A series of unexpected ferromagnetic behaviors based on the surface-vacancy state: an insight into NiO nanoparticles with a core-shell structure
A series of unexpected ferromagnetic behaviors based on the surface-vacancy state: an insight into NiO nanoparticles with a core-shell structure
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基于表面空位态的一系列意想不到的铁磁行为：深入了解具有核壳结构的NiO纳米颗粒
DOI：
10.1039/c4ra06472k
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发表时间：
2014
期刊：
影响因子：
3.9
通讯作者：
Xue Desheng
中科院分区：
文献类型：
--
作者：
Yang Zhaolong;Gao Daqiang;Tao Kun;Zhang Jing;Shi Zhenhua;Xu Qiang;Shi Shoupeng;Xue Desheng
由于强量子效应和表面效应，作为终极低维材料的反铁磁纳米颗粒具有不同于其体形的新型磁性。在此，我们提出观察到的NiO纳米颗粒的异常铁磁行为是由于形成了与氧空位相关的铁磁粒子壳。一种新的自洽饱和磁化估计进一步证实了我们的建议。通过热分解方法合成的样品表现出不同的异常铁磁行为，如迟滞曲线，大矫顽力，交换偏置和自旋玻璃行为。6 nm NiO样品的饱和磁化强度为0.536 emu g−1，且随晶粒尺寸的增大而减小。通过x射线光电子、x射线衍射和选择区域电子衍射测量，样品中未发现杂质元素，也未发现价态变化。x射线光电子能谱拟合结果和拉曼光谱结果证实了材料中存在大量的氧空位。由于表面缺陷增强，后氢退火工艺强烈地提高了铁磁有序度。此外，我们基于第一性原理计算结果估计的饱和磁化强度与实验结论是一致的，这揭示了表面态在低维反铁磁材料中介导异常磁性能的重要性。
Antiferromagnetic nanoparticles as ultimate low-dimensional materials potentially give novel magnetic properties that differ from their bulk form due to strong quantum and surface effects. Herein, we propose that the observed anomalous ferromagnetic behavior of NiO nanoparticles is due to the formation of a ferromagnetic particle shell that is oxygen-vacancy related. A novel self-consistent estimation of the saturation magnetization further confirmed our proposal. The samples, synthesized by a thermal decomposition method, exhibit diversely anomalous ferromagnetic behavior, such as hysteresis curves, large coercivities, exchange bias and spin-glass behavior. A large saturation magnetization of 0.536 emu g−1 exists in the 6 nm NiO sample and it is found to decrease with increasing crystal size. Neither impurity element nor change of valence state has been found in the samples, through X-ray photoelectron, X-ray diffraction or selected-area electron diffraction measurements. Remarkably, a large amount of oxygen vacancies exists, which was verified by X-ray photoelectron spectrum fitting results and Raman spectroscopy. A post-hydrogen-annealing process strongly elevates ferromagnetic ordering, as a result of surface defect enhancement. Moreover, our estimation of the saturation magnetization based on first principle calculation results is in agreement with the experimental conclusion, which reveals the significance of surface states in mediating anomalous magnetic properties in low-dimensional antiferromagnetic materials.