dc and high frequency magnetic properties of nanopatterned CoFe2O4arrays fabricated using sol-gel precursors

Goutam Sheet,1,a� Alexandra R. Cunliffe,1 Erik J. Offerman,1 Chad M. Folkman,2

Chang-Beom Eom,2 and Venkat Chandrasekhar1,a�

1Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, Illinois60208, USA2Department of Materials Science and Engineering Physics, University of Wisconsin–Madison, 2164Engineering Centers Building, 1550 Engineering Drive Madison, Wisconsin 53706, USA

�Received 7 January 2010; accepted 22 March 2010; published online 20 May 2010�

Nanostructures of ferromagnetic oxides having Curie temperatures above room temperature havepotential for applications in memory devices and future spin-based electronic applications. In thisarticle, we report on the dc and high frequency magnetic properties of arrays of elliptical CoFe2O4

nanopillars, covering a large area, fabricated by combined electron beam lithography, and a sol-gelbased chemical route. The nanopillars were successfully fabricated on insulating oxidized siliconsubstrates and on epitaxial thin films of ferroelectric BiFeO3. We performed magnetic forcemicroscopy and ferromagnetic resonance spectroscopy on the arrays to probe their magneticproperties. Due to the possible existence of dominant pinning sites, the CoFe2O4 nanopillars arenot single-domain even at nanometer size scales. © 2010 American Institute of Physics.�doi:10.1063/1.3393745�

I. INTRODUCTION

Nanometer scale structures of novel magnetic materialshave gained substantial attention in the emerging fields ofnanotechnology and spintronics in the last several years.Low-dimensional magnetic structures of elemental magnetslike cobalt,1,2 nickel,3,4 alloys like permalloy,5,6 andmetal-oxides7,8 have been studied extensively in the past.One unavoidable difficulty with such systems is that below acertain length scale they become superparamagnetic, limitingtheir application potential. On the other hand, several ferro-magnetic complex oxides remain ferromagnetic even belowthe length scale of 100 nm. In addition, there are severaloxide ferromagnets that show large spin-polarization ap-proaching 100 percent, such as Fe3O4,9 CrO2,10 andLa0.7Sr0.3MnO3,11 making them particularly suitable for spin-tronic applications. More recently, there has been growinginterest in the fabrication of multiferroic12 materials made upof composite ferromagnets and ferroelectrics. Such multifer-roics possess enormous potential for both technological ap-plications and fundamental research. For example, compositeferroelectric/ferromagnetic materials exhibit the magneto-electric effect,13 and therefore, efficient switching of the fer-romagnetic domains is expected to be achieved and con-trolled by an external electric field. In such nanostructurecomposites, it is potentially possible to attain single ferro-magnetic domains leading to superior switching properties.

In this article, we report on the fabrication and magneticproperties of small-scale structures of a ferrimagneticoxide system, CoFe2O4, which has a Curie temperature��400 °C� well above room temperature. The bulk proper-

ties of this material have been studied for several decades.14

This material has an inverse spinel structure and it is anelectrically insulating and magnetically hard material. Owingto almost perfect lattice matching with ferroelectric BiFeO3,it can grow epitaxially on BiFeO3.15 Therefore, single do-main nanostructures of ferrimagnetic CoFe2O4 grown onferroelectric BiFeO3 may be exploited as an ideal system forachieving novel magnetoelectric properties, the idea beingthat an electric field can be applied to the BiFeO3 to controlthe orientation of the magnetization of the magneticCoFe2O4 nanostructure. Traditionally, procedures involvingchemical etching or ion-beam milling are followed to fabri-cate nanoscale structures. Such processes are not only com-plicated but also damage the substrate considerably, some-times rendering the surface of the substrate conducting. Here,we report on the dc and high-frequency magnetic properties

a�Authors to whom correspondence should be addressed. Electronic ad-dresses: g-sheet@northwestern.edu and v-chandrasekhar@northwestern.edu.

FIG. 1. Grazing incidence x-ray diffraction spectrum on the thin film ofCoFe2O4. The Miller indices corresponding to the characteristic CoFe2O4

peaks are indicated.

JOURNAL OF APPLIED PHYSICS 107, 104309 �2010�

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of CoFe2O4 nanopillars fabricated following a simple tech-nique that involves no etching of the materials.

II. EXPERIMENTS

The experiments including the sample preparation, char-acterizations, and measurements will be discussed in Secs.III and IV. First, the thin films of the material were fabricatedusing a sol-gel precursor in order to characterize the stoichi-ometry. The patterning using e-beam lithography and thefabrication of the arrays of nanopillar were done afterwards.After the arrays were fabricated, magnetic force microscopy�MFM� and ferromagnetic resonance �FMR� spectroscopywere performed in order to characterize their magnetic state.

A. Thin film fabrication by sol-gel and characterization

We followed a sol-gel-based chemical route for the syn-thesis of CoFe2O4 with correct stoichiometry.16 To synthe-size the sol precursor of CoFe2O4, 1 g of 2-methoxyethanolwas added to 30 ml of diethanolamine. After that, 74.7 mg�1/200 mol� of Co�CH3CO2�2 ·4H2O and 242.4 mg �1/100mol� of Fe�NO3�3 were added to the solution and stirred untilthe ingredients fully dissolved. The container was thenplaced in a water bath and the solution was refluxed at 70 °Cfor 4 h. In order to characterize the physical properties of thematerial synthesized by the sol-gel route, we first fabricated athin film of the material on an oxidized Si substrate. Weperformed grazing incidence x-ray scattering on the thin film

FIG. 2. �Color� �a� Magnetic moment as a function of magnetic field �M −H� of a CoFe2O4 film on an oxidized Si substrate, with the field perpendicular tothe plane of the film. �b� SEM image of the surface of the same film as in �a�. Crystallites of CoFe2O4 are clearly visible. �c� Topographic AFM image of theCoFe2O4 film surface. �d� MFM image of the surface.

104309-2 Sheet et al. J. Appl. Phys. 107, 104309 �2010�

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of the material to ensure that the material was grown in thecorrect stoichiometry. In Fig. 1, the x-ray spectrum is shown.The Miller indices corresponding to the CoFe2O4 character-istic peaks are indicated in the figure. The spectrum confirmsthat the material forms in a single CoFe2O4 phase, and alsoindicates the polycrystalline nature of the film.

In order to confirm the magnetic state of the material, wemeasured the magnetization �M� as a function of magneticfield �H� at room temperature using a quantum design super-conducting quantum interference device magnetometer. TheM −H curve �Fig. 2�a�� shows hysteresis with a coercive fieldof 500 Oe. This is much larger than the coercive field of thinfilms of elemental magnets like cobalt17 and alloys likepermalloy.18 The high value of coercive field confirms thatCoFe2O4 in its two-dimensional thin film form exhibits hardferromagnetism unlike the elemental ferromagnets. Scanningelectron microscopy �SEM� on the CoFe2O4 film reveals thatthe film is polycrystalline, with small crystallites having welldefined facets clearly visible �Fig. 2�b��.

Topographic atomic force microscopy �AFM� images ofthe thin films of CoFe2O4 grown on oxidized Si substrates asdisplayed in Fig. 2�c� reveal that the surface is granular withan average roughness of 2 nm. However, when the surface isscanned with a magnetic cantilever, as shown in Fig. 2�d�,we can observe large domains ��200 nm� that show up asregions of light and dark contrast. Observation of these mag-netic domains in the thin films of CoFe2O4 motivated us tofabricate nanostructures of CoFe2O4 down to a length scaleof 200 nm, with the expectation that these nanostructureswould be single domain. In order to confirm the expecteddomain structure, we have also performed micromagneticsimulations using object oriented micromagnetic framework�OOMMF� and found that the calculated hysteresis for theelliptical nanodots with major axes �300 nm and minoraxes �200 nm were square shaped indicating single mag-netic domains.

B. Fabrication of nanopillar arrays

To fabricate arrays of CoFe2O4 nanopillars on both oxi-dized Si substrates and epitaxial thin films of ferroelectricBiFeO3, we employed a combined electron beam lithographyand sol-gel based chemical route.19,20 To fabricate theCoFe2O4 pillars, polymethyl methacrylate �PMMA� was firstspin-coated on a clean oxidized Si substrate at 4000 rpm for60 s. The wafer was then baked at 170 °C for 30 min. Arraysof elliptical dots covering a total area of 1 mm2 were pat-terned on the PMMA using electron beam lithography. Thedots were made elliptical to introduce shape anisotropy,which is expected to dominate over crystalline anisotropy atthis scale. The major axes of the elliptical pillars were 300nm and the minor axes were 200 nm long. The sol-gel pre-cursor was then deposited on the wafers by spinning at 1500rpm for 30 s. The CoFe2O4 /SiO2 sample was then baked at120 °C for 5 min in order to promote the gelation of theCoFe2O4 nanostructures. The PMMA was then lifted off us-ing warm acetone. After that, the wafer was annealed at600 °C for 5 h. The procedure was repeated on an epitaxial�001� BiFeO3 thin film15 grown by magnetron sputtering on asingle crystal �001� SrTiO3 substrate. In this case, the anneal-ing was done in the presence of bismuth powder in order tocompensate for any loss of bismuth. A dark field optical im-age of the CoFe2O4 �CFO� arrays is displayed in Fig. 5�a�.

C. AFM and MFM on the arrays

Figure 3�a� shows an AFM image illustrating the unifor-mity of the dots grown following the technique outlinedabove. The corresponding line profiles shown in Fig. 3�b�confirm that all the dots belonging to an array have almostidentical height and width. CoFe2O4 nanoparticles on oxi-dized Si are expected to be polycrystalline. In order to ob-serve single crystalline CoFe2O4 nanoparticles, we have alsofabricated CoFe2O4 on epitaxial BiFeO3, which has good

FIG. 3. �Color� �a� AFM images of CoFe2O4 nanopillar arrays fabricated onoxidized Si. The scan range is 15.5 �m by 15.5 �m. �b� Line profilescorresponding to the green and red lines in �a�, showing the uniformity ofthe patterned structures. �c� Topographic image of a single elliptical pillar onoxidized Si, and �d� MFM image on the same pillar as in �c�.

FIG. 4. �Color� �a� AFM images of CoFe2O4 nanopillar arrays fabricated onepitaxial BiFeO3. �b� Line profiles corresponding to the green and red linesin �a�. �c� A surface image of the epitaxial BiFeO3 film. �d� 3-D image of asingle CoFe2O4 pillar on epitaxial BiFeO3. The height of the pillar is 200nm.

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lattice matching with CoFe2O4. However, due to the surfacemorphology of BiFeO3 �Fig. 3�c��, the cross-section of thepillars turned out to be irregular.

Figure 4�a� shows an AFM image of a CoFe2O4 nano-pillar array grown on BiFeO3. As shown in the line profilesin Fig. 4�b�, the variation in the height of the pillars is muchlarger than that of the CoFe2O4 pillars on oxidized Si. Thislarge variation in height originates from the roughness in theunderlying BiFeO3 layer. Also, in this case, the height of thepillars is greater than 200 nm while the thickness of thePMMA layer was significantly less than this. This may arisefrom the fact that the CoFe2O4 layer follows the crystalstructure of the BiFeO3, resulting in growth beyond thethickness of the patterned PMMA. Figure 4�d� shows a threedimensional image of a single CoFe2O4 pillar grown onBiFeO3, demonstrating this overgrowth. The triangular shapeof the over-grown crystal might indicate faceted growth.

Ideally, for a single domain elliptical nanostructure,where the shape anisotopy dominates, the magnetization isexpected to exist in the plane of the ellipse and should beparallel to the major axis of the ellipse. In this case oneshould observe a dark spot and a bright spot at the two endsof the ellipse. In fact, such dark-bright contrast has been seenin elliptical nanodots of simple magnetic systems like

permalloy.21,22 However, when MFM was performed on theCoFe2O4 arrays on oxidized Si, it was found that the ellipti-cal dots �Fig. 3�c�� have complex magnetic domain struc-tures. As shown in Fig. 3�d�, in the MFM image of our dotsthe inner region appears dark surrounded by a bright region.This indicates the existence of an attractive interaction be-tween the magnetic tip and the sample in the central regionwhile the interaction is repulsive close to the edges of thestructures. The irregular shape of the central dark region con-firms that even at this small length scale CoFe2O4 does notform single domain structures. The above observation clearlysuggests that the domain size in the CoFe2O4 is stronglydependent on the magnetic environment, i.e., though themagnetic domain size is large �200 nm� in a thin film of thematerial, isolated nanostructures of comparable size are mul-tidomain. In ferrite thin films, such effect was discussed inthe past by van der Zaag et al.23,24 However, as evident fromthe micromagnetic simulations that we will discuss later,even for individual CFO nanoparticles of this size scale,single domain magnetic structure should be achieved. Thefact that we do not observe single domain behavior experi-mentally could be attributed to microscopic pinning sites thatare not taken into account for the simulations.

FIG. 5. �Color� �a� Dark field optical image of the nanopillar arrays. �b� Side view of the pillars in SEM. �c� FMR spectrum with varying direction of themagnetic field with respect to the major axis of the elliptical pillars. The magnetic field is applied in the plane of the substrate where the nanopillars are grown.The two peaks in the spectra are indicated by two arrows. �d� Zoomed view of the FMR spectrum between 3000 and 4500 Oe. For �c� and �d� the curves havebeen offset from each other for visual clarity. The orientation of the magnetic field corresponding to each curve is shown in the index.

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D. FMR spectroscopy

In order to measure the dynamic magnetic properties ofthe arrays of the elliptical nanopillars of CoFe2O4, we haveperformed FMR spectroscopy on the arrays grown on oxi-dized Si. The samples were measured at room temperature ina Varian electron-spin-resonance cavity spectrometer with anoperating frequency of 9.37 GHz, and the orientation of thestatic dc field could be varied with respect to the samplesubstrate. An external dc magnetic field was swept from 0 to6 kOe. The field was always applied parallel to the plane ofthe sample having the nanopillars. Figure 5 shows the ab-sorption spectra of the sample grown on SiO2 as a functionof the magnetic field. The resonance is clearly visible as apeak at 3500 Oe. When the sample is rotated in the plane ofthe substrate with respect to the direction of the magneticfield, the peaks of the resonance spectrum do not shift. Thisfurther indicates the polycrystalline nature of the ellipticalferromagnetic dots and that the shape anisotropy in this casefails to dominate over the crystalline anisotropy of the dots.This observation is different from the FMR data on bulkCoFe2O4 �Refs. 14, 25, and 26� where the position of theresonance peak was seen to shift with the field direction.However, as displayed in Fig. 5, a double peak structure inthe FMR spectrum is present which is also observed in thecase of bulk CoFe2O4.25 This double peak structure in theabsorption curve is an intrinsic property of CoFe2O4. FMRspectroscopy could not be performed on the arrays grown onepitaxial BiFeO3, as BiFeO3 is a strong absorber of the mi-crowave radiation and overloads the microwave cavity.

III. MICROMAGNETIC SIMULATIONS

In order to identify the expected magnetic domain struc-tures theoretically, we have done micromagnetic simulationsusing OOMMF for elliptical nanostructures with major axis=300 nm, minor axis=200 nm, and thickness=60 nm.For this simulation we have used previously reported param-eters for CoFe2O4 �Refs. 27 and 28�: saturation magnet-ization Ms=5�105 A /m, crystalline anisotropy A=12�10−12 J /m, and magnetocrystalline anisotropy K=3�105 J /m3.

The M −H curves generated from simulations �shown inFig. 6� are square shaped both for elliptical and circularnanostructures indicating single domain behavior. However,the fact that we do not observe the single domain behavior inthe experiments might point to the existence of dominantpinning sites in the nanostructures.

IV. CONCLUSION

To conclude, we have grown complex ferromagneticCoFe2O4 nanopillars covering a large area of 1 mm2 on oxi-dized Si and BiFeO3 wafers employing a combined electronbeam lithography and sol-gel based chemical route. The pil-lars grow in polycrystalline form on oxidized Si with com-plex magnetic domain structures. MFM and FMR measure-ments indicate that even at this size scale, single domainmagnetic structures are not achieved possibly due to the ex-istence of dominant pinning sites.

ACKNOWLEDGMENTS

This work was supported by the U.S. Department of En-ergy through Grant No. DE-FG02-06ER46346. We thank D.A. Dikin for help with the SEM imaging.

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FIG. 6. M −H curves for the CoFe2O4 nanostructures generated by micro-magnetic simulations using OOMMF. The solid and the dotted lines repre-sent the M −H curves for the circular and elliptic nanoparticles respectively.

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