Structural and magnetic characteristics of monodispersed Fe and oxide-coated Fe cluster assemblies
Structural and magnetic characteristics of monodispersed Fe and oxide-coated Fe cluster assemblies
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DOI：
10.1063/1.1501754
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发表时间：
2002-09-15
影响因子：
3.2
通讯作者：
Morikawa, H
中科院分区：
文献类型：
--
作者：
Peng, DL;Hihara, T;Morikawa, H
我们系统地研究了平均团簇尺寸为7-16 nm的单分散Fe团簇和氧化物包裹Fe团簇的结构和磁性。透射电子显微镜和扫描电子显微镜(SEM)观察表明，在室温下，组装中的Fe团簇保持了原来的尺寸。在扫描电子显微镜图像中，观察到Fe团簇的随机堆积和低团簇堆积分数约为25%的多孔结构。对于Fe团簇组件，随着团簇平均尺寸从7.3 nm增加到16.3 nm，室温下的磁矫顽力(H-c)从4×10(1)增加到4×10(2)Oe。利用T大于或等于100K的矫顽力实验值和阻塞温度T-B from H-c=H-c0[1-(T/T-B)(1/2)]的拟合值，我们估计了T-B=KV/25K(B)的磁各向异性常数K的数量级为10(6)erg/cm(3)，比块体Fe的值(5×10(5)erg/cm(3))大一个数量级。在T大于或等于100K时出现如此大的有效各向异性，归因于小团簇的大表面各向异性效应和Fe团簇的低团簇堆积分数。对于氧化物包覆的Fe团簇样品，其矫顽力强烈地依赖于沉积过程中氧气的流量、团簇的大小和温度。在高氧气流量(即高表面氧化团簇)的情况下，铁磁性氧化物壳晶也影响T>；50K的矫顽力：磁滞回线移动消失，导致矫顽力的复杂变化和有效各向异性常数的增加。(C)2002年美国物理研究所。
We systematically studied structural and magnetic characteristics of size- monodispersed Fe and oxide-coated Fe cluster assemblies with the mean cluster sizes of 7-16 nm. Transmission electron microscopy and scanning electron microscopy (SEM) observations show that the Fe clusters in the assemblies maintain their original size at room temperature. In the SEM images, a random stacking of the Fe clusters and a porous structure with a low cluster packing fraction of about 25% are observed. For the Fe cluster assemblies, magnetic coercivity (H-c) at room temperature increases from 4x10(1) to 4x10(2) Oe by increasing the mean cluster size from 7.3 to 16.3 nm. Using the experimental values of the coercivity at Tgreater than or equal to100 K and the fitting values of blocking temperature T-B from H-c=H-c0[1-(T/T-B)(1/2)], we estimated the values of magnetic anisotropy constant K of the order of 10(6) erg/cm(3) from T-B=KV/25k(B), which is larger by an order of magnitude than the bulk Fe value (5x10(5) erg/cm(3)). Such a large effective anisotropy at Tgreater than or equal to100 K is ascribed to the large surface anisotropy effects of the small clusters and the low cluster-packing fraction of the Fe cluster assemblies. For the oxide-coated Fe cluster samples, the coercivity strongly depends on the oxygen gas flow rate during deposition, cluster size, and temperature. In the case of a high oxygen gas flow rate (namely high surface-oxidized clusters), the ferrimagnetic oxide shell crystallites also affect the coercivity at T>50 K: The hysteresis loop shift disappears, leading to a complex change in the coercivity and an enhancement of the effective anisotropy constant. (C) 2002 American Institute of Physics.