IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011 2515

Comprehensive Studies on the Magnetic Reversal Properties of BilayeredMagnetic Anti-Dot Lattices

N. G. Deshpande�, M. S. Seo�, J. M. Kim�, S. J. Lee�, J. Y. Rhee�, K. W. Kim�, and Y. P. Lee�

q-Psi and Department of Physics, Hanyang University, Seoul, KoreaDepartment of Physics, Sungkyunkwan University, Suwon, Korea

Department of Information Display, Sunmoon University, Ansan, Korea

In this work, we report the results of our study on the magnetization-reversal properties in a bilayered magnetic antidot lattice (BMAL)system consisting of upper perforated thick Co layer of 40 nm and lower continuous thin Ni layer of 5 nm, probed by using a super-conducting-quantum-interference-device (SQUID) magnetometer and by magnetic-force microscopy (MFM). Such a BMAL structurewas fabricated by using photolithography and controlled wet-etching processes. A systematic study on the in-plane anisotropy, and theswitching-field properties was carried out. The atomic-force-microscopy (AFM) image clearly indicated that the anti-dot array structuresare well defined, and the local element composition was confirmed by using the energy dispersive spectra (EDS). The room-temperaturehysteresis curves, taken along different directions of the applied magnetic-field, were proved to be useful to understand the magneticanisotropy in the sample. A kind of uniaxial anisotropy with easy axis along 0 and hard axis along 90 of applied field direction wasobserved. To get a comprehensive knowledge about the domain configuration, we performed the MFM imaging along the easy and hardaxis of the lattice. The MFM images revealed well-defined periodic domain networks which can be ascribed to the anisotropies such asmagnetic uniaxial anisotropy, configurational anisotropy, etc. The observed changes in the magnetic properties are closely related to thepatterning that pins the domains as well as to the magneto-anisotropic BMAL structure.

Index Terms—Magnetic array, magnetic-domain reversal, magnetic-force microscopy, magneto-optical kerr effects.

I. INTRODUCTION

R ECENTLY, magnetic anti-dot lattice (MAL), where ahole mesh is embedded in the contiguous magnetic film,

has attracted a lot of attention [1]–[3]. It has been discussed andreported that one can tune the overall magnetic properties onlyby changing the anti-dot dimensions, and lattice arrangement[4]–[7]. The anti-dot pattern induces strong shape anisotropyin the film, which can control the magnetic properties suchas coercivity, permeability, magnetization reversal process,and magneto-resistance [8]–[10]. It is a valuable system toinvestigate domain nucleation as well as domain-wall pinning.Technologically, the prime motivation of such a MAL liesin applications such as spintronics, magnetic sensing, andultrahigh-density magnetic recording.

Many methods/techniques for the reliable fabrication of mag-netic nano/micro-structures are currently being used, includingtop-down lithographic approaches and bottom-up self-assemblystrategies. Self-assembled lithography methods, especially, an-odized alumina or block-copolymer templates, have been em-ployed to make patterns with small nm-size pores (anti-dots),but obtaining the long-range order by using these methods islimited. Additionally, the anti-dot shape cannot be controlledprecisely [11]–[13]. On the other hand, lithographic techniquessuch as electron-beam, x-ray, imprint, and interference, providelarge-area uniform patterning of sample with deep micron/sub-micron size and shapes [14]–[16]. Lithographically fabricated

Manuscript received February 16, 2011; revised April 28, 2011; acceptedMay 07, 2011. Date of current version September 23, 2011. Corresponding au-thor: Y. P. Lee (e-mail: yplee@hanyang.ac.kr).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2011.2155038

single-layer magnetic nano/micro-structures have been widelystudied to reveal the magnetization state and switching fieldof the elements as a function of shape, size and composition.Especially, particular attention is given on permalloy anti-dotsystems. Notwithstanding this, these techniques are even fur-ther employed in patterning the bi/multi-layer magnetic nano/micro-structures to study various interesting phenomena like ex-change-bias and spin-valve mechanisms, etc. [17], [18]. In suchstructures, both the magnetostatic and the exchange couplingbetween patterned structures and the inter-layer/faces play animportant role, thereby affecting the switching process and thecoercive field. Additionally, one can study the reversal of indi-vidual magnetic layers in the elements using various character-ization techniques. As a result, such a structure gives an addi-tional degree of freedom to tune the overall magnetic propertiesas compared to single-layer structures and/or uniform contin-uous thin magnetic films.

Hence, considering all the advantages led by such bilay-ered magnetic antidot lattice (BMAL) systems and also thelithographic techniques, we employed photolithography withcontrolled wet-etching process to fabricate BMAL systemsconsisting of upper perforated thick “cobalt (Co)” layer of40 nm and lower continuous thin “nickel (Ni)” layer of 5nm thick. A systematic study on the magnetization-reversalproperties, the in-plane anisotropy, and the switching-fieldproperties were carried out and discussed. In particular, themagnetic switching (M-H) and the remanent-state magneticforce microscopy (MFM) properties of the BMAL system, asa function of the angle between the lattice-symmetry and theapplied-magnetic-field directions, are reported.

II. EXPERIMENTAL DETAILS

The micro-patterning of anti-dot arrays (2 mm 2 mm) wascarried out by using the photo-lithography technique with a KrFstepper for creating the designed patterns [14]. 5 nm-thin Ni

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2516 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011

Fig. 1. Schematic of various steps (process flow) involved in the fabrication ofCo/Ni bilayer with micro-patterned rhomboidal anti-dot array.

layer and 40 nm-thick Co layer were sequentially deposited ona commercially available 6-inch Si wafer. The metal depositionwas performed by using a dc-sputtering system with a minimumdeposition rate of 140 and 193 for Co and Ni, respec-tively. The base pressure used was Torr, while theworking pressure was 2 mTorr. Rhomboidal-lattice pattern fora mask was designed in such a way that the anti-dot size waskept smaller than the anti-dot center-to-center distance. Beforethe exposure process, the top of the Co-deposited wafer wascoated with a bottom anti-reflective coating (BARC) and a pos-itive photo-resist (PR) layer, whose thicknesses were around 60and 680 nm, respectively. After the exposure and the develop-ment, the exposed holes of each pattern were etched out by thewet etching process, using a diluted metal etchant. An appro-priate metal etchant (nitric acid with low dilution) was chosen toetch only the Co layer to ensure that it did not affect the beneath‘Ni’ layer. Finally, to remove the PR residuals and to clean theetched surface, the ashing process was carried out by employingan plasma. Optimization of the etching process was neces-sary to get well-defined and large-area uniformly patterned ar-rays in the desired size, shape and geometry. For a more clearview, details of the fabrication method and the process flow areschematically represented in Fig. 1.

The overall dimensions (size, shape etc.) of the obtainedBMAL structure were measured by field-emission scan-ning-electron microscopy (FE-SEM: Hitachi, S-4800) andAFM. The FE-SEM instrument was used to acquire the energydispersive spectrum (EDS) at various selected areas. The in-duced magneto-anisotropy was studied by applying magneticfield in different azimuthal directions and observing the changein the hysteresis curve obtained by SQUID magnetometer. Themagnetic-domain configuration in the remanent state has beeninvestigated by using a magnetic-force microscope (PSIA,XE-100), which is essentially an atomic-force microscope(AFM), equipped with magnetic tips (Nanosensors) that arecoated with a Co alloy and have a radius less than 50 nm.Interleave mode at a lift-scan height of 50 nm was used tomeasure the magnetic-force-microscopy (MFM) images. TheMFM images were acquired several times to make sure to havethe same magnetic-domain states in the acquired image so that

Fig. 2. (a) Schematic representation of the bilayered Co/Ni MAL and (b) AFMimage ���� �� �� � of the bilayered Co/Ni MAL with the axis representingthe lattice direction.

Fig. 3. EDS spectra for Co/Ni anti-dot array: (a) the entire area, and (b) theanti-dot area.

the image is not affected by the stray field originating from thetip itself.

III. RESULTS AND DISCUSSION

Fig. 2(a) displays a schematic view of the fabricated BMALwith anti-dot lattice information viz. shape, size, dimensions,thickness, lattice periodicity and symmetry, while the corre-sponding sample (real) AFM image is shownin Fig. 2(b). AFM image [Fig. 2(b)] clearly indicates that theanti-dot array structures after magnetic-material deposition, pat-terning, and wet-etching process are well defined. We madeEDS analysis at selected areas of our samples, and the resultsare shown in Fig. 3(a) and 3(b). Fig. 3(a) shows the spectrumfor the entire anti-dot region, which indicates the presence ofboth Co and Ni. To confirm the presence of Ni in the vacanthole, we selected that particular area whose spectrum shows thepresence of Ni only [see Fig. 3(b)].

DESHPANDE et al.: STUDIES ON THE MAGNETIC REVERSAL PROPERTIES OF BILAYERED MAGNETIC ANTI-DOT LATTICES 2517

Fig. 4. Room-temperature hysteresis curves for (a) continuous thin film, (b)BMAL system taken along 0 of applied external magnetic-field direction withrespect to the lattice symmetry and (c) BMAL system taken along 90 of appliedexternal magnetic-field direction with respect to the lattice symmetry.

To get knowledge about how this BMAL structure behaveswith the application of external magnetic field along differentdirections, we acquired M-H curves at room temperature.The 40-nm-thick continuous ‘Co’ film displayed a uniaxialanisotropy with the easy axis of the magnetization lying in thefilm plane [see Fig. 4(a)]. The origin of this anisotropy might beattributed to the deposition conditions. For the BMAL sample,the measurements indicate that the easy axis of magnetizationalso lies in the film plane. However, the hysteresis loops takenalong different lattice planes of the rhomboid lattice showthe dominant role of the anti-dots in the reversal behaviorover the intrinsic anisotropy of the Co film [see Fig. 4(b)and 4(c)]. These anisotropically-pinned domain due to thepatterning results in the hysteresis loops that resemble easy-and hard-axis-like characteristics along the application of themagnetic field with respect to the different lattice planes in therhomboid structure. We found out a kind of uniaxial in-planeanisotropy with the easy axis along 0 and the hard axis along90 as seen in Fig. 4(b) and 4(c), respectively. Interestingly, itwas also seen that the coercivity was strongly affected in suchstructures.

A comparative investigation on continuous Co thin filmand single-layer Co anti-dot patterned arrays was alsoperformed. Fig. 5 shows the comparative histogram ofcoercivity for thin film, Co anti-dot and Co/Ni bilayeranti-dots. The coercivity (in our case) seems to follow arule as Coercivity Coercivity

. Hence this helps one in manipulatingthe magnetic properties. Usually, in dot/anti-dot systems onecan reduce the coercivity by adjusting the dot/anti-dot distance,thereby changing the magnetostatic energy, while in our case weshow that using a BMAL system one can reduce the coercivitywithout affecting the magnetization saturation of the sample.Such effect may be attributed to the fact that an exchangecoupling between the interfaces changes the overall magnetic

Fig. 5. Comparison of coercivity between continuous Co thin film, single-layerCo anti-dot patterned arrays, and bilayer Co/Ni anti-dot patterned arrays.

Fig. 6. Remenant-state MFM images ��� � �� �� � of patterned anti-dotarray taken after saturating the sample along (a) the easy and (b) the hard axis.The anti-dot is also marked.

properties of either the continuous thin film or single-layermagnetic anti-dot system of same material.

MFM analysis is employed to understand the magnetic-do-main configuration in the BMAL system. These measurementswere performed at room temperature with images taken at onlyremanent states, i.e., at zero field after saturating the samplealong a particular direction. Fig. 6(a) and 6(b) show the rema-nent-state MFM images taken along the easy and the hard axis,respectively. Both images clearly show the periodic domain net-works with regular bright and dark contrast. This is one of thebest features in the anti-dots, which enables us to control orconfine the domains within the lattice period, which is not thecase with the un-patterned films, where domains are extendedin lengths [19].

In Fig. 6(a), which exhibits, the remanent image along theeasy axis, one can see three important domain regions. There aretwo regions with wedge like domains between nearest-neighborholes. Such kinds of domains are similar to the “onion-like” con-figuration [indicated by square frame in Fig. 6(a)] which formsaround the holes. In addition to this, there is also a third region,which is like a diamond shape bounded by four holes were themagnetization is aligned parallel to the vector connecting holecenters {indicated by rhomboidal frame [Fig. 6(a)]} [20].

On the other hand, in Fig. 6(b) (the remanent image along thehard axis), one can observe a more complex domain configu-ration [marked by yellow and red circle/ellipse]. The basic do-main configuration as observed in Fig. 6(a) also remains intactin Fig. 6(b) with different magnetization orientation. However,more distorted features, for instance, the formation of “vortex”

2518 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011

(marked in the MFM image with red circle), can be observed.Please note here that the indicated “vortex” by red-circle inFig. 6(b) is not the anti-dot. Formation of such vortex may resultfrom the fact that domains tend to orient themselves away fromthe hard axis towards the easy axis. Additionally, at the anti-dotintersections there is a strong possibility of formation of the 90domain walls (vertically or horizontally oriented) and/or 180domain walls which may lead to such complex domain struc-tures. The magnetic anisotropy (i.e., uniaxial as well as con-figurational) is responsible for different magnetic-domain con-figuration corresponding to the easy and hard axis. It should,therefore, be possible to trap the vortex state (at zero fields)by simply applying the field along a particular direction in amagneto-anisotropic BMAL system. It should be noted herethat having stable and controlled vortex state is a key require-ment for the successful implementation of ferromagnetic struc-tures in various logic and memory devices. More elaborationand detailed understanding can be done with various studieson these systems, in particular, using field-dependent MFM im-ages, MOKE magnetometry, etc. which are under progress.

IV. CONCLUSION

We fabricated uniform and large-area BMAL systems of Coanti-dots with rhomboidal lattice geometry on a thin Ni un-derlayer by photolithography and wet-etching techniques. Thelarge-area uniform patterning was confirmed by AFM, while thecompositional studies verified the presence of both Co and Ni.The SQUID magnetometry revealed uniaxial anisotropy witheasy and hard axis along 0 and 90 of the lattice symmetry,respectively. The underlayer is found to be useful in reducingthe coercivity without affecting the high saturation. The MFMstudies show formation of the periodic domain network. Morecomplex domains (especially, vortex) were formed at the rema-nent state along the hard axis. Our study provides insight on howthe artificially-fabricated BMAL structures (more specifically,having perforated anti-dot ferromagnetic material over uniformferromagnetic thin layer) can be used to tune the coercivity andalso pin the vortex state at zero field along a particular direction,thereby providing a new way to manipulate the magnetic prop-erties which is a prerequisite for developing advanced devices.

ACKNOWLEDGMENT

This work was supported by MEST/NRF through theQuantum Photonic Science Research Center, Korea and isalso supported by Priority Research Center Program throughthe National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology(2010-0029700).

REFERENCES

[1] C. C. Wang, A. O. Adeyeye, N. Singh, Y. S. Huang, and Y. H. Wu,“Magnetoresistance behavior of nanoscale antidot arrays,” Phys. Rev.B., vol. 72, p. 174426, 2005.

[2] C. T. Yu, H. Jaing, L. Shen, P. J. Flanders, and G. J. Mankey, “Themagnetic anisotropy and domain structure of permalloy antidot arrays,”J. Appl. Phys., vol. 87, p. 6322, 2000.

[3] L. J. Heyderman, S. Czekaj, F. Nolthing, D. H. Kim, and P. Fischer,“Cobalt antidot arrays on membranes: Fabrication and investigationswith transmission X-ray microscopy,” J. Magn. Magn. Mater., vol. 316,p. 99, 2007.

[4] C. C. Wang, A. O. Adeyeye, and N. Singh, “Magnetic antidot nanos-tructures: Effect of lattice geometry,” Nanotechnology, vol. 17, p. 1629,2006.

[5] N. G. Deshpande, W. C. Nam, M. S. Seo, X. R. Jin, S. J. Lee, Y. P. Lee,and J. Y. Rhee, “Studies on magnetization reversal in 2-dimensionalspin photonic crystals of cobalt anti-dot arrays,” J. Korean Phys. Soc.,vol. 56, p. 1345, 2010.

[6] C. Gao, K. Chen, L. Lu, J. Zhao, and P. Chen, “Out-of-plane coercivefield of �� �� antidot arrays,” J. Magn. Magn. Mater., vol. 322, p.3278, 2010.

[7] N. G. Deshpande, M. S. Seo, X. R. Jin, S. J. Lee, Y. P. Lee, J. Y. Rhee,and K. W. Kim, “Tailoring of magnetic properties of patterned cobaltantidot by simple manipulation of lattice symmetry,” Appl. Phys. Lett.,vol. 96, p. 122503, 2010.

[8] A. Chiolerio, P. Allia, E. Celasco, P. Martino, F. Spizzo, and F. Cel-egato, “Magnetoresistance anisotropy in a hexagonal lattice of cobaltantidots obtained by thermal evaporation,” J. Magn. Magn. Mater., vol.322, p. 1409, 2010.

[9] F. J. Castano, K. Nielsch, C. A. Ross, J. W. A. Robinson, and R. Kr-ishnan, “Anisotropy and magnetotransport in ordered magnetic anti-dote arrays,” Appl. Phys. Lett., vol. 85, p. 2872, 2004.

[10] C. C. Wang, A. O. Adeyeye, and Y. H. Wun, “Magnetic properties ofasymmetric antirectangular �� �� arrays,” J. Appl. Phys., vol. 94,p. 6644, 2003.

[11] P. Tiberto, L. Boarino, F. Celegato, M. Coisson, N. D. Leo, F. Vinai,and P. Allia, “Magnetic and magnetotransport properties of arraysof nanostructured antidots obtained by self-assembling polystyrenenanosphere lithography,” J. Appl. Phys., vol. 107, p. 09B502, 2010.

[12] G. A. B. Confalonieri, K. R. Pirota, M. Vazquez, N. M. Nemes, M.G. Hernandez, M. Knobel, and F. Batallian, “Magnetic and transportproperties in ordered arrays of permalloy antidots and thin films,” J.Appl. Phys., vol. 107, p. 083918, 2010.

[13] J. A. Barnard, H. Fujiwara, V. R. Inturi, J. D. Jarratt, J. W. Scharf, andJ. L. Weston, “Nanostructured magnetic networks,” Appl. Phys. Lett.,vol. 69, p. 2758, 1998.

[14] A. O. Adeyeye and N. Singh, “Large area patterned magnetic nanos-tructures,” J. Phys. D. Appl. Phys., vol. 41, p. 153001, 2008.

[15] E. Mengotti, L. J. Heyderman, F. Nolthing, B. R. Craig, J. N. Chapman,L. Lopez-Diaz, R. J. Matelon, U. G. Volkmann, M. Klaui, U. Rudiger,C. A. F. Vaz, and J. A. C. Bland, “Easy axis magnetization reversals incobalt antidot arrays,” J. Appl. Phys., vol. 103, p. 07D509, 2008.

[16] Y. Luo and V. Misra, “Large-area longe-range ordered anisotropicmagnetic nanostructure fabrication by photolithography,” Nanotech-nology, vol. 17, p. 4909, 2006.

[17] D. Tripathy, A. O. Adeyeye, and N. Singh, “Exchange bias in nanoscaleantidot arrays,” Appl. Phys. Lett., vol. 93, p. 022502, 2008.

[18] R. Cheng, B. L. Justus, A. Rosenberg, D. N. Mcllroy, Z. Holman,D. Zhang, and Y. Kranov, “The magnetic domain configurationin Co/Ni/Co nanoscale antidot arrays,” J. Appl. Phys., vol. 108, p.086110, 2010.

[19] J. B. Wedding, M. Li, and G. C. Wang, “Magnetization reversal of a thinpolycrystalline cobalt film measured by the magneto-optic Kerr effect(MOKE) technique and field-dependent magnetic force microscopy,”J. Magn. Magn. Mater., vol. 204, p. 79, 1999.

[20] G. Ctistic, E. Papaioannou, P. Patoka, J. Gutek, P. Fumagalli, and M.Giersig, “Optical and magnetic properties of hexagonal arrays of sub-wavelegnth holes in optically thin cobalt films,” Nanoletters, vol. 9, p.1, 2009.