The propagation of a magnetic domain wall (DW) in a submicron magnetic wire
consisting of a magnetic/nonmagnetic/magnetic trilayered structure with
asymmetric notches was investigated by utilizing the giant magnetoresistance
effect. The propagation direction of a DW was controlled by a pulsed local
magnetic field, which nucleates the DW at one of the two ends of the wire. It
was found that the depinning field of the DW from the notch depends on the
propagation direction of the DW.Comment: 12 pages, 3 figure

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Himeno, A.

Kasai, S.

Mibu, Ko

Nasu, S.

Okuno, T.

Ono, T.

Shinjo, T.

Journal of Applied Physics

English

arXiv

The propagation of a magnetic domain wall (DW) in a submicron magnetic wire consisting of a magnetic/nonmagnetic/magnetic trilayered structure with asymmetric notches was investigated by utilizing the giant magnetoresistance effect. The propagation direction of a DW was controlled by a pulsed local magnetic field, which nucleates the DW at one of the two ends of the wire. It was found that the depinning field of the DW from the notch depends on the propagation direction of the DW.journal articl

Nagoya Institute of Technology Repository System

Propagation of a magnetic domain wall in magnetic wireswith asymmetric notchesA. Himeno,a! T. Okuno, S. Kasai, and T. OnoInstitute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, JapanS. NasuGraduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, JapanK. MibuResearch Center for Low Temperature and Materials Sciences, Kyoto University, Uji,Kyoto 611-0011, JapanT. ShinjoInternational Institute for Advanced Studies, Soraku-gun, Kyoto 619-0225, JapansReceived 25 June 2004; accepted 21 December 2004; published online 4 March 2005dThe propagation of a magnetic domain wallsDWd in a submicron magnetic wire consisting of amagnetic/nonmagnetic/magnetic trilayered structure with asymmetric notches was investigated byutilizing the giant magnetoresistance effect. The propagation direction of a DW was controlled bya pulsed local magnetic field, which nucleates the DW at one of the two ends of the wire. Itwas found that the depinning field of the DW from the notch depends on the propagation directionof the DW. © 2005 American Institute of Physics. fDOI: 10.1063/1.1861505gThe magnetic domain structure and the magnetizationreversal process in magnetic nanostructures are controllableby modifying the sample shape. In a magnetic wire withsubmicron width, the magnetization is restricted to be paral-lel to the wire axis due to the magnetic shape anisotropy, andthe magnetization reversal takes place by nucleation andpropagation of a magnetic domain wallsDWd. Therefore, themagnetic wire is a simple model system to investigate thepropagation of a magnetic DW. Several attempts have beenreported to control nucleation and propagation of a DW.1–5We present a study on the propagation of a magneticDW in a submicron magnetic wire with asymmetric notches.The propagation of the DW was detected by utilizing thegiant magnetoresistance effect.2 We show that the depinningfield of the DW from the asymmetric notch depends on thepropagation direction of the DW. We also show that the DWmoves notch by notch by applying a pulsed local magneticfield.The samples were fabricated onto thermally oxidizedSi substrates by means ofe-beam lithography and lift-off method. Figure 1 shows a schematic illustration of a topview of the whole sample. A magnetic wire has a tri-layered structure consisting of Ni81Fe19s5 nmd /Cus20 nmd /Ni81Fe19s20 nmd. The main body of the magnetic wire hasfour notches with asymmetric shape. The sample has fourcurrent–voltage probes made of a nonmagnetic material, Cu.Further, it has two narrow Cu wires crossing the ends of themagnetic wire and a wide Cu wire covering the notched partof the magnetic wire. These Cu wires are electrically insu-lated from the magnetic wire by SiO2 layers of 50 nm inthickness. A flow of a pulsed electric current in each Cu wirecan generate local magnetic fields,HL, HM, or HR, which actson the left end, the main part with notches, or the right end ofthe magnetic wire, respectively.HLsHRd can trigger thenucleation of a DW at the leftsrightd end of the magneticwire. Thus, the propagation direction of a DW can be con-trolled by HLsHRd. The resistance was measured by using afour-point dc technique, and an external magnetic fieldsHextdwas applied along the wire axis. All magnetoresistancesMRdmeasurements were performed at room temperature.adAuthor to whom correspondence should be addressed; FAX:181-774-38-3109; electronic mail: himeno@ssc1.kuicr.kyoto-u.ac.jpFIG. 1. Schematic illustration of a top view of the whole sample. Theblack part is a trilayered magnetic wire consisting of NiFes5 nmd /Cus20 nmd /NiFes20 nmd. The main body of the magnetic wire has fourasymmetric notches. A flow of an electric current in the Cu wire crossing themagnetic wire generates a local magnetic field, which is used to nucleate amagnetic DW.JOURNAL OF APPLIED PHYSICS97, 066101s2005d0021-8979/2005/97~6!/066101/3/$22.50 © 2005 American Institute of Physics97, 066101-1Downloaded 02 Sep 2010 to 133.68.192.97. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsFigure 2sad shows a typical resistance change of thetrilayerd magnetic wire with asymmetric notches as a func-tion of Hext. Before the measurement,Hext of −200 Oe wasapplied in order to align the magnetizations of two NiFelayers of the trilayered wire in the same direction. Then, MRmeasurement was performed whileHext was swept up to+200 Oe. WhenHext was negative, the resistance showed thesmallest value due to the parallel magnetization alignment oftwo NiFe layers. Then, the resistance suddenly increased andshowed the largest value withHext between 10 and 162 Oe.We have an evidence from a previous study on NiFe wires ofvarious thicknesses that a thicker NiFe layer has a largercoercive force than the thinner one.6 Therefore, this resis-tance increase corresponds to the magnetization reversal ofthe NiFes5 nmd layer, and an antiparallel magnetizationalignment was realized while the resistance showed the larg-est value. At 162 Oe, the resistance abruptly decreased fromthe largest value to the smallest value, which corresponds tothe overall magnetization reversal of the NiFes20 nmd layer.Thus, the magnetic DW was not pinned by asymmetricnotches during the magnetization reversals of NiFes20 nmdlayer because of its large nucleation field.In order to nucleate a DW in the NiFes20 nmd layer atsmallerHext and to pin the DW at the notch, we utilized themethod generating a pulsed local magnetic field at an end ofthe magnetic wire.5 Figure 2sbd shows the result of the DWinjection into the NiFes20 nmd layer of the magnetic wire byHL. The measurement procedure is the same as that of themeasurement shown in Fig. 2sad, except applyingHL whenHext=60 Oe. The magnitude and the duration of the pulsedHL were 200 Oe and 100 ns, respectively. The resistanceabruptly decreased after the application ofHL and stayed at avalue between the largest and the smallest values. The valueof the resistance indicates that a DW injected from the left byHL was pinned at the first notch. By increasingHext after theinjection of the DW, the resistance abruptly decreased to thesmallest value at 98 Oe, which indicates that the DW propa-gated to the right end of the wire through the asymmetricnotches. Thus, the depinning field for the rightward propaga-tion of the DW can be determined to be 98 Oe. On the otherhand, the depinning field for the leftward propagation of theDW can be determined from the result shown in Fig. 2scd,which shows the MR measurement when the DW was in-jected from the right end of the wire byHR. In this case, thedepinning field was 54 Oe. Thus, the depinning field for therightward propagation is much larger than that for the right-ward propagation. This result indicates that the asymmetricnotch in the magnetic wire works as an asymmetric pinningpotential against the DW propagation.As seen in Figs. 2sbd and 2scd, when Hext exceeds thepinning field of the first notch, the DW propagates to the endof the wire and cannot be pinned at the successive notches.This indicates that all notches are identical from the view-point of the pinning potential. Here, we demonstrate that acombination ofHext and pulsedHM can move the DW fromone notch to the neighboring one. This is shown in Fig. 3.The horizontal and the vertical axes show the elapsed timefrom the measurement start and the resistance of the sample,respectively. In the beginning of the measurement,Hext of−200 Oe was applied, and thenHext was swept up to 30 OeFIG. 2. Resistance change of the trilayered magnetic wire with asymmetricnotches as a function of the external magnetic field. The magnetic domainstructures inferred from the resistance measurement are schematicallyshown.sad Typical MR curve of the trilayered system.sbd The magnetic DWwas injected into the NiFes20 nmd layer from the left end of the magneticwire by the pulsedHL. scd The DW was injected into the NiFes20 nmd layerfrom the right end of the magnetic wire by the pulsedHR.FIG. 3. Resistance change of the MR measurement of the trilayered mag-netic wire with asymmetric notches as a function of the elapsed time. Themagnetic domain structures of the NiFes20 nmd layer inferred from the re-sistance change are schematically shown.066101-2 Himeno et al. J. Appl. Phys. 97, 066101 ~2005!Downloaded 02 Sep 2010 to 133.68.192.97. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsand kept at 30 Oe until the measurement finished. WhenHextreached 30 Oe, the resistance showed the largest value andthe magnetizations of two NiFe layers became antiparallel.By the application of the pulsedHR, a DW was injected intothe NiFes20 nmd layer and it was pinned at the first notch,which results in the first abrupt decrease in resistance. Then,a pulsedHM was applied to the same direction asHext. Themagnitude and the duration of the pulsedHM were 20 Oeand 50 ns, respectively. EachHM triggered an abrupt de-crease in resistance. The value of each resistance changecorresponded to the length between the notches. Thus, theDW moved notch by notch by the pulsedHM. We also con-firmed that the DW moved notch by notch to the rightwarddirection by the pulsedHM with the opposite polarity.The interpretation why the depinning field depends onthe propagation direction is the following. Since the DWenergy is proportional to its area, the DW at wider position inthe wire has larger energy. This change in the DW energyalong the wire axis produces the pinning potential for theDW. The force to move the DW against the potential is givenby the derivative of the DW energy with respect to the DWposition, which is proportional to the slope of the notch. Inour magnetic wire shown in Fig. 1, the slope of the notch issteeper for the rightward propagation, resulting in the largerdepinning field for the rightward propagation.In conclusion, pinning and depinning of a magnetic DWin a submicron magnetic wire with asymmetric notches wereinvestigated by utilizing the giant magnetoresistancesGMRdeffect. The DW was nucleated and injected from an end ofthe magnetic wire by a pulsed local magnetic field. The de-pinning field of the DW from the notch depends on thepropagation direction of the DW. This indicates that theasymmetric structures constructed into the magnetic wirework as asymmetric pinning potential against the DW propa-gation.The present work was partly supported by the Ministryof Education, Culture, Sports, Science and Technology ofJapansMEXTd through the Grants-in-Aid for COE ResearchsGrant Nos. 10CE2004 and 12CE2005d and MEXT SpecialCoordination Funds for Promoting Science and TechnologysNanospintronics Design and Realization, NDRd and by the21st Century COE Program by Japan Society for the Promo-tion of Science.1T. Ono, H. Miyajima, K. Shigeto, and T. Shinjo, Appl. Phys. Lett.72,1116 s1998d.2T. Ono, H. Miyajima, K. Shigeto, K. Mibu, N. Hosoito, and T. Shinjo,Science284, 468 s1999d.3K. Shigeto, T. Shinjo, and T. Ono, Appl. Phys. Lett.75, 2815s1999d.4D. A. Allwood, N. Vernier, G. Xiong, M. D. Cooke, D. Atkinson, C. C.Faulkner, and R. P. Cowburn, Appl. Phys. Lett.81, 4005s2002d.5A. Himeno, T. Ono, S. Nasu, K. Shigeto, K. Mibu, and T. Shinjo, J. Appl.Phys. 93, 8430s2003d.6Y. Sugita, T. Ono, and T. Shinjosunpublished.066101-3 Himeno et al. J. Appl. Phys. 97, 066101 ~2005!Downloaded 02 Sep 2010 to 133.68.192.97. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

Propagation  of a magnetic domain wall in magnetic wires with asymmetric notches

A. Himeno

T. Okuno

S. Kasai

T. Ono

S. Nasu

K. Mibu

T. Shinjo

Crossref

Propagation of a magnetic domain wall in magnetic wires with asymmetric notches

Himeno, A

Okuno, T

Kasai, S

Ono, T

Nasu, S

Mibu, K

Shinjo, T

Kyoto University Research Information Repository

Title Propagation of a magnetic domain wall in magnetic wires withasymmetric notchesAuthor(s)Himeno, A; Okuno, T; Kasai, S; Ono, T; Nasu, S; Mibu, K;Shinjo, TCitationJOURNAL OF APPLIED PHYSICS (2005), 97(6)Issue Date2005-03-15URL http://hdl.handle.net/2433/50348RightCopyright 2005 American Institute of Physics. This article maybe downloaded for personal use only. Any other use requiresprior permission of the author and the American Institute ofPhysics.Type Journal ArticleTextversionpublisherKyoto UniversityPropagation of a magnetic domain wall in magnetic wireswith asymmetric notchesA. Himeno,a! T. Okuno, S. Kasai, and T. OnoInstitute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, JapanS. NasuGraduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, JapanK. MibuResearch Center for Low Temperature and Materials Sciences, Kyoto University, Uji,Kyoto 611-0011, JapanT. ShinjoInternational Institute for Advanced Studies, Soraku-gun, Kyoto 619-0225, JapansReceived 25 June 2004; accepted 21 December 2004; published online 4 March 2005dThe propagation of a magnetic domain wall sDWd in a submicron magnetic wire consisting of amagnetic/nonmagnetic/magnetic trilayered structure with asymmetric notches was investigated byutilizing the giant magnetoresistance effect. The propagation direction of a DW was controlled bya pulsed local magnetic field, which nucleates the DW at one of the two ends of the wire. Itwas found that the depinning field of the DW from the notch depends on the propagation directionof the DW. © 2005 American Institute of Physics. fDOI: 10.1063/1.1861505gThe magnetic domain structure and the magnetizationreversal process in magnetic nanostructures are controllableby modifying the sample shape. In a magnetic wire withsubmicron width, the magnetization is restricted to be paral-lel to the wire axis due to the magnetic shape anisotropy, andthe magnetization reversal takes place by nucleation andpropagation of a magnetic domain wall sDWd. Therefore, themagnetic wire is a simple model system to investigate thepropagation of a magnetic DW. Several attempts have beenreported to control nucleation and propagation of a DW.1–5We present a study on the propagation of a magneticDW in a submicron magnetic wire with asymmetric notches.The propagation of the DW was detected by utilizing thegiant magnetoresistance effect.2 We show that the depinningfield of the DW from the asymmetric notch depends on thepropagation direction of the DW. We also show that the DWmoves notch by notch by applying a pulsed local magneticfield.The samples were fabricated onto thermally oxidizedSi substrates by means of e-beam lithography and lift-off method. Figure 1 shows a schematic illustration of a topview of the whole sample. A magnetic wire has a tri-layered structure consisting of Ni81Fe19s5 nmd /Cus20 nmd /Ni81Fe19s20 nmd. The main body of the magnetic wire hasfour notches with asymmetric shape. The sample has fourcurrent–voltage probes made of a nonmagnetic material, Cu.Further, it has two narrow Cu wires crossing the ends of themagnetic wire and a wide Cu wire covering the notched partof the magnetic wire. These Cu wires are electrically insu-lated from the magnetic wire by SiO2 layers of 50 nm inthickness. A flow of a pulsed electric current in each Cu wirecan generate local magnetic fields, HL, HM, or HR, which actson the left end, the main part with notches, or the right end ofthe magnetic wire, respectively. HLsHRd can trigger thenucleation of a DW at the left srightd end of the magneticwire. Thus, the propagation direction of a DW can be con-trolled by HLsHRd. The resistance was measured by using afour-point dc technique, and an external magnetic field sHextdwas applied along the wire axis. All magnetoresistance sMRdmeasurements were performed at room temperature.adAuthor to whom correspondence should be addressed; FAX: 181-774-38-3109; electronic mail: himeno@ssc1.kuicr.kyoto-u.ac.jpFIG. 1. Schematic illustration of a top view of the whole sample. Theblack part is a trilayered magnetic wire consisting of NiFes5 nmd /Cus20 nmd /NiFes20 nmd. The main body of the magnetic wire has fourasymmetric notches. A flow of an electric current in the Cu wire crossing themagnetic wire generates a local magnetic field, which is used to nucleate amagnetic DW.JOURNAL OF APPLIED PHYSICS 97, 066101 s2005d0021-8979/2005/97~6!/066101/3/$22.50 © 2005 American Institute of Physics97, 066101-1Downloaded 06 Mar 2008 to 130.54.110.22. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jspFigure 2sad shows a typical resistance change of thetrilayerd magnetic wire with asymmetric notches as a func-tion of Hext. Before the measurement, Hext of −200 Oe wasapplied in order to align the magnetizations of two NiFelayers of the trilayered wire in the same direction. Then, MRmeasurement was performed while Hext was swept up to+200 Oe. When Hext was negative, the resistance showed thesmallest value due to the parallel magnetization alignment oftwo NiFe layers. Then, the resistance suddenly increased andshowed the largest value with Hext between 10 and 162 Oe.We have an evidence from a previous study on NiFe wires ofvarious thicknesses that a thicker NiFe layer has a largercoercive force than the thinner one.6 Therefore, this resis-tance increase corresponds to the magnetization reversal ofthe NiFes5 nmd layer, and an antiparallel magnetizationalignment was realized while the resistance showed the larg-est value. At 162 Oe, the resistance abruptly decreased fromthe largest value to the smallest value, which corresponds tothe overall magnetization reversal of the NiFes20 nmd layer.Thus, the magnetic DW was not pinned by asymmetricnotches during the magnetization reversals of NiFes20 nmdlayer because of its large nucleation field.In order to nucleate a DW in the NiFes20 nmd layer atsmaller Hext and to pin the DW at the notch, we utilized themethod generating a pulsed local magnetic field at an end ofthe magnetic wire.5 Figure 2sbd shows the result of the DWinjection into the NiFes20 nmd layer of the magnetic wire byHL. The measurement procedure is the same as that of themeasurement shown in Fig. 2sad, except applying HL whenHext=60 Oe. The magnitude and the duration of the pulsedHL were 200 Oe and 100 ns, respectively. The resistanceabruptly decreased after the application of HL and stayed at avalue between the largest and the smallest values. The valueof the resistance indicates that a DW injected from the left byHL was pinned at the first notch. By increasing Hext after theinjection of the DW, the resistance abruptly decreased to thesmallest value at 98 Oe, which indicates that the DW propa-gated to the right end of the wire through the asymmetricnotches. Thus, the depinning field for the rightward propaga-tion of the DW can be determined to be 98 Oe. On the otherhand, the depinning field for the leftward propagation of theDW can be determined from the result shown in Fig. 2scd,which shows the MR measurement when the DW was in-jected from the right end of the wire by HR. In this case, thedepinning field was 54 Oe. Thus, the depinning field for therightward propagation is much larger than that for the right-ward propagation. This result indicates that the asymmetricnotch in the magnetic wire works as an asymmetric pinningpotential against the DW propagation.As seen in Figs. 2sbd and 2scd, when Hext exceeds thepinning field of the first notch, the DW propagates to the endof the wire and cannot be pinned at the successive notches.This indicates that all notches are identical from the view-point of the pinning potential. Here, we demonstrate that acombination of Hext and pulsed HM can move the DW fromone notch to the neighboring one. This is shown in Fig. 3.The horizontal and the vertical axes show the elapsed timefrom the measurement start and the resistance of the sample,respectively. In the beginning of the measurement, Hext of−200 Oe was applied, and then Hext was swept up to 30 OeFIG. 2. Resistance change of the trilayered magnetic wire with asymmetricnotches as a function of the external magnetic field. The magnetic domainstructures inferred from the resistance measurement are schematicallyshown. sad Typical MR curve of the trilayered system. sbd The magnetic DWwas injected into the NiFes20 nmd layer from the left end of the magneticwire by the pulsed HL. scd The DW was injected into the NiFes20 nmd layerfrom the right end of the magnetic wire by the pulsed HR.FIG. 3. Resistance change of the MR measurement of the trilayered mag-netic wire with asymmetric notches as a function of the elapsed time. Themagnetic domain structures of the NiFes20 nmd layer inferred from the re-sistance change are schematically shown.066101-2 Himeno et al. J. Appl. Phys. 97, 066101 ~2005!Downloaded 06 Mar 2008 to 130.54.110.22. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jspand kept at 30 Oe until the measurement finished. When Hextreached 30 Oe, the resistance showed the largest value andthe magnetizations of two NiFe layers became antiparallel.By the application of the pulsed HR, a DW was injected intothe NiFes20 nmd layer and it was pinned at the first notch,which results in the first abrupt decrease in resistance. Then,a pulsed HM was applied to the same direction as Hext. Themagnitude and the duration of the pulsed HM were 20 Oeand 50 ns, respectively. Each HM triggered an abrupt de-crease in resistance. The value of each resistance changecorresponded to the length between the notches. Thus, theDW moved notch by notch by the pulsed HM. We also con-firmed that the DW moved notch by notch to the rightwarddirection by the pulsed HM with the opposite polarity.The interpretation why the depinning field depends onthe propagation direction is the following. Since the DWenergy is proportional to its area, the DW at wider position inthe wire has larger energy. This change in the DW energyalong the wire axis produces the pinning potential for theDW. The force to move the DW against the potential is givenby the derivative of the DW energy with respect to the DWposition, which is proportional to the slope of the notch. Inour magnetic wire shown in Fig. 1, the slope of the notch issteeper for the rightward propagation, resulting in the largerdepinning field for the rightward propagation.In conclusion, pinning and depinning of a magnetic DWin a submicron magnetic wire with asymmetric notches wereinvestigated by utilizing the giant magnetoresistance sGMRdeffect. The DW was nucleated and injected from an end ofthe magnetic wire by a pulsed local magnetic field. The de-pinning field of the DW from the notch depends on thepropagation direction of the DW. This indicates that theasymmetric structures constructed into the magnetic wirework as asymmetric pinning potential against the DW propa-gation.The present work was partly supported by the Ministryof Education, Culture, Sports, Science and Technology ofJapan sMEXTd through the Grants-in-Aid for COE ResearchsGrant Nos. 10CE2004 and 12CE2005d and MEXT SpecialCoordination Funds for Promoting Science and TechnologysNanospintronics Design and Realization, NDRd and by the21st Century COE Program by Japan Society for the Promo-tion of Science.1T. Ono, H. Miyajima, K. Shigeto, and T. Shinjo, Appl. Phys. Lett. 72,1116 s1998d.2T. Ono, H. Miyajima, K. Shigeto, K. Mibu, N. Hosoito, and T. Shinjo,Science 284, 468 s1999d.3K. Shigeto, T. Shinjo, and T. Ono, Appl. Phys. Lett. 75, 2815 s1999d.4D. A. Allwood, N. Vernier, G. Xiong, M. D. Cooke, D. Atkinson, C. C.Faulkner, and R. P. Cowburn, Appl. Phys. Lett. 81, 4005 s2002d.5A. Himeno, T. Ono, S. Nasu, K. Shigeto, K. Mibu, and T. Shinjo, J. Appl.Phys. 93, 8430 s2003d.6Y. Sugita, T. Ono, and T. Shinjo sunpublishedd.066101-3 Himeno et al. J. Appl. Phys. 97, 066101 ~2005!Downloaded 06 Mar 2008 to 130.54.110.22. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp

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Propagation of a magnetic domain wall in magnetic wires with asymmetric
  notches

Propagation of a magnetic domain wall in magnetic wires with asymmetric notches

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