内表现出三个磁贡献。在 170K 以下,BiFeO3/CuO 纳米复合材料表现出交换偏置效应。在 5K 下,磁场冷却 50kOe 时,x=50 时的最大交换偏置场 HEB 为 1841Oe。交换偏置耦合导致在 5K 时矫顽力增加 1934Oe。由于 BiFeO3 纳米晶与 CuO 纳米片之间的强磁交换耦合,界面区域内的被钉扎自旋导致纳米复合材料中出现显著的交换偏置效应。