Control of magnetic vortex circulation in one-side-flat nanodisk pairs by in-plane magnetic filed
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摘要: 铁磁纳米盘中的磁涡旋态因稳定性高, 并且其面内磁化的旋转方向具有天然的二向性(顺时针(CW)和逆时针(CCW)), 可以作为信息存储的一个比特单元而成为最近研究的热点. 基于磁涡旋旋性的信息存储要求人们能够独立地控制磁涡旋的旋转方向. 从旋性的角度考虑, 在一对纳米盘中可能出现四种磁涡旋基态, 即(CCW, CCW), (CCW, CW), (CW, CCW)和(CW, CW). 本文通过引入厚度不同且切边的纳米磁盘对, 并对其施加面内磁场来实现对四种涡旋基态的独立控制, 并通过微磁学模拟证明了这种方法的可行性.Abstract: In a nanodisk made of soft ferromagnet, the magnetic vortex structure are highly stabilized, and the circulation directions of the vortices are naturally binary (either clockwise (CW) or counter-clockwise (CCW)), which can be associated with one bit of information, and thus the magnetic vortices have been of great interest recently. A vortex-circulation-based memory requires the perfect controllability of the circulation direction. From the circulation point of view, there are four possible ground states in a nanodisk pair: (CCW, CCW), (CCW, CW), (CW, CCW) and (CW, CW). In a perfect circular nanodisk, CW and CCW states are degenerate because of the high symmetry of the system. However, the circulation of the magnetic vortex is known to be controlled by introducing the asymmetry. It has been reported that the magnetic vortices with opposite (the same) circulations are realized in one-side-flat disk pair. That means in one-side-flat nanodisk pair only the control of two of these four ground states is possible, eg., (CCW, CW), (CW, CCW) or (CCW, CCW), (CW, CW). We found that the reversal of the magnetic vortex circulation is affected by the nanodisk thickness as well. By further introducing another asymmetry, different thickness, the control of the four circulation ground states is achieved in a nanodisk pair. In this work, the controllability of the four ground states in a nanodisk pair was numerically investigated via micromagnetic simulations. The results show that in a single one-side-flat nanodisk, there exists a preferred rotational sense at the remanent state after the nanodisk is saturated by the external magnetic field, applied parallel to the flat edge of the nanodisk. The shape anisotropy is the primary cause of this phenomenon. We further found that the obtained rotational senses of the magnetization in the vortex state in nanodisks with the same geometrical parameters but different thickness (20 nm and 50 nm) are opposite for the same direction of the externally applied field. This is attributed to the competition between the demagnetization field energy and the exchange energy during the vortex formation. The method we proposed provides a simple means of controlling the vortex state that can thus become a useful tool for designing vortex-based devices.
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Key words:
- magnetic vortex /
- circulation reversal /
- magnetic nanodisk /
- micromagnetic simulation
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图 1 厚度t不同的两对切边的纳米盘的形状和尺寸
Figure 1. Geometry and dimension of one-side-flat nanodisk pairs with different thickness t.
图 2 厚度(a) t = 50 nm和(b) t = 20 nm的纳米盘的磁滞回线. 图中的颜色和箭头代表xy平面内的磁化方向, 黑色和白色的点分别代表方向朝下和朝上的磁涡旋核. 当磁感应强度从150 mT减小至0 mT时, 厚度为(c) 50 nm和(d) 20 nm的纳米盘的能量密度的变化
Figure 2. Hysteresis loops of (a) t = 50 nm and (b) t = 20 nm nanodisks. The color map as well as the arrows inside the nanodisks represents the magnetization directions in xy plane, and the black and white dots represent downward and upward magnetic vortex core, respectively. Variation of the energy density for (c) t = 50 nm and (d) t = 20 nm nanodisks when the magnetic filed is swept from 150 mT to 0 mT.
图 3 (a) Pair A和(b) Pair B的磁滞回线. (c) Pair A和(d) Pair B在不同磁感应强度下的磁化分布图
Figure 3. Hysteresis loops of (a) nanodisk Pair A and (b) nanodisk Pair B. The snapshots of local magnetization distribution of nanodisk (c) Pair A and (d) Pair B under different external magnetic field.
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