Random and exchange anisotropy in consolidated nanostructured Fe and Ni: Role of grain size and trace oxides on the magnetic properties
Random and exchange anisotropy in consolidated nanostructured Fe and Ni: Role of grain size and trace oxides on the magnetic properties
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DOI：
10.1103/physrevb.57.2915
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发表时间：
1998-02
影响因子：
3.7
通讯作者：
J. F. Löffler;J. Meier;B. Doudin;J. Ansermet;W. Wagner
中科院分区：
文献类型：
--
作者：
J. F. Löffler;J. Meier;B. Doudin;J. Ansermet;W. Wagner
介绍了惰性气体冷凝法制备纳米铁和纳米镍晶体的磁化测量结果。在100 ~ 1000℃的温度下，样品的矫顽力场随晶粒尺寸的增大而增大，晶粒尺寸约为10 ~ 20 nm。矫顽力场随晶粒尺寸的增大而增大，在30 nm左右达到最大值，在较小晶粒尺寸时急剧减小。在随机各向异性模型的基础上讨论了矫顽力，该模型预测通过对磁耦合晶粒进行平均可以降低有效各向异性常数。只要晶粒尺寸小于有效体畴壁宽度，就可以观察到这种行为。该模型还考虑了铁在纳米结构中接近饱和时的屈服值与晶粒的铁磁相关长度和各向异性常数的关系。后者大约是铁的体积值的4倍。磁场冷却后5 K的磁滞测量表明，铁和镍的磁滞回线都发生了位移和变宽，这是由于铁磁晶粒与反铁磁或铁磁（氧化物）界面相之间的交换耦合。随着晶粒尺寸的增大，磁滞位移逐渐减小，最终消失。这表明了氧化物的重组，铁样品的矫顽力场证实了这一点，在大约120k时，由fe3o4的相变引起了一个台阶。随着晶粒尺寸的进一步增大，该台阶再次消失。Ni样品的饱和磁化强度随退火温度的升高而增加，这也归因于氧化物的演化。
Results of magnetization measurements on nanocrystalline Fe and Ni produced by inert-gas condensation are presented. The grain size, which is about 10 to 20 nm in the as-prepared state, is increased by annealing the samples incrementally from 100 C to 1000 C. The coercive field shows a pronounced variation with grain size, with a maximum at around 30 nm and a steep decrease for smaller grain sizes. The coercivity is discussed on the basis of the random-anisotropy model that predicts that the effective anisotropy constant is reduced by averaging over magnetically coupled grains. This behavior is observed as long as the grain size is smaller than the effective bulk domain-wall width. The model also accounts for the approach to saturation in nanostructured Fe yielding values for the ferromagnetic correlation length and the anisotropy constant of the grains. The latter is about four times higher than the bulk value of Fe. Hysteresis measurements at 5 K after field cooling show a shift and broadening of the hysteresis loops for both Fe and Ni, which is attributed to an exchange coupling between the ferromagnetic grains and antiferromagnetic or ferrimagnetic (oxide) interfacial phases. The hysteresis shift decreases and finally vanishes with increasing grain size. This is indicative of a restructuring of the oxides, which is confirmed by the coercive field of the Fe samples showing a step at about 120 K caused by a phase transition of Fe 3 O 4. The step vanishes again with further increasing grain size. The saturation magnetization of the Ni samples increases with increasing annealing temperature, a fact that is attributed to the evolution of the oxides also.