2778 IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006

Effect of Magnetic Field Annealing Upon Co-RichNanowires

R. Sharif1, X. Q. Zhang1, S. Shahzadi1, L. X. Jiang1, X.-F. Han1, and Y. K. Kim2

State Key Laboratory for Magnetism, Institute of Physics, Chinese Academy of Science, Beijing 100080 ChinaDepartment of Materials Science and Engineering Korea University, Seoul 136-713 Korea

The magnetic field (MF) annealing of dc electrodeposited ferromagnetic nanowire arrays of compositions Co94Fe06, Co91Fe07B2 wasdone in the direction of their easy axis of magnetization (in plane) at 2650C under 800 Oe applied magnetic field for 2 h. An inducedmagnetic anisotropy (MA) and a large increase in saturation magnetization ( ) were manifested by the changes in hysteresis loopsof the nanowire samples before and after the annealing process. The structural changes were investigated using X-ray diffraction andtransmission electron microscopy (TEM). The shape of nanowires, the predicted natural texture with c-axis in the plane of membrane(because of high concentration of cobalt) and diffusional pair ordering of unlike atoms along the direction of external applied field werethought to be the causes of induced MA. The reannealing of samples in absence of MF at 400 C and 600 C produced no significantchanges in hysteresis loops instead of removing the MA but with evolution of crystalline phase in both the samples. Therefore, the shapeof nanowire was concluded to play the major role in persistence of induced MA in nanowire samples after high temperature annealing.

Index Terms—Diffusional pair ordering, ferromagnetic nanowires, induced anisotropy, magnetic field (MF) annealing.

I. INTRODUCTION

TWO intrinsic properties of materials such as magneticanisotropy and magnetostriction have been investigated

for long to understand and tailor these properties according tothe required device applications. Especially the modeling ofanisotropy is of technological interest since it can broaden theapplications of materials in devices. The fabrication of magneticrecording heads using ferromagnetic amorphous alloy (CoFeB)have been described by Liao [1], [2]. He applied magnetic field(MF) during plating in orthogonal directions and tailored themagnetic anisotropy and permeability proportionately.

The MF annealing/stress to induce uniaxial anisotropy innanocrystalline materials [3], [4], and amorphous materials [5]have also been extensively searched. During last few decades,researchers have paid much attention towards CoFe alloys be-cause they possess high saturation magnetization and elevatedcurie temperatures. In case of bulk CoFe alloy compositiondiagram, maximum intrinsic induction and permeability arefound for cobalt contents approximately equal to 50% andgreater than 90% [6].

The primary purpose of this paper was to fabricate thenanowires by adding boron in the high Co content CoFe alloy,i.e., [Co Fe ] B for to 15 and to investigatetheir intrinsic properties. Magnetic field annealing is one ofthe established experimental techniques for inducing mag-netic anisotropy in a chosen direction in both amorphous andcrystalline materials [4]. Currently, there is limited report onsystematic investigations of induced magnetic anisotropy forelectrodeposited ferromagnetic nanowires. Here we report theeffect of MF annealing upon Co Fe & Co Fe B nanowiresamples and persistence of induced MA, even after reannealingat high temperatures (400 C and 600 C) with evolution ofcrystalline phase.

Digital Object Identifier 10.1109/TMAG.2006.880459

II. EXPERIMENTAL METHOD

The above-mentioned ferromagnetic alloys were dc elec-trodeposited in the commercially available alumina membranes(0.02 m, Whatman Ltd., Inc) with variable pore size as dis-cussed in [7]. The electrolytes: cobalt sulphate, iron sulphateand dimethyle aminoborane were used as source of Co, Fe,and B, respectively. Their concentrations were varied to get therequired alloy compositions. Other contents were: boric acidas buffer agent, sodium citrate to prevent oxidation of Fe toFe , sodium lauryl sulphate (NaLs) as surfactant and thioureato decrease stress during deposition of nanowires [7]. Thedc electrochemical deposition experiments were performedat and room temperature (RT). The nanowiresCo Fe & Co Fe B , were selected for present work. TheX-ray diffractrometory was performed for structure analysis.Also the transmission electron microscopy (TEM) images andselected area electron diffraction (SAED) patterns of nanowiressamples were taken after releasing the nanowires from aluminamembrane by dissolving it for about 12 h in 1M NaOH solution.The compositions of nanowires were determined using induc-tion coupled plasma atomic emission spectrometry (ICP-AES).Magnetic properties were investigated using vibrating samplemagnetometer (VSM). The MF annealing of two selected sam-ples was carried at 265 C (below crystallization temperature

) under 800 Oe of in plane MF for 2 hr. Also reannealing wasperformed at higher temperatures (400 C and 600 C) withoutapplied magnetic field.

III. RESULTS AND DISCUSSION

The RT magnetic hysteresis curves under in-plane appliedfield for (a) Co Fe , and (b) Co Fe B nanowire arrays withlengths of 13 m and 18 m, respectively, are shown in Fig. 1.The slanted hysteresis loops indicate the strong interparticleinteractions in nanowire samples [8]. It can be observed that afterMF annealing, there was a large increase in , as compared tomodest increase in coercivity for both samples. During MF

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SHARIF et al.: EFFECT OF MAGNETIC FIELD ANNEALING UPON Co-RICH NANOWIRES 2779

Fig. 1. (a) (b) show the changes in magnetization before and after MF annealingand reannealing without field at 400 C and 600 C of Co Fe , Co Fe Bnanowire arrays. (Color version available online at http://ieeexplore.ieee.org.)

annealing, temperature was kept at 265 C, which can producestress relief but avoids crystallization of the samples.

The large increase in , due to MF annealing or withoutfield annealing was also previously observed [9]. The magneti-zation behavior of nanowire arrays can be explained accordingto the model of prolate ellipsoid chains as described in [9].

According to this model can be written as

(1)

where

and n is the number of ellipsoids in one wire, a is the lengthof short axis of the ellipsoids and diameter of the wire, b is thelength of long axis of the ellipsoid and the distance betweenthe centers of two ellipsoids, and and are demagneti-zation factors of the ellipsoid perpendicular and parallel to the

TABLE ICHANGES INH (Oe) FROM VSM DATA FOR THE NANOWIRE ARRAYS

nanowire axis which depend on the aspect ratio of the ellipsoid[9]. Equation (1) can be adopted to explore our results. Thisequation suggests that will increase as increases andwill decrease as wire diameter decreases. Note that this equationdoes not show the dependence of on interpore distance [9].

The changes in values of our samples are summarized inTable I. There was an increase in but this increase was notso large as compared to the increase in because dependsupon the demagnetizing factor and aspect ratios, according to(1). Due to external applied field there would be alignment ofmagnetic moments or production of ordered atomic pairs of Co,Fe, and B atoms along the direction of external applied field [4].This affected the demagnetization factors and , thus pro-ducing small increase in values as compared to values.Similarly, smaller increase in value of Co Fe B sampleas compared to Co Fe can be interpreted by the length differ-ence: based on (1), length of Co Fe B nanowires (19 m),represented as “b” in (1), is large compared to that of Co Fe(13 m). Therefore, (1) satisfies this result also. Small incre-ment in and accordingly in after 400 C and 600 Creannealing can be attributed to an increase in grain size athigh temperatures. But that increase is limited due to the shapeof nanowire samples. Fig. 2 shows the XRD patterns of (a)Co Fe and (b) Co Fe B nanowires with the evolution ofcrystalline phases for the increasing temperatures. It is obviousthat no significant peak appeared before and after MF annealing.On the contrary, TEM results showed dominant (110) peak be-fore annealing for Co Fe as depicted in Fig. 3. This behavioris similar to that of Co nanowires for which (110) planes tend tolie along the wire axis without applied external field [10]. How-ever, after 400 C and 600 C reannealing without applied field,(111) peak becomes dominant which shows the fcc phase, thatcan also be observed by heating the bulk Co from 300 C to600 C [11].

The shape of nanowires, the predicted natural texture withc-axis in the plane of membrane (because of high concentrationof cobalt hcp phase) and diffusional pair ordering of unlike atomsalong the direction of external applied field were thought to bethe causes of induced anisotropy. The purpose of reannealing wasto erase the effect of MF annealing (atomic pair ordering) and tofind the actual cause of induced anisotropy. Since there was nosignificant change in hystersis loops and XRD patterns showed(111) peak (change of phase from hcp to fcc) after 400 C and600 C reannealing without field, therefore, the persistence ofinduced anisotropy can be ascribed due to shape of the nanowireother than diffusional atomic pair ordering and hcp phase.

The SAED patterns of both Co Fe , and Co Fe B sam-ples were found to be the same. The diameters of the nanowiresobserved from TEM image lie between 30–150 nm with grain

2780 IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006

Fig. 2. (a) and (b) shows the XRD of Co Fe , Co Fe B nanowire arraysbefore and after MF annealing and reannealing without field at 400 C and600 C. (Color version available online at http://ieeexplore.ieee.org.)

Fig. 3. (a) TEM image of Co Fe and the inset shows the SAED patternof Co Fe and (b) shows nanocrystalline grains of Co Fe B beforeannealing.

size of about 2–8 nm. An amorphous phase was observed [12] inTb-Fe films composed of nanometer-sized grains embedded inthe amorphous matrix which were not detected by XRD exper-iments but considered to affect the magnetic properties [12]. Ithas been observed that presence of sodium citrate can decreasethe grain size [13]. It is thought that it has also affected the grainsof our nanowire samples resulting in appearing no significantpeak in XRD patterns before and after MF annealing of samples.The reannealing at 400 C and 600 C increased the grain size

and then these grains become detectable by X-ray diffractome-tory showing (111) peak after high temperature reannealing.

IV. CONCLUSION

The CoFe and CoFeB nanowires were fabricated usingporous alumina membrane by D.C electrodeposition. Thecoercivity behavior was interpreted based on the model ofprolate ellipsoid chains. The induced magnetic anisotropy inthe nanowires was produced after MF annealing and persistedeven after reannealing of the samples at 400 C and 600 C inabsence of field, instead of erasing the magnetic properties withevolution of crystalline phase in both the samples. Therefore, itcan be concluded that shape of nanowire played the major rolein persistence of induced anisotropy. Microstructural character-izations by XRD and TEM also suggested that nanocrystallinegrains were embedded in the amorphous matrix.

ACKNOWLEDGMENT

This work was supported by the Chinese National NaturalScience Foundation under Grants 10274103, 50371102, and50271081 and its Outstanding Young Researcher Foundationunder Grant 50325104.

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Manuscript received March 13, 2006; (e-mail: xfhan@aphy.iphy.ac.cn;ykim97@korea.ac.kr).