Magnetic force microscopy of signature erasure in magnetic recording media

Magnetic force microscopy of signature erasure in magnetic recording media Hsia-Po V. Kuo and E.D. Dahlberg Department of Physics, University of Minnesota Minneapolis, MN 55455 Magnetic force microscope (MFM) studies of the erasure of bit transitions in highdensity particulate magnetic recording media have been investigated. Qualitative analysis found the erasure to be similar to the avalanche dynamics described by Walsh, Austvold, and Proksch [J. Appl. Phys. 84, 5709 (1998)]. For this particular media, the magnetic signatures became most susceptible to erasure at an applied field of 1.4 koe, where large scale reversal was observed. These occur mostly around regions which had irregular magnetic boundaries. I. Introduction Nature is filled with systems exhibiting dynamical behavior that is not typified by exponential decay. One such example is the slow dynamics of ferromagnetic materials. When a sample of ferromagnetic material is initially prepared in some magnetic state, disorder and the competition between exchange, magnetostatic, thermal and other interactions cause a slow relaxation of the magnetization, observable on times scales up to the patience of experimentalists. This relaxation is referred to as magnetic viscosity, the magnet after effect, or slow magnetic relaxation and has been the subject of interest for more than a century. 1 In addition to its fundamental interest for statistical physics and materials science, an understanding of the dynamics of slow magnetic relaxation is becoming ever more technologically critical in the magnetic recording industry. As data densities increase, the effects of slow relaxation will begin to represent the dominant noise source in the recording system. 2 Previous work in the MFM of avalanche dynamics in magnetic media was done by Walsh, Austvold, and Proksch. 3 They observed that the magnetic avalanches obeyed a power law distribution in a manner consistent with Bak, Tang, and Weisenfeld s concept of self-organized criticality. 3 The 50 nm sputtered CoPtCr alloy thin film studied in their work is commonly used in longitudinal hard disk recording systems. Extensive numerical analysis allowed Walsh, Austvold, and Proksch to identify the critical exponent of ~2 in the distribution. Their study provided the frame work for image analysis of another type of magnetic recording media. In doing our study, an iomega 100 MB Zip TM disk was used. After the disk was filled with arbitrary data, the recording media was sectioned off into small, circular pieces for use in a Nanoscope III multimode magnetic force microscope (MFM). 4 The MFM 1

and the sample media sat in an externally applied field. This external field was achieved using an electromagnet which could generate magnetic fields between -4 koe to +4 koe. 5 One can see from the hysteresis curve 6 (Figure 1) that larger fields are not necessary. The greatest amount of bit erasure occurs at fields between 1 koe and 2 koe. This region was the primary field range in this study. The magnetic images were made using a Nanoscope operating in tapping-lift mode 7, which allows the sample topography to be separated from the magnetic structure; two passes are made over each scan line. In the first pass, the MFM cantilever is oscillated near its resonant frequency very close to the sample so that it lightly taps the surface. The oscillation amplitude is used as the feedback signal. The second pass is made with the feedback turned off and the computer repeating the topographic motion recorded during the first pass plus an added vertical offset. During this second pass the oscillation amplitude and phase of the cantilever can be monitored. Both the phase and amplitude can be used to image the magnetic force gradient acting between the tip and the sample. All the magnetic images in this study are of the phase. 8 The magnetic moments of the bits are large enough that the initially aligned (with the applied field) regions can be easily identified. 4 As shown in Figure 2, the magnetization of the cantilever tip allows for the signal detected by the MFM to be fairly regular. 5 It was found that none of the signals observed in this study were anywhere near this level of clarity (Figure 3). 4 As the applied field increased, the situation became more favorable for the bit moments to align with the applied field. This caused parts of the initially antiparallel bit moments to become parallel to the applied field. In addition, we have previously found that fields greater than 300-400 Oe realign the moment of the MFM tip. Although this alters the sensed field gradients, the images are still measures of the bit structure. Consequently, the cantilever tip experienced more attractive forces 9, and the images showed more dark regions (Figure 4). 4 The images all show gray spots of irregular shapes. These have been determined as the regions initially aligned with the applied field, parallel and anti-parallel. 2

II. Experimental Results The image in Fig. 5(A) was made in a 375 Oe applied field. The characteristics of the bit transitions are as would be expected with a MFM tip where the magnetization is primarily oriented in the z axis (Figure 2). 10 The heights of the attractive transitions were different than the heights of the repulsive transitions, at least in part because the phase response of the DI Nanoscope was not linear. 6 The bit transitions are not sharp, with the edges of the data bits very jagged. However, the structure of the bit transitions appears to be largely unaffected at this field value. This image, as are all other images present in this study, is a 10 µm scan at a scan height of 35 nm. The image in Fig. 5(B) was made in a 730 Oe applied field. Already one can see the moments aligning. New alignments occur around regions which previously had been gray (aligned). This shows that the region most likely to undergo magnetic flipping (alignment) is one around where magnetic moments were previously aligned. The images in Fig. 5(C) and 5(D) were made in 970 Oe and 1.21 koe applied fields. Here the magnetic reversals penetrate further into the bright and dark regions. Connecting with areas that were previously gray, they make up large regions of the media newly aligned with the applied field. However, with such a large field difference, one can hardly distinguish between the two images Figures 5(E) - 5(G) show images that were made in an applied field of 1.44, 1.56, and 1.68 koe, respectively. There exist large scale penetration of aligned regions into previously bright and dark areas. As a result, no clearly identifiable data bits wider than 1 m (length of black bar located at lower right hand corner of images) on a side are visible. Figures 5(H) - 5(O) show images that were made in an applied field of 1.80, 1.92, 2.04, 2.16, 2.28, 2.40, 2.64, and 3.10 koe, respectively. The bits continue to degenerate in this series of images until the magnetic signal at the end resemble that of noise. Imaging in a zero applied field after this point continued to exhibit magnetic structures of such sporadic distribution. This configuration may be the result of the cantilever tip picking up remnant moments on the surface of the media. Closer examination suggested to us that the degeneration of the data bits were due mostly to the regions where magnetic domains were initially irregular; the likelihood of bit degeneration rests largely on whether it is energetically favorable for the bits to align with the externally applied field. From the images in Figure 5 one can see that within the squared off sections, the dark region grew as the applied field increased. The section in image A had a slight 3

irregularity along the right boundary. Subsequent images show that this is where the bit degeneration proceeded to grow. To confirm this, we subtracted each image from its previous image. 11 The differences in the magnetic signature were manifested as light bands against a dark background; the dark background being the region where the signature did not change as the applied field increased. However, this is very difficult to see. Therefore, we inverted the color table to get the negative contrast. The images in Figure 6 were done this way. Now the dark colors are the regions where magnetic alignment occurred as the applied field increased. As one can see, the dark bands around the previously squared off regions grew larger in subsequent subtractions, verifying our preliminary conclusion. The magnetic-topography overlay images in Figure 7 were made with 50% transparency of the magnetic image on top of 100% opacity of the corresponding topography underneath. It was done to observe the possible relation between the magnetic signature with its erasure behavior and the composition of the media. These images showed very slight correlation between the location of the initial bit reversal and the surface structure of the recording media. However, the structure of the media material did not have any visible effect on the way the data bits were originally written onto the media. In the squared-off region, same as before, one can see the degeneration of the magnetic signature on top of the topography of the media. When compared to the images in Figure 5, the underlying topography can be easily identified. Within the region of interest, there appears to be a large chunk of media material at a lower elevation than the surrounding region. Further analysis revealed that this area indeed represents a much lower height in the MFM image. The initial alignment of magnetic signature in this region may be attributed to the domain walls created by this valley and its surrounding peaks. Attempts to identify similar characteristics in other areas proved to be difficult due to the extreme closeness of the peaks and valleys in the topography, and the extremely irregular patterns of the degenerating magnetic signature. However, along the top part of Figure 8 there exists numerous regions of peaks and valleys. Upon closer inspection, one can see that the magnetic signatures all degenerate in these regions as well. This further confirms the notion established earlier. 4

III. Conclusion Bit erasure in magnetic recording media has been studied using magnetic force microscopy with increasing external magnetic fields. Several analysis have been done in an attempt to determine the relationship between the signature erasure and the structure of the recording media. It was found that the initial bit alignment to the externally applied field was primarily due to the unstable domain walls created by the high and low regions present in the recording media. Once the initial erasure began, it became energetically more favorable for the neighboring regions to align. For this particular media, the magnetic signatures became most susceptible to alignment at an applied field of 1.4 koe. As seen in the study, once the applied field exceeds 1.4 koe, the magnetic bits all exhibit large scale magnetization reversal. These developments occur mostly around regions where previously had irregular magnetic boundaries. These boundaries served as a guide where the erasure of magnetic signature will most likely take place. Continued increase in the externally applied field yielded expansions in the signature degeneration, protruding into the previously established data bits. Once the signature erasure began, the regions around the initial erasure started to follow suit. Small changes in the applied field created large amounts of degeneration. This phenomenon started at an applied field of around 1 koe, and continued beyond 2 koe, until the magnetic signature initially written can no longer be recognized. This is consistent with the findings of Walsh, Austvold, and Proksch, who in the Journal of Applied Physics published the observation of avalanche dynamics of signature erasure in magnetic recording media. 3 After the applied field increased beyond 3 koe, the magnetic image resembled that of noise, where it should have saturated. It is our belief that the MFM tip most likely was picking up residual magnetism from the media at this point. Furthermore, the electromagnets in this study were unable to create a sufficiently large enough external field for saturation. However, comparable future experiments can treat this as background noise, thus utilize it to obtain a systematic error analysis. 5

Acknowledgements I d like to thank Chris Merton for his guidance throughout the length of the study. 1 2 3 4 5 6 7 8 9 10 11 Lord Rayleigh, Philos. Mag. 23, 225 (1887). J.-G. Zhu, IEEE Trans. Magn. MAG-27, 5040 (1992). P. Bak, C. Tang, and K. Weisenfeld, Phys. Rev. Lett. 59, 381 (1987); Phys. Rev. A 38, 364 (1988). Digital Instruments, Santa Barbara, CA, 93117 R. Proksch, E. Runge, P.K. Hansma, S. Foss, and B. Walsh, J. Appl. Phys. 78, 3303 (1995). M. Sharrock, Imation Corp., Data Storage and Information Management, 1999 Q. Zhong, D. Inniss, K. Kjoller, and V.B. Elings, Surf. Sci. Lett. 290, L688 (1993). B. Walsh, S. Austvold, and R. Proksch, J. Appl. Phys. 84, 5709 (1998). U. Hartmann, J. Appl. Phys. 64, 1561 (1988). D. rugar, H.J. Mamin, P. Guenther, S.E. Lambert, J.E. stern, I. McFayden, and T. Yogi, J. Appl. Phys. 68, 1169 (1990). J. Wittborn, K.V. Rao, R. Proksch, I. Revenko, E.D. Dalhberg, and D.A. Bazylinski (unpublished). 6

5E-3 M (em u) 0-5E-3-5E+ 3 0 5E+3 H (Oe) Figure 1. The bulk m agnetization curve vs applied field. This hysteresis loop w as done at 25 C, with field increm ents of 50 O e in an average tim e of 200 m s per increm ent. signal β α γ H M cantilev er z Figure 2. T he m agnetic m om ents of the bits in a sm all applied field are aligned at the bottom. T he signal obtained by M F M should be in the form show n at the top. If the bit transition is a boundary of tw o north poles, the tip feels an attractive force, thus im ages a valley (darker signal) at α. If the bit transition is a boundary of tw o south poles, the tip feels a repulsive force and im ages a peak (brighter signal) at β. T he initially alig ned ( anti-alig ned) r e g ion is γ. 7

+ 5.0 deg 0.0 5.0 µ m 10.0 µ m - 5.0 deg Figure 3. C ross section analysis of a 10 µ m scan of the sam ple m edia. P eaks and valleys are distinguishable. H ow ever, the data bits are tightly spaced, and the regions of initial alignm ent w ere not apparent. The M FM tip used had a resolution of about 0.2 µ m. signal H cantilever M Figure 4. The magnetic moments of the bits in a slightly magnetizing applied field. 8

A F K B G L C H M D I N E J O Figure 5: Degenerating bit patterns in a magnetic recording media as the applied field increased. The images A, B, C, D, E, F, G, H, I, J, K, L, M, N, O were made in applied fields of 375, 730, 970, 1210, 1440, 1560, 1680, 1800, 1920, 2040, 2160, 2280, 2400, 2640, and 3100 Oe, respectively. 9

B-A C-B D-C Figure 8: Topographical mapping of the magnetic recording media. E-D Figure 6: Resultant images from B-A, C-B, D-C, and E-D. The letters represent the same magnetic images from Figure 5. 10

A D G J M Figure 7: Images of magnetic signature on top of its topography. The letters represent the same images as in Figures 5. 11