Nonstandard imaging in magnetic force microscopy: new probes and methods

Abstract

Despite decades of advances in magnetic imaging, obtaining direct, quantitative information with high spatial resolution remains an outstanding challenge. The imaging technique most widely used for local characterization of magnetic nanostructures is the Magnetic Force Microscope (MFM), which is indeed a very active topic of investigation. Advantages of MFM include relatively high spatial resolution, simplicity in operation as well as sample preparation, and the capability to applied in situ magnetic fields to study magnetization processes. Recently we have also demonstrated the possibility of operating the MFM systems in different environments including liquid media. Such development opens up the possibility of characterizing biomagnetic samples. In this work, we present the advantage of use nonstandard homemade probes as well as new MFM-based methods to partially solve the challenges in MFM as the limited spatial resolution, the sensitivity and the chance to achieve quantitative measurements. Three main parameters can be controlled: the magnetic coating, the geometry of the tip and the mechanical properties of the cantilever. High-performance MFM probes with sub-10 nm topographic lateral resolution and sub-25 nm magnetic lateral resolution are obtained by using easy low-cost approach consisting of selecting the accurate partial coating of the probe. This allows one to not only customize the tip stray field, avoiding tip-induced changes in the sample magnetization, but also to optimize the MFM imaging in vacuum or liquid media by choosing tips mounted on cantilevers mechanically hard or soft respectively, a product that is currently not available on the market. In Figure 1 we show an example of the advantages of tuning the mechanical properties of the cantilever by comparing MFM images of a reference sample -a commercial high density hard disk- acquired with commercial and home-made MFM probes. Figures 1a and 1b correspond to images obtained with a commercial probe under ambient conditions and in liquid. Notice the poor signal/noise ratio of the MFM image in liquid (Figure 1b). However, the custom-made probe (Figure 1 c and d) with lower force constant allows us to obtain MFM images under ambient conditions and in liquid environment with higher signal/noise ratio (see Figure 1d). It is well known that due to the viscosity of the liquid media there is a decrease in the quality factor of the cantilever i.e. an increase of the noise in the MFM signal is expected. By using specific customized MFM probes the sensitivity can be enhanced in about a factor of 10 improving significantly the quality of the images. Such contrast improvement is an opportunity to address the study of low moment materials. In particular, the use of customized MFM probes in combination with new MFM methods based on alternative feedback control allow us to image biological materials in physiological conditions. Figure 2 shows the topography and the magnetic signal of magnetotactic bacteria Magnetospirillum gryphiswaldense acquired with custom made MFM tips and using special feedback mode. Furthermore, the idea of exploring new MFM probe architectures allows us to focus some of the challenges of the technique as the lack of quantitative information. Here we propose a combination of MFM advanced methods and electron holographic tomography techniques to obtain quantitative measurements with nanometric resolution.