Parker Audrey C, Maryon Olivia O, Kaffash Mojtaba T, Jungfleisch M Benjamin, Davis Paul H
Micron School of Materials Science & Engineering, Boise State University.
Department of Physics and Astronomy, University of Delaware.
J Vis Exp. 2022 Jul 20(185). doi: 10.3791/64180.
Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force microscopy (AFM) probe whose tip has been magnetized vertically (i.e., perpendicular to the probe cantilever) is oscillated at a fixed height above the sample surface. The resultant shifts in the oscillation phase or frequency, which are proportional to the magnitude and sign of the vertical magnetic force gradient at each pixel location, are then tracked and mapped. Although the spatial resolution and sensitivity of the technique increases with decreasing lift height above the surface, this seemingly straightforward path to improved MFM images is complicated by considerations such as minimizing topographical artifacts due to shorter range van der Waals forces, increasing the oscillation amplitude to further improve sensitivity, and the presence of surface contaminants (in particular water due to humidity under ambient conditions). In addition, due to the orientation of the probe's magnetic dipole moment, MFM is intrinsically more sensitive to samples with an out-of-plane magnetization vector. Here, high-resolution topographical and magnetic phase images of single and bicomponent nanomagnet artificial spin-ice (ASI) arrays obtained in an inert (argon) atmosphere glovebox with <0.1 ppm O2 and H2O are reported. Optimization of lift height and drive amplitude for high resolution and sensitivity while simultaneously avoiding the introduction of topographical artifacts is discussed, and detection of the stray magnetic fields emanating from either end of the nanoscale bar magnets (~250 nm long and <100 nm wide) aligned in the plane of the ASI sample surface is shown. Likewise, using the example of a Ni-Mn-Ga magnetic shape memory alloy (MSMA), MFM is demonstrated in an inert atmosphere with magnetic phase sensitivity capable of resolving a series of adjacent magnetic domains each ~200 nm wide.
磁力显微镜（MFM）能够以纳米级分辨率绘制样品表面的局部磁场。为了进行MFM，将一个尖端已垂直磁化（即垂直于探针悬臂）的原子力显微镜（AFM）探针在样品表面上方固定高度处振荡。然后跟踪并绘制振荡相位或频率的结果变化，这些变化与每个像素位置处垂直磁力梯度的大小和符号成正比。尽管该技术的空间分辨率和灵敏度会随着表面上方提升高度的降低而提高，但这条看似直接通往改进MFM图像的路径却因多种因素而变得复杂，比如要尽量减少由于较短程范德华力引起的形貌伪像、增加振荡幅度以进一步提高灵敏度以及存在表面污染物（特别是在环境条件下由于湿度产生的水）。此外，由于探针磁偶极矩的取向，MFM本质上对具有面外磁化矢量的样品更敏感。在此，报告了在氧气和水含量均小于0.1 ppm的惰性（氩气）气氛手套箱中获得的单组分和双组分纳米磁体人工自旋冰（ASI）阵列的高分辨率形貌和磁相图像。讨论了为实现高分辨率和灵敏度同时避免引入形貌伪像而对提升高度和驱动幅度进行的优化，并展示了对ASI样品表面平面内排列的纳米级条形磁体（长约250 nm，宽小于100 nm）两端发出的杂散磁场的检测。同样，以镍 - 锰 - 镓磁性形状记忆合金（MSMA）为例，展示了在惰性气氛中具有磁相灵敏度的MFM，其能够分辨一系列宽度约为200 nm的相邻磁畴。
