Anomalous Temperature Dependence of Magnetic Anisotropy in Gradient-Composition Sputterred Thin Films
Anomalous Temperature Dependence of Magnetic Anisotropy in Gradient-Composition Sputterred Thin Films
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DOI：
10.1002/adma.201203995
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发表时间：
2013-02-20
影响因子：
29.4
通讯作者：
Ong, C. K.
中科院分区：
文献类型：
--
作者：
Phuoc, Nguyen N.;Ong, C. K.
对铁磁体中磁各向异性的温度依赖性的研究是磁学中最早的主题之一[1-3]，但它仍然是一个有争议的问题[4，5]，甚至现在也引起了科学家的极大兴趣。[4，5]对磁各向异性的温度依赖性机制的透彻理解不仅对磁学的基础知识[4]有用，而且对各种应用也是非常宝贵的。[4-6]例如，用于信息存储的基于磁隧道结的自旋电子存储单元需要远高于室温的良好热稳定性。[6，7]另一个值得注意的应用实例是使用磁性材料的高频器件[8-10]，如微波噪声滤波器，微波吸收器，了解磁各向异性的温度依赖性的行为也有利于研究人员实现使用磁性材料的新技术，如热-辅助磁记录[11在文献中，通常已知磁各向异性随着测量温度的升高而降低[5]，并且这种降低可以直观地理解如下。随着温度的升高，局域磁化方向在平均磁化方向附近的一定角度范围内扩展，易磁化轴能量升高，难磁化轴能量降低，磁各向异性减小。[3，5]虽然这种定性的理解可以提供磁各向异性随温度降低的起源的合理描述，但这种行为的定量细节是相当有争议的，尚未完全理解。[4]同时，迄今为止，从未发现存在磁各向异性随着测量温度而增加的磁性薄膜。在本文中，我们报告了在通过梯度成分沉积技术[13-15]制备的FeCoHf薄膜中磁各向异性随温度增加的发现（参见图1（a）和实验部分），并通过考虑这种特殊制备方法引起的应力诱导效应起源从不同的角度进行了讨论。如图1（B）所示，通过能量色散X射线光谱仪（EDS）对我们的膜进行的组成分析表明，Fe和Co的浓度降低，而Hf的浓度增加。然而，所有组成的变化相当小，在β= 0和β= 15（β是图1（a）中定义的沉积角）下沉积的样品之间小于1.5%。我们还进行了膜内不同点的成分分析，结果表明，沿着难磁化轴（在图1（a）中表示为HA），存在Hf成分的梯度分布，其中在更靠近Hf靶的位置处的成分更高。例如，具有β= 0的FeCoHf膜的Hf组分沿着难磁化轴（具有10 mm的边缘尺寸）从7.5%端到端地改变到8.5%。然而，沿着边缘尺寸为5 mm的易轴（在图1（a）中表示为EA），Hf的组成是相当均匀的。这一观察结果类似于文献中的报告，从其他小组的薄膜制造相同的技术。[13 Hf的梯度组成与图1（c）中呈现的X射线衍射（XRD）结果一致。如图1（c）的插图所示，XRD图谱显示FeCo bcc（100）的单个主峰，并且该峰向衍射角2 θ的较低范围偏移，表明晶格间距扩大[14]，如图1（c）所示。根据此前…
The study on temperature dependence of magnetic anisotropy in ferromagnets is one of the earliest themes [1–3] in magnetism but it is still a controversial issue [4, 5] and generates much interest from scientists, even now.[4, 5] A thorough understanding of the mechanism of temperature dependence of magnetic anisotropy is not only useful for fundamental knowledge [4] of magnetism but also invaluable for various applications.[4–6] A spintronics memory cell based on magnetic tunnel junctions used in information storage, for instance, requires a good thermal stability well beyond room temperature.[6, 7] Another notable example of applications in which thermal stability is a crucially important parameter for the perfomance is high-frequency devices employing magnetic materials [8–10] such as microwave noise filters, microwave absorbers, inductors etc. Understanding the behavior of the temperature dependence of magnetic anisotropy is also beneficial for researchers to realize novel technology using magnetic materials such as heat-assisted magnetic recording.[11, 12] In the literature, magnetic anisotropy is normally known to decrease as the measured temperature is increased [5] and this decreasing can be intuitively understood as follows. With the increasing of temperature, the local magnetization direction spreads over a range of angles around the mean directions which results in the raising of energy of the easy axis and lowering the energy of the hard axis leading to the decrement of magnetic anisotropy.[3, 5] While this qualitative understanding may provide a reasonable description of the origin of the reduction of magnetic anisotropy with temperature, quantitative details of this behavior are rather controversial and not yet fully understood.[4] Meanwhile, magnetic thin films whose magnetic anisotropy increases with the measured temperature have never been found to exist thus far. In the present paper, we report the discovery of an increment of magnetic anisotropy with temperature in the FeCoHf films fabricated by gradient-composition deposition technique [13–15](See Figure 1 (a) and Experimental section) and discuss it from a different perspective by considering the stress-induced effect origin arising from this special fabrication method. The composition analysis of our films performed by energy dispersive x-ray spectroscope (EDS) as in Figure 1 (b) shows that the concentrations of Fe and Co are reduced while that of Hf is increased. However, the change of all the compositions is quite small, less than 1.5% between the samples deposited at β= 0 and at β= 15 (β is the deposition angle defined in Figure 1 (a)). We also carried out the composition analysis of different spots within the film and the result reveals that along the hard axis (denoted as HA in Figure 1 (a)) there is a gradient distribution of Hf composition where the composition at the position closer to Hf target is higher. For example, the Hf compositon of the FeCoHf film with β= 0 is changed from 7.5% to 8.5% end-to-end along the hard axis (with the edge size of 10 mm). However, along the easy axis with the edge size of 5 mm (denoted as EA in Figure 1 (a)), the composition of Hf is quite uniform. This observation is similar to reports in the literature from other groups for thin films fabricated by the same technique.[13, 14, 15] The gradient composition of Hf is consistent with the x-ray diffraction (XRD) result presented in Figure 1 (c). As observed in the inset of Figure 1 (c), the XRD patterns show a single prominent peak of FeCo bcc (100) and the peak is shifted to lower range of diffraction angle 2 θ indicating that the lattice spacing is expanded [14] as in Figure 1 (c). According to previous …