Surface magnetization processes in soft magnetic nanowiresN. Lupu, M. Lostun, and H. Chiriac Citation: J. Appl. Phys. 107, 09E315 (2010); doi: 10.1063/1.3360209 View online: http://dx.doi.org/10.1063/1.3360209 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v107/i9 Published by the American Institute of Physics. Related ArticlesAnomalous magnetic properties of 7nm single-crystal Co3O4 nanowires J. Appl. Phys. 111, 013910 (2012) High field-gradient dysprosium tips for magnetic resonance force microscopy Appl. Phys. Lett. 100, 013102 (2012) Dipolar interactions in magnetic nanowire aggregates J. Appl. Phys. 110, 123924 (2011) Dielectric and spin relaxation behaviour in DyFeO3 nanocrystals J. Appl. Phys. 110, 124301 (2011) Finite size versus surface effects on magnetic properties of antiferromagnetic particles Appl. Phys. Lett. 99, 232507 (2011) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Surface magnetization processes in soft magnetic nanowiresN. Lupu,1,a� M. Lostun,1,2 and H. Chiriac1

1National Institute of Research and Development for Technical Physics, Iasi, Romania2“Alexandru Ioan Cuza” University, Faculty of Physics, Iasi, Romania

�Presented 21 January 2010; received 31 October 2009; accepted 18 February 2010;published online 11 May 2010�

The surface magnetization processes taking place in simple permalloy �Py� and FeGa nanowires,Py/Cu, CoFeB/Cu, CoNiP/Cu, FeGa/Py, and FeGa/CoFeB multilayered nanowires have beenstudied by magneto-optical Kerr effect �MOKE� magnetometry. The results indicate a strongcorrelation between the direction of the anisotropy axis relative to the direction of the applied fieldand the plane of incidence of the laser spot, as well as the effect of dipolar interactions between thenanowires or between the ferromagnetic layers on the magnetization reversal. The larger laser spotsare inducing more noise in the MOKE hysteresis loops because of the dimensional imperfectionsalong the nanowires. © 2010 American Institute of Physics. �doi:10.1063/1.3360209�

I. INTRODUCTION

Recently, magnetic nanowires were extensively studiedbecause of their specific properties and multiple potentialapplications.1–3 A special emphasis is laid on the nanowiresprepared by electrochemical deposition in the nanopores ofdifferent templates, due to the efficiency of this preparationmethod.2,3

One of the most used soft magnetic materials for nano-wires, either single or multilayered, is permalloy �Py�.2 Incombination with nonmagnetic Cu, it offers one of the bestsolutions for spin devices. Among other soft magnetic mate-rials, the amorphous nanowires4,5 offer a viable alternative toPy. Due to the lack of magnetocrystalline anisotropy, thesoftness of amorphous nanowires is enhanced. Very recently,Fe–Ga nanowires as well as combinations with Py or CoFeBamorphous alloys have been fabricated for the first time byusing electrodeposition into nanoporous anodic aluminamembranes.6–8 Such nanowires have a strong potential to beused in sensors for different medical applications, mainlybecause of their magnetoelastic properties they can mimicdifferent cilia from the human body.6

Due to their small size, the magnetic properties of thenanowires are difficult to measure with conventional magne-tometers, mainly when it comes to single nanowires. A morepowerful tool used widely in the recent years to characterizethe magnetic nanowires, including the domain walls move-ment, spin dynamics or magnetic domain imaging, is the�magneto-optical Kerr effect� MOKE magnetometry.9 Be-cause of the relative high penetration depth �about 30 nm ormore, depending on the laser power�, comparable with thenanowires thickness/diameter, MOKE magnetometry is asuitable method to characterize both the surface and volumemagnetization reversal processes.

Here, we present comparatively our results on the sur-face magnetization processes taking place in different softmagnetic nanowires: simple permalloy �Py� �Ni80Fe20=Py�and FeGa nanowires, Py/Cu, CoFeB/Cu and CoNiP/Cu,

FeGa/Py, and FeGa/CoFeB multilayered nanowires, whereCoFeB and CoNiP are showing amorphous structures andFeGa is a magnetostrictive alloy.

II. EXPERIMENT

All nanowires have been prepared by electrodepositioninto the alumina templates with nanopores diameters rangingfrom 35 nm to 300 nm, respectively, using single bath com-plex solutions of the corresponding salts.4,7,8,10 The followingnominal compositions have been used to prepare simple ormultilayered nanowires: Ni80Fe20=Py, Co75Fe10B15=CoFeB,�Co,Ni�80P20=CoNiP, and Fe80Ga20=FeGa. The nanowireshave been released from the template10 using a concentratesolution of NaOH, followed by final cleaning by ultrasonica-tion. The surface magnetic loops have been measured bymeans of longitudinal MOKE effect �the rotation of the planeof polarization is proportional with the magnetization com-ponent parallel with the plane of incidence�, using a na-noMOKE II magnetometer, produced by Durham MagnetoOptics, Ltd. A polarized light generated by a He–Ne laserwas reflected from the nanowire to the detector. The diameterof the light beam varied between 2 and 10 �m. For the2 �m laser spot the maximum magnetic field is created by aquadrupole magnet and can reach a maximum value of about150 Oe, whereas for the 10 �m laser spot the maximumobtained field with an electromagnet is 5000 Oe. The planeof incidence was parallel with the nanowires long axis.Nanowires with lengths of 1–2 cm have been studied. Theexperiments were carried out on the central part of a singlenanowire, at room temperature. Additionally, measurementson 2, 3, and 4 nanowires placed parallel in the same plane ofthe sample stage have been done to estimate the interactionbetween nanowires. External magnetic field was applied par-allel to both the nanowires and the plane of incidence. Thehysteresis loops were obtained from the simple and multilay-ered nanowires by averaging 3000 to 5000 individual loops.a�Electronic mail: nicole@phys-iasi.ro.

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III. RESULTS AND DISCUSSION

The increase in the simple Py nanowires diameter causesfirst the decrease in the coercive field, followed by an in-crease for larger diameter nanowires due to the depth pen-etration of the laser spot and the demagnetizing effects �Fig.1�a��. The decrease in the coercive field for the individualPy/Cu multilayered nanowires of 35 nm in diameter com-pared with the simple Py nanowires is the result of the re-duced axial magnetic anisotropy, whereas for larger diametermultilayered nanowires the variation in the coercive field isinfluenced by both the reduced axial magnetic anisotropyand the increase in the circumferential magnetic permeabilityin the surface layer due to the presence of nonmagnetic Culayers10 �Fig. 1�b��. Additionally, the coercive field decreasesfor Py/Cu multilayered nanowires with the same diameteronce the thickness of the magnetic Py layer increases, whilethe Cu layer thickness remains unchanged. This behavior ismost probably caused by the increased pinning role playedby the nonmagnetic Cu layers separating thicker magneticlayers of Py. One can notice the squared hysteresis loops ofsimple Py nanowires, regardless of their diameter, as well asof multilayered �Py�30�/Cu�10�� nanowires, associated with asingle large Barkhausen jump between two stable remanentmagnetic configurations. The situation is different for themultilayered nanowires with the Py layer of 50 nm, in agree-ment with the ac magnetic measurements reported in Ref. 10.Moreover, the hysteresis curves of all multilayered �Py/Cu�nanowires presents a number of Barkhausen jumps associ-ated with the number of ferromagnetic sequences �Py� be-cause each Py layer creates a dipolar field which influencesthe neighboring Py layers.11

The same effect is evidenced when 2, 3, or 4 nanowiresof Py are placed near each other, parallel between them and

with the plane of incidence �Fig. 2�. In this case, theBarkhausen jumps are correlated perfectly with the numberof nanowires measured together. Therefore, each Barkhausenjump is the result of the magnetization reversal in each nano-wire. The number of jumps increases with the number ofnanowires and the hysteresis loop remains squared. Theswitching field increases first with the number of nanowires,and decreases suddenly for four parallel nanowires. Eachnanowire produces a dipolar field, which affects the neigh-boring nanowires additional to the external magnetic field.Thus, a complicated spatial magnetic field distribution is cre-ated, which affects very strongly the response of the wholesystem12,13 and causes the reduction in the switching field forfour nanowires configuration.

The multilayered nanowires containing CoFeB andCoNiP magnetic amorphous layers and non-magnetic Cu lay-ers are behaving in a totally different manner, and showmuch smaller coercive fields, due to the reduced magneticanisotropy of the amorphous CoFeB or CoNiP alloys com-pared with Py �Fig. 3�. The larger saturation magnetic fieldsrequired for CoFeB/Cu and CoNiP/Cu multilayered nano-wires can be explained by the larger circumferential mag-netic anisotropy existing in such materials compared with Pylayers.14 However, no clear Barkhausen jumps are observedfor these multilayered nanowires, most probably because theanisotropy axis is oriented along the nanowires and parallelwith the external applied field, compared with �Py/Cu� mul-tilayered nanowires for which the anisotropy is oriented per-pendicular relative to the long nanowires axis direction, i.e.,in the plane of the Py layers.2

FIG. 1. Surface magnetic hysteresis curves measured by means of MOKEeffect for: �a� Py simple nanowires and �b� Py/Cu multilayered nanowires.

FIG. 2. MOKE hysteresis loops for 1, 2, 3, and 4 Py nanowires, placed neareach other and parallel with the laser beam incidence plane. The diameter ofeach nanowire is 120 nm.

09E315-2 Lupu, Lostun, and Chiriac J. Appl. Phys. 107, 09E315 �2010�

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Magnetostrictive FeGa alloys are less soft magnetic ma-terials compared with Py but still the coercive field is re-duced and the saturation field low enough �below 0.1 T�. Thecombination with soft magnetic Py or amorphous CoFeBlayers is softening the multilayered nanowires, as one can seein Fig. 4, and reduces the switching field at which the mag-netization reversal is achieved. However, the surface hyster-esis loops for FeGa-based multilayered nanowires are behav-ing in an opposite manner compared with their volumehysteresis loops counterparts,7,8 which shows a softer mag-netic behavior for FeGa/CoFeB compared with FeGa/Pymultilayered nanowires. The reason for such a different be-havior might be the local structure of the CoFeB amorphousalloys, which behaves differently at short range order. Thesmall jumps existing in the MOKE hysteresis loops of bothFeGa simple nanowires and FeGa/Py or FeGa/CoFeB multi-layered nanowires are given the most probably by the dimen-

sional imperfections of the nanowires along their length �thehigh resolution scanning electron microscopy �HR-SEM� im-ages have shown the existence of a very thin superficial layerof alumina still existing on the surface7,8�, considering that inthis case the laser spot was of 10 �m and thus a longer partof the nanowires is averaged, compared with simple Py andPy/Cu multilayered nanowires for which a 2 �m in diameterlaser spot was used.

IV. CONCLUSIONS

We have shown that MOKE is a powerful tool for thecharacterization of magnetic nanowires prepared by elec-trodeposition technique. The surface hysteresis loops arestrongly dependent on the diameter of the nanowires, theircomposition, the thickness of the layers composing the mul-tilayered nanowires and the number of nanowires measuredat once. The results are relevant to understand the intrinsicmechanisms governing the magnetic behavior of magneticnanowires with different compositions, mainly because themacroscopic methods of measurement are not always rel-evant for such nanomaterials, and to future design of mag-netic multilayer devices using nanowires with various com-positions and magnetic characteristics.

ACKNOWLEDGMENTS

Support from the PN II – Partnerships Programme �Con-tract No. 11-072/2007, NANOBIODET, and Contract No.12-114/2008, SANAM�, and NUCLEU Programme �Con-tract No. 09-43 01 02� is highly acknowledged.

1R. Cowburn and D. Petit, Nature Mater. 4, 721 �2005�.2A. Fert and L. Piraux, J. Magn. Magn. Mater. 200, 338 �1999�.3J.-E. Wegrowe, D. Kelly, A. Franck, S. E. Gilbert, and J.-Ph. Ansermet,Phys. Rev. Lett. 82, 3681 �1999�.

4H. Chiriac, A. E. Moga, M. Urse, I. Paduraru, and N. Lupu, J. Magn.Magn. Mater. 272–276, 1678 �2004�.

5M. Ciureanu, F. Beron, L. Clime, P. Ciureanu, A. Yelon, T. A. Ovari, R. W.Cochrane, F. Normandin, and T. Veres, Electrochim. Acta 50, 4487 �2005�.

6P. D. McGary, L. Tan, J. Zou, and B. J. H. Stadler, “Magnetic nanowiresfor acoustic sensors,” J. Appl. Phys. 99, 08B310 �2006�.

7N. Lupu, H. Chiriac, and P. Pascariu, J. Appl. Phys. 103, 07B511 �2008�.8N. Lupu, P. Pascariu, C. Gherasim, and H. Chiriac, IEEE Trans. Magn. 44,3005 �2008�.

9D. A. Allwood, G. Xiong, M. D. Cooke, and R. P. Cowburn, J. Phys. D 36,2175 �2003�.

10H. Chiriac, O.-G. Dragos, M. Grigoras, G. Ababei, and N. Lupu, IEEETrans. Magn. 45, 4077 �2009�.

11H. Chiriac, T.A. Óvári, and P. Pascariu, J. Appl. Phys. 103, 07D919�2008�.

12H. Chiriac, S. Corodeanu, and T.-A. Óvári, IEEE Trans. Magn. 44, 479�2008�.

13A. Chizhik, A. Zhukov, J. M. Blanco, and J. Gonzalez, J. Non-Cryst.Solids 287, 374 �2001�.

14H. Chiriac and T. A. Óvári, Prog. Mater. Sci. 40, 333 �1996�.

FIG. 3. Surface magnetic hysteresis curves for CoFeB/Cu and CoNiP/Cumultilayered nanowires.

FIG. 4. MOKE magnetic hysteresis curves for simple FeGa nanowires and,respectively, FeGa/Py and FeGa/CoFeB multilayered nanowires.

09E315-3 Lupu, Lostun, and Chiriac J. Appl. Phys. 107, 09E315 �2010�

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