Journal of Magnetism and Magnetic Materials 316 (2007) e44–e47

Fabrication and magnetic properties of hexagonal arrays of NiFeelongated nanomagnets

S. Moralejoa, F.J. Castanob, C. Redondoa, R. Jic, K. Nielschc, C.A. Rossb, F. Castanoa,!

aFacultad de Ciencia y Tecnologıa, University of the Basque Country (UPV/EHU), 48080 Bilbao, SpainbDepartment of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Av., Cambridge, MA 02139, USA

cBMBF* Nanotechnology Research Group, Max Planck Institute of Microstructure Physics, Weinberg 1, 06120 Halle, Germany

Available online 25 February 2007

Abstract

Large area ð"cm2Þ periodic arrays of 20 nm-thick NiFe nanomagnets have been fabricated using templates with undercut side-wallscreated by interference lithography (IL) and liftoff processing. Hexagonal arrays of 195$ 690 nm2 and 190$ 320nm2 nanomagnets wereinvestigated using both spatially resolved magneto-optical Kerr effect (MOKE) and alternating gradient magnetometry (AGM). The in-plane magnetization reversal is dominated by shape anisotropy and elements in both arrays reverse between single-domain states viaincoherent rotation, either through a single switch or a series of switching events. These results are attributed to an increase in themagnetostatic interactions, which generates locally correlated magnetic configurations through the reversal of the array.r 2007 Elsevier B.V. All rights reserved.

PACS: 75.75.+a; 75.60.Jk; 75.30.Gw

Keywords: Interference lithography; Nanomagnet arrays; Permalloy; Magnetostatic interactions

1. Introduction

The magnetic properties of periodic arrays of thin filmelements with deep-sub-micron lateral dimensions (‘nano-magnets’) have recently attracted considerable attentiondue to their applications in magnetic access randommemories (MRAMs) and other magneto-electronic devices[1]. For high-density MRAM devices to be competitivethese small structures have to be patterned in closeproximity and therefore studies focused on the effect ofmagnetostatic coupling on the magnetization reversal ofnanomagnets are of great current interest.

From the fabrication point of view, arrays of NiFenanomagnets can be made over small areas using electron-beam lithography [2,3], however to create large-areað"cm2Þ arrays alternative approaches such as interferencelithography (IL) are more suitable. Most reported workusing IL [4–7] is focused on subtractive fabrication

approaches which involve patterning of a hard maskfollowed by pattern transfer into a magnetic thin film usingion-milling. On the other hand, reported additive (liftoff)fabrication approaches [8,9] used templates with verticalside-walls created using a trilayer-resist-stack [6] andmultiple etching steps for pattern transfer. Templates withundercut profiles are desirable for additive patterning andthe present work reports on a simple additive fabricationprocess to create deep-sub-micron nanomagnet arraysusing IL templates with undercut side-wall profiles.Previous work on the magnetic properties of large-area

arrays of nanomagnets [5–9] has been focused on structuressufficiently spaced to consider magnetostatic couplingbetween elements negligible. IL allows patterning periodicarrays with a variety of symmetries, which in turn providefor ideal model structures in which to investigate the effectsof increasing the coupling between these small magneticsolids. In this work the magnetic properties of arrays of190 nm-wide NiFe elongated elements with hexagonalsymmetries are investigated and the effects of increasingthe magnetostatic interactions among elements discussed.

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0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jmmm.2007.02.022

!Corresponding author. Tel.: +3494 601 2533; fax: +34 94 601 3500.E-mail address: f.castano@ehu.es (F. Castano).

2. Experimental methods

The IL system included a He-Cd TEM00 laser (KimmonElectric Co.) operating at 325 nm and a Lloyds-mirror.Using two consecutive exposures at angles of 25% and 60%,respectively, grid patterns were made in a (200 nm-thickTSMR-iN027 negative-tone-resist, Ohka Co.)/(80 nm-thickWiDE-8B anti-reflection-coating, Brewer Science, Inc.)bylayer stack, spun onto thermally oxidized Si(1 0 0)wafers. First the WiDE-ARC was spun and baked at 100and 180 %C for 40 and 60 s, respectively, followed byspinning of the resist and baking the stack at 90 %C for 90 s.The advantages of using WiDE-ARC instead of conven-tional ARC are that it etches in the resist developer and itdisplays a broad etch rate variation depending on thebaking temperature, which in turn allows for undercutprofiles [10].

After exposure, the templates were baked at 110 %Cduring 90 s and developed in AZ726 MIF (Microchemi-cals), followed by a short O2 etch to ensure that the WiDE-ARC had cleared. The templates, (Fig. 1(a) and (b)),present undercut profiles (Fig. 1(c)) and were coated with20 nm-thick polycrystalline NiFe films deposited using

electron-beam evaporation. Liftoff was achieved by im-mersing the samples in 1-methyl-2pyrrolidinone at 120 %C,followed by an ultrasonic bath.The present article described the magnetic properties of

the arrays shown in Fig. 1(d) and (e) (labelled as arrays Iand II), comprising NiFe nanomagnets with similar widthsand aspect ratios of 3.6 and 1.6, respectively. Roomtemperature hysteresis loops were obtained in applied fieldsup to 700Oe using spatially resolved magneto-optical Kerrmagnetometry (nanoMOKEII, Durham Magneto OpticsLtd), as well as by MicroMagt2900 (Princeton MeasuringCorporation) alternating gradient magnetometry (AGM).While spatially resolved measurements allow the magneti-zation to be probed by averaging "1000 hysteresis loopmeasurements resulting from a 2:8 mm-diameter area of thearray (around 27–30 elements in arrays I and II), AGMmeasurements average the signal over 5$ 8mm2 pieces ofthe arrays, containing on the order of 109 nanomagnets.

3. Results and discussion

The MOKE and AGM hysteresis loops for both arrays(Fig. 2) suggest an in-plane magnetization reversal domi-nated by shape anisotropy, resulting in an easy magnetiza-tion axis when the field is applied parallel to the longer sideof the elements and a hard axis loop when the field is parallel

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Fig. 1. (a) and (b) Plan-view scanning electron micrographs (SEMs) of thetemplates prior to metallization, (c) side-view SEM showing the undercuttemplate profile, (d) and (e) plan-view SEMs of the 20 nm-thick NiFehexagonal element arrays. (d) 190$ 690nm2 (array I) (e) 195$ 320nm2

(array II).

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Fig. 2. Hysteresis loops obtained with MOKE and AGM and appliedmagnetic fields along the long and short axis of the elements in arrays I (a)and II (b).

S. Moralejo et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e44–e47 e45

to short axis. Along the easy axis, while AGM and MOKEdata are in good agreement for array II, AGM loops forarray I show a lower coercivity than MOKE data, as well asa small low-field jump. Minor AGM loops (not shown) ofthis low-field switch suggest a coercivity of 25Oe. Cuttingthe sample into smaller pieces changed the relative height ofthe low-field step, which suggests that this switch in theAGM data is due to small patches of unpatterned NiFe filmin this array. On the other hand, MOKE measurementsscanning different areas of both arrays with applied fieldsalong the easy axis did not display this low-field transition,showing the advantages of spatially resolved measurementsin the characterization of nanomagnet arrays.

For applied fields along the elements’ longer axis, array Ishows a single step and array II displays a softer loop withthree switching events (evident in both AGM and MOKEdata). The coercivities for arrays I and II (320 and 90Oe,respectively) are smaller than what would be expected [11]for coherent rotation of ellipsoids with the same dimensions(792 and 543Oe, respectively). Therefore incoherent rotationis expected in both arrays due to the large size of the elementscompared to the exchange length. The center-to-centernearest neighbor interaction fields ðHIntÞ calculated using adipolar approximation for elements in arrays I and II areonly of a few tens of Oe [12], which is comparable to thecoercivity of array II. Hence, magnetostatic interactions arelikely to be of more importance in the reversal of array IIsince the elements switch at lower applied fields.

In order to further understand the magnetizationreversal, we have measured the angular dependence ofthe hysteresis loops in both arrays using spatially resolvedMOKE and applied fields between 0% and 360% (the 0%-direction corresponds to the long axis of the elements forboth arrays). From these results, the angular dependence ofthe coercivity ðHcÞ for each array is depicted in Fig. 3.

Both arrays show a 2-fold symmetry in the angulardependence of Hc, which suggests that the uniaxial shapeanisotropy of the elements dominates the magnetizationreversal. For array I the hysteresis loops show a singleswitch accompanied by a decrease of the remanence onincreasing the angle between the applied field and the longaxis of the elements. Conversely, all the loops of array IIfor applied fields away from the short axis show threedistinct steps and, additionally, minor MOKE loops (Fig.3(b)) also show these three steps. Similar plateaus havebeen predicted by micromagnetic simulations [13] onallowing for either side-to-side or end-to-end interactionsbetween NiFe patterned elements and therefore the steps inthe loops of array II may be attributed to locally correlatedmagnetic configurations resulting from magnetostaticinteractions. Overall, magnetostatic interactions are greaterin array II than in array I, and the elements in both arraysreverse between single-domain states via a single switch(array I) or a series of locally correlated stable configura-tions resulting from magnetostatic interactions (array II).

In summary, we have developed a simple additiveprocess to fabricate arrays of NiFe nanomagnets with

deep-sub-micron lateral dimensions using templates withundercut side-wall profiles made by IL. The magneticproperties of arrays of 190 nm-wide NiFe nanomagnetswith different lengths (690 nm, array I and 320 nm, arrayII) and hexagonal symmetries are dominated by shapeanisotropy. The hysteresis loops of array I show a singleswitch and the loops from array II display several stepswhich may be attributed to the magnetostatic interactionsresulting from the reduced spacing between elements,suggesting the existence of local magnetic correlationsaffecting the magnetization reversal of the array.

Acknowledgments

We thank H.I. Smith for use of nanofabricationfacilities at MIT. This work was supported by the

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Fig. 3. (a) Angular dependence of the coercivity for array I (full circles)and array II (open circles). (b) Major loop (700Oe) and minor loops (425and 350Oe) MOKE hysteresis loops for array II along the 40% direction(data is measured in the same area of the array and normalized withrespect to the Kerr signal at 700Oe).

S. Moralejo et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e44–e47e46

Spanish-Science-ministry (MEC) under Grants BQU2001-0511 and CTQ2004-07188/BQU.

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