磁性纳米颗粒系统的磁学性质及偶极相互作用研究
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摘要
由于具有广泛的应用前景和丰富的物理内涵,磁性纳米颗粒正吸引着国内外众多科学家的目光,成为纳米科技领域中最热门的方向之一。本论文从材料制备和颗粒间偶极相互作用的角度去研究磁性纳米颗粒的性质,其中偶极相互作用研究是本论文的重点。
在材料制备方面,采用金属蒸发真空电弧离子注入机(MEVVA)把大剂量的铁离子分别注入到(100)、(110)和(111)晶向的单晶氧化镁中以形成磁性纳米颗粒薄膜,分别通过RBS和SQUID测量薄膜的成分和磁学性质。磁学测量说明在氧化镁母体中形成了尺寸分布范围很宽的磁性纳米颗粒。测量结果也显示该颗粒薄膜在5K和300K温度下都出现类似于铁磁性的回线;而且不同晶向样品的矫顽力HC的大小有以下规律:HC(110) 在磁性纳米颗粒系统的偶极相互作用研究方面,主要采用蒙特卡罗模拟的局域动力学模型对各向异性轴随机均匀分布的单畴单轴铁磁性纳米颗粒的单分散体系进行了系统的研究。研究综合考虑了偶极相互作用强度、各向异性强度、以及颗粒的空间排列结构等因素的影响,通过模拟测量ZFC/FC磁化强度曲线,M-H磁化曲线以及热剩磁曲线,来定性地探索各种因素对系统的有效能垒分布、磁化过程以及磁关联的影响。该部分主要研究了四个问题:
(1)偶极相互作用对系统有效能垒分布的影响
通过对不同各向异性强度和颗粒浓度的磁性纳米颗粒系统的ZFC/FC曲线的模拟研究发现,系统的阻塞温度TB随着偶极相互作用强度的增强而显著提高,说明偶极相互作用提高磁性纳米颗粒系统的平均有效能垒。同时ZFC/FC曲线模拟结果也表明了偶极相互作用使有效能垒分布的宽度增加。这些结论对当前关于偶极相互作用在改变磁性纳米颗粒系统的有效能垒中起何种作用的争论,具有一定的参考意义。
(2)颗粒的空间排列结构对偶极相互作用系统性质的影响
通过对颗粒空间排列为立方排列和无序排列两种系统的ZFC/FC曲线、M-H磁化曲线的模拟研究及关联函数的学习发现,在偶极相互作用的磁性纳米颗粒系统中,颗粒的空间排列结构是一个非常重要的影响因素,不同空间排列结构的系统的磁关联状态、有效能垒分布和磁化过程差别很大。
由于空间排列结构的影响,在一定强度的磁场下,高浓度系统的阻塞温度会比低浓度系统的阻塞温度低,因此在用ZFC/FC曲线分析偶极相互作用系统的有效能垒时,应该考虑系统中颗粒可能形成的空间排列状态,这个结论为偶极相互作用系统有效能垒的争论提供一种新的思路。
此外,本文也提出由于颗粒的空间排列效应和各向异性效应的共同作用,导致偶极相互作用体系在超顺磁温区的磁化过程不能简单地用T*模型描述。
(3)偶极相互作用系统中的磁关联
通过研究经过ZFC/FC过程后的系统的磁关联函数发现,偶极相互作用使系统形成包含铁磁性和反铁磁性排列的磁关联结构;这种关联结构与磁性纳米颗粒的空间排列、偶极相互作用强度、系统温度及外磁场的关系密切。磁关联是影响系统的有效能垒分布和磁化过程性质的主要原因之一;由于磁关联的存在,不能直接应用非相互作用的纳米颗粒系统的热剩磁模型分析来偶极相互作用系统的有效能垒分布。此外,本文通过在朗之万函数中引入场相关的关联长度,定性地解释了高密度的磁性纳米颗粒系统隧穿磁阻的H/T标度问题。
(4)外磁场在偶极相互作用系统中扮演的角色
通过对ZFC/FC曲线、M-H磁化曲线和热剩磁曲线的模拟研究发现,外磁场与偶极相互作用形成一种竞争关系,表现在外磁场降低偶极相互作用系统的ZFC曲线的峰值温度,并减弱系统的内秉磁关联。
Magnetic nanoparticles have received considerable interests, due to both their important potential applications and their rich experimental behaviors in physics. In this thesis, interested in both the preparations and the dipolar interactions of the magnetic nanoparticle assembly, we focused more on the investigation of the latter.
We firstly studied the magnetic properties of the Fe-implanted MgO samples. Single crystals of MgO with (100), (110) and (111) orientations were implanted with Fe ions at high dose by using metal-vapor vacuum arc ion source (MEVVA). The magnetic properties were investigated by a superconducting quantum interference device magnetometer (SQUID) and Rutherford backscattering spectrometry (RBS) was used to analyze the Fe concentration and distribution. The presence and the wide size distribution of magnetic nanoparticles in MgO matrix was verified by magnetization measurements. Results show that all the samples behave as ferromagnetism at 5K and 300K, and the coercive field, HC, well follows the relation: HC(110) We then devoted to the investigation of dipolar interactions influence on the magnetic properties of the magnetic nanoparticle systems by using Monte Carlo simulations with local dynamics model. Monodispersed single-domain ferromagnetic nanoparticle systems with randomly oriented anisotropic axis were studied systematically. Three important effects including the dipolar interaction intensity, the anisotropy intensity and the nanoparticles’spatial arrangement were considered carefully. By simulating the zero field cooling (ZFC)/field cooling (FC) magnetization (susceptibility) curves, M-H magnetization curves and the thermal remanence curves, the influence of dipolar interactions on the magnetization process, the effective energy barrier of the systems and the magnetic correlation were evaluated. There are four parts in this dissertation:
1. Influence of dipolar interactions on the effective energy barrier distribution.
From the ZFC/FC magnetization simulations for systems with different dipolar interactions and anisotropy intensity, we observe that the blocking temperature TB clearly increases with interaction strength, indicating that the dipolar interactions enhance the average effective energy barrier. It is also clear from the ZFC/FC magnetization simulations that the increase in dipolar interactions gives rise to an increase in the width of the effective energy barrier distribution.
2. The nanoparticle’s spatial arrangement effect on the magnetic properties of the dipolar interacting systems.
From the ZFC/FC magnetization simulations and the M-H magnetization curves simulations for the randomly arranged and cubic arranged systems,we find that systems with different particle spatial arrangement have different effective energy barrier distribution, magnetization process and magnetic correlation. The spatial arrangement effect associated with the anisotropy will result in the invalidity of the T* model. More over, the spatial arrangement effect may cause the decrease of blocking temperature with magnetic particle concentration in proper applied field. Consequently, we point out that the spatial arrangement effect should be taken into account in analyzing the effective energy barrier of dipolar interacting systems using ZFC/FC magnetization curves.
3. Magnetic correlation in the dipolar interacting systems.
From the correlation function obtained after ZFC or FC procedure, we clarify that the dipolar interactions cause the formation of short-range order of the particle moments. This short-range order consists of both antiferromagnetic and ferromagnetic array of the particle moments, and depends crucially on the spatial arrangement, the strength of the dipolar interactions, the temperature and the applied field. The magnetic correlation has important effect on the effective energy barrier distribution and magnetic process. The thermal remanence thus can not be directly employed to study the effective energy barrier distribution of the dipolar interacting systems.
4. Role of the applied field on the dipolar interacting systems.
From the ZFC/FC magnetization, M-H curves and the reversed thermal remanence curves, we conclude that the applied field competes with the dipolar interactions. This is manifested in decreasing the ZFC peak temperature with applied field, as well as the weakening of the magnetic correlation strength.
在材料制备方面,采用金属蒸发真空电弧离子注入机(MEVVA)把大剂量的铁离子分别注入到(100)、(110)和(111)晶向的单晶氧化镁中以形成磁性纳米颗粒薄膜,分别通过RBS和SQUID测量薄膜的成分和磁学性质。磁学测量说明在氧化镁母体中形成了尺寸分布范围很宽的磁性纳米颗粒。测量结果也显示该颗粒薄膜在5K和300K温度下都出现类似于铁磁性的回线;而且不同晶向样品的矫顽力HC的大小有以下规律:HC(110)
(1)偶极相互作用对系统有效能垒分布的影响
通过对不同各向异性强度和颗粒浓度的磁性纳米颗粒系统的ZFC/FC曲线的模拟研究发现,系统的阻塞温度TB随着偶极相互作用强度的增强而显著提高,说明偶极相互作用提高磁性纳米颗粒系统的平均有效能垒。同时ZFC/FC曲线模拟结果也表明了偶极相互作用使有效能垒分布的宽度增加。这些结论对当前关于偶极相互作用在改变磁性纳米颗粒系统的有效能垒中起何种作用的争论,具有一定的参考意义。
(2)颗粒的空间排列结构对偶极相互作用系统性质的影响
通过对颗粒空间排列为立方排列和无序排列两种系统的ZFC/FC曲线、M-H磁化曲线的模拟研究及关联函数的学习发现,在偶极相互作用的磁性纳米颗粒系统中,颗粒的空间排列结构是一个非常重要的影响因素,不同空间排列结构的系统的磁关联状态、有效能垒分布和磁化过程差别很大。
由于空间排列结构的影响,在一定强度的磁场下,高浓度系统的阻塞温度会比低浓度系统的阻塞温度低,因此在用ZFC/FC曲线分析偶极相互作用系统的有效能垒时,应该考虑系统中颗粒可能形成的空间排列状态,这个结论为偶极相互作用系统有效能垒的争论提供一种新的思路。
此外,本文也提出由于颗粒的空间排列效应和各向异性效应的共同作用,导致偶极相互作用体系在超顺磁温区的磁化过程不能简单地用T*模型描述。
(3)偶极相互作用系统中的磁关联
通过研究经过ZFC/FC过程后的系统的磁关联函数发现,偶极相互作用使系统形成包含铁磁性和反铁磁性排列的磁关联结构;这种关联结构与磁性纳米颗粒的空间排列、偶极相互作用强度、系统温度及外磁场的关系密切。磁关联是影响系统的有效能垒分布和磁化过程性质的主要原因之一;由于磁关联的存在,不能直接应用非相互作用的纳米颗粒系统的热剩磁模型分析来偶极相互作用系统的有效能垒分布。此外,本文通过在朗之万函数中引入场相关的关联长度,定性地解释了高密度的磁性纳米颗粒系统隧穿磁阻的H/T标度问题。
(4)外磁场在偶极相互作用系统中扮演的角色
通过对ZFC/FC曲线、M-H磁化曲线和热剩磁曲线的模拟研究发现,外磁场与偶极相互作用形成一种竞争关系,表现在外磁场降低偶极相互作用系统的ZFC曲线的峰值温度,并减弱系统的内秉磁关联。
Magnetic nanoparticles have received considerable interests, due to both their important potential applications and their rich experimental behaviors in physics. In this thesis, interested in both the preparations and the dipolar interactions of the magnetic nanoparticle assembly, we focused more on the investigation of the latter.
We firstly studied the magnetic properties of the Fe-implanted MgO samples. Single crystals of MgO with (100), (110) and (111) orientations were implanted with Fe ions at high dose by using metal-vapor vacuum arc ion source (MEVVA). The magnetic properties were investigated by a superconducting quantum interference device magnetometer (SQUID) and Rutherford backscattering spectrometry (RBS) was used to analyze the Fe concentration and distribution. The presence and the wide size distribution of magnetic nanoparticles in MgO matrix was verified by magnetization measurements. Results show that all the samples behave as ferromagnetism at 5K and 300K, and the coercive field, HC, well follows the relation: HC(110)
1. Influence of dipolar interactions on the effective energy barrier distribution.
From the ZFC/FC magnetization simulations for systems with different dipolar interactions and anisotropy intensity, we observe that the blocking temperature TB clearly increases with interaction strength, indicating that the dipolar interactions enhance the average effective energy barrier. It is also clear from the ZFC/FC magnetization simulations that the increase in dipolar interactions gives rise to an increase in the width of the effective energy barrier distribution.
2. The nanoparticle’s spatial arrangement effect on the magnetic properties of the dipolar interacting systems.
From the ZFC/FC magnetization simulations and the M-H magnetization curves simulations for the randomly arranged and cubic arranged systems,we find that systems with different particle spatial arrangement have different effective energy barrier distribution, magnetization process and magnetic correlation. The spatial arrangement effect associated with the anisotropy will result in the invalidity of the T* model. More over, the spatial arrangement effect may cause the decrease of blocking temperature with magnetic particle concentration in proper applied field. Consequently, we point out that the spatial arrangement effect should be taken into account in analyzing the effective energy barrier of dipolar interacting systems using ZFC/FC magnetization curves.
3. Magnetic correlation in the dipolar interacting systems.
From the correlation function obtained after ZFC or FC procedure, we clarify that the dipolar interactions cause the formation of short-range order of the particle moments. This short-range order consists of both antiferromagnetic and ferromagnetic array of the particle moments, and depends crucially on the spatial arrangement, the strength of the dipolar interactions, the temperature and the applied field. The magnetic correlation has important effect on the effective energy barrier distribution and magnetic process. The thermal remanence thus can not be directly employed to study the effective energy barrier distribution of the dipolar interacting systems.
4. Role of the applied field on the dipolar interacting systems.
From the ZFC/FC magnetization, M-H curves and the reversed thermal remanence curves, we conclude that the applied field competes with the dipolar interactions. This is manifested in decreasing the ZFC peak temperature with applied field, as well as the weakening of the magnetic correlation strength.
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