纳米球帽结构铁磁/反铁磁双层膜的交换偏置研究
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摘要
纳米结构的交换偏置系统,其磁学特性与结构的尺寸、纵横比、形状等因素有很大关联。然而到目前为止,特定研究体系中的参数(如铁磁结构尺寸、反铁磁材料、铁磁/反铁磁结构类型、形状等)的多变性使得人们对交换偏置的尺度效应的研究很难得出一个统一性的结论。真正理解纳米尺度下的交换偏置效应仍是一个巨大的挑战。
我们选择了球帽型纳米结构的FeNi/FeMn双层膜磁体阵列作为交换偏置研究系统,观察了这种纳米球帽的几何构型、维度尺寸等微观结构因素对交换偏置特性的影响。
利用胶体模板法,以自组织的单层密排列聚苯乙烯(Polystyrene,PS)微球阵列为形貌模板,采用磁控溅射制备了Cu/FeNi/FeMn/Cu成分的纳米球帽阵列。用扫描电子显微镜(SEM)直接观察到了制备出的有序纳米球帽结构,并利用振动样品磁强计(VSM)研究了样品的磁特性。
首先,在适当的铁磁/反铁磁厚度比条件下,纳米球帽结构的磁滞回线其下降支与上升支出现不对称性。
其次,纳米球帽结构导致交换偏置效应的减弱。但是,纳米球帽结构和连续膜结构的H_E对反铁磁厚度的依赖关系有着明显区别。随着反铁磁厚度的递增,连续膜H_E在某一临界厚度时出现一个极陡的上升,随后进入一个相对平坦的平台缓增区间。而对于纳米球帽结构,在我们的实验中的反铁磁厚度范围内,其交换偏置始终保持一个稳定渐增的趋势。
同样,对于矫顽力H_C的反铁磁厚度响应关系,连续膜和纳米球帽结构也表现出了不同的变化趋势。连续膜的H_C在特定临界厚度达到峰值,然后随着反铁磁厚度继续匀?H_C开始下降。而纳米球帽的H_C在整个反铁磁厚度范围内基本保持增加的趋势,但是其增加率不断减小。
而对于H_E与铁磁层厚度的变化趋势,两种结构都符合了H_E∝1/t_(FM)的经典规律。
我们认为系统的交换偏置展现出的一系列新性状,都是由于纳米球帽的构型和尺寸因素而产生的,但是通过运用交换偏置的基本物理图像,我们能对这些新的特性做出合理的解释。这说明了纳米结构的各种条件参数导致人们对交换偏置的尺寸效应的出许多不同的结论,但究其内禀机理,纳米结构中和连续膜中的交换偏置都是有相同的物理原理所主宰的。
The magnetic properties of nano-structured exchange bias systems are closely related to their lateral size,aspect ratio or shape.Due to the complicated system parameters, it is still hard to give a general description of the size effects of exchange bias nanostructures.
We chose the sphere cap-shaped ferromagnetic/antiferromagnetic(FM/AFM) nanomagnets arrays as the study system,investigated the influence of nanostructure morphology in Exchange Bias effect.
Close-packed 2 dimensional(2D) Polystyrene(Ps) beads arrays were used as the templates to fabricate the nanocap Exchange Bias Cu/FeNi/FeMn/Cu structures. Scanning Electron Microscope(SEM) gives us direct view of the nanocap structure. Vibrating sample magnetometer(VSM) are used to collect the magnetic information of the samples.
Pronounce asymmetry evolution of the hysteresis loops was observed through tailoring the thickness ratio of AFM to FM in nanocap EB structures.
The hysteresis loop shift of nanocap structures is smaller than that of continuous films with the same composition,whereas the exchange bias field(H_E) evolution trends depending on AFM thickness are different for the two kinds of samples.The nanocap structures can cont??te to either a reduction or an enhancement of the coercivity (H_c),depending on the AFM layer thickness.
H_E of both nanocap structures and the continuous films samples follows the 1/t_(FM) law with different slopes.
Tne effects different from those of continuous films are ascribed to the influences of nanocap morphology.
我们选择了球帽型纳米结构的FeNi/FeMn双层膜磁体阵列作为交换偏置研究系统,观察了这种纳米球帽的几何构型、维度尺寸等微观结构因素对交换偏置特性的影响。
利用胶体模板法,以自组织的单层密排列聚苯乙烯(Polystyrene,PS)微球阵列为形貌模板,采用磁控溅射制备了Cu/FeNi/FeMn/Cu成分的纳米球帽阵列。用扫描电子显微镜(SEM)直接观察到了制备出的有序纳米球帽结构,并利用振动样品磁强计(VSM)研究了样品的磁特性。
首先,在适当的铁磁/反铁磁厚度比条件下,纳米球帽结构的磁滞回线其下降支与上升支出现不对称性。
其次,纳米球帽结构导致交换偏置效应的减弱。但是,纳米球帽结构和连续膜结构的H_E对反铁磁厚度的依赖关系有着明显区别。随着反铁磁厚度的递增,连续膜H_E在某一临界厚度时出现一个极陡的上升,随后进入一个相对平坦的平台缓增区间。而对于纳米球帽结构,在我们的实验中的反铁磁厚度范围内,其交换偏置始终保持一个稳定渐增的趋势。
同样,对于矫顽力H_C的反铁磁厚度响应关系,连续膜和纳米球帽结构也表现出了不同的变化趋势。连续膜的H_C在特定临界厚度达到峰值,然后随着反铁磁厚度继续匀?H_C开始下降。而纳米球帽的H_C在整个反铁磁厚度范围内基本保持增加的趋势,但是其增加率不断减小。
而对于H_E与铁磁层厚度的变化趋势,两种结构都符合了H_E∝1/t_(FM)的经典规律。
我们认为系统的交换偏置展现出的一系列新性状,都是由于纳米球帽的构型和尺寸因素而产生的,但是通过运用交换偏置的基本物理图像,我们能对这些新的特性做出合理的解释。这说明了纳米结构的各种条件参数导致人们对交换偏置的尺寸效应的出许多不同的结论,但究其内禀机理,纳米结构中和连续膜中的交换偏置都是有相同的物理原理所主宰的。
The magnetic properties of nano-structured exchange bias systems are closely related to their lateral size,aspect ratio or shape.Due to the complicated system parameters, it is still hard to give a general description of the size effects of exchange bias nanostructures.
We chose the sphere cap-shaped ferromagnetic/antiferromagnetic(FM/AFM) nanomagnets arrays as the study system,investigated the influence of nanostructure morphology in Exchange Bias effect.
Close-packed 2 dimensional(2D) Polystyrene(Ps) beads arrays were used as the templates to fabricate the nanocap Exchange Bias Cu/FeNi/FeMn/Cu structures. Scanning Electron Microscope(SEM) gives us direct view of the nanocap structure. Vibrating sample magnetometer(VSM) are used to collect the magnetic information of the samples.
Pronounce asymmetry evolution of the hysteresis loops was observed through tailoring the thickness ratio of AFM to FM in nanocap EB structures.
The hysteresis loop shift of nanocap structures is smaller than that of continuous films with the same composition,whereas the exchange bias field(H_E) evolution trends depending on AFM thickness are different for the two kinds of samples.The nanocap structures can cont??te to either a reduction or an enhancement of the coercivity (H_c),depending on the AFM layer thickness.
H_E of both nanocap structures and the continuous films samples follows the 1/t_(FM) law with different slopes.
Tne effects different from those of continuous films are ascribed to the influences of nanocap morphology.
引文
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