Using a self-assembly double template method, we have prepared highly ordered magnetic dot arrays of Ni on sub-micron scales. For the template preparation we have used colloidal suspensions of polystyrene latex spheres with different diameters ranging from 200 to 700 nm combined with secondary templates of the conducting polymer polypyrrole. By variation of the template parameters and the deposition charge the material shape can be modified significantly from isolated dots to connected 3D networks. Samples have been characterized by magnetometry. Magnetic properties were found to depend strongly on dot size and connectivity leading to a rich variety of magnetic behaviors

A. Goncharov

A.A. Zhukov

Cowburn

Ghanem

H. Fangohr

Helm

Lebib

M.A. Ghanem

Martin

Mikrajuddin

P.A.J. de Groot

P.N. Bartlett

R. Boardman

Sander

Shinjo

English

Crossref

Goncharov, A.

Zhukov, A.A.

Bartlett, P.N.

Ghanem, M.A.

Boardman, R.

Fangohr, H.

de Groot, P.A.J.

Southampton (e-Prints Soton)

ARTICLE IN PRESSJournal of Magnetism and Magnetic Materials 286 (2005) 1–40304-8853/$doi:10.1016Correspfax: +4423E-mail awww.elsevier.com/locate/jmmmOrdered sub-micron magnetic dot arrays using self-assemblytemplate methodA. Goncharova,, A.A. Zhukova, P.N. Bartlettb, M.A. Ghanemb, R. Boardmanc,H. Fangohrc, P.A.J. de GrootaaSchool of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UKbSchool of Chemistry, University of Southampton, Southampton, SO17 1BJ, UKcComputational Engineering and Design Group, School of Engineering Sciences, University of Southampton, SO17 1BJ, UKAvailable online 8 October 2004AbstractUsing a self-assembly double template method, we have prepared highly ordered magnetic dot arrays of Ni on sub-micron scales. For the template preparation we have used colloidal suspensions of polystyrene latex spheres withdifferent diameters ranging from 200 to 700 nm combined with secondary templates of the conducting polymerpolypyrrole. By variation of the template parameters and the deposition charge the material shape can be modiﬁedsigniﬁcantly from isolated dots to connected 3D networks. Samples have been characterized by magnetometry.Magnetic properties were found to depend strongly on dot size and connectivity leading to a rich variety of magneticbehaviors.r 2004 Published by Elsevier B.V.PACS: 75.50.SsKeywords: Magnetic arrays; Nanomagnetics; Self-assembly; Template methods; Vortex1. IntroductionRecent achievements in fabricating sub-micronmagnetic patterned materials offer excellent per-spectives for magnetic applications [1,2]. In parti-cular, they hold the promise of breaking the- see front matter r 2004 Published by Elsevier B.V./j.jmmm.2004.09.014onding author. Tel.: +442380592104;80593910.ddress: gubbiotti@ﬁsica.unipg.it (A. Goncharov).superparamagnetic limit in data storage. However,the cost of lithographically prepared arraysbecomes extremely high for small structures withlateral dimensions below 200 nm. Developingrelatively cheap methods for fabricating nano-scale structures is one of the most important issuesin nano-physics today. We present results onmagnetic dot arrays prepared using a noveldouble-template electrodeposition method. Thismethod allowed us to grow dot arrays made of NiARTICLE IN PRESSA. Goncharov et al. / Journal of Magnetism and Magnetic Materials 286 (2005) 1–42with periods ranging from 200 to 700 nm. Thediameter of the dots can be set from 20 nm. Thequasi-spherical shape of dots resulting from ourfabricating process has been found to be ofimportance for the magnetic properties.Fig. 1. SEM image of polypyrrole macroporous template ﬁlm.2. Sample preparationAt present there are several methods used tocreate ordered sub-micron magnetic structuressuch as e-beam lithography, X-ray and interfer-ence lithography and others [3]. Recently, muchattention has been directed towards templatemethods [4–6]. In our case the electro-depositionthrough hard self-assembly templates has beenused for preparation of highly regular nano-scaledot arrays. The templates were prepared by self-assembly of polystyrene latex spheres on a gold-coated glass substrate. For this purpose a 0.5wt%water suspension of the polystyrene spheres wasspread on the substrate. Subsequently, the waterwas allowed to evaporate slowly for 3–5 days.During evaporation the spheres self-assemble bycapillary and electrostatic forces into a 3D closepacked structure. This ordered structure of spheresforms the primary template. This primary templatewas used for the electrodeposition of the conduct-ing polymer polypyrrole resulting in a macro-porous polymer template. After the deposition ofpolypyrrole the polystyrene spheres were dissolvedin toluene leaving a secondary polymer template(Fig. 1). The polypyrrole was then converted intoan insulator either by over-oxidation or by un-doping at a sufﬁciently negative potential. Thisinsulating structure was used as the template forelectrochemical deposition of magnetic material.At the point where the polystyrene spheres—priorto being removed—touched the substrate, disc-shaped areas with a diameter of d/4 of goldsurface are exposed. Electrodeposition of magneticmaterials starts at these points gradually ﬁlling thespherical cavities of the polypyrrole template.More details about the preparation of secondarytemplates are given in Ref. [7]. Using thistechnique, we have prepared ordered arrays ofdots with quasi-spherical geometry. The thicknessof the dots t can be controlled by varying theamount of charge passed during electrodeposition.For small values of t these structures form orderedarrays of disconnected dots but if the amount ofmaterial deposited is increased these dots join upforming ordered magnetic networks. In the formercase the dots have a near-spherical or ‘‘droplet’’shape with a bottom part reproducing the sphe-rical shape of the template cavity and a top part,which is dome-shaped due to the growth process.The dot array has a period d, which is equal to thediameter of the polystyrene spheres used intemplate preparation. The morphology of theﬁlms and their nano-structure were studied usingelectron microscopy. Fig. 2 shows an SEM imageof connected and isolated Ni dot arrays preparedby the double template deposition method. Fig. 2balso demonstrates the ‘‘droplet’’ shape of dots.Magnetic measurements were performed at roomtemperature using a vibrating sample magnet-ometer and a SQUID magnetometer.3. Results and discussionThe magnetic properties of Ni nano-dot arraysproduced by the above-described method havebeen studied. In particular, we investigated arraysof isolated dots with a ratio t/d=13prepared fromspheres of diameters d in a range from 200 to700 nm. It is known that for lithographicallyprepared nano-scale dots in a particular size rangethe most favorable state is a vortex state [8–10]: theARTICLE IN PRESSFig. 2. SEM image of isolated (a) and interconnected (b) Ni dotarrays prepared using polystyrene spheres of 500 nm diameter.-4000 -2000 0 2000 4000-15-10-5051015-8-6-4-202468(b)4H (Oe)(a)M (104 emu) M (104 emu) Fig. 3. Magnetization loops, measured by VSM, for an array ofNi dots prepared from 350 nm (a) and 500nm (b) spheres.A. Goncharov et al. / Journal of Magnetism and Magnetic Materials 286 (2005) 1–4 3magneto-static energy can be lowered by introdu-cing a circular conﬁguration in the distribution ofmagnetic moments. In the case of large particlesthis decrease in magnetic energy dominates theincrease of exchange energy but in small dots theformation of vortices costs too much in exchangeenergy and a single-domain state becomes themost stable state. The critical size for the transitionbetween vortex and single-domain behavior will beaffected by dot shape, inter-dot magnetic dipolarinteractions and the magneto-crystalline anisotro-py of the dots. We have found that for unpat-terned Ni ﬁlms produced by the sameelectrochemical method, the behavior is magneti-cally soft. In addition, in the arrays investigated,the dots are well-separated. Hence the magneticproperties are dominated by exchange interactionsand intra-dot magnetostatic energies. Our mea-surements on the dot arrays allow us to study howmagnetization reversal mechanism depends on dotsize and at which size the cross-over betweenvortex and single-domain state occurs in dots witha quasi-spherical shape. For large dots withd4200 nm the magnetic measurements, performedusing the VSM, showed nearly reversible magne-tization curves. Our ﬁndings suggest that for dbetween 200 and 700 nm the dots undergo vortex-like magnetization processes [11]. In Fig. 3, wepresent magnetization curves for dots withd=350 nm (a) and 500 nm (b) measured with themagnetic ﬁeld applied in the plane of the arrayﬁlm. The magnetization reaches saturation inmagnetic ﬁelds 1 kOe and demonstrates reversi-ble behavior within the resolution of our magnet-ometer. But in arrays of smaller dots the magneticbehavior changes. Magnetization curves for a dotarray with d=200 nm are shown in Fig. 4. It canbe seen that for the magnetic ﬁeld applied in theplane of the array the magnetization reversal issimilar to a single-domain behavior—relativelylarge hysteresis with a value of the coercive ﬁeldof about 720Oe. The squareness of the loopARTICLE IN PRESS-10000 -5000 0 5000 10000-3-2-10123-2.0-1.5-1.0-0.50.00.51.01.52.0(b)M (104 emu)M (104 emu)H (Oe) (a)HHFig. 4. Magnetization curves for the ﬁeld applied parallel to thearray plane (a) and transverse to the plane (b) of Ni dotsprepared using a template of 200 nm spheres.A. Goncharov et al. / Journal of Magnetism and Magnetic Materials 286 (2005) 1–44Mrem/Msat is 0.45. For lithographically preparednano-discs of thickness 10 and 15 nm the transitionbetween vortex and single-domain behavior takesplace for somewhat smaller diameters: 150–200 nm[10]. This disagreement may be caused by thedifference in materials of the discs and our dots,dot shape and thickness, which in our case was70 nm. In contrast to the case for ﬁelds in thearray plane, the behavior for magnetic ﬁeldsapplied transverse to the array reveals a magneti-zation curve, which remains reversible. We inter-pret this in terms of the dot shape, which has anaspect ratio smaller than one: the dot diameter islarger than its thickness. For ﬁelds in the arrayplane it is energetically unfavorable for a vortex toform. Due to the smaller length scale over whichthe magnetization would have to turn, the increasein exchange energy inhibits the formation of avortex. For magnetic ﬁelds perpendicular to thearray plane, the vortex is set up in the circularcross-section of the dot, which has a lower cost inexchange energy. These ﬁndings were found to bein agreement with results of 3-D micromagneticcalculations for small dots of the same shape [12];giving values for the critical size of about 140 nm.The somewhat smaller critical value of found d inthe simulations is probably related to the uncer-tainty in the value of the exchange parameter forNi. Several other factors like details of dot shapeand interactions between dots may also contributeto this difference.4. ConclusionWe have described a novel method for thefabrication of highly ordered arrays of magneticdots. Results of magnetization measurementsshow that the magnetic properties change depend-ing on nano-structuring. We have found evidencefor the existence of vortex and single-domainstates in small dots with quasi-spherical shapedepending on dot size and ﬁeld direction.AcknowledegmentsThis work has been supported by the EPSRC(Grant GR/29396). A.G. also acknowledges thesupport of the Institute of Physics Magnetismgroup and the C R Barber Trust.References[1] M. Helm, et al., Science 272 (1996) 1782.[2] R.P. Cowburn, J. Magn. Magn. Mater 242–245 (2002) 505.[3] J.I. Martin, et al., J. Magn. Magn. Mat. 256 (2003)449–501.[4] R. Zhu, et al., Chem. Phys. Lett. 368 (2003) 696–700.[5] Mikrajuddin, et al., Adv. Mat. 14 (2002) 930–933.[6] M.S. Sander, et al., Adv. Func. Mat. 13 (2003) 393–397.[7] M.A. Ghanem, et al., J. Mat. Chem. 11 (2001) 849.[8] T. Shinjo, et al., Science 289 (2000) 930.[9] A. Lebib, et al., J. Appl. Phys 89 (2001) 3892.[10] R.P. Cowburn, et al., Phys. Rev. Lett. 83 (1999)1042–1045.[11] A. A. Zhukov, et al., J. Magn. Magn. Mater, in print.[12] R. Boardman, et al., J. Appl. Phys., in print.

Phys., in print.

Ordered sub-micron magnetic dot arrays using self-assembly template method

https://eprints.soton.ac.uk/57279/1/Gonc_05pp.pdf

Ordered sub-micron magnetic dot arrays using self-assembly template method

Abstract

Similar works

Full text

Available Versions

Crossref

Southampton (e-Prints Soton)