Magnetic imaging of magnetic recording media using transmission electron microscopy [electronic resource]
Keywords: Materials Science and Engineering.

The information is stored as magnetic "bits" in HDD, and these bits comprises of grains with a diameter of less than about 10 nm. To study this granular structure, a technique with high resolution capability is necessary. For this reason, transmission electron microscopy (TEM) is widely used. However, no magnetic information of these grains are obtained in conventional TEM. To overcome this limitation and directly correlate the magnetic bits to the grains, the magnetic imaging mode of TEM called Lorentz TEM is used in this dissertation to study HDD media. The first half of the dissertation discusses hard disk media, magnetic imaging techniques, and TEM specimen preparation. The overall history of how HDD was developed and its improvement over the past decades is introduced along with the future direction of HDD. For the magnetic imaging, several techniques are compared with their advantages and disadvantages. These techniques include magnetic force microscopy (MFM), spin-polarized scanning electron microscopy, Lorentz TEM, and electron holography. Then various TEM specimen preparation methods are introduced. A conventional preparation, focused ion beam (FIB) milling, and nanoparticle methods are all used in the experiments throughout this dissertation. In the second half, different TEM experiments are applied in different situations. The first sample is a Co/Pd multilayer thin film with perpendicular magnetic anisotropy. This sample was chosen for its simple stripe magnetic domain structure. To verify the feasibility of using Lorentz TEM on a more complicated hard disk media sample, the stripe domains were imaged in both plan view and cross-sectional view. The plan view image shows stripe patterns similar to that of MFM images, and the cross-sectional image showed magnetic contrast extending into the vacuum region from the thin film consistent with the domain structure. After successfully imaging stripe domains, a perpendicular magnetic recording (PMR) medium, commercially used in current HDD, was imaged in plan view. A pattern of bits was recorded for this experiment and confirmed using MFM. In order to understand the Lorentz TEM results, image simulation was used and compared to the experimental data. Different sizes of bits were successfully identified and a few bits were selected to directly correlate to the magnetic grains comprising them. After the PMR media, the next generation heat-assisted magnetic recording (HAMR) media was studied. For this experiment, a FIB was used to image in cross-section a single recorded track. Some magnetic contrast was observed using the Fresnel (defocus) Lorentz imaging method, and was compared to the targeted bits using the MFM image. This experiment shows that some bits survived the FIB specimen preparation and imaging bits in cross-section is possible. In addition to the Fresnel mode imaging, two examples of using electron holography on magnetic nanoparticle samples are briefly introduced. Although this technique has challenges to be applied to the hard disk media, future studies may find a way to benefit from the quantitative magnetic imaging of electron holography. Finally, the dissertation is summarized with suggestions on future work to be followed. The TEM is demonstrated to be very promising in imaging magnetic domains in hard disk media. Although, further improvement is necessary to characterize the most interesting regions of the bit transition that is only about a few nanometers in dimension, the characterization studies introduced in this dissertation will act as a basis for the future studies.