High Temperature Stability of Magnetic Clusters

Abstract

Summary of the most important results (1) Enhancement of Coercivity in Nanometer-Size CoPt Crystallites Many magnetic devices are required to have high magnetic coercivity (He), such as magnetic bias films of magneto-resistive elements, magnetic tips for magnetic force microscopy, high-density magnetic recording media and magneto-optic recording media. The CoPt binary alloy is an excellent system because of its chemical stability and high magnetic anisotropy. The anisotropy of CoPt compounds is as high as 4xl0 7 ergs/cm 3 and the saturation magnetization is about 800 emu/cm 3 . For this study, we showed that CoPt thin films with well-separated crystallites and well- ordered fct structure were achieved by controlling the annealing time and annealing temperature. From atomic force microscopy and magnetic force microscopy studies, the magnetic single domain size of CoPt is in the range of 100 to 200 nm. The high He is likely due to the well- separated nanometer-size crystallites and the well-ordered fct phase of CoPt alloy. For the first time, we have enhanced the magnetic coercivity of a CoPt film to 37 kOe. (2) CoPt Magnetic Force Microscopy (MFM) Tips The magnetization direction of a magnetic force microscopy (MFM) tip will change during measurement if the magnetic coercivity (He) of the MFM tip is lower than the magnetic stray field of the sample. As a result, the magnetic images are difficult to interpret. This is also a problem in the determination of the magnetization direction of the MFM tip, when the demagnetization field can not be neglected due to the shape of the tip. These problems can be resolved by using high He MFM tips because its He is higher than the demagnetization field. To our knowledge, we have fabricated CoPt MFM tips with high He for the first time. We demonstrated that the He of these MFM tips is higher than the gap field of a typical thin-film recording head. The magnetic transitions and magnetic grains in a recording disk were also clearly observed. This was likely due to the better defined magnetization direction and very small size of the high He CoPt MFM tip. There are possibly many other advantages in the application of the high He tips. For example, the high He tips may be used in the study of magnetic domain images in the presence of an external magnetic field. It has also been shown by Proksch et al. that the power dissipation in the magnetic image can be eliminated by using a high He CoPt MFM tip which will result in improved images. (3) Advanced Magnetic Force Microscopy Tips We have developed several methods to fabricate advanced MFM tips for magnetic images. In this work, we used our combined resources in thin film preparation, tip microfabrication techniques and expertise in magnetic force microscopy. Magnetic force microscopy (MFM) is a useful imaging tool for studying a wide variety of local magnetic phenomena. It allows the direct visualization of magnetic domains and provides the experimental basis for theoretical modeling. The technique measures changes in the interaction force between a magnetized probe and the local stray magnetic field from the sample,