*Corresponding author. Fax: #1 612 624 4578; e-mail:schm0127@gold.tc.umn.edu.

Journal of Magnetism and Magnetic Materials 190 (1998) 108—113

Localized magnetostrictive magnetization reversal usingscanning probe tips

Jake Schmidt!,*, Sheryl Foss", George D. Skidmore!, 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 55414, USA

Abstract

Localized magnetic reversal of a perpendicular anisotropy thin film has been performed using the magnetostrictiveresponse of the film to a force applied by probe tips of scanning force microscope cantilevers. Non-magnetic andmagnetic cantilever tips were used to apply local stresses which alter the local magnetization through magnetostriction.The magnetic field of the tip, if any, and the local demagnetizing field of the film reverse the stressed area for stresses exceedinga critical value. These findings were in agreement with a simple model. ( 1998 Elsevier Science B.V. All rights reserved.

Keywords: Magnetization reversal; Magnetostriction; Anisotropy constant

1. Introduction

The magnetic force microscope (MFM) employsa sharp magnetic tip scanned in close proximity toa surface to image micromagnetic structures withlateral resolution down to tens of nanometers [1].The magnetic field of an MFM tip, considereda bane to the understanding of conventional MFMimages, has been used to locally manipulate themagnetization of several samples [2—5,12]. Alongthe same avenue, the force microscope tip has beenused to alter the magnetization of a specimenthrough magnetostriction [6]. In the work byManalis and coworkers on a similar system to that

described here [5], these effects were not con-sidered. In the present work, local stress on a mag-netic thin film applied with a scanning probe tipprovided nucleation sites which, when combinedwith the local demagnetizing field, caused the mag-netization to reverse in the vicinity of the contactpoint. This can be understood as a local change inthe effective anisotropy constant of the material,since the magnetostriction energy has the samefunctional dependence on the angle of the magnet-ization with the preferred axis as the magneto-crystalline anisotropy energy.

2. Experimental

The sample investigated in this work was anamorphous Tb

17Fe

66Co

6Gd

11alloy film 540 As

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 5 - 3

Fig. 1. A hysteresis loop of the film taken normal to the filmplane. The loop is square, indicative of a strong perpendicularanisotropy. The coercivity and saturation magnetization ob-tained from this loop were 920 Oe and 130 emu/cc, respectively.A separate hysteresis loop with the field applied in the plane ofthe sample gave a measure of the anisotropy constant of7.4]105 erg/cc.

thick. This rare-earth/transition metal (RE/TM)film, encapsulated with 90 As thick SiC layers, wassputter-deposited onto a silicon wafer at room tem-perature. The magnetic properties of the film weremeasured with an alternating gradient force mag-netometer. From a magnetic hysteresis loop per-pendicular to the film plane (seen in Fig. 1) thecoercivity, H

#, was found to be approximately

920 Oe, the saturation magnetization, M4, was

130 emu/cc and the ratio of the remanent magneti-zation to the saturation magnetization, M

3/M

4, was

nearly 1. This implies that the film will have a uni-formly magnetized remanent state after a saturat-ing field perpendicular to the film plane has beenapplied. A measure of the uniaxial anisotropyconstant of 7.4]105 erg/cc was obtained froman in-plane hysteresis loop of the sample. Thisyields a ‘quality factor’, Q"K/2pM2

4, of 7.1, consis-

tent with magnetization perpendicular to the filmplane.

The experiments were performed using a Dimen-sion 3000 scanning probe microscope controlledby a NanoscopeTM IIIa [7]. Cantilevers of length225 lm, sputter-coated with a 15 nm CoCr thinfilm, were used for imaging. For force application(described below) 125 lm long uncoated cantileverswere used as well as coated and uncoated 225 lmlong cantilevers. The coated tips were magnetizedapproximately perpendicular to the sample plane

and anti-parallel to the bulk film sample magneti-zation for both the imaging and force application.

The sample was first saturated in the field ofa NdFeB permanent magnet (B

3&4 kG). Sub-

sequent MFM images were featureless, i.e., no re-versed domains were observed, as expected fromthe perpendicular hysteresis loop (Fig. 1). Thesample was scanned in contact mode, and the inter-action force between the tip and sample was speci-fied for different experimental runs. Subsequentchanges in the sample magnetic structure were ob-served at locations where the interaction forcebetween the tip and the sample was increasedsignificantly. This was done with both magneticand non-magnetic cantilevers, demonstrating thatthis was a purely magnetostrictive effect. This wasnot surprising considering the high rare-earth con-tent of the films.

The cantilevers were used to apply localized for-ces to the sample by specifying a cantilever deflec-tion signal (via the feedback setpoint) in contactmode. In this mode, electronic feedback is used tomaintain a constant deflection of the cantileverand hence, a constant force. For the experimentsdescribed below, the following procedure wasperformed: the cantilevers were moved to a pre-programmed position above the sample. The tipwas then brought into contact with the sample andpressed in until it reached the specified deflectionvalue. Either the tip was removed at this time (towrite a single dot domain), or the cantilever wasmoved in a straight line for a specified distancebefore it was withdrawn (to write a single linedomain). Subsequently, the cantilever was movedto a new position and the process repeated, ifdesired.

Shown in Fig. 2 is a 7.5 lm square MFM imageof the sample after a writing experiment. This is theresult of moving the tip of a 225 lm long magneticcantilever to a certain point, increasing the setpointuntil a reverse domain nucleated (this setpointvalue was determined after a number of experi-mental trials), and then tracing out the letter ‘M’before reducing the setpoint and moving the tip toanother location. The magnetization of the tipwas parallel to the magnetization of the newlyformed domain and the tip field (&500 Oe atthe location of the sample [8]) certainly aided in

J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 108—113 109

Fig. 2. A 7.5 lm square MFM image of a magnetic domainshaped like the letter ‘M’, made through the magnetostrictionprocess described in the text. The letter is approximately 5.5 lmsquare and the domain comprising it is 300 nm wide. The roughperimeter of the domain is indicative of a large number ofdomain wall pinning sites and a large domain wall pinningenergy.

Fig. 3. A 5.2]8.4 lm MFM scan of ten magnetic domainsproduced by selectively increasing the contact force between thetip and sample at the location of each of the domains. Eachdomain is approximately 300 nm in diameter and all the do-mains appear to have the same characteristic shape. This islikely indicative of the stress profile produced by the tip which isclosely related to the tip shape.

domain formation. The letter is approximately5.5 lm square and the width of the line comprisingthe letter is on average 300 nm. The domain walljaggedness is characteristic of these RE/TM sput-tered thin films due to the large pinning site density[9,13].

After each writing experiment, topographic andmagnetic images were taken of the stressed area.The topographic images showed no alteration ofthe sample surface where the domains were written.In some instances, when uncoated tips were used,fragments of the tips were left on the sample surface(as is discussed in more detail below). Changes inthe tips’ shapes resulting from use can be seen in theSEM images in Fig. 4.

In another experiment, an array of domains waswritten by moving a magnetized tip to a specifiedlocation, increasing the deflection setpoint to a cer-tain amount for a short time (&1 s), reducing thesetpoint and moving to a new location to repeat.

These dot domains all have roughly the same shapewhich is probably the shape of the area stressedbeyond the critical value (discussed below) and isintimately related to the shape of the tip. Thesmallest domain written and observed with thismethod was a circular domain of diameter approx-imately 100 nm, corresponding closely to the size ofthe magnetic tip end.

The array pictured in Fig. 3 was intended to befive rows (each offset towards the right with respectto the previous row starting at the top) of five dotdomains written under identical conditions. How-ever, the writing process did not always produceconsistent results. Much more consistent resultswere obtained when linear domains were attem-pted. We believe that this was the case because oflocal inhomogeneities in the sample. In stressinga linear area, if reversal does not occur at the initialpoint of contact between the tip and sample, the tipwill sample a large area for an extended time so theprobability of reversed domain nucleation is high-er. If a domain is nucleated at any point alonga line, the domain can be enlarged relatively easilyby the tip dragging along the domain wall, andin this way, a line domain can be formed. For thepoint domains, if the reversal does not occur, thedomain will not be written at that location and the

110 J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 108—113

Fig. 4. Six SEM micrographs of the tips used in this experi-ments. Images (a) and (b) show the tip used with the magneticcoating before and after the experiment, respectively. The scalebar is 150 nm for both images. The tip appears no different afterthe experiment than before. Images (c) and (d) show the un-coated tip on the 225 lm cantilever before and after the experi-ment, respectively. The scale bar on both of these images is150 nm as well. The sharp end of the tip has broken off leavinga blunt, flat tip face. Images (e) and (f) show the uncoated tip onthe 125 lm cantilever before and after the experiment, respec-tively. The scale bar of (e) is 150 nm, whereas the scale bar of (f) is600 nm, as a large amount of the tip has been sheared off.

tip is removed from contact and placed in a differ-ent location. This is also consistent with a largenumber of domain wall pinning sites as well, whichcould be the result of local composition inhomo-geneities or defects in the microstructure.

The tips of the cantilevers used in the experimentwere examined in a scanning electron microscope(SEM) before and after the experiments. Examples

of SEM images measured of the different tips usedin this work are shown in Fig. 4. The images in theleft column of the figure were taken before theexperiments were conducted and the images inthe right column were taken afterwards. The tip ofthe cantilever with a magnetic coating (Fig. 4a andFig. 4b) showed significantly less wear than theuncoated cantilevers (Fig. 4c—f). The tip of the can-tilever with the magnetic coating was unchangedwith use, maintaining a tip radius of 50 nm. Whilethe two non-magnetic tips which started with endradii under 10 nm (Fig. 4c and Fig. 4e) ended withflat tip ends of width 200 and 800 nm, respectively(Fig. 4d and Fig. 4f). The tips of the non-magneticcantilevers appear to have been sheared off ata crystal face as the resulting face was quite flat andparallel to the length of the cantilever which isa [1 0 0] plane. The magnetic coating appears tomake the cantilever tip much more robust than theuncoated silicon tip. The last sets of experimentswith the non-magnetic cantilevers showed signs ofthe wear on the tips as the topographic imagestaken after writing displayed features indicative ofparticles left on the sample surface at the locationsof the dot domains at the ends of each line. Regard-less of the type of tip used, the width of the linedomains was on average 300 nm and the dotdomains written at the end of each line by applyingan extremely large force for a significantly longerperiod of time were all &600 nm in diameter.

3. Simple model

A simple physical model of this process consistsof a sphere with magnetocrystalline anisotropywhose magnetization coherently rotates in re-sponse to an external stress or magnetic field (dueto the demagnetizing field of the film and the tipfield, if any). The magnetostatic energy of thesphere in the external field is given by

E&*%-$

"M4(4pM

4#H

5*1) cos h,

where M4

is the saturation magnetization of thefilm, 4pM

4the magnetic field from the rest of

the film, H5*1

the field of the tip at the locationof the sphere, and h the angle the magnetization of

J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 108—113 111

Fig. 5. A magnetic image of an area where an array of lines waswritten with a non-magnetic tip at varying tip forces, increasingfrom bottom to top in the image. A dot domain at the right endof each line was written to mark the line position. At thethreshold for creating a reversed line domain (in this case, at thefourth line attempt in the array), the effective anisotropy con-stant, K

%&&, discussed in the text became equal to 2pM2.

the sphere makes with the original magnetizationdirection normal to the film plane. The anisotropyenergy,

E!/*4

"K6sin2 h,

is assumed to be the standard uniaxial form. Themagnetostriction energy can be written as

E453*#5*0/

"!32j4p cos2 h,

where j4

is the saturation magnetostrictionconstant, p the stress applied to the sphere, andh defined as above [10]. This anisotropy and mag-netostriction energies can be combined to form aneffective anisotropy energy,

E%&&!/*4

"const.#K%&&

sin2 h,

where

K%&&"K

6!3

2j4p,

because p is negative (compressive stress). IfK

%&&'2pM2

4, the sphere will maintain its original

magnetization. However, if K%&&"2pM2

4, there is

no energy barrier opposing magnetization reversal.For the film studied here, a state of uniform mag-netization normal to the film plane is a stableconfiguration at remanence with no stress applied,implying K

%&&'2pM2

4. With stress applied, the ef-

fective anisotropy becomes smaller until it ulti-mately becomes equal to 2pM2 at which pointreversal occurs.

4. Results and discussion

An experimental illustration of the above modelis shown in Fig. 5. Using a relatively short (125 lmlong), non-magnetic cantilever, ten lines werestressed with increasing force as the tip progressedtowards the top of the image area. Scanning incontact, the cantilever movement was as follows:the cantilever was moved to the lower left of theimage area; the deflection was increased to applythe writing force; the cantilever was moved 10 lmto the right, intending to write a single line domain;to mark the location of the line, the deflection wasthen increased to a large amount which almostinvariably wrote a single dot domain; the deflection

was then decreased to its initial value; and thecantilever was moved to new place to start anotherline. The process was then repeated with a slightlyincreased deflection for writing the next line do-main. After the experiment was completed, a canti-lever with magnetic coating was used to image thestressed area, shown in the figure. There are threeround domains at the bottom of the array andseven line domains of slightly varying length andincreased width at the right end (where the setpointwas increased). The setpoint used for each lineincreases from bottom to top. This indicates a de-flection threshold for writing near that used for thefourth line. Below the fourth line no magnetic re-versal occurs and above this it occurs almost entire-ly. At this threshold, K

%&&"2pM2, and since a

non-magnetic cantilever was used, the anisotropybarrier to reversal has been eliminated entirelythrough magnetostriction.

112 J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 108—113

5. Conclusions

A technique has been presented for using mag-netostrictive effects to locally nucleate reversedomains in a saturated perpendicular anisotropythin film. Non-magnetic and magnetic cantilevertips were used to apply localized stresses on a speci-men. The applied stress modifies the local magnet-ization of the sample by magnetostriction. Forsufficiently large stresses, the magnetization in thevicinity of the applied stress will reverse due to thedemagnetizing field. The addition of an externalmagnetic field assists in this reversal. By applyinga series of controlled stresses, the stress thresholdbelow which magnetization reversal did not occur,but above which it did, could be determined. Thistechnique can be extended to perform localmeasurements of the magnetostriction [11].

Acknowledgements

The authors gratefully acknowledge WilliamChallener for preparation of the thin film samplesused for 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 graduateschool for financial support.

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J. Schmidt et al. / Journal of Magnetism and Magnetic Materials 190 (1998) 108—113 113