Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA.
Nano Lett. 2010 Sep 8;10(9):3796-803. doi: 10.1021/nl102861e.
Magnetization reversal is an important topic of research in the fields of both basic and applied ferromagnetism. For the study of magnetization reversal dynamics and magnetic domain wall (DW) motion in ferromagnetic thin films, imaging techniques are indispensable. Here, we report 4D imaging of DWs by the out-of-focus Fresnel method in Lorentz ultrafast electron microscopy (UEM), with in situ spatial and temporal resolutions. The temporal change in magnetization, as revealed by changes in image contrast, is clocked using an impulsive optical field to produce structural deformation of the specimen, thus modulating magnetic field components in the specimen plane. Directly visualized are DW nucleation and subsequent annihilation and oscillatory reappearance (periods of 32 and 45 ns) in nickel films on two different substrates. For the case of Ni films on a Ti/Si(3)N(4) substrate, under conditions of minimum residual external magnetic field, the oscillation is associated with a unique traveling wave train of periodic magnetization reversal. The velocity of DW propagation in this wave train is measured to be 172 m/s with a wavelength of 7.8 microm. The success of this study demonstrates the promise of Lorentz UEM for real-space imaging of spin switching, ferromagnetic resonance, and laser-induced demagnetization in ferromagnetic nanostructures.
磁化反转是基础和应用铁磁学领域的一个重要研究课题。对于铁磁薄膜中磁化反转动力学和磁畴壁（DW）运动的研究，成像技术是必不可少的。在这里，我们通过洛伦兹超快电子显微镜（UEM）中的离焦菲涅耳方法报告了 DW 的 4D 成像，具有原位空间和时间分辨率。通过改变图像对比度揭示的磁化的时间变化，通过脉冲光场产生样品的结构变形来计时，从而调制样品平面中的磁场分量。在两个不同的衬底上的镍薄膜中，直接观察到了 DW 的成核以及随后的湮灭和振荡再现（周期为 32 和 45 ns）。对于 Ni 薄膜在 Ti/Si(3)N(4)衬底上的情况，在最小残余外磁场的条件下，该振荡与周期性磁化反转的独特行波列车相关联。在该波列中 DW 传播的速度被测量为 172 m/s，波长为 7.8 微米。这项研究的成功证明了洛伦兹 UEM 用于铁磁纳米结构中自旋开关、铁磁共振和激光诱导退磁的实空间成像的前景。
