magnetization reversal processes in perpendicular anisotropy thin films observed with magnetic force...

*Corresponding author. Fax: #1 612 624 4578; e-mail:[email protected]

Journal of Magnetism and Magnetic Materials 190 (1998) 81—88

Magnetization reversal processes in perpendicular anisotropythin 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 withperpendicular 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 valuescomprising a complete hysteresis loop. This microscopic hysteresis loop was found to be nearly identical to a bulkhysteresis loop. Changes in the magnetization of the film as the hysteresis loop was traversed can be linked to individualmicroscopic domain changes evident in the MFM images. These studies show that the magnetization of this film wascharacterized by a two-stage process — fast and slow rates of change of magnetization with applied field. A secondexperiment in which the film was incompletely saturated and brought back to zero field showed that domain nucleationwas not responsible for the rate of the fast process, but rather all magnetization changes were primarily limited by the lowdomain wall mobility. These observations are linked to previous work on magnetization processes in similar magneticsystems. ( 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 totheir use as magneto-optic recording media. Thesefilms are typically deposited by sputtering whichresults in an amorphous microstructure [1,2].

The usefulness of these films is due in part to theirhigh perpendicular anisotropy, reported as high as3]106 ergs/cc [3,4]. The origin of this anisotropyis still debated; inverse magnetostriction [3,5],shape anisotropy from columnar growth or voids[4], single-ion anisotropy [1], and pair ordering orexchange anisotropy [2] are proposed contributingmechanisms. The domain structures in unsaturatedperpendicular films, depending on the amount ofdomain 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

Experimental investigation of the magnetizationreversal process in a perpendicular thin film wasfirst reported by Kooy and Enz in which the rever-sal of a barium ferrite platelet was studied withFaraday microscopy and parts of the hysteresisloop were correlated with general microscopicchanges in the domain pattern [6]. There it wasobserved that the application of a sufficiently largemagnetic field perpendicular to the sample planesaturated the specimen. Decreasing the field event-ually resulted in the nucleation of a reverse domainin a microscopic area of the film. The nucleationfield was not the same for each experimental runsince the nucleation sites were in different locationson the film and local inhomogeneities of the speci-men undoubtedly existed. The nucleation event ledto an irreversible, fast process in which the reverseddomain, after nucleating at a point, grew by do-main wall propagation throughout the film. Im-mediately after overcoming this nucleation barrier,the entire film consisted of stripe domains of twodifferent widths. The reversal continues through theslow, reversible process marked by changes in therelative fractions of the two domain types. Theirtheoretical model involving the minimization of thesum of the magnetostatic and domain wall energiesin the appropriate applied field explained the mea-sured hysteresis loop quite well.

The magnetization reversal process in RE—TMfilms is controlled by two coercivities [8]: the do-main nucleation coercivity and the coercivity ofdomain wall motion. Investigations into thesetwo coercivities or energies have been done usingtwo types of remanence curves, the isothermal re-manence curve (IRM) and the demagnetizationremanence curve (DCD). These curves can showwhether the limiting process to reversal is due todomain nucleation or domain wall motion; thesecurves also give the distribution of energies asso-ciated with each process. By examination of themagnetic microstructure at different stages of thereversal process, understanding of the mechanismsgoverning the reversal can be gained.

Here we report the results of an experiment inwhich we studied the microscopic magnetizationchanges of a RE—TM thin film with low domainwall mobility, correlating changes in the magneticstructure on a micron to sub-micron scale to

changes in the bulk magnetization curve. Correla-tions between micromagnetic structure and bulkmagnetic behavior are essential for a complete un-derstanding of the magnetization process, and havebeen done in other systems [9]. We observe that forthe TbGdFeCo film studied, the microscopic hys-teresis loop is almost identical to the bulk hysteresisloop which has a form similar to that described byKooy and Enz, but with a much higher field atwhich the onset of the fast process occurs. For thisfilm, we show that the fast process, though verysteep, is governed not by domain nucleation, but bydomain wall motion. The distribution of coercivi-ties to domain wall motion is thus relatively nar-row, since magnetization reversal occurs abruptlyonce a certain field is exceeded. The slow processalso shows many similarities to the work of Kooyand Enz; the mode of introduction of new lineardomains, the characteristic widths of these do-mains, and the mode of disappearance of thesedomains are shared by both systems.

2. Experiment

The experiments were performed on a 580 Asthick amorphous thin film of Tb

11Gd

21Fe

65Co

3encapsulated in 90 As of SiC and sputter-coated ona Si wafer. A bulk hysteresis loop, perpendicular tothe plane of the film (Fig. 1), was obtained using analternating gradient force magnetometer (AGFM)showing a coercive field, H

#, of 300 Oe and a satu-

ration magnetization, M4, of 173 emu/cc. This per-

pendicular loop was square, with the ratio of theremanent magnetization to the saturation magnet-ization, M

3/M

4, approximately equal to 1, indicat-

ing that the sample should have an approximatelyuniform magnetization in zero field after exposureto a field sufficient to saturate it.

The sample was imaged with tapping/liftTM modeusing a Digital Instruments Dimension 3000 scan-ning probe microscope [10] equipped with phasedetection. We used 225 lm long microfabricatedsilicon cantilevers with integrated tips obtainedcommercially and sputtered with 150 As of Cr and150 As of CoCr. This coating was observed to beconsiderably less perturbative to the sample mag-netization than a tip with a 450 As thick coating of

82 J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 81—88

Fig. 1. Magnetization versus applied magnetic field for the TbGdFeCo film. The bulk hysteresis loop, shown by the thick line, wasmeasured on an alternating force gradient magnetometer. The curve plotted by the triangles was obtained by plotting the fractionalnumber of black pixels on a threshold 20 lm MFM image versus field, after the sample had been initially exposed to a 6 kOe magneticfield. The squares represent a similar MFM experiment, except that the sample was not fully saturated prior to the application ofa reversing magnetic field.

CoCr. The tips used in this work were alwaysmagnetized perpendicular to the sample surface.Our microscope was modified so that a large elec-tromagnet could be placed under the sample andscanning probe. This electromagnet can producea maximum field of $2 kOe. The field was ob-served to be uniform to 1% over a 1 cm diameterarea in the center of the rod. The sample was placedon a mount which insulates the sample from anyvibrations of the magnet. Using this apparatus, insitu microscopic hysteresis loops can be performedon the film as described below. Other experimentshave also been performed using MFM with appliedfield capability as well [11—13].

Since the film has a uniaxial perpendicular an-isotropy, the domains in the film are oriented eitherinto or out of the plane and are either parallel oranti-parallel to the tip magnetization. They appearin the MFM images as areas of different contrast,

dark or light, as seen in Figs. 2 and 3. Imageprocessing can be applied to the images of thesedomains by imposing a threshold on the gray scaleof the image, mapping all pixels with values abovea certain cutoff to black, and those below the cutoffto white. Then, for a given domain structure, therelative amounts of each domain orientation maybe calculated by simply counting the fractionalnumber of pixels corresponding to each domaintype. This fractional value is related to the magnet-ization of the scanned area. By repeating this pro-cess at different field values, a hysteresis loop ofa microscopic area can be done. The loops areinitiated by setting the electromagnet at 2 kOe andholding a permanent magnet (B

3&4 kG) directly

above it, thus saturating the film in a field of 6 kOe.After removing the permanent magnet, the film canthen be imaged as a function of field for valuesstarting at 2000 Oe and decreasing through zero to

J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 81—88 83

b

Fig. 2. A sequence of 20 lm MFM images measured in increasing magnetic field: (a) shows a branched reverse domain of thecharacteristic width, taken at an applied field of !158 Oe; the previous image (taken at !153 Oe) showed no domains, (b) taken at anapplied 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 nowbecoming the characteristic width; (d) (!990 Oe) shows all the white domains of the same width; the reversal process continues here viaa shortening of the white domains; (e) (!1215 Oe) shows the white line domains significantly decreased in length; some lines have evenshrunk to a length which is the characteristic width and have become bubbles. In (f) (!1328 Oe), there are almost no lines left andbubbles appear to be the dominant domain shape. For the images in this figure, the tip magnetization is parallel to the magnetization ofthe black areas and parallel to the applied field.

!2000 Oe, so that major hysteresis loops can beperformed on this sample.

Microscopic hysteresis loops were performedand one is shown in Fig. 1. The threshold chosenfor all images in this series was halfway between themaximum and the minimum allowed data values.The shape of the microscopic loop is very similar tothe bulk loop, the only difference being that the fastmagnetization process occurs at a smaller negativefield for the microscopic loop than for the bulkloop. There is also some deviation from the bulkbehavior at larger negative fields, but at this pointin the hysteresis loop the domains which still re-mained were quite small and the noise was fraction-ally much greater than for lower field values, so it isbelieved that this deviation is spurious, and couldbe eliminated by filtering and manipulation of thethreshold. Although the figure shows only half ofa microscopic loop, complete loops were taken andthe hysteresis loop was symmetric with respect toapplied field.

A second experiment was performed in which thefilm was subjected to a 2000 Oe field and thenimaged. According to the bulk hysteresis loop, thefilm should be saturated at this field. However,a few isolated bubble domains remained, indicatingthat the film was not entirely saturated. The fieldwas then decreased to 1300 Oe, and images showedthat those bubbles were unchanged. At 0 Oe, im-ages showed that the majority of the bubbles in theimage were in the same locations as in previousimages, but several bubbles had begun to forma branched domain structure similar to the pre-vious experiment, although with a limited spatialextent. As the field was decreased further in smallsteps, the images showed growth of the branchdomain structures until significant reversal occur-red. Applying the same image processing tech-

niques discussed above yielded essentially the be-ginning of a minor hysteresis loop. There wasa steep change in the magnetization similar to thatof the bulk and MFM major hysteresis loops atapproximately the same applied field value as in theprevious experiment. This experiment was then re-peated with negative field values to see if the bubbledomains which remained at !2000 Oe were inthe same location as those which remained at2000 Oe; they were not. The field was increased inthe same manner as above, and the steep changein magnetization with field was observed again,at approximately the same magnitude of field asbefore.

3. Analysis

The study by Kooy and Enz [6] of the perpen-dicular magnetization process of single-crystal bar-ium ferrite platelets is a system dissimilar to thepresent system in a number of ways. Because of theextreme uniformity of the platelets, the domain wallpinning energy was close to zero making the entiresystem a global energy minimization problem. Inthe present system, the films are inhomogenouswith voids between amorphous columns of mater-ial [4]. It is likely that the local magnetizationand anisotropy vary such that the propagationof a domain is governed by local and not globalconditions. This can be seen by comparison of thedomain structures: the domains in the bariumferrite system were linear stripes which maximizethe packing of the domains in the film therebylowering the global energy; whereas the domainsin the TbGdFeCo system are branched structures,the shape of which determined by the local condi-tions and domain wall pinning sites.


b

Fig. 3. A sequence of 20 lm MFM images measured in increas-ing 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, butfour of them have expanded slightly; (c) (!140 Oe) shows thefamiliar branching domain structure observed from the previousexperiment. For the images in this figure, the magnetization ofthe tip is anti-parallel to the magnetization of the white areasand anti-parallel to the applied field.

It is interesting to compare the four stages ofreversal in barium ferrite suggested by Kooy andEnz in contrast to those found in the present work:

(1) The nucleation regime, in which the film goesfrom a saturated state to a state in which stripescover the sample, occurs through the nucleation ofa reversed bubble. This is immediately followed bydomain wall motion where the reversed stripe do-mains propagate outward from the bubble. Thesereversed stripes all have the same width whichchanges very little through the remainder of themagnetization process. It was never possible toobserve the reversal event directly, but by viewingthe film immediately following reversal, the stripescould be traced back to their origins from whichthey radiated. Such a spot was deduced to be a nu-cleating bubble. Although the nucleation processdid occur in the TbGdFeCo film, the second experi-ment showed isolated bubbles existing at zero field(see Fig. 3b) which rules out nucleation as thelimiting factor in the reversal process. The reverseddomains also maintained a characteristic widthwhich did not significantly change through the re-versal process (see Fig. 2e), just as did the stripes inthe barium ferrite platelets.

(2) Upon further decreasing the applied field, theslow process begins as the unreversed stripe do-mains reversibly adjust their width adding newreversed stripes (of the characteristic width) as ne-cessary until equal stripe widths are found at zerofield. In the TbGdFeCo film, the reversal processdid not proceed past ‘nucleation’ by adjustingthe width of the reversed domains, but rather byadding domains into the unreversed areas. Thedifference between the number and magnitude ofthe domain wall pinning sites of the two filmsbecomes evident here by examining the shapes ofthe reversed domains. In the TbGdFeCo film, the


reversed domains appear branched and fractal innature (see Fig. 2c and Fig. 2d), in sharp contrast tothe linear stripes of the barium ferrite system.

(3) The reversal continues until the unreversedareas become the characteristic width. Then, whilemaintaining that width, these regions decrease inlength until they have become bubbles. Similarbehavior was observed in the present system (seeFig. 2e and Fig. 2f) at high fields where shortsegments of domains and isolated bubbles are seen.

(4) The reversal finishes as the bubbles are event-ually reversed and the film slowly approachessaturation. Again, the high field behavior in theTbGdFeCo is very similar.

For the TbGdFeCo film in the regime of the fastmagnetization process it is possible to see indi-vidual bubble domains before they have grown intoa branched domain structure (see Fig. 3). With thefilm starting in a nearly saturated state, the reversedbubbles persist to the steep part of the hysteresiscurve (Fig. 3a and Fig. 3b). This indicates that thereversal barrier of greatest importance is not do-main nucleation, but domain growth. This wascorroborated by rare observations of bubble do-mains in the MFM images of the film taken at zerofield after the film had been subjected to a 6 kOefield. This indicates that the nucleation field for thisfilm is smaller in magnitude than the field at whichthe fast process begins. Therefore, the primary bar-rier to magnetic reversal is associated with domainwall motion. Images in Fig. 2a, Fig. 2b, and Fig. 3cwere taken in the fast part of the magnetizationreversal. This regime persists to approxi-mately !250 Oe where the second regime begins.An interesting feature of this film is that eventhough the reversal-limiting process is domain wallmotion, it is a fast process. This seems to indicatethat there is a relatively small spread in the distri-bution of domain wall pinning energies. This is seenby the fact that the hysteresis loop has such a sharpcorner in both experiments and is very steepimmediately after that corner.

In the present system, the microscopic loop andthe bulk loop are very similar with the only majordifference being that the fast process begins ata field value smaller than that of the bulk value.There was observed to be a variation of this fieldbetween successive experimental runs which could

reflect different local values of the domain wallcoercive field for different sites on the film. Thedecreased value of this field could also be due toperturbation of the sample micromagnetic struc-ture by the tip field which would tend to aid thereversal. This was observed for tip orientationsboth parallel and anti-parallel to the nucleatingdomains. Even with the added tip field assisting thereversal, the microscopic loop still appears to bea good indicator of the state of the bulk magneti-zation.

4. Conclusions

The magnetic reversal in a perpendicular anisot-ropy thin film of TbGdFeCo occurs by a two-stagereversal process where the domain nucleation en-ergy distribution is centered about a smaller fieldthan the domain wall pinning energy distribution.Nucleated cylindrical bubble domains can be pres-ent in the film up until the domain wall pinningenergy is overcome, at which point the fast part ofthe two-stage reversal begins. The fast process con-tinues until the global energy minimization require-ments are met with the additional restrictionsimposed by limited domain wall mobility. The do-mains are branched in nature, but are otherwisesimilar to those found in films with very smalldomain wall pinning. The reversal continues bygrowing one domain type and shrinking the otherfirst to a characteristic width and then to cylin-drical bubbles before eliminating them altogether.

Acknowledgements

The authors thank William Challener of ImationCorporation for preparation of the thin film sam-ples studied and Matt Dugas for the coated tipsused in this work. This work was supported byImation Corporation and grants dN00014-94-1-0123 and dN00014-95-1-0799 from the Office ofNaval Research. One of the authors (J.S.) wouldlike to thank the University of Minnesota Grad-uate School for financial support.


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, IEEETrans. 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...

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