摘要
由于磁性纳米环具有热稳定性好,没有高能的涡旋核,几乎没有杂散场且可以形成稳定的涡旋结构等特点,在高密度磁记录器件及微波传感器件等领域都有潜在的应用价值。因而深刻理解其磁化反转机制及高频特性成了近来研究的热点。而微磁学模拟是研究磁性物质磁化反转过程和高频动态响应的有力工具,因此在本论文中,我们用微磁学模拟软件OOMMF (3D)研究了磁性纳米环的静态及动态磁学特性。具体内容如下：
对内半径50 nm,外半径100 nm,不同厚度的FCC结构金属Co纳米环,主要研究了在不同方向加外场时,体系的磁化反转过程及对应的反转模式。我们发现形状各向异性强烈的影响着磁体的反转机制,并且外场的方向也影响着磁滞回线的形状及磁矩分布。在环的面内加场时,厚度为10 nm的纳米环有较大的矫顽力与剩磁,剩磁态为onion态月.反转过程为两步反转：onion- vortex-onion。随着厚度的增大,纳米环的矫顽力与剩磁减小至0,反转过程为伴随着畴壁移动的两步反转,零场时形成vortex态。对于零场时形成vortex结构的纳米环,vortex态的成核场随着厚度的增大而增大。在厚度方向加场时,厚度为20 nm的磁体的反转过程为一致反转且剩磁态为vortex态。当厚度从200 nm增大到400 nm时,随着形状各向异性的增大,矫顽力与饱和场逐渐增大。反转过程较复杂,伴随着四个涡旋核的形成与移动。
对内半径为9 nm,外半径15 nm的环状坡莫体系,研究了不同厚度对应的剩磁态及高频磁特性。当环的厚度从1nm增大至40 nm时,体系最稳定的平衡磁化结构分别为水平态、涡旋态、H态和轴向态。分别研究了沿面内和轴向加微波场时体系的共振峰数量、共振模式、共振频率和磁化率虚部与厚度的关系。对于平衡磁化结构为水平态的体系,在环面内加场时只得到一个共振峰,对应于体共振模式；在轴向加场时,得到两个共振峰且低频共振峰的频率与在面内加场时有同样的值,其高频共振峰对应于形状共振模式。对于平衡磁化结构为涡旋态的体系,在不同方向加场时分别只得到一个峰,对应于体共振模式。对于平衡磁化结构为轴向态的体系,在环面内加激发场时只得到一个共振峰,对应于形状共振模式,共振频率的值与变化趋势均符合基泰尔公式的计算结果。
Magnetic nanorings in flux closure states with negligible stray fields have significant advantage in advanced storage media, spintronic and advanced microwave technologies, as the highly energetic vortex core is removed. The key issue from both fundamental and technological viewpoint is to understand the magnetization reversal mechanisms and the high-frequency response. Micromagnetic simulation is suitable for investigating the magnetic reversal process and dynamic evolution of ferromagnetic materials. In this paper, the static and dynamic magnetic properties of nanorings are studied using 3D object oriented micromagnetic framwork OOMMF.
The magnetic configrations and magnetization reversal mechanism in FCC-Co nanorings (R0=100 nm, Ri=50 nm) were investigated. It is found that the shape anisotropy has great influence on magnetization reversal mechanism, and the applied field directions have effect on the magnetic hysteresis and remanent states. As the applied field is in the plane of the ring, there are two stable onion states in the thin ring (t=10 nm). At the remanence, the ring is magnetized in the onion state. The simulated hysteresis loop showed a coercivity of 893 Oe and a remanent ratio of 0.833. The magnetic reversal process is a typical two-steps reversal. For the thicker rings (t=200,400 nm), the simulation results indicate that the reversal mechanism is different from that of thin rings, the reversal is via the nucleation and annihilation of vortex structure and a reversed domain wall, when the applied field is 0, the vortex structure is the stable state. The coercivity and remanent ratio reduced to 0 with the increase of thickness. When the applied field is parallel to the normal axis of the ring, the reversal mode of the thin ring (t=20 nm) is the coherent reversal mode. The coercivity and the remanent ratio increase with the augment of the thickness. For thicker rings(t=200,400 nm), the reversal mechanism is via the nucleation and annihilation of four vortex cores.
For the small Permalloy ring (R0=15 nm, Ri=9 nm), There are four states for rings depending on the thickness:in-plane, vortex, H, and out-of-plane state. The emphasis is placed on the effect of ring thickness on the dynamic susceptibility spectra of each state. For the in-plane state, when a exponentially pulse is applied in the plane of the ring, there is only one resonance peak. When the exponentially pulse is along the normal axis, the system shows two resonance peaks. The main resonance frequencies have the same value. The high resonance peak corresponds to the shape mode. For the vortex state, there is only one resonance peak corresponding to the volume mode. For out-of-plane state, the investigated systems show one major resonance mode (shape mode) corresponding to the resonance frequency. The frequency of the shape mode converges to that of the Kittel prediction.
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