Co0.45Fe0.55 alloy nanowires with 12 to 35 nm diameter and 12 μm length were fabricated by electrodeposition in porous anodic alumina templates. The initial magnetization curves reveal that the zero magnetization state is not unique and is determined by the field history (acdemagnetization process) leading to the zero average moment state. For acdemagnetization processes with the field applied parallel to the nanowire axis, the subsequent magnetization curves suggest that an individual nanowire behaves as a single domain with neighboring nanowires being antiparallel to each other in the zero magnetization state. However, for a demagnetization process with the field applied perpendicular to the nanowires, a different zero magnetization state is created in which the individual nanowires consist of multidomains having opposite axial orientations. These results are consistent with the asymmetric (symmetric) behavior found in the minor hysteresis loops measured after perpendicular (parallel) acdemagnetization on these nanowire arrays

Fodor, Petru S.

Tsoi, Georgy M.

Wenger, Lowell E.

English

Cleveland-Marshall College of Law

Cleveland State UniversityEngagedScholarship@CSUPhysics Faculty Publications Physics Department2003Zero Magnetization States in ElectrodepositedCo0.45Fe0.55 Nanowire ArraysPetru S. FodorCleveland State University, p.fodor@csuohio.eduGeorgy M. TsoiWayne State UniversityLowell E. WengerWayne State University, wenger@physics.wayne.eduFollow this and additional works at: https://engagedscholarship.csuohio.edu/sciphysics_facpubPart of the Physics CommonsHow does access to this work benefit you? Let us know!Publisher's Statement© 2003 American Institute of Physics.This Article is brought to you for free and open access by the Physics Department at EngagedScholarship@CSU. It has been accepted for inclusion inPhysics Faculty Publications by an authorized administrator of EngagedScholarship@CSU. For more information, please contactlibrary.es@csuohio.edu.Repository CitationFodor, Petru S.; Tsoi, Georgy M.; and Wenger, Lowell E., "Zero Magnetization States in Electrodeposited Co0.45Fe0.55 NanowireArrays" (2003). Physics Faculty Publications. 210.https://engagedscholarship.csuohio.edu/sciphysics_facpub/210Zero magnetization states in electrodeposited  nanowire arraysP. S. Fodor, G. M. Tsoi, and L. E. WengerCitation:  93, 7035 (2003); doi: 10.1063/1.1557400View online: http://dx.doi.org/10.1063/1.1557400View Table of Contents: http://aip.scitation.org/toc/jap/93/10Published by the American Institute of PhysicsArticles you may be interested inLong-range ordered nanoaperture array with uniform diameter and interpore spacing87, 173116 (2005); 10.1063/1.2117608A growth pathway for highly ordered quantum dot arrays85, (2004); 10.1063/1.1834987In situ fabrication of single-crystal Fe nanomagnet arrays85, (2004); 10.1063/1.1803622Periodic array of uniform ZnO nanorods by second-order self-assembly84, (2004); 10.1063/1.1728298Zero magnetization states in electrodeposited Co0.45Fe0.55 nanowire arraysP. S. Fodor, G. M. Tsoi, and L. E. Wengera)Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201~Presented on 13 November 2002!Co0.45Fe0.55 alloy nanowires with 12 to 35 nm diameter and 12 mm length were fabricated byelectrodeposition in porous anodic alumina templates. The initial magnetization curves reveal thatthe zero magnetization state is not unique and is determined by the field history ~ac demagnetizationprocess! leading to the zero average moment state. For ac demagnetization processes with the fieldapplied parallel to the nanowire axis, the subsequent magnetization curves suggest that an individualnanowire behaves as a single domain with neighboring nanowires being antiparallel to each other inthe zero magnetization state. However, for a demagnetization process with the field appliedperpendicular to the nanowires, a different zero magnetization state is created in which theindividual nanowires consist of multidomains having opposite axial orientations. These results areconsistent with the asymmetric ~symmetric! behavior found in the minor hysteresis loops measuredafter perpendicular ~parallel! ac demagnetization on these nanowire arrays. © 2003 AmericanInstitute of Physics. @DOI: 10.1063/1.1557400#I. INTRODUCTIONThe synthesis and characterization of magnetic nano-structures have been extensively investigated during the lastdecade due to an increasing demand for miniaturization andhigher levels of performance capability in numerous applica-tions. In addition to the more conventional techniques suchas x-ray and e-beam lithography which are able to producestate-of-the-art structures,1 a multitude of more novel nano-fabrication methods have been developed including colloidchemical methods,2 atomic layer-by-layer deposition,3 self-assembly of polymers,4 and porous-membrane-basedsynthesis.5,6 From a more fundamental point of view, theinterest focuses on the study of the new magnetic propertiesthat emerge when the size of the nanostructures becomescomparable with the exchange lengths and domain wallthicknesses. For example, structures with large lateral con-finement such as nanowires are expected to behave as singledomain particles;7 however, recent magnetic force micros-copy studies8 have shown that intricate domain structurescan be created in a single nanowire.In this paper we investigate the magnetic characteristicsof electrodeposited Co0.45Fe0.55 alloy nanowire arrays in po-rous anodic alumina templates. Our previous studies on mi-crometer long Co-Fe alloy nanowires electrodeposited in alu-mina templates have shown that the magnetization reversal isa localized process.10 Once the magnetization switchingtakes place in a volume much smaller than the nanowirevolume, there are no energy barriers to prevent the propaga-tion of the reversal through the entire nanowire. As a conse-quence, the inhomogeneous magnetization states during thereversal process are short lived and the only stable magneti-zation states are single domains along the nanowire length,which is the easy axis resulting from the strong shape anisot-ropy. In this study evidence for stable multidomain structuresin the individual nanowires is presented on the basis of themagnetization curves measured from different zero magneticmoment states and minor hysteresis loops.II. EXPERIMENTAL DETAILSThe electrodeposition of the magnetic transition metalsand their alloys in porous membranes has proved to be a verysimple and low-cost route for fabricating large arrays ofmagnetic nanowires.5,6,8–11 All of the samples reported inthis study were prepared in porous anodic alumina templatessince these templates can be produced with very uniformpore sizes and spacing. The Co0.45Fe0.55 nanowire arrayswere fabricated in these hexagonally ordered templates by anac electrodeposition technique as described more fully inRef. 9. The pore spacing Di was fixed at 55 nm while thepore diameter Dp was adjusted from 12 to 35 nm with thelength of the nanowires being 12 mm.All magnetic measurements were performed using a su-perconducting quantum interference device ~SQUID! magne-tometer. Both the initial magnetization curves and the minorhysteresis loops were measured with the field along thenanowires axis ~normal to the Al substrate! starting fromdifferent initial demagnetized states. The demagnetized stateswere obtained through ac demagnetization processes begin-ning with fields larger than the coercive field; however, twodifferent demagnetization ~DM! processes were used. One ofthe demagnetization processes ~parallel DM! had the demag-netizing field applied parallel to the length of the nanowireswhile the other ~perpendicular DM! had the demagnetizingfield applied perpendicular to the nanowire axis before mag-netization measurements along the nanowire axis.III. RESULTS AND DISCUSSIONFigure 1 shows the initial magnetization curves along thenanowire axis after the two demagnetization processes forCo0.45Fe0.55 alloy nanowire arrays with wire diameter Dpa!Electronic mail: wenger@physics.wayne.eduJOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 10 15 MAY 200370350021-8979/2003/93(10)/7035/3/$20.00 © 2003 American Institute of Physics512 and 19 nm. Since different initial magnetizations areobserved depending on the field history, it is evident that thezero magnetic moment state is degenerate and depends onthe field history as well. The magnetostatic interactions be-tween the nanowires in such arrays are quite strong and adegeneracy of the zero moment state is expected because ofthe magnetic frustration present in this hexagonal structure.The situation is analogous to an antiferromagnetic Isingmodel on a triangular lattice where no configuration can bedesigned so that all the antiferromagnetic interactions arefully satisfied. As a result, there are a multitude of lowest-energy configurations for a zero moment state. However,since our magnetization measurements are performed on avery large number of nanowires representing a statistical av-erage over a large number of possible configurations, no de-generacy should be expected. Therefore the large differencesobserved between the initial magnetization curves arise fromdifferences in the magnetization structure of the individualnanowires that compose the arrays.For samples demagnetized with the magnetic field par-allel to the nanowire axis, the magnetization curves can besimply explained by considering each nanowire as a singledomain with only two possible states: parallel ~up! and anti-parallel ~down! to the nanowire axis.10 Clearly, the numberof nanowires in the ‘‘up’’ state in a zero moment state isequal to the number of nanowires in the ‘‘down’’ state. Asthe external magnetic field is applied, no dramatic change inthe magnetization state occurs until fields approach the valueof the coercivity field. Then the internal magnetic fields arestrong enough to activate the magnetization switching. Incomparison, a large change in magnetization occurs even forsmall magnetic field values for samples demagnetized nor-mal to the nanowires. This can be understood if multidomainmagnetization states are initially present in each nanowire,which require no energy from the external field to createthem. Thus applied fields much smaller than the coercivityfield can change the multidomain magnetization states in ananowire to a single-domain state and correspondingly a siz-able magnetic response is measured. A recent magnetic forcemicroscopy study on electrodeposited magnetic nanowires8of similar diameter has shown that the magnetization struc-ture for a single nanowire in the remanent state after thesaturation field was applied normal to the nanowires consistsof several axial domains separated by 180° domain walls.This observation is a direct result of the large shape anisot-ropy and magnetization reversal localization. As the perpen-dicular magnetic field is removed, the magnetization in dif-ferent parts of the nanowire aligns with the easy axisalthough there is no preferential orientation, and domainwalls are formed at the boundaries between single domains.The net moment of an individual nanowire would not neces-sarily have to be zero. A similar process is thought to occurin the nanowire array structures during the demagnetizationprocess when the field is applied perpendicular to the nano-wire axis. The only substantive difference is that, since thenanowires in the array structures are magnetostaticallycoupled, these structures reach a local energy minimum in azero moment state by cycling the demagnetization field.Minor hysteresis loops were measured for these nano-wire array structures in order to gain further insight into thedynamics of the states produced through the two differentdemagnetization processes. For parallel demagnetizedsamples, the minor hysteresis loops shown in Fig. 2 are sym-metrical around the origin and reproducible for any recyclingof the magnetic field. This result is not surprising for a sys-tem of quasi-identical magnetic units ~single-domain nano-wires! even at magnetic fields smaller than the saturationfield. However, the behavior is dramatically different forsamples demagnetized perpendicularly. Not only is the initialmagnetic response larger, but the first (N51) minor hyster-FIG. 1. Magnetization curves along the long axis for Co0.45Fe0.55 nanowirearrays with Dp5(a) 12 and ~b! 19 nm. The samples were demagnetizedwith the magnetic field applied parallel ~d! and perpendicular ~s! to thenanowire length. The dashed line represents part of the total hysteresis loop.FIG. 2. Minor hysteresis loops for a Co0.45Fe0.55 nanowire array with Dp512 nm for Hmax53000 Oe. ~a! Parallel and ~b! perpendicular demagneti-zation.7036 J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Fodor, Tsoi, and Wengeresis loop is not closed. It is only after a few magnetic fieldcycles that a closed hysteresis loop is achieved, with thecenter of the hysteresis loop showing a noticeable shift to-ward positive values of magnetization. Figure 3 more clearlydisplays this shift ~midpoint of the hysteresis loop! as a func-tion of the number of cycles and maximum applied field. Webelieve that this behavior arises from the strong irreversibil-ity associated with the state of the system. While manynanowires having inhomogeneous ~multidomain! states canbe easily magnetized even in small fields, once the nanow-ires are in a single-domain state their magnetization can bereversed only if the applied field is larger than its coercivityfield. Likewise, nanowires still remaining in inhomogeneousstates after the initial maximum field Hmax is applied willexperience a magnetostatic interaction field from the sur-rounding fully magnetized ~single-domain! nanowires thatwill tend to enhance the ability of these inhomogeneousnanowires to become fully magnetized in the opposite direc-tion upon the reversal of the applied magnetic field. Thisprocess of fully magnetizing the nanowires into single do-mains continues through each cycle until all inhomogeneousstates are essentially removed. Eventually, the coercivity ofthe individual nanowires and the magnetostatic interactionsbetween the nanowires will dominate the characteristics ofthe minor hysteresis loops, just as in the parallel demagne-tized case. In fact, the final closed minor hysteresis loops forthe perpendicular demagnetized case are identical to thoseloops produced after the parallel demagnetization for thesame Hmax . The shift in the ‘‘center’’ of the initial hysteresisloops for Hmax,Hc is related to the number of fully magne-tized nanowires oriented ‘‘upward’’ during the first half ofthe cycle minus those oriented ‘‘downward’’ during the sec-ond half of each cycle. Not surprisingly, the maximum shiftin these hysteresis loops occurs for loops having Hmax ap-proaching the measured coercivity, as approximately half ofthe nanowires with inhomogeneous states are fully magne-tized during the first half of the first cycle for Hmax’Hc@M’(1/2)M s# and the other half magnetized in the oppositedirection for Hmax’2Hc (M’0), or a maximum shift ofabout (1/4)M s . For Hmax.Hc , more than half of the nano-wires are fully magnetized during the first half of the initialcycle, and the remainder are fully magnetized in the oppositedirection during the subsequent second half of the cycle. Inaddition, a large fraction of the fully magnetized nanowiresoriented upward have their moments reversed in the secondhalf of the cycle since the ‘‘downward’’ applied field exceedsthe coercivity field, thus lowering the center of the minorhysteresis loop toward zero.In summary, evidence for inhomogeneous magnetizationstates in microscopic nanowires was found by performingmacroscopic magnetization measurements. These states wereobtained through a demagnetization process with the fieldapplied perpendicular to the easy magnetization axis. Thestates are metastable, and small magnetic fields can induceirreversible changes toward the energetically favorablesingle-domain states.ACKNOWLEDGMENTThis work is supported in part by the National ScienceFoundation through Grant No. DGE-9870720.1 S. Y. Chou, IEEE Trans. Magn. 85, 652 ~1997!.2 C. R. Martin, Science 266, 296 ~1997!.3 A. Gupta, Curr. Opin. Solid State Mater. Sci. 2, 23 ~1997!.4 S. I. Stupp, V. LeBonheur, K. Walker, L. S. Li, K. E. Huggins, M. Keser,and A. Amstutz, Science 276, 384 ~1997!.5 S. Dubois, C. Marchal, J. M. Beuken, L. Piraux, J. L. Duvail, A. Fert, J.M. George, and J. L. Maurice, Appl. Phys. Lett. 70, 396 ~1997!.6 H. Masuda, H. Asoh, M. Watanabe, K. Nishio, M. Nakao, and T. Tama-mura, Adv. Mater. 13, 189 ~2001!.7 E. H. Frei, S. Shtrikman, and D. Treves, Phys. Rev. 106, 446 ~1957!.8 Y. Henry, K. Ounadjela, L. Piraux, S. Dubois, J. M. George, and J. L.Duvail, Eur. Phys. J. B 20, 35 ~2001!.9 P. S. Fodor, G. M. Tsoi, and L. E. Wenger, J. Appl. Phys. 91, 8879 ~2002!.10 P. S. Fodor, G. M. Tsoi, and L. E. Wenger ~submitted for publication!.11 L. Menon, S. Bandyopadhyay, Y. Liu, H. Zeng, and D. J. Sellmyer, J.Nanosci. Nanotechnol. 1, 149 ~2001!.FIG. 3. Dependence of the center of the hysteresis loops for the Dp512 nm array on the maximum applied magnetic field Hmax and the numberof cycles N . The lines are Gaussian fits to the data.7037J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Fodor, Tsoi, and Wenger

Zero Magnetization States in Electrodeposited Co0.45Fe0.55 Nanowire Arrays

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Zero Magnetization States in Electrodeposited Co0.45Fe0.55 Nanowire Arrays

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