Development of advanced magnetic force microscopy tips

The goal of this part of the research is to obtain higher resolution and sensitivity in MFM images for magnetic characterization of nanometer size magnetic features.

Since Magnetic force microscopy (MFM) development in 1987, it has emerged as a powerful tool for studying a wide variety of local magnetic phenomena, which reveals magnetic processes with unprecedented clarity, resolution and ease. It allows the direct visualization of magnetic domains and provides the experimental basis for theoretical modeling. The technique measures change of the interaction force between a magnetized probe and the local stray magnetic field from the sample, point by point, as the probe is scanned across the surface. The probe is typically a cantilever made from silicon or silicon nitride, with a ferromagnetic tip on the free end. The inherent resolution depends upon the confinement of the interaction at the end of the probe and sensitivity depends upon the ratio of the cantilever spring constant and the magnetic moment. At present, commercial MFM probes resolves about 10-100 nm features at force constant of about 0.01 N/m---roughtly equivalent to resolving the field gradients from a 10^-12 emu source at a distance of 50 nm.

Despite the impressive performance and widespread use of the MFM, there are important probe-related limitations that need to be overcome to realize its full potential. First of these is the enhancement of resolution and sensitivity. As is well known from microscopy, in order to measure something at a given scale it is necessary to have a probe whose fundamental size is well below the size of the object to be measured. In the case of magnetic force microscopy (MFM) the force between the probe and the sample is carried by the magnetic field. Obviously, the smaller the magnetically active area of the probe, the less it will be affected by areas from far away since the dipole nature of the field causes it to diminish rapidly with distance. Therefore, in order to make a high resolution MFM it would be necessary to create an extremely small magnetic probe. The smaller volume of the magnetic probe, on the other hand, will result in a lower magnetic moment and a smaller interaction volume and thus a weaker force. Hence, the lateral resolution of the MFM probe will also be limited by its sensitivity (e.g. the spring constant of the cantilever). The improvement in resolution would have to be complemented with an enhancement of the probe sensitivity. Fig. 1 shows a few examples of the advanced magnetic force microscopy tips that were made in our laboratory and collaborators by different methods.

Fig. 1 Advanced magnetic force microscopy tips made by three different methods (a) by ion milling, (b) electron beam deposition, (c) focused ion beam milling. Collaborate with Jon Orloff at the University of Maryland

Second is the development of specialized probes whose properties are optimized for a given specimen and free from instrument-induced distortions. Since the MFM relies on a mutual interaction, it is inherently invasive. Thus, the measurement process itself could cause irreversible changes to the system and the measured image may not reflect the intrinsic state of the sample. Conversely, the probe’s moment itself may change as it moves in varying fields, which would cause nonlinearities in the instrument response. This would render the image interpretation to become complicated and equivocal. To overcome these problems, we have developed a low moment with very high coercivity MFM probe.

As shown in Fig.2, we have demonstrated a better resolution by using an electron beam deposition MFM tip (it has a low moment with very high coercivity and a very small magnetically active area). Here, we shall note that the results shown at Fig. 2(a) is similar to that of measured by B. D. Terris et al.[Appl. Phys. Lett. 1999].

Fig. 2 Magnetic images of an inhomogeneous sample, a Co/Pt multilayer (10 periods of 3 ÅCo and 10 Å Pt deposited on Si), patterned by irradiated N⁺ at 700 keV through a silicon stencil mask having about 1.3 micrometer diameters holes. (a) use thin film coated MFM tips (b) use an electron beam deposition MFM tip. Sample provided by B. D. Terris, L. Folks and D. Weller at IBM

Finally, the MFM requires the fundamental understanding of the magnetic characteristics of the probes themselves, which can be incorporated into theoretical models of image interpretation. At present the generally accepted model for MFM assumes a point dipole at the tip apex. This picture is adequate in qualitative descriptions which treat the images as representations of the distribution of magnetic charges from the divergence of the volume magnetization or the normal component of the surface magnetization. Several sophisticated theoretical descriptions for image representation have been proposed in the literature which take into consideration the finite volume of the tip. Unfortunately, because of the absence of direct experimental evidence of the probe’s magnetization distribution, the models simply provide possible explanations of observed contrast formation rather than offer precise magnetization reconstruction.

The goal of this work is to address the aforementioned limitations of conventional MFM probes, by improving the resolution and sensitivity, by developing processes to tailor probes with predefined moment and coercivity, and by developing characterization and calibration methods for incorporation into theoretical models of image reconstruction. In this work, we will use our combined resources in thin film preparation, tip microfabrication techniques and expertise in magnetic force microscopy.