Fabrication and magnetic properties of long Ni wires of submicron width

Ordered arrays of 250 μm long submicrometric Ni wires have been fabricated on silicon substrates by electron beam lithography. The center-to-center distance between two adjacent parallel wires is 900 nm, while the linewidth is 200 nm. The array is
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  See discussions, stats, and author profiles for this publication at: Fabrication and magnetic propertiesof long Ni wires of submicron width  Article   in  Journal of Magnetism and Magnetic Materials · November 2000 DOI: 10.1016/S0304-8853(00)00441-8 CITATIONS 14 READS 16 4 authors , including:José Luis Costa-KrämerSpanish National Research Council 140   PUBLICATIONS   2,304   CITATIONS   SEE PROFILE F. BrionesSpanish National Research Council 223   PUBLICATIONS   2,772   CITATIONS   SEE PROFILE All content following this page was uploaded by J. L. Vicent on 15 January 2014. The user has requested enhancement of the downloaded file.  * Correspondingauthor. Tel.: # 34-91-3944559;fax: # 34-91-3944547.  E-mail address:  jlvicent @ (J.L. Vicent).  Present address: Departamento F m  H sica, Fac. Ciencias,Universidad de Oviedo, 33007 Oviedo, Spain.Journal of Magnetism and Magnetic Materials 221 (2000) 215 } 218 Fabrication and magnetic properties of long Ni wiresof submicron width J.I. Mart m  H n   , J.L. Costa-Kra   K mer  , F. Briones  , J.L. Vicent   *   Departamento. F  n &      sica de Materiales, C.C. F  n &      sicas, Uni v ersidad Complutense, A v . Complutense s /  n, 28040 Madrid, Spain   Instituto de Microelectro &     nica de Madrid, CNM, CSIC, Isaac Newton 8 (PTM), Tres Cantos, 28760 Madrid, Spain Abstract Ordered arrays of 250  m long submicrometric Ni wires have been fabricated on silicon substrates by electron beamlithography. The center-to-center distance between two adjacent parallel wires is 900nm, while the linewidth is 200nm.The array is prepared over a total area of 250  250  m  , so that the magneticbehavior can be studied by magnetoopticalKerr e !  ect. The results show an increment in the coercive  " eld  H    of these submicrometric patterned structures incomparison with the continuous  " lms. The analysis of the angular dependence of   H    reveals that the magnetizationreversal process of the Ni wires is well consistent with an incoherent rotation by curling.    2000 Elsevier Science B.V.All rights reserved.  Keywords:  Electron beam lithography; Magnetic wires; Kerr e !  ect; Magnetization reversal 1. Introduction The study of magnetic properties of ordered fer-romagnetic structures of submicrometric size isa very challenging  " eld of material science. First of all, it has been receiving a lot of attention in the lastfew years due to the recent advances in lithographyprocesses as well as other controlled fabricationtechniques. In particular, di !  erent techniques aselectron beam lithography [1], X-ray lithography[2] or laser interference lithography [3] haveallowed the preparation of ferromagnetic arrays of particles with high-quality shape [4 } 6] and goodcrystalline properties [7 } 9].One of the main problems that can be studied inthese magnetic ordered structures is the modi " ca-tion of the magnetization reversal processes as thelateral dimensions of the ferromagnetic particlesare reduced down to sizes below 1  m. In this way,several works have shown [7,10] that the magneticbehavior of the unpatterned samples is stronglymodi " ed by only reducing one lateral dimension tofabricate wires, that, for example, present an impor-tant change in the coercive  " eld. Then, it is interest-ing to study the properties of well structured arraysof ferromagnetic lines and analyze how the funda-mental magnetic behavior of the material ismodi " ed in a controlled way, when the sampledimensions are reduced to become comparable 0304-8853/00/$-see front matter    2000 Elsevier Science B.V. All rights reserved.PII: S0 3 0 4 -8 85 3 (0 0 ) 00 4 4 1- 8  Fig. 1. SEM micrograph of an array of nickel long lines fab-ricated by electron beam lithography. The center-to-center dis-tance is 900nm. with the characteristic magnetic lengths such as theexchange length.In this work, we have focused on the fabricationand study of arrays of long Ni wires of submic-rometric width. Their magnetic behavior has beenanalyzed by magnetooptical measurements andcompared with that of the unpatterned material.The magnetization reversal of the wires is discussedin terms of incoherent rotation processes. 2. Experimental procedure The patterned magnetic structures have beenfabricated by using electron beam lithographycombined with a lift-o !   technique [6,11]. Sum-marizing, a 300nm thick electron sensitive PMMAlayer is prepared on top of a Si substrate; then, theelectron beam of a  " eld emission Hitachi S-800scanning electron microscope (SEM) is used todraw the pattern on the resist. Typical workingconditions are 25kV and a beam current of 30 pA.In this way, a template with submicrometric holesin the illuminated regions can be obtained in thePMMA layer when the sample is developed. Afterthis step, a 40nm thick Ni  " lm is grown on top of the sample by DC magnetron sputtering. Finally,the desired magnetic structure is produced by re-moving the PMMA layer in acetone; with thislift-o !   procedure, only the nickel grown on theholes of the resist remains on the Si substrate. Afterthis process, arrays of well parallel and straight Niwires, 200nm in width and 250  m in length, havebeen obtained over a total area of 250  250  m  .The center-to-center distance between lines is0.9  m. A SEM image of these submicrometricelements is shown in Fig. 1.The magnetic properties of the Ni samples havebeen analyzed by magnetooptical measurements.In particular, the experimental setup has been con- " gurated to study the transverse Kerr e !  ect of thesamples illuminating with white light and applyinga magnetic  " eld up to 215Oe. 3. Results and discussion Fig. 2 shows the comparison of the magnetoopti-cal Kerr e !  ect (MOKE) hysteresis loops of an arrayof nickel wires and a reference Ni  " lm grown in thesame conditions; in the former the magnetic  " eldhas been applied parallel to the lines direction. Theunpatterned sample presents a coercivity of   H   " 17Oe (Fig. 2(a)). However, as it has alreadybeen observed in arrays of iron [7] and permalloylines [10] of similar dimensions, the coercive  " eld  H   " 91Oe is much higher in the patterned Niwires (Fig. 2(b)). This value is comparable with theresults found in isolated Ni bars of submicrometricwidth [12]; and it is considerably smaller than thevalues found in 100nm width short Ni bars [13],or in individual cylinder-like Ni particles of 40 } 100nm diameter fabricated by electrochemi-cally  " lling the nanopores of polycarbonate mem-branes [14].It is important to note that Adeyeye et al. haveobserved in experiments with permalloy wires thatthe magnetic interaction between adjacent lines isgoverned by the characteristic lengths of the 216  J.I. Mart  n &     n et al.  /   Journal of Magnetism and Magnetic Materials 221 (2000) 215 } 218  Fig. 2. Normalized MOKE hysteresis loops in di !  erent nickelsamples: (a) reference unpatterned Ni  " lm; (b) array of 200nmwidth Ni lines with the applied  " eld parallel to theline direction.Fig. 3. Normalized MOKE hysteresis loops of the array of Niwires for di !  erent values of the angle    between the appliedmagnetic  " eld and the line direction.Fig. 4. Angular dependence of the coercive  " eld in the array of Ni lines (hollow circles). The solid line is a  " t to the curlingmodel represented by Eq. (1). The  " tting parameter is a "! 0.309. patterned microstructure [15]. In particular, themagnetic array can be considered as a group of non-interacting single wires when the relation be-tween the width ( w ) of the lines and their separation( s ) is  s /  w * 1. In our case  s " 700nm and  w " 200nm, so that this relation can be estimated as s /  w " 3.5, which is far larger than this limit. Itindicates that we can neglect the interactions be-tweenelements and consider them as isolatedwires.As a consequence the coercive  " elds of the hyster-esis loops can be interpreted as the switching  " elds H   of the Ni lines.Also, it has been proposed [16], after micromag-netic simulations, that magnetic particles with sizeseveral times larger than the exchange length   " A  /  M   should present an incoherent mag-netization reversal process ( A  is the exchange con-stant and  M   is the saturation magnetization). Inparticular, a typical estimated value of this lengthin nickel is    " 20.7nm [16]. Then, it is expectedthat some kind of incoherent switching takes placein our Ni wires.To study this behavior, the angular dependenceof the coercive  " eld has been measured. In Fig. 3,several MOKE hysteresis loops are shown, corre-sponding to di !  erent values of the angle  formedby the applied  " eld and the lines direction ( H  isalwaysapplied inthe substrateplane).Asthe " eld istilted away from the easy direction an increase in H   is observed. The coercive  " eld angular depend-ence is plotted in Fig. 4. An increment of 40% isfound when   " 50 3 ; the study at larger angles islimited by the experimental maximum availablevalue of the applied  " eld.In order to understand the magnetic switching of the wires, and taking into account the high aspect  J.I. Mart  n &     n et al.  /   Journal of Magnetism and Magnetic Materials 221 (2000) 215 } 218  217  ratio 1000:1 of the long Ni lines, our experimentaldata can be compared with the theoretical predic-tions of the curling magnetization reversal modefor an in " nite cylinder [17,18]. For this model, theequation for the angular dependence of the switch-ing  " eld can be written as H  " M  2 a (1 # a )   a  # (1 # 2 a ) cos   , (1)where  a "! 1.08 (2   /  d )   and  d  is the diameter of the cylinder. This expression can be  " tted reason-ably well to the measured  H  (  ) values of the longNi wires, as it is shown in Fig. 4. In particular, thededuced  " tting parameter is  a "! 0.309 $ 0.007.Estimating the e !  ective value of the diameter of thelines as the geometrical average of the thickness ( t  )and the width,  d "   ( t  w ) " 89 nm, we obtaina Ni exchange length    " ( d /2)   ( ! a /1.08) " 24 $ 1 nm. This value is close to the usually con-sidered    " 20.7nm [16], indicating that the curl-ing model is consistent with the experimentalmagnetic behavior found in our arrays of Ni lines. 4. Conclusions In summary, an important increment in the co-ercive  " eld respect to the reference unpatternedsamples has been found in arrays of long Ni wiresfabricated by electron beam lithography. The mag-netic behavior obtained by MOKE measurementsis consistent with the image of non-interactingwires with a magnetization reversal process thatcan be understood in terms of the curling model.The deduced exchange length for the Ni,    " 24nm, is in good agreement with the commonlyestimated value. Acknowledgements This work has been supported by the SpanishCICYT (contract MAT99-0724). References [1] J.F. Smyth, S. Schultz, D.R. Fredkin, D.P. Kern, S.A.Rishton, H. Schmid, M. Cali, T.R. Koehler, J. Appl. Phys.69 (1991) 5262.[2] M. Hehn, K. Ounadjela, J.P. Bucher, F. Rousseaux, D.Decanini, B. Bartenlian, C. Chappert, Science 272 (1996)1782.[3] J.P. Spallas, A.M. Hawryluk, D.R. Kania, J. Vac. Sci.Technol. B 13 (1995) 1973.[4] C. Miramond, C. Fermon, F. Rousseaux, D. Decanini,F. Carcenac, J. Magn. Magn. Mater. 165 (1997) 500.[5] E.F. Wassermann, M. Thielen, S. Kirsch, A. Pollmann,H. Weinforth, A. Carl, J. Appl. Phys. 83 (1998) 1753.[6] J.I. Mart m  H n, Y. Jaccard, A. Ho !  mann, J. Nogues, J.M.George, J.L. Vicent, I.K. Schuller, J. Appl. Phys. 84 (1998)411.[7] C. Shearwood, S.J. Blundell, M.J. Baird, J.A.C. Bland, M.Gester, H. Ahmed, H.P. Hughes, J. Appl. Phys. 75 (1994)5249.[8] J. Yu, U. Ru   K diger, L. Thomas, S.S.P. Parkin, A.D. Kent,J. Appl. Phys. 85 (1999) 5501.[9] M. Hanson, C. Johansson, B. Nilsson, P. Isberg, R. Wa   K pp-ling, J. Appl. Phys. 85 (1999) 2793.[10] A.O. Adeyeye, J.A.C. Bland, C. Daboo, J. Magn. Magn.Mater. 188 (1998) L1.[11] J.I. Mart m  H n, J.L. Vicent, J.V. Anguita, F. Briones, J. Magn.Magn. Mater. 203 (1999) 156.[12] M.S. Wei, S.Y. Chou, J. Appl. Phys. 76 (1994) 6679.[13] L. Kong, S.Y. Chou, J. Appl. Phys. 80 (1996) 5205.[14] W. Wernsdorfer, B. Doudin, D. Mailly, K. Hasselbach, A.Benoit, J. Meier, J.-Ph. Ansermet, B. Barbara, Phys. Rev.Lett. 77 (1996) 1873.[15] A.O. Adeyeye, J.A.C. Bland, C. Daboo, D.G. Hasko, Phys.Rev. B 56 (1997) 3265.[16] M.E. Schabes, J. Magn. Magn. Mater. 95 (1991) 249.[17] W.F. Brown, Phys. Rev. 105 (1957) 1479.[18] E.H. Frei, S. Shtrikman, D. Treves, Phys. Rev. 106 (1957)446.218  J.I. Mart  n &     n et al.  /   Journal of Magnetism and Magnetic Materials 221 (2000) 215 } 218
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