3862 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009

Composition-Dependent Crystal Structure and Magnetism inNanocrystalline Co-Rich Alloy

B. S. Chun���, J. U. Cho�, S. D. Kim�, Y. S. Kim�, J. Y. Hwang���, S. S. Kim�, J. R. Rhee�, T. W. Kim�,J. P. Hong�, M. H. Jung�, and Y. K. Kim�

Department of Materials Science and Engineering, Korea University, Seoul 136-713, KoreaDepartment of Physics, University of Sookmyung Women’s, Seoul 140-742, Korea

Department of Nanotechnology and Advanced Materials Engineering, University of Sejong, Seoul 143-747, KoreaDepartment of Physics, Sogang University, Seoul 121-742, KoreaDepartment of Physics, Hanyang University, Seoul 133-791Korea

CRANN, School of Physics, Trinity College, Dublin 2, IrelandDepartment of Electronics and Communication Engineering,Hanyang University, Seoul 133-791 Korea

We report the magnetism of a ferromagnetic two phase mixture system by investigating the microstructural evolution and relevantproperty changes as a function of alloy composition in CoFeSiB film. The crystal structure evolved from an amorphous to hcp Conanocrystal embedded two-phase mixture when Co concentration exceeded the critical amount ( 75 at.%) confirmed through TEMand XRD. Very low coercivity value was observed in the amorphous samples. The soft magnetic properties are due to the fact that themagnetic moments of Co-rich phase are exchange-coupled via the amorphous matrix, as a consequence the magnetic anisotropies areaveraged out in the amorphous system. Meanwhile, beyond 75 at.% of Co, an abrupt increase in coercivity was observed in the nanocrys-talline samples where highly anisotropic Co nanocrystals were precipitated. The hard magnetic properties resulted from the magneticdecoupling (breaking of the nanocrystal-amorphous matrix-nanocrystal coupling chain) because the exchange forces are not able toovercome the magnetocrystaline anisotropy. We also found that antiferromagnetic exchange coupling existed at the nanocrystal—amor-phous matrix interfaces.

Index Terms—Co-alloy, nanocrystalline, negative remanence.

I. INTRODUCTION

I N recent years, ferromagnetic nanocomposite materialsconsisting of nanocrystals embedded in an amorphous ma-

trix have received both theoretical and experimental attentionsbecause they exhibit unique magnetic, magnetoelectronic, andmechanical properties. Up to now, the magnetic characteristicsof isolated magnetic nanocrystals (or nanoparticles) have beenwell understood [1]–[3]. However, when the nanocrystals areembedded in an amorphous matrix to form a nanocomposite,the magnetic properties significantly differ from the individualand bulk ones. This is mainly because the magnetic momentsof the nanocrystals interact with each other through exchangecoupling via the amorphous matrix. In addition, due to strongdependence of nanocrystalline properties on the size anddistribution of the nanocrystals as well as the portion of theamorphous matrix in the structure, it is uneasy to characterizethe magnetic behaviors of this type of material system bothexperimentally and theoretically.

Several previous investigations have been reported on themagnetic properties of ferromagnetic nanocrystalline materials[4]–[8]. It has been observed that their magnetic properties werecharacterized to be either soft or hard, depending on the sizeand magnetic anisotropy property of the nanocrystals. In addi-tion, the intergranular amorphous matrix also plays a criticalrole. For example, the behavior of the magnetic nanocrystals ina nonmagnetic intergranular material is governed by a single

Manuscript received March 05, 2009; revised May 02, 2009. Current ver-sion published September 18, 2009. Corresponding author: Y. K. Kim (e-mail:ykim97@korea.ac.kr).

Digital Object Identifier 10.1109/TMAG.2009.2024540

domain particle model, while that in the magnetic intergranularphase shows much more complicated behavior [9]. Therefore,a thorough understanding of the exchange interaction betweenthe nanocrystal and matrix is necessary for designing a new ma-terial for potential device applications. In this paper, we attemptto clarify the correlation between the structural and magneticproperties of CoFeSiB alloy films with various compositions.

We have chosen the Co Fe Si B alloy as a modelsystem because it has been employed in the giant mag-netoimpedance sensors, and magnetic tunnel junctions formagnetic random access memory devices. When the Co con-centration was below 75 at.%, the alloy had an amorphousphase and was magnetically soft. Meanwhile, beyond 75 at.%,the structure consisted of nanocrystals precipitated in theamorphous matrix, and became magnetically hard.

II. EXPERIMENT

A series of CoFeSiB thin films were deposited on Si (100)substrates with 100 nm thick thermally oxidized SiO layersusing a six-target dc magnetron sputtering system under atypical base pressure of less than Torr. A magneticfield of 100 Oe was applied during the CoFeSiB film depositionto induce uniaxial magnetic anisotropy. A Co Fe Si Btarget on which small Co chips (5 mm 5 mm 2 mm) wereplaced to control the Co concentration of the CoFeSiB films.Typical film thickness was about 430 nm. The composition ofthe CoFeSiB thin films was confirmed by energy dispersivex-ray spectroscopy (EDX) and inductively coupled plasmaatomic emission spectrometry (ICP-AES). The microstructurewas characterized by high-resolution transmission electron

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CHUN et al.: COMPOSITION-DEPENDENT CRYSTAL STRUCTURE AND MAGNETISM IN NANOCRYSTALLINE CO-RICH ALLOY 3863

Fig. 1. In plane TEM images and SAD pattern of the CoFeSiB films. It isclearly shown that the microstructure of (a) Co Fe Si B is amorphouswith (b) SAD pattern of Co Fe Si B with broad halo ring. As Co con-centration increases, nanocrystals precipitated in the residual amorphous ma-trix are apparent as shown in (c) Co Fe Si B . (d) Dotted SAD pattern ofCo Fe Si B with six-fold symmetry indicating that the lattice structure ofthe nanocrystal is hcp (002).

microscopy (HRTEM) and x-ray diffraction (XRD). The mag-netic properties of the CoFeSiB films at 5 K and 300 K werecharacterized by using a superconducting quantum interferencedevice (SQUID).

III. RESULTS AND DISCUSSION

We have carried out transmission electron microscopy (TEM)and x-ray diffraction (XRD) studies. Both offer complemen-tary (i.e., global and local) information regarding microstruc-tural evolution. A series of in-plane TEM images and selectedarea diffraction (SAD) patterns is displayed in Fig. 1. Fig. 1(a)shows the in-plane TEM images of the Co Fe Si B , and(b) is an SAD pattern of the Co Fe Si B film. Basically,these films with at.% are in amorphous phase.

Fig. 1(c) shows the TEM images of the Co Fe Si B , and(d) the SAD pattern of the Co Fe Si B film. Unlike previoussamples with Co-concentration below 75 at.%, nanocrystallinestructures are apparent. When a solid solution system is super-saturated, it becomes thermodynamically unstable, and, to avoidthat, this system precipitates out the supersaturated solute. In ourcase, the precipitates are most likely Co nanocrystals.

The SAD pattern (see Fig. 1(d)) shows a six-fold symmetrymeaning that the nanocrystal structure is most likely the hcp Co.The lattice structure of the nanocrystals is important in termsof determining the magnetic properties because the hcp Costructure has higher magnetic anisotropy (related with highercoercivity) compared to the fcc one. In overall, the films withCo-concentration above 75 at.% can be regarded as nanocom-posites consisted of a mixture of precipitated nanocrystalsembedded in the amorphous matrix. The TEM result suggests

Fig. 2. Magnetic hysteresis curves in amorphous (a) Co Fe Si B and(b) Co Fe Si B . The magnetic field was applied on the easy direction andthe measurement was performed at 300 K.

that, therefore, the crystal structure of CoFeSiB samples variesfrom an amorphous to a nanocrystal one when the Co con-centration exceeds a certain critical value ( at.%). Inorder to estimate the grain size, at least semi-quantitatively, weused the Scherrer equation [10]. According to this equation,the estimated sizes of Co nanocrystals in the Co Fe Si B ,Co Fe Si B , and Co Fe Si B films were 20 nm, 24 nmand 25 nm, respectively. By increasing the Co concentration,therefore, the precipitates Co nanocrystal size is increased.

Fig. 2 shows the magnetic hysteresis of the amorphous phase(a) Co Fe Si B and (b) Co Fe Si B films measuredat RT.

The values in the amorphous phase appear to be verylow ( Oe) because the magnetic anisotropy arises from theshort-range order. The soft magnetic properties of amorphousmaterials were explained by Herzer within the framework of socalled the random magnetic anisotropy model [6]. Accordingto this model, when the exchange correlation length ex-ceeds the orientation fluctuation length, the magnetic momentsare strongly exchange-coupled via the amorphous matrix. Be-cause of this, the anisotropy energy of the Co-rich phase can betransmitted to each other. Hence, excellent soft magnetic prop-erties have been observed due to the fact that all of the mag-netic anisotropies were averaged out in the amorphous system[6]–[8]. Moreover, the Co-based amorphous alloys exhibit nearzero magnetostriction [11].

The magnetic characteristics of the nanocomposite, wherenanocrystals are embedded in the amorphous matrix, arediscussed. Fig. 3(a) and (b) shows the M-H hysteresis of thenanocrystal Co Fe Si B sample which has a different shapefrom that of the amorphous one. The M-H curve representsa linear magnetization switching part with an abrupt increasein . The magnetic hysteresis curves of the Co Fe Si Bsample are displayed in Fig. 3(a) and (b) for high and lowfield sweeps, respectively. The saturation field of this sample

3864 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009

Fig. 3. Magnetic hysteresis curve of the nanocrystalline Co Fe Si Bsample. This sample exhibits (a) linear magnetization switching with a highsaturation magnetization field (full M-H curve) and (b) high coercivity withpositive remanence (reduced M-H curve).

is around 2.5 kOe (see Fig. 3(a)). This large saturation field ispresumed to be due to an antiferromagnetic exchange couplingbetween the Co nanocrystals and residual amorphous matrixat the interface. As an attempt to confirm this hypothesis, wemeasured (not shown here) the applied magnetic field depen-dence of the Néel temperature . A decrease in thevalue with increasing magnetic field is a typical characteristicof antiferromagnetic exchange coupling [12].

In this case, the exchange correlation length can not exceedsthe orientation fluctuation length and hence, the magnetic de-coupling occurs between amorphous matrix and Co nanocrys-tals (broken of the nanocrystal-amorphous matrix-nanocrystalcoupling chain). And, as a consequence, it brings a significantincrease in the (see Fig. 3(b)). The magnetic decoupling wasalso observed at temperatures above the Curie temperature ofthe residual intergranular amorphous phase [13]. Another originof the increase in may also be due to magnetoelastic effectresulted from precipitation of Co nanoparticles. The presenceof internal stress along with a large magnetostriction constantof results in the development of magnetoelasticanisotropy [9].

Fig. 4 shows the valve as a function of Co content inCoFeSiB film. When the Co concentration was below 75 at.%,the alloy was magnetically soft. Meanwhile, beyond 75 at.% ofCo the alloy became magnetically hard.

IV. CONCLUSION

We have investigated the microstructural evolution andchange in the relevant magnetic properties upon the addition ofCo to an amorphous ferromagnetic CoFeSiB alloy. The crystalstructure evolved from amorphous to nanocrystal-embeddedtwo-phase mixture system when the amount of Co additionexceeded the critical amount ( 75 at.%). Very low coercivity

Fig. 4. � valve as a function of Co content in CoFeSiB film.

values took places in the amorphous state due to the averagingout of magnetic anisotropy. Meanwhile, a drastic magnetichardening and antiferromagnetic exchange coupling effectsappeared for higher Co concentration samples due to the pre-cipitation of highly anisotropic Co nanocrystals.

ACKNOWLEDGMENT

This work was supported by the Korea Science and Engi-neering Foundation through the National Research LaboratoryProgram (No. M60501000045-05A0100-04510). B. S. Chunacknowledges financial support from by the Korea ResearchFoundation Grant (KRF-2006- 352-D00091).

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