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Magnetization reversal processes in perpendicular anisotropy thin films observed with magnetic force microscopy
Journal of Magnetism and Magnetic Materials 190 (1998) 81—88
Magnetization reversal processes in perpendicular anisotropy thin films observed with magnetic force microscopy Jake Schmidt *, George Skidmore , Sheryl Foss, E. Dan Dahlberg , Chris Merton Magnetic Microscopy Center, School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA Imation Corporation, St. Paul, MN 55128, USA
Abstract We have carried out studies of the magnetic reversal process of a rare earth—transition metal thin film with perpendicular magnetic anisotropy using a magnetic force microscope (MFM) capable of applying in situ magnetic fields. The magnetization of the microscopic area shown in MFM images was determined for a number of field values comprising a complete hysteresis loop. This microscopic hysteresis loop was found to be nearly identical to a bulk hysteresis loop. Changes in the magnetization of the film as the hysteresis loop was traversed can be linked to individual microscopic domain changes evident in the MFM images. These studies show that the magnetization of this film was characterized by a two-stage process — fast and slow rates of change of magnetization with applied field. A second experiment in which the film was incompletely saturated and brought back to zero field showed that domain nucleation was not responsible for the rate of the fast process, but rather all magnetization changes were primarily limited by the low domain wall mobility. These observations are linked to previous work on magnetization processes in similar magnetic systems. 1998 Elsevier Science B.V. All rights reserved. Keywords: Magnetic reversal; Thin films; Domain wall motion
1. Introduction Perpendicular rare earth—transition metal (RE— TM) thin films are a subject of great interest due to their use as magneto-optic recording media. These films are typically deposited by sputtering which results in an amorphous microstructure [1,2].
* Corresponding author. Fax: #1 612 624 4578; e-mail: [email protected]
The usefulness of these films is due in part to their high perpendicular anisotropy, reported as high as 3;10 ergs/cc [3,4]. The origin of this anisotropy is still debated; inverse magnetostriction [3,5], shape anisotropy from columnar growth or voids [4], single-ion anisotropy [1], and pair ordering or exchange anisotropy [2] are proposed contributing mechanisms. The domain structures in unsaturated perpendicular films, depending on the amount of domain wall pinning, will vary from linear stripes [6] to highly convoluted fractal patterns [7].
0304-8853/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 2 7 7 - 7
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Experimental investigation of the magnetization reversal process in a perpendicular thin film was first reported by Kooy and Enz in which the reversal of a barium ferrite platelet was studied with Faraday microscopy and parts of the hysteresis loop were correlated with general microscopic changes in the domain pattern [6]. There it was observed that the application of a sufficiently large magnetic field perpendicular to the sample plane saturated the specimen. Decreasing the field eventually resulted in the nucleation of a reverse domain in a microscopic area of the film. The nucleation field was not the same for each experimental run since the nucleation sites were in different locations on the film and local inhomogeneities of the specimen undoubtedly existed. The nucleation event led to an irreversible, fast process in which the reversed domain, after nucleating at a point, grew by domain wall propagation throughout the film. Immediately after overcoming this nucleation barrier, the entire film consisted of stripe domains of two different widths. The reversal continues through the slow, reversible process marked by changes in the relative fractions of the two domain types. Their theoretical model involving the minimization of the sum of the magnetostatic and domain wall energies in the appropriate applied field explained the measured hysteresis loop quite well. The magnetization reversal process in RE—TM films is controlled by two coercivities [8]: the domain nucleation coercivity and the coercivity of domain wall motion. Investigations into these two coercivities or energies have been done using two types of remanence curves, the isothermal remanence curve (IRM) and the demagnetization remanence curve (DCD). These curves can show whether the limiting process to reversal is due to domain nucleation or domain wall motion; these curves also give the distribution of energies associated with each process. By examination of the magnetic microstructure at different stages of the reversal process, understanding of the mechanisms governing the reversal can be gained. Here we report the results of an experiment in which we studied the microscopic magnetization changes of a RE—TM thin film with low domain wall mobility, correlating changes in the magnetic structure on a micron to sub-micron scale to
changes in the bulk magnetization curve. Correlations between micromagnetic structure and bulk magnetic behavior are essential for a complete understanding of the magnetization process, and have been done in other systems [9]. We observe that for the TbGdFeCo film studied, the microscopic hysteresis loop is almost identical to the bulk hysteresis loop which has a form similar to that described by Kooy and Enz, but with a much higher field at which the onset of the fast process occurs. For this film, we show that the fast process, though very steep, is governed not by domain nucleation, but by domain wall motion. The distribution of coercivities to domain wall motion is thus relatively narrow, since magnetization reversal occurs abruptly once a certain field is exceeded. The slow process also shows many similarities to the work of Kooy and Enz; the mode of introduction of new linear domains, the characteristic widths of these domains, and the mode of disappearance of these domains are shared by both systems.
2. Experiment The experiments were performed on a 580 As thick amorphous thin film of Tb Gd Fe Co encapsulated in 90 As of SiC and sputter-coated on a Si wafer. A bulk hysteresis loop, perpendicular to the plane of the film (Fig. 1), was obtained using an alternating gradient force magnetometer (AGFM) showing a coercive field, H , of 300 Oe and a satu ration magnetization, M , of 173 emu/cc. This per pendicular loop was square, with the ratio of the remanent magnetization to the saturation magnetization, M /M , approximately equal to 1, indicat ing that the sample should have an approximately uniform magnetization in zero field after exposure to a field sufficient to saturate it. The sample was imaged with tapping/lift2+ mode using a Digital Instruments Dimension 3000 scanning probe microscope [10] equipped with phase detection. We used 225 lm long microfabricated silicon cantilevers with integrated tips obtained commercially and sputtered with 150 As of Cr and 150 As of CoCr. This coating was observed to be considerably less perturbative to the sample magnetization than a tip with a 450 As thick coating of
J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 81—88
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Fig. 1. Magnetization versus applied magnetic field for the TbGdFeCo film. The bulk hysteresis loop, shown by the thick line, was measured on an alternating force gradient magnetometer. The curve plotted by the triangles was obtained by plotting the fractional number of black pixels on a threshold 20 lm MFM image versus field, after the sample had been initially exposed to a 6 kOe magnetic field. The squares represent a similar MFM experiment, except that the sample was not fully saturated prior to the application of a reversing magnetic field.
CoCr. The tips used in this work were always magnetized perpendicular to the sample surface. Our microscope was modified so that a large electromagnet could be placed under the sample and scanning probe. This electromagnet can produce a maximum field of $2 kOe. The field was observed to be uniform to 1% over a 1 cm diameter area in the center of the rod. The sample was placed on a mount which insulates the sample from any vibrations of the magnet. Using this apparatus, in situ microscopic hysteresis loops can be performed on the film as described below. Other experiments have also been performed using MFM with applied field capability as well [11—13]. Since the film has a uniaxial perpendicular anisotropy, the domains in the film are oriented either into or out of the plane and are either parallel or anti-parallel to the tip magnetization. They appear in the MFM images as areas of different contrast,
dark or light, as seen in Figs. 2 and 3. Image processing can be applied to the images of these domains by imposing a threshold on the gray scale of the image, mapping all pixels with values above a certain cutoff to black, and those below the cutoff to white. Then, for a given domain structure, the relative amounts of each domain orientation may be calculated by simply counting the fractional number of pixels corresponding to each domain type. This fractional value is related to the magnetization of the scanned area. By repeating this process at different field values, a hysteresis loop of a microscopic area can be done. The loops are initiated by setting the electromagnet at 2 kOe and holding a permanent magnet (B &4 kG) directly above it, thus saturating the film in a field of 6 kOe. After removing the permanent magnet, the film can then be imaged as a function of field for values starting at 2000 Oe and decreasing through zero to
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J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 81—88
!2000 Oe, so that major hysteresis loops can be performed on this sample. Microscopic hysteresis loops were performed and one is shown in Fig. 1. The threshold chosen for all images in this series was halfway between the maximum and the minimum allowed data values. The shape of the microscopic loop is very similar to the bulk loop, the only difference being that the fast magnetization process occurs at a smaller negative field for the microscopic loop than for the bulk loop. There is also some deviation from the bulk behavior at larger negative fields, but at this point in the hysteresis loop the domains which still remained were quite small and the noise was fractionally much greater than for lower field values, so it is believed that this deviation is spurious, and could be eliminated by filtering and manipulation of the threshold. Although the figure shows only half of a microscopic loop, complete loops were taken and the hysteresis loop was symmetric with respect to applied field. A second experiment was performed in which the film was subjected to a 2000 Oe field and then imaged. According to the bulk hysteresis loop, the film should be saturated at this field. However, a few isolated bubble domains remained, indicating that the film was not entirely saturated. The field was then decreased to 1300 Oe, and images showed that those bubbles were unchanged. At 0 Oe, images showed that the majority of the bubbles in the image were in the same locations as in previous images, but several bubbles had begun to form a branched domain structure similar to the previous experiment, although with a limited spatial extent. As the field was decreased further in small steps, the images showed growth of the branch domain structures until significant reversal occurred. Applying the same image processing tech-
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niques discussed above yielded essentially the beginning of a minor hysteresis loop. There was a steep change in the magnetization similar to that of the bulk and MFM major hysteresis loops at approximately the same applied field value as in the previous experiment. This experiment was then repeated with negative field values to see if the bubble domains which remained at !2000 Oe were in the same location as those which remained at 2000 Oe; they were not. The field was increased in the same manner as above, and the steep change in magnetization with field was observed again, at approximately the same magnitude of field as before.
3. Analysis The study by Kooy and Enz [6] of the perpendicular magnetization process of single-crystal barium ferrite platelets is a system dissimilar to the present system in a number of ways. Because of the extreme uniformity of the platelets, the domain wall pinning energy was close to zero making the entire system a global energy minimization problem. In the present system, the films are inhomogenous with voids between amorphous columns of material [4]. It is likely that the local magnetization and anisotropy vary such that the propagation of a domain is governed by local and not global conditions. This can be seen by comparison of the domain structures: the domains in the barium ferrite system were linear stripes which maximize the packing of the domains in the film thereby lowering the global energy; whereas the domains in the TbGdFeCo system are branched structures, the shape of which determined by the local conditions and domain wall pinning sites.
䉳 Fig. 2. A sequence of 20 lm MFM images measured in increasing magnetic field: (a) shows a branched reverse domain of the characteristic width, taken at an applied field of !158 Oe; the previous image (taken at !153 Oe) showed no domains, (b) taken at an applied field of !200 Oe, shows the reverse domains throughout the image where the fast magnetization process is nearly complete. In (c) (!455 Oe), the reverse (black) domains are getting wider than the characteristic width and the unreversed (white) domains are now becoming the characteristic width; (d) (!990 Oe) shows all the white domains of the same width; the reversal process continues here via a shortening of the white domains; (e) (!1215 Oe) shows the white line domains significantly decreased in length; some lines have even shrunk to a length which is the characteristic width and have become bubbles. In (f) (!1328 Oe), there are almost no lines left and bubbles appear to be the dominant domain shape. For the images in this figure, the tip magnetization is parallel to the magnetization of the black areas and parallel to the applied field.
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It is interesting to compare the four stages of reversal in barium ferrite suggested by Kooy and Enz in contrast to those found in the present work: (1) The nucleation regime, in which the film goes from a saturated state to a state in which stripes cover the sample, occurs through the nucleation of a reversed bubble. This is immediately followed by domain wall motion where the reversed stripe domains propagate outward from the bubble. These reversed stripes all have the same width which changes very little through the remainder of the magnetization process. It was never possible to observe the reversal event directly, but by viewing the film immediately following reversal, the stripes could be traced back to their origins from which they radiated. Such a spot was deduced to be a nucleating bubble. Although the nucleation process did occur in the TbGdFeCo film, the second experiment showed isolated bubbles existing at zero field (see Fig. 3b) which rules out nucleation as the limiting factor in the reversal process. The reversed domains also maintained a characteristic width which did not significantly change through the reversal process (see Fig. 2e), just as did the stripes in the barium ferrite platelets. (2) Upon further decreasing the applied field, the slow process begins as the unreversed stripe domains reversibly adjust their width adding new reversed stripes (of the characteristic width) as necessary until equal stripe widths are found at zero field. In the TbGdFeCo film, the reversal process did not proceed past ‘nucleation’ by adjusting the width of the reversed domains, but rather by adding domains into the unreversed areas. The difference between the number and magnitude of the domain wall pinning sites of the two films becomes evident here by examining the shapes of the reversed domains. In the TbGdFeCo film, the 䉳 Fig. 3. A sequence of 20 lm MFM images measured in increasing magnetic field after incomplete saturation of the film: (a) (#1300 Oe) shows the bubble domains which have remained; (b) (0 Oe) shows that these bubbles are largely unchanged, but four of them have expanded slightly; (c) (!140 Oe) shows the familiar branching domain structure observed from the previous experiment. For the images in this figure, the magnetization of the tip is anti-parallel to the magnetization of the white areas and anti-parallel to the applied field.
J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 81—88
reversed domains appear branched and fractal in nature (see Fig. 2c and Fig. 2d), in sharp contrast to the linear stripes of the barium ferrite system. (3) The reversal continues until the unreversed areas become the characteristic width. Then, while maintaining that width, these regions decrease in length until they have become bubbles. Similar behavior was observed in the present system (see Fig. 2e and Fig. 2f) at high fields where short segments of domains and isolated bubbles are seen. (4) The reversal finishes as the bubbles are eventually reversed and the film slowly approaches saturation. Again, the high field behavior in the TbGdFeCo is very similar. For the TbGdFeCo film in the regime of the fast magnetization process it is possible to see individual bubble domains before they have grown into a branched domain structure (see Fig. 3). With the film starting in a nearly saturated state, the reversed bubbles persist to the steep part of the hysteresis curve (Fig. 3a and Fig. 3b). This indicates that the reversal barrier of greatest importance is not domain nucleation, but domain growth. This was corroborated by rare observations of bubble domains in the MFM images of the film taken at zero field after the film had been subjected to a 6 kOe field. This indicates that the nucleation field for this film is smaller in magnitude than the field at which the fast process begins. Therefore, the primary barrier to magnetic reversal is associated with domain wall motion. Images in Fig. 2a, Fig. 2b, and Fig. 3c were taken in the fast part of the magnetization reversal. This regime persists to approximately !250 Oe where the second regime begins. An interesting feature of this film is that even though the reversal-limiting process is domain wall motion, it is a fast process. This seems to indicate that there is a relatively small spread in the distribution of domain wall pinning energies. This is seen by the fact that the hysteresis loop has such a sharp corner in both experiments and is very steep immediately after that corner. In the present system, the microscopic loop and the bulk loop are very similar with the only major difference being that the fast process begins at a field value smaller than that of the bulk value. There was observed to be a variation of this field between successive experimental runs which could
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reflect different local values of the domain wall coercive field for different sites on the film. The decreased value of this field could also be due to perturbation of the sample micromagnetic structure by the tip field which would tend to aid the reversal. This was observed for tip orientations both parallel and anti-parallel to the nucleating domains. Even with the added tip field assisting the reversal, the microscopic loop still appears to be a good indicator of the state of the bulk magnetization.
4. Conclusions The magnetic reversal in a perpendicular anisotropy thin film of TbGdFeCo occurs by a two-stage reversal process where the domain nucleation energy distribution is centered about a smaller field than the domain wall pinning energy distribution. Nucleated cylindrical bubble domains can be present in the film up until the domain wall pinning energy is overcome, at which point the fast part of the two-stage reversal begins. The fast process continues until the global energy minimization requirements are met with the additional restrictions imposed by limited domain wall mobility. The domains are branched in nature, but are otherwise similar to those found in films with very small domain wall pinning. The reversal continues by growing one domain type and shrinking the other first to a characteristic width and then to cylindrical bubbles before eliminating them altogether.
Acknowledgements The authors thank William Challener of Imation Corporation for preparation of the thin film samples studied and Matt Dugas for the coated tips used in this work. This work was supported by Imation Corporation and grants CN00014-94-10123 and CN00014-95-1-0799 from the Office of Naval Research. One of the authors (J.S.) would like to thank the University of Minnesota Graduate School for financial support.
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References [1] T. Egami, C.D. Graham Jr., W. Dmowski, P. Zhou, P.J. Flanders, IEEE Trans. Magn. 23 (1987) 2269. [2] W.B. Meiklejohn, F.E. Luborsky, P.G. Frischmann, IEEE Trans. Magn. 23 (1987) 2273. [3] S.-C. Cheng, M.H. Kryder, J. Appl. Phys. 69 (1991) 7202. [4] D. Roy Callaby, R.D. Lorentz, S. Yatsuya, J. Appl. Phys. 75 (1994) 6843. [5] S. Hashimoto, Y. Ochiai, K. Aso, IEEE Trans. Magn. 23 (1987) 2278. [6] C. Kooy, U. Enz, Philips Res. Rep. 15 (1960) 7.
[7] G.V. Sayko, A.K. Zvezdin, T.G. Pokhil, B.S. Vvedensky, E.N. Nikolaev, IEEE Trans. Magn. 28 (1992) 2931. [8] T. Thomson, K. O’Grady, J. Phys. D 30 (1997) 1566. [9] R.J. Celotta, D.T. Pierce, Science 234 (1986) 333. [10] Digital Instruments, Santa Barbara, CA. [11] G.A. Gibson, J.F. Smyth, D.P. Kern, S. Schultz, IEEE Trans. Magn. 27 (1991) 5187. [12] R.D. Gomez, I.D. Mayergoyz, E.R. Burke, IEEE Trans. Magn. 31 (1995) 3346. [13] R. Proksch, E. Runge, P.K. Hansma, S. Foss, B. Walsh, J. Appl. Phys. 78 (1995) 3303.
Magnetization reversal processes in perpendicular anisotropy thin films observed with magnetic force microscopy Jake Schmidt *, George Skidmore , Sheryl Foss, E. Dan Dahlberg , Chris Merton Magnetic Microscopy Center, School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA Imation Corporation, St. Paul, MN 55128, USA
Abstract We have carried out studies of the magnetic reversal process of a rare earth—transition metal thin film with perpendicular magnetic anisotropy using a magnetic force microscope (MFM) capable of applying in situ magnetic fields. The magnetization of the microscopic area shown in MFM images was determined for a number of field values comprising a complete hysteresis loop. This microscopic hysteresis loop was found to be nearly identical to a bulk hysteresis loop. Changes in the magnetization of the film as the hysteresis loop was traversed can be linked to individual microscopic domain changes evident in the MFM images. These studies show that the magnetization of this film was characterized by a two-stage process — fast and slow rates of change of magnetization with applied field. A second experiment in which the film was incompletely saturated and brought back to zero field showed that domain nucleation was not responsible for the rate of the fast process, but rather all magnetization changes were primarily limited by the low domain wall mobility. These observations are linked to previous work on magnetization processes in similar magnetic systems. 1998 Elsevier Science B.V. All rights reserved. Keywords: Magnetic reversal; Thin films; Domain wall motion
1. Introduction Perpendicular rare earth—transition metal (RE— TM) thin films are a subject of great interest due to their use as magneto-optic recording media. These films are typically deposited by sputtering which results in an amorphous microstructure [1,2].
* Corresponding author. Fax: #1 612 624 4578; e-mail: [email protected]
The usefulness of these films is due in part to their high perpendicular anisotropy, reported as high as 3;10 ergs/cc [3,4]. The origin of this anisotropy is still debated; inverse magnetostriction [3,5], shape anisotropy from columnar growth or voids [4], single-ion anisotropy [1], and pair ordering or exchange anisotropy [2] are proposed contributing mechanisms. The domain structures in unsaturated perpendicular films, depending on the amount of domain wall pinning, will vary from linear stripes [6] to highly convoluted fractal patterns [7].
0304-8853/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 2 7 7 - 7
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Experimental investigation of the magnetization reversal process in a perpendicular thin film was first reported by Kooy and Enz in which the reversal of a barium ferrite platelet was studied with Faraday microscopy and parts of the hysteresis loop were correlated with general microscopic changes in the domain pattern [6]. There it was observed that the application of a sufficiently large magnetic field perpendicular to the sample plane saturated the specimen. Decreasing the field eventually resulted in the nucleation of a reverse domain in a microscopic area of the film. The nucleation field was not the same for each experimental run since the nucleation sites were in different locations on the film and local inhomogeneities of the specimen undoubtedly existed. The nucleation event led to an irreversible, fast process in which the reversed domain, after nucleating at a point, grew by domain wall propagation throughout the film. Immediately after overcoming this nucleation barrier, the entire film consisted of stripe domains of two different widths. The reversal continues through the slow, reversible process marked by changes in the relative fractions of the two domain types. Their theoretical model involving the minimization of the sum of the magnetostatic and domain wall energies in the appropriate applied field explained the measured hysteresis loop quite well. The magnetization reversal process in RE—TM films is controlled by two coercivities [8]: the domain nucleation coercivity and the coercivity of domain wall motion. Investigations into these two coercivities or energies have been done using two types of remanence curves, the isothermal remanence curve (IRM) and the demagnetization remanence curve (DCD). These curves can show whether the limiting process to reversal is due to domain nucleation or domain wall motion; these curves also give the distribution of energies associated with each process. By examination of the magnetic microstructure at different stages of the reversal process, understanding of the mechanisms governing the reversal can be gained. Here we report the results of an experiment in which we studied the microscopic magnetization changes of a RE—TM thin film with low domain wall mobility, correlating changes in the magnetic structure on a micron to sub-micron scale to
changes in the bulk magnetization curve. Correlations between micromagnetic structure and bulk magnetic behavior are essential for a complete understanding of the magnetization process, and have been done in other systems [9]. We observe that for the TbGdFeCo film studied, the microscopic hysteresis loop is almost identical to the bulk hysteresis loop which has a form similar to that described by Kooy and Enz, but with a much higher field at which the onset of the fast process occurs. For this film, we show that the fast process, though very steep, is governed not by domain nucleation, but by domain wall motion. The distribution of coercivities to domain wall motion is thus relatively narrow, since magnetization reversal occurs abruptly once a certain field is exceeded. The slow process also shows many similarities to the work of Kooy and Enz; the mode of introduction of new linear domains, the characteristic widths of these domains, and the mode of disappearance of these domains are shared by both systems.
2. Experiment The experiments were performed on a 580 As thick amorphous thin film of Tb Gd Fe Co encapsulated in 90 As of SiC and sputter-coated on a Si wafer. A bulk hysteresis loop, perpendicular to the plane of the film (Fig. 1), was obtained using an alternating gradient force magnetometer (AGFM) showing a coercive field, H , of 300 Oe and a satu ration magnetization, M , of 173 emu/cc. This per pendicular loop was square, with the ratio of the remanent magnetization to the saturation magnetization, M /M , approximately equal to 1, indicat ing that the sample should have an approximately uniform magnetization in zero field after exposure to a field sufficient to saturate it. The sample was imaged with tapping/lift2+ mode using a Digital Instruments Dimension 3000 scanning probe microscope [10] equipped with phase detection. We used 225 lm long microfabricated silicon cantilevers with integrated tips obtained commercially and sputtered with 150 As of Cr and 150 As of CoCr. This coating was observed to be considerably less perturbative to the sample magnetization than a tip with a 450 As thick coating of
J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 81—88
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Fig. 1. Magnetization versus applied magnetic field for the TbGdFeCo film. The bulk hysteresis loop, shown by the thick line, was measured on an alternating force gradient magnetometer. The curve plotted by the triangles was obtained by plotting the fractional number of black pixels on a threshold 20 lm MFM image versus field, after the sample had been initially exposed to a 6 kOe magnetic field. The squares represent a similar MFM experiment, except that the sample was not fully saturated prior to the application of a reversing magnetic field.
CoCr. The tips used in this work were always magnetized perpendicular to the sample surface. Our microscope was modified so that a large electromagnet could be placed under the sample and scanning probe. This electromagnet can produce a maximum field of $2 kOe. The field was observed to be uniform to 1% over a 1 cm diameter area in the center of the rod. The sample was placed on a mount which insulates the sample from any vibrations of the magnet. Using this apparatus, in situ microscopic hysteresis loops can be performed on the film as described below. Other experiments have also been performed using MFM with applied field capability as well [11—13]. Since the film has a uniaxial perpendicular anisotropy, the domains in the film are oriented either into or out of the plane and are either parallel or anti-parallel to the tip magnetization. They appear in the MFM images as areas of different contrast,
dark or light, as seen in Figs. 2 and 3. Image processing can be applied to the images of these domains by imposing a threshold on the gray scale of the image, mapping all pixels with values above a certain cutoff to black, and those below the cutoff to white. Then, for a given domain structure, the relative amounts of each domain orientation may be calculated by simply counting the fractional number of pixels corresponding to each domain type. This fractional value is related to the magnetization of the scanned area. By repeating this process at different field values, a hysteresis loop of a microscopic area can be done. The loops are initiated by setting the electromagnet at 2 kOe and holding a permanent magnet (B &4 kG) directly above it, thus saturating the film in a field of 6 kOe. After removing the permanent magnet, the film can then be imaged as a function of field for values starting at 2000 Oe and decreasing through zero to
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J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 81—88
!2000 Oe, so that major hysteresis loops can be performed on this sample. Microscopic hysteresis loops were performed and one is shown in Fig. 1. The threshold chosen for all images in this series was halfway between the maximum and the minimum allowed data values. The shape of the microscopic loop is very similar to the bulk loop, the only difference being that the fast magnetization process occurs at a smaller negative field for the microscopic loop than for the bulk loop. There is also some deviation from the bulk behavior at larger negative fields, but at this point in the hysteresis loop the domains which still remained were quite small and the noise was fractionally much greater than for lower field values, so it is believed that this deviation is spurious, and could be eliminated by filtering and manipulation of the threshold. Although the figure shows only half of a microscopic loop, complete loops were taken and the hysteresis loop was symmetric with respect to applied field. A second experiment was performed in which the film was subjected to a 2000 Oe field and then imaged. According to the bulk hysteresis loop, the film should be saturated at this field. However, a few isolated bubble domains remained, indicating that the film was not entirely saturated. The field was then decreased to 1300 Oe, and images showed that those bubbles were unchanged. At 0 Oe, images showed that the majority of the bubbles in the image were in the same locations as in previous images, but several bubbles had begun to form a branched domain structure similar to the previous experiment, although with a limited spatial extent. As the field was decreased further in small steps, the images showed growth of the branch domain structures until significant reversal occurred. Applying the same image processing tech-
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niques discussed above yielded essentially the beginning of a minor hysteresis loop. There was a steep change in the magnetization similar to that of the bulk and MFM major hysteresis loops at approximately the same applied field value as in the previous experiment. This experiment was then repeated with negative field values to see if the bubble domains which remained at !2000 Oe were in the same location as those which remained at 2000 Oe; they were not. The field was increased in the same manner as above, and the steep change in magnetization with field was observed again, at approximately the same magnitude of field as before.
3. Analysis The study by Kooy and Enz [6] of the perpendicular magnetization process of single-crystal barium ferrite platelets is a system dissimilar to the present system in a number of ways. Because of the extreme uniformity of the platelets, the domain wall pinning energy was close to zero making the entire system a global energy minimization problem. In the present system, the films are inhomogenous with voids between amorphous columns of material [4]. It is likely that the local magnetization and anisotropy vary such that the propagation of a domain is governed by local and not global conditions. This can be seen by comparison of the domain structures: the domains in the barium ferrite system were linear stripes which maximize the packing of the domains in the film thereby lowering the global energy; whereas the domains in the TbGdFeCo system are branched structures, the shape of which determined by the local conditions and domain wall pinning sites.
䉳 Fig. 2. A sequence of 20 lm MFM images measured in increasing magnetic field: (a) shows a branched reverse domain of the characteristic width, taken at an applied field of !158 Oe; the previous image (taken at !153 Oe) showed no domains, (b) taken at an applied field of !200 Oe, shows the reverse domains throughout the image where the fast magnetization process is nearly complete. In (c) (!455 Oe), the reverse (black) domains are getting wider than the characteristic width and the unreversed (white) domains are now becoming the characteristic width; (d) (!990 Oe) shows all the white domains of the same width; the reversal process continues here via a shortening of the white domains; (e) (!1215 Oe) shows the white line domains significantly decreased in length; some lines have even shrunk to a length which is the characteristic width and have become bubbles. In (f) (!1328 Oe), there are almost no lines left and bubbles appear to be the dominant domain shape. For the images in this figure, the tip magnetization is parallel to the magnetization of the black areas and parallel to the applied field.
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It is interesting to compare the four stages of reversal in barium ferrite suggested by Kooy and Enz in contrast to those found in the present work: (1) The nucleation regime, in which the film goes from a saturated state to a state in which stripes cover the sample, occurs through the nucleation of a reversed bubble. This is immediately followed by domain wall motion where the reversed stripe domains propagate outward from the bubble. These reversed stripes all have the same width which changes very little through the remainder of the magnetization process. It was never possible to observe the reversal event directly, but by viewing the film immediately following reversal, the stripes could be traced back to their origins from which they radiated. Such a spot was deduced to be a nucleating bubble. Although the nucleation process did occur in the TbGdFeCo film, the second experiment showed isolated bubbles existing at zero field (see Fig. 3b) which rules out nucleation as the limiting factor in the reversal process. The reversed domains also maintained a characteristic width which did not significantly change through the reversal process (see Fig. 2e), just as did the stripes in the barium ferrite platelets. (2) Upon further decreasing the applied field, the slow process begins as the unreversed stripe domains reversibly adjust their width adding new reversed stripes (of the characteristic width) as necessary until equal stripe widths are found at zero field. In the TbGdFeCo film, the reversal process did not proceed past ‘nucleation’ by adjusting the width of the reversed domains, but rather by adding domains into the unreversed areas. The difference between the number and magnitude of the domain wall pinning sites of the two films becomes evident here by examining the shapes of the reversed domains. In the TbGdFeCo film, the 䉳 Fig. 3. A sequence of 20 lm MFM images measured in increasing magnetic field after incomplete saturation of the film: (a) (#1300 Oe) shows the bubble domains which have remained; (b) (0 Oe) shows that these bubbles are largely unchanged, but four of them have expanded slightly; (c) (!140 Oe) shows the familiar branching domain structure observed from the previous experiment. For the images in this figure, the magnetization of the tip is anti-parallel to the magnetization of the white areas and anti-parallel to the applied field.
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reversed domains appear branched and fractal in nature (see Fig. 2c and Fig. 2d), in sharp contrast to the linear stripes of the barium ferrite system. (3) The reversal continues until the unreversed areas become the characteristic width. Then, while maintaining that width, these regions decrease in length until they have become bubbles. Similar behavior was observed in the present system (see Fig. 2e and Fig. 2f) at high fields where short segments of domains and isolated bubbles are seen. (4) The reversal finishes as the bubbles are eventually reversed and the film slowly approaches saturation. Again, the high field behavior in the TbGdFeCo is very similar. For the TbGdFeCo film in the regime of the fast magnetization process it is possible to see individual bubble domains before they have grown into a branched domain structure (see Fig. 3). With the film starting in a nearly saturated state, the reversed bubbles persist to the steep part of the hysteresis curve (Fig. 3a and Fig. 3b). This indicates that the reversal barrier of greatest importance is not domain nucleation, but domain growth. This was corroborated by rare observations of bubble domains in the MFM images of the film taken at zero field after the film had been subjected to a 6 kOe field. This indicates that the nucleation field for this film is smaller in magnitude than the field at which the fast process begins. Therefore, the primary barrier to magnetic reversal is associated with domain wall motion. Images in Fig. 2a, Fig. 2b, and Fig. 3c were taken in the fast part of the magnetization reversal. This regime persists to approximately !250 Oe where the second regime begins. An interesting feature of this film is that even though the reversal-limiting process is domain wall motion, it is a fast process. This seems to indicate that there is a relatively small spread in the distribution of domain wall pinning energies. This is seen by the fact that the hysteresis loop has such a sharp corner in both experiments and is very steep immediately after that corner. In the present system, the microscopic loop and the bulk loop are very similar with the only major difference being that the fast process begins at a field value smaller than that of the bulk value. There was observed to be a variation of this field between successive experimental runs which could
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reflect different local values of the domain wall coercive field for different sites on the film. The decreased value of this field could also be due to perturbation of the sample micromagnetic structure by the tip field which would tend to aid the reversal. This was observed for tip orientations both parallel and anti-parallel to the nucleating domains. Even with the added tip field assisting the reversal, the microscopic loop still appears to be a good indicator of the state of the bulk magnetization.
4. Conclusions The magnetic reversal in a perpendicular anisotropy thin film of TbGdFeCo occurs by a two-stage reversal process where the domain nucleation energy distribution is centered about a smaller field than the domain wall pinning energy distribution. Nucleated cylindrical bubble domains can be present in the film up until the domain wall pinning energy is overcome, at which point the fast part of the two-stage reversal begins. The fast process continues until the global energy minimization requirements are met with the additional restrictions imposed by limited domain wall mobility. The domains are branched in nature, but are otherwise similar to those found in films with very small domain wall pinning. The reversal continues by growing one domain type and shrinking the other first to a characteristic width and then to cylindrical bubbles before eliminating them altogether.
Acknowledgements The authors thank William Challener of Imation Corporation for preparation of the thin film samples studied and Matt Dugas for the coated tips used in this work. This work was supported by Imation Corporation and grants CN00014-94-10123 and CN00014-95-1-0799 from the Office of Naval Research. One of the authors (J.S.) would like to thank the University of Minnesota Graduate School for financial support.
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References [1] T. Egami, C.D. Graham Jr., W. Dmowski, P. Zhou, P.J. Flanders, IEEE Trans. Magn. 23 (1987) 2269. [2] W.B. Meiklejohn, F.E. Luborsky, P.G. Frischmann, IEEE Trans. Magn. 23 (1987) 2273. [3] S.-C. Cheng, M.H. Kryder, J. Appl. Phys. 69 (1991) 7202. [4] D. Roy Callaby, R.D. Lorentz, S. Yatsuya, J. Appl. Phys. 75 (1994) 6843. [5] S. Hashimoto, Y. Ochiai, K. Aso, IEEE Trans. Magn. 23 (1987) 2278. [6] C. Kooy, U. Enz, Philips Res. Rep. 15 (1960) 7.
[7] G.V. Sayko, A.K. Zvezdin, T.G. Pokhil, B.S. Vvedensky, E.N. Nikolaev, IEEE Trans. Magn. 28 (1992) 2931. [8] T. Thomson, K. O’Grady, J. Phys. D 30 (1997) 1566. [9] R.J. Celotta, D.T. Pierce, Science 234 (1986) 333. [10] Digital Instruments, Santa Barbara, CA. [11] G.A. Gibson, J.F. Smyth, D.P. Kern, S. Schultz, IEEE Trans. Magn. 27 (1991) 5187. [12] R.D. Gomez, I.D. Mayergoyz, E.R. Burke, IEEE Trans. Magn. 31 (1995) 3346. [13] R. Proksch, E. Runge, P.K. Hansma, S. Foss, B. Walsh, J. Appl. Phys. 78 (1995) 3303.