【作者】 白宇浩；

【导师】 云国宏；

【作者基本信息】 内蒙古大学 ， 理论物理， 2010， 博士

【摘要】 铁磁／反铁磁体系的交换偏置效应在磁电阻传感器和自旋阀方面具有重要的用途,因而也得到了人们的广泛关注.最近一些实验证实：在测量交换偏置场与矫顽场随磁化角度的变化曲线中(以下简称为交换偏置的角度依赖关系)可以观测到阶跃现象.另外,铁磁／反铁磁体系在磁化反转过程中可以表现出半平面转动和全平面转动两种磁化模式.但是,人们并没有说明这些新现象背后的物理机制.本文提出了一种新的方法,详细地分析了铁磁／反铁磁体系的磁化过程,解释了体系在磁化反转过程中出现两种转动磁化模式的机制,也解释了交换偏置角度依赖关系中出现阶跃现象的原因.根据不施加外磁场时体系的能量与铁磁层磁化强度方向之间的关系,我们将铁磁／反铁磁体系的起始磁化状态分为单稳态、双稳态、三稳态及四稳态共四种不同的状态.将体系出现能量极小与极大时,铁磁层磁化强度的方向分别定义为内禀易轴和内禀难轴.根据内禀易轴与内禀难轴的具体位置将整个磁化方向划分为若干个角度区域.分角度区域地研究体系的磁化过程可以发现,当磁化强度在磁化循环中越过全部的内禀难轴时,铁磁／反铁磁体系在磁化反转过程中将出现全平面转动的磁化模式；当磁化强度在磁化循环中只能越过部分内禀难轴或者根本没有越过任何一个内禀难轴时,铁磁／反铁磁体系在磁化反转过程中将出现半平面转动的磁化模式.外磁场沿内禀易轴和内禀难轴方向磁化时,一支转换场发生突变而另一支转换场则保持连续,最终导致交换偏置场和矫顽场出现阶跃行为.本文利用这种分析方法讨论了各种类型的各向异性对铁磁／反铁磁体系磁化过程的影响,研究了交换偏置场与矫顽场随外磁场磁化角度的变化关系.本论文中的主要结果有：一、对于仅存在单轴各向异性的铁磁／反铁磁体系.研究表明,通过调节交换各向异性的大小及方向,体系可以在单稳态和双稳态之间相互转换,并且导致交换偏置出现不同形状的角度依赖关系曲线.外磁场沿内禀易轴和内禀难轴方向磁化时,交换偏置场和矫顽场出现了明显的阶跃现象.数值计算表明,交换偏置场和矫顽场在阶跃点处均具有较大的数值.在内禀易轴的阶跃点,矫顽场可以达到最大；在内禀难轴的阶跃点,交换偏置场达到最大而矫顽场可以突然消失.体系在磁化反转过程中可出现半平面转动和全平面转动两种磁化模式,内禀易轴与内禀难轴所在的方向恰是两种转动磁化模式的分界磁化角度.二、对于存在外应力的铁磁／反铁磁体系.研究表明,调节外应力的大小和方向可以使体系在单稳态和双稳态之间相互转换并导致交换偏置的角度依赖关系发生显著变化.体系存在外应力时,交换偏置的角度依赖关系中仍然存在阶跃现象.外应力可作为一种有效的外部手段来控制和调节铁磁／反铁磁体系的交换偏置,这种调控作用对磁致伸缩系数较大的铁磁层材料将更为明显.三、对于同时存在单轴和立方各向异性的铁磁／反铁磁体系.研究表明,通过调节立方各向异性的大小和方向,体系将出现单稳态、双稳态、三稳态以及四稳态共计四种状态.存在立方各向异性时,交换偏置将出现复杂的角度依赖关系,表现在交换偏置的角度依赖关系中可以出现非常剧烈的振荡行为.交换偏置场和矫顽场的阶跃行为依然可以在交换偏置的角度依赖关系中出现.体系从单稳态向四稳态变化时,交换偏置角度依赖关系中的阶跃次数逐渐增多；同时发现,增强立方各向异性可以显著提高体系的矫顽力.另外,体系在磁化反转时仍可出现半平面转动和全平面转动两种磁化模式.四、以平行界面的畴壁模型为基础,研究了反铁磁层的畴壁能对交换偏置角度依赖关系的影响.研究发现,考虑了反铁磁层的畴壁结构之后,交换偏置的角度依赖关系中仍可以出现阶跃现象,同时铁磁／反铁磁体系在磁化过程中仍可以出现全平面转动和半平面转动两种磁化模式.反铁磁层的畴壁能较大时,钉扎角的变化范围较小,反铁磁层可为铁磁层的磁化反转提供稳定的钉扎效果.反铁磁层的畴壁能趋于无穷大时,平行界面的畴壁模型与M-B模型一致.反铁磁层的畴壁能较小时,钉扎角的变化范围较大,反铁磁层对铁磁层的钉扎效果减弱.反铁磁层的畴壁能趋于无穷小时,反铁磁层磁化强度将随铁磁层磁化强度同步转动,铁磁／反铁磁交换偏置体系的磁化性质将与单层铁磁材料相同.本文是在国家自然科学基金(批准号：10762001),教育部新世纪优秀人才计划基金(批准号：2005-0272),教育部科学技术研究重大项目(批准号：2006024)和高等学校博士学科点专项科研基金(批准号：200801260003)的支持下完成的.更多还原

【Abstract】 The exchange bias between ferromagnetic/antiferromagnetic systems has received much attention in recent years for its technological importance in magnetoresistive sensors and spin valves. Recently, it has been confirmed by experiments that a jump phenomenon will be emerged in the angular dependence of the exchange bias. Additionally, it is found that the ferromagnetic/antiferromagnetic systems will exhibt the full-plane and half-plane rotation modes in the magnetization reversal processes. However, the mechanism for these new features was not interpreted in these works. In this thesis, a new method was proposed to analyze the magnetization processes of the systems. The magnetization reversal processes of the ferromagnetic/ antiferromagnetic systems have been investigated in detail. The jump phenomenon together with the full-plane and half-plane rotation modes have been explained by this method.According to the energy spectrum of the systems, it is found that the ferromagnetic/antiferromagnetic systems will be in monostable, bistable, tristable and quadristable states when the applied field is absent at the initial magnetization state. Furthermore, the orientations of ferromagnetic magnetization for the energy minima and energy maxima in the initial magnetization state are defined as the intrinsic easy axes and intrinsic hard axes, respectively. Based on the positions of the intrinsic easy and hard axes, the whole angular range of the magnetization can be divided into several angular regions. The magnetization processes are analyzed when the external field is applied in every angular region. It is found that when the ferromagnetic magnetization crosses the whole of the intrinsic hard axes during the magnetization cycles, the systems will exhibit a full-plane rotation mode during the magnetization reversal processes. Otherwise, the magnetization reversal will show a half-plane rotation mode when the ferromagnetic magnetization crosses partial intrinsic hard axes or it does not cross any intrinsic hard axis in the magnetization cycles. In addition, when the external field is applied along the intrinsic easy and hard axes, it is found that one of the switching fields at the descending or ascending branch of the hysteresis loop makes an abrupt change, while the other switching field keeps continuity, and consequently the exchange bias field and the coercivity will show the jump phenomenon in the angular dependence of the exchange bias. Based on this method, the effect of various anisotropies on the magnetization processes for ferromagnetic/antiferromagnetic systems has been investigated. The dependence of the exchange bias field and the coercivity on the orientation of the applied field has also been investigated in the thesis. Some main results are generalized as follows:Firstly, only the uniaxial anisotropy is considered in the ferromagnetic/antiferromagnetic systems. By tuning the magnitude and the orientation of the exchange anisotropy, the initial magnetization state of the systems can be divided into monostable state and bistable state, which determine the angular dependence of exchange bias immediately. When the external field is applied along the orientations of the intrinsic easy and hard axes, the exchange bias field and the coercivity will show a jump phenomenon obviously. The numerical calculations indicate that both the exchange bias field and the coercivity are larger in the magnitude at the points of the jump. At the jumping points of the intrinsic easy axes, the coercivity reaches the maximum; at the jumping points of the intrinsic hard axes, the exchange bias field will reach the maximum, at the meantime the coercivity vanishes itself suddenly. The half-plane and full-plane rotation modes will be shown in the magnetization reversal processes, the orientations of the intrinsic easy axes and hard axes are just the critical magnetization angle of these two rotation modes.Secondly, the effect of the external stress on the exchange bias has been investigated.’Our results demonstrate that both the magnitude and orientation of the external stress will affect the angular dependence of the exchange bias significantly by making a transition between monostable state and bistable state in the systems. The jump phenomenon is also existent in the angular dependence of the exchange bias. The external stress is a viable way to control and tune the exchange bias of the ferromagnetic/antiferromagnetic systems. The effect of the applied stress on the exchange bias will be more remarkable when the ferromagnetic materials have a larger magnetostriction. Thirdly, both the uniaxial and cubic anisotropies are considered in the ferromagnetic/antiferromagnetic systems. By tuning the relative magnitude and orientation of the cubic anisotropy, the systems will be in monostable, bistable, tristable and quadristable states. It displaces a complex angular dependence of exchange bias in the systems, which manifests itself by a visible oscillation in the curves. The jump phenomenon is also existent in the angular dependence of the exchange bias. Additionally, the half-plane rotation and full-plane rotation modes are also shown in the magnetization reversal processes. The numerical calculations indicate that the coercivity of the system was enhanced significantly by reinforcing the cubic anisotropy, and the times of the jumps are increased in the curves when the systems make the transition from monostable state to quadristable state.Lastly, the effect of domain wall in the antiferromagnet on the angular dependence of the exchange bias has been investigated based on the planar domain wall model. It is shown that the jump phenomenon is also existent in the angular dependence of the exchange bias. Additionally, the half-plane rotation and full-plane rotation modes are also displaced in the magnetization reversal processes. When the energy of the antiferromagnetic domain wall is larger, the changing range of pinning angle is narrower, which indicates that the better pinning effect of the antiferromagnetic layer is exerted on the ferromagnetic layer. When the energy of the domain wall tends to infinity, the planar domain wall model coincides with the M-B model. On the contrary, if the energy of the antiferromagnetic domain wall is weaker, the changing range of pinning angle is wider, which means a weaker pinning effect exists in the systems. When reducing the energy of the domain wall without limit, the antiferromagnetic magnetization will rotate synchronously with the ferromagnetic magnetization. Therefore, the magnetizing behavior of the exchange-biased ferromagnetic/antiferromagnetic systems will be same to the single ferromagnetic layer.This work was supported by the National Natural Science Foundation of China (Grant No.10762001), the Program for New Century Excellent Talents in University of China (Grant No.2005-0272), the key project of Chinese ministry of education (grant No.2006024).and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No.200801260003).更多还原