The magnetization reversal process in mesoscopic permalloy (Ni80Fe20) wire structures has been investigated using scanning Kerr microscopy, magnetic force microscopy (MFM) and micromagnetic calculations. We find that the junction offers a site for reversed domain wall nucleation in the narrow part of the wires. As a consequence, the switching field is dominated by the domain nucleation field and the junction region initiates reversal by the wall motion following the nucleation of domains. Our results suggest the possibility of designing structures that can be used to “launch” reverse domains in narrow wires within a controlled field rang

Bland, J.A.C.

Gardiner, S.M.

Guertler, C.M.

Hirohata, A.

Lee, W.Y.

Leung, H.T.

Xu, Y.B.

Yao, C.C.

IEEE Transactions on Magnetics

English

White Rose Research Online

This is a repository copy of Magnetization reversal in mesoscopic Ni80Fe20 wires: A magnetic domain launching device.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/1856/Article:Lee, W.Y., Hirohata, A. orcid.org/0000-0001-9107-2330, Leung, H.T. et al. (5 more authors)(2000) Magnetization reversal in mesoscopic Ni80Fe20 wires: A magnetic domain launching device. IEEE Transactions on Magnetics. pp. 3018-3020. ISSN 1941-0069 https://doi.org/10.1109/20.908661eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. 3018 IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 5, SEPTEMBER 2000Magnetization Reversal in Mesoscopic Ni80Fe20Wires: A Magnetic Domain Launching DeviceW. Y. Lee, A. Hirohata, H. T. Leung, Y. B. Xu, S. M. Gardiner, C. C. Yao, and J. A. C. BlandAbstract—The magnetization reversal process in mesoscopicpermalloy (Ni80Fe20) wire structures has been investigated usingscanning Kerr microscopy, magnetic force microscopy (MFM)and micromagnetic calculations. We find that the junction offersa site for reversed domain wall nucleation in the narrow partof the wires. As a consequence, the switching field is dominatedby the domain nucleation field and the junction region initiatesreversal by the wall motion following the nucleation of domains.Our results suggest the possibility of designing structures that canbe used to “launch” reverse domains in narrow wires within acontrolled field range.Index Terms—Magnetization reversal, mesoscopic wires, nucle-ation process, switching field.I. INTRODUCTIONIN recent years, there has been great interest in the magnetiza-tion reversal and magnetoresistance (MR) behavior in ferro-magnetic mesoscopic wires because of their importance in bothMR devices and spin electronic devices [1]–[9]. It is well recog-nized that the switching field and magnetization reversal processdepend strongly on the end shape as well as the width of ferro-magnetic wire [1]–[4]. The effect of the end shape is attributedto the formation of end domains or edge domains, which arecrucial in the magnetization reversal process [1]–[4]. Our pre-vious work demonstrated that the shape of a wire structure has adecisive influence on the magnetic properties in the mesoscopicrange [5]–[6].In this paper, we present the effect of the junction geometryin extended permalloy (Ni Fe ) wire structures for launchingreverse domains in the wires. We demonstrate that it is possibleto control the switching field by introducing a junction that fa-cilitates magnetization reversal in the narrow part. Our resultssuggest the possibility of designing structures that can be usedto “launch” reverse domains in narrow wires within a controlledfield range. We discuss the geometrical effect of the junction onthe magnetization reversal and switching field associated withdomain configurations inferred from MFM imaging and micro-magnetic calculations.II. EXPERIMENTA permalloy film was deposited at room temperature at arate of 2 Å/min by electron-beam evaporation in an ultrahighManuscript received February 14, 2000. This work was supported by theEPSRC (UK) and the ESPRIT (EU) programs (‘Spider’ and ‘Massdots’). Thework of W. Y. Lee was supported by the British Council Korea.The authors are with the Cavendish Laboratory, University of Cambridge,Cambridge CB3 0HE, UK.Publisher Item Identifier S 0018-9464(00)08410-7.Fig. 1. The variation of the switching field as a function of position for thewire junction structure (w = 1 m, w = 5 m), measured every 10 m alongthe wire length. The arrow indicates H in the fixed width wire (w = w =1 m) obtained by MR measurement.vacuum (UHV) chamber with a pressure of 6 10 mbarand then annealed at 120 C for 30 min to remove the uniaxialanisotropy induced during deposition. A combination of elec-tron beam lithography and a lift-off process has been utilizedto fabricate wire array structures from a continuous film of30 Å Au/300 Å Ni Fe /GaAs(001). Each wire consists ofa narrow part ( = 1 m) and a wide part ( = 5 m): thewidths of the wires change abruptly at the midpoint to createtwo 200 m length regions with = 1 m and = 5 m (seethe inset of Fig. 1). We also fabricated bridge-wire structures asshown in Fig. 3(a). The bridge wire is a single wire ( = 205 mand = 10 m) which has a narrow region of length = 5 mand width = 0.5–10 m introduced in the center of the wire.The local hysteresis loops were obtained by scanning Kerrmicroscopy [7], [8]. Two objective lenses ( 20, NumericalAperture: 0.55, 50, NA: 0.85) were used to focus the probinglaser beam ( 3 m, 1 m spot size, respectively) on thewires. Magnetic force microscopy (MFM, Digital Instruments)has been carried out in order to observe domain configurations.The calculated magnetization pattern was also obtained bymicromagnetic calculation based on the Labonte algorithm[10].III. RESULTS AND DISCUSSIONScanning Kerr microscopy has been employed to measuremicroscopic MOKE hysteresis loops for the wire junction inorder to investigate the magnetization reversal behavior andswitching field, when magnetic fields were applied parallelto the wire axis. Fig. 1 presents that the switching fields ofthe narrow ( = 1 m) and wide part ( = 5 m) of the0018–9464/00$10.00 © 2000 IEEELEE et al.: MAGNETIZATION REVERSAL IN MESOSCOPIC Ni Fe WIRES: A MAGNETIC DOMAIN LAUNCHING DEVICE 3019Fig. 2. MOKE hysteresis loops for the wire junction structure obtained at fourdifferent positions near the junction area. The inset shows the calculated andschematic domain patterns of the wire junction structure in the remanent state.In the calculated pattern, the magnitude of the divergence of the magnetizationis shown by the gray scale.wire junction structure are in the range 22–26 Oe, indicatingthat the magnetization reversal of the complete structure isdetermined by the wider region ( = 5 m). This behavioris in contrast to that of fixed width wires [4] in which theswitching field is dependent upon the width of ferromagneticwires due to buckling of the magnetization perpendicular to thewire, leading to the formation of domain walls perpendicular tothe wire [4]. Previous MR measurements have shown thatof a single wire ( = 1 m) is 100 Oe [11]. From thesimilar values of the switching fields in the wide and narrowparts we infer that the threshold field required for nucleation inthe wire junction structure is much smaller than that of a singlewire, while the field for domain wall motion is almost identicalto that of the single wire and independent of the variable wirewidth in the micron range.Fig. 2 presents local MOKE hysteresis loops obtainednear the junction area and an inset showing the calculatedand schematic domain patterns of the wire junction in theremanent state. The calculated results exhibit a zigzag domainpattern and “piling up” of domain walls on the wide part,while the narrow part remains single domain. This zigzagpattern is believed to result from the combined influence of thegeometrical shape of the junction and a strong demagnetizingfield favoring closure domains despite the abrupt transitionin the width. A double-jump switching is clearly observed inthe junction of the wire: = 12 Oe and = 22 Oe, asin Fig. 2(b). On travelling across the wire from the wide partduring magnetization reversal, domain walls are trapped in thejunction, and cannot move further until a field of = 22 Oeis reached, which corresponds to the wall motion field ( ) inFig. 3. (a) Schematics of the bridge structures, which have a narrow regionwith a length l = 5 m and a width w = 0.5–10 m introduced in the center ofthe wire. The dots on the schematics denote the fixed positionsA (50 m awayfrom the end of a wide wire) andB (center of the narrow wire) of the laser beamspots. (b) The variation in the switching field (H ) against the wire width of thenarrow region.the narrow part and which is much smaller than the nucleationfield ( = 100 Oe). This implies that is greater thanin the narrow part. The double-jump switching disappears asthe probing beam spot moves away from the junction by 5 mand 10 m as shown in Fig. 2(c) and (d), respectively.These results show that the junction in the wire junctionstructure facilitates magnetization reversal in the narrow part,in contrast to symmetrically modified ends such as pointed [1],[2] and rounded [1], [3] ends which suppress the formation ofend domains in rectangular elements and increase the switchingfield. Multidomains form in the wide part in the vicinity of thejunction. The junction offers a more favorable site for reverseddomain nucleation than the corners of the wide part. In thisway, the initiation of magnetization reversal occurs, and henceswitching responds to a much smaller applied field than for afixed width structure. Similar reversal behavior has been veryrecently demonstrated using a NiFe/Cu/Co submicron wire,where a domain wall nucleates initially in a larger square padconnected to one end of the wire and is injected into the wireby a field, less than the switching field of the wire [12].To better understand the effect of the junction geometry onmagnetization reversal in the permalloy wires, we also inves-tigated bridge wires. In Fig. 3, we present the variation in theswitching field ( ) against the wire width of the narrow re-gion. The dots on the schematics in Fig. 3(a) denote the fixedpositions (50 m away from the end of a wide wire) and(center of the narrow wire) of the laser beam spots whose sizewere controlled according to the wire width. It is clearly seenthat does not vary with decreasing wire width but is iden-tical ( 10 Oe) in both regions. These results support theview that the magnetization reversal mechanism is dominatedby domain wall nucleation rather than domain wall motion inthe permalloy wires and, more importantly, that the junction is3020 IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 5, SEPTEMBER 2000Fig. 4. MFM images of the bridge structure with (a) w = 1 m and (b) w =0.5 m in the demagnetized state. The narrow regions withw = 0.5, 1 m showa single domain state, while multidomains are seen in the wide wires.crucial in initiating reversal. This is consistent with the results ofMR measurements [13] in a bridge structure with = 0.5 m,which shows a single switching.MFM images indicate that narrow regions with 5 maccommodate domain walls, whereas narrow regions with2 m show a single domain state [see Fig. 4(a): = 1 m, (b):0.5 (m]. MFM images also show multidomains appearing in thewide wires. The MFM images are in good agreement with thedomain patterns obtained by micromangetic simulations [14].This illustrates that the junction offers a site for reversed do-main wall nucleation in the narrow part and hence switchingresponds to a much smaller applied field than for a fixed widthstructure. Our results suggest the possibility of designing struc-tures which can be used to “launch” reverse domains in narrowwires within a controlled field range for applications such asspin electronic devices or for fundamental experiments in mag-netization reversal dynamics.IV. CONCLUSIONWe find that the junction in both the wire junction andbridge wire structures offers a site for reversed domain wallnucleation in the narrow part and hence switching occurs for amuch smaller applied field than for a fixed width wire. Theseresults illustrate the possibility of lowering the switching fieldin narrow wires by introducing wider regions that nucleatereverse domains, in contrast to symmetrically modified endsthat suppress the formation of end domains in rectangularelements.REFERENCES[1] J. McCord et al., “Domain observation on magnetoresistive sensor ele-ments,” IEEE Trans. Mag., vol. 32, pp. 4806–4808, 1996.[2] K. J. Kirk, J. N. Chapman, and C. D. W. Wilkinson, “Switching fieldsand magnetostatic interaction of thin film magnetic nanoelements,”Appl. Phys. Lett., vol. 71, pp. 539–541, July 1997.[3] J. Shi et al., “End domain state and magnetization reversal in submi-cron magnetic structures,” IEEE Trans. Mag., vol. 34, pp. 997–999, July1998.[4] A. O. Adeyeye et al., “Magnetoresistance behavior of submicronNi Fe wires,” Appl. Phys. Lett., vol. 70, pp. 1046–1048, Feb. 1997.[5] C. C. Yao, D. G. Hasko, X. B. Xu, W. Y. Lee, and J. A. C. Bland, “Shapeeffect on magnetoresistance and magnetization reversal in permalloywires,” J. Appl. Phys., vol. 85, pp. 1689–1692, Feb. 1999.[6] Y. B. Xu et al., “Domain wall trapping probed by MR and MFM insubmicron ferromagnetic wire structures,” J. Appl. Phys., vol. 85, pp.6178–6810, Apr. 1999.[7] W. Y. Lee et al., “Effect of junction geometry on switching field andreversal behaviorf in permalloy wires,” IEEE Trans. Mag., vol. 35, pp.3883–3885, Sept. 1999.[8] , “Magnetization reversal and magnetoresistance in a lateral spininjection device,” J. Appl. Phys., vol. 85, pp. 6682–6685, May 1999.[9] F. G. Monzon and M. L. Roukes, “Spin injection and the ocal Hall effectin InAs quantum wells,” J. Magn. Magn. Mater., vol. 199, pp. 632–635,June 1999.[10] A. E. Labonte, “Two-dimensional Bloch-type domain walls in ferromag-netic films,” J. Appl. Phys., vol. 40, pp. 2450–2458, May 1969.[11] W. Y. Lee et al., “Domain nucleation processes in mesoscopic Ni Fewire junctions,” J. Appl. Phys., vol. 87, pp. 3032–3036, Mar. 2000.[12] K. Shigeto, T. Shinjo, and T. Ono, “Injection of a magnetic domainwall into a submicron magnetic wire,” Appl. Phys. Lett., vol. 75, pp.2815–2817, Nov. 1997.[13] A. O. Adeyeye, R. P. Cowburn, and M. E. Welland, “Magnetoresistanceof constricted ferromagnetic wires,” J. Appl. Phys., vol. 87, pp. 299–302,Jan. 2000.[14] A. Hirohata, Y. B. Xu, C. C. Yao, W. Y. Lee, H. T. Leung, and J. A. C.Bland, “Domain wall trapping in controlled submicron ferromagneticelements,” J. Appl. Phys., vol. 87, pp. 4727–4729, May 2000.

Magnetization reversal in mesoscopic Ni80Fe20 wires: A magnetic domain launching device

이우영

A. Hirohata

H.T. Leung

Y.B. Xu

S.M. Gardiner

C.C. Yao

J.A.C. Bland

Oak Repository-KIST

Magnetization reversal in mesoscopic Ni80Fe20 wires : a magnetic domain launching device

https://eprints.whiterose.ac.uk/1856/1/xuyb16.pdf

Magnetization reversal in mesoscopic Ni80Fe20 wires: A magnetic domain launching device

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