高垂直各向异性磁性材料的光热辅助磁矩反转动力学

Magnetic materials with large perpendicular magnetic anisotropy (PMA) are required for magnetic recording to meet the demanding of increasing higher density. However, these recording mediums with high PMA exhbit very high coercivity, so reducing the magnetization switching field is a challengling issue for their applications in the hard disk and other magnetic recording systems. Currently, opto-thermal assisted recording is a promising approach because it has been demonstrated that laser interaction may dramatically reduce the switching field. But the switching dynamics in the opto-thermal assisted switching has not been identified yet. Understanding such dynamics is important to improve the efficiency of opto-thermal assisted magnetization switching and extend the lifetime of the recording medium. In this project, we propose to investigate the fast magnetization switching dynamics of magnetic materilas with high PMA. By employing the single-group laser pluses pump and single pulse probe magneto-optical Kerr effect, we will study the complete magnetization switching dynamics of FePt and MnGa films and micrometer size bits to clarify the magnetization reversal time and mode under the effect of opto-thermal interaction. Our work will be helpful to establish the physical model of the opto-thermal assisted magnetization switching dynamics, and to seek solutions for improving the efficiency of magnetization switching of magnetic mediums with high PMA at low magnetic fields.

进一步提高磁存储密度将采用高垂直各向异性磁性材料，但是这些材料通常具有很高的矫顽场，因此如何降低反转磁场是当前面临的重要课题，目前的技术表明采用光热辅助可以有效降低反转场，但是由于相应的磁矩反转动力学过程并不清楚，很大程度上限制了光热辅助在磁矩反转过程中的作用效率，影响了介质的重复使用寿命。本项目将开展具有高垂直各向异性磁性材料在光热辅助下的快速磁矩反转动力学研究，我们将利用单组脉冲泵浦和探针磁光技术，记录FePt以及MnGa等垂直各向异性材料的薄膜与单晶纳米颗粒在瞬态光热作用下磁矩反转的完整过程，明确光脉冲作用下的磁矩快速反转方式和速度，从而建立光热辅助磁矩反转的动力学模型，为改善光热辅助磁矩反转效率提供依据。

开展了高垂直磁性各向异性介质L10 FePt的光热辅助磁矩翻转动力学研究，研究了不同化学有序度FePt薄膜在1 KHz重复频率的飞秒光脉冲、单个飞秒光脉冲、以及不同时间延迟的两个飞秒光脉冲作用下的磁矩翻转，确立了单个飞秒光脉冲可以在远低于静态矫顽场的外加磁场下驱动高化学有序度L10 FePt薄膜的磁矩翻转，通过双脉冲实验发现矫顽场随着双脉冲时间延迟增加而迅速增加，时间常数为1ps量级，从而明确了超快退磁峰值对于磁矩翻转的决定性影响，自旋翻转成畴的时间开始于皮秒时间尺度，并且揭示了局域化学无序可以形成钉扎中心阻碍光辅助的磁矩翻转。由于超快退磁峰值由自旋温度决定，且自旋温度上升比晶格温度上升更快，温升幅度更大，因此采用飞秒光脉冲实现光热辅助磁矩翻转相较于使用连续激光进行光热辅助磁矩翻转具有减少总的热注入和材料热劳损以及提高磁矩翻转速度的优势。此外，利用时间分辨泵浦探针磁光谱确立了垂直磁性各向异性与自旋轨道耦合强度平方成正比，Gilbert阻尼因子与化学无序度关联的电子散射率成正比关系。

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