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Intermetallics 16 (2008) 636e641www.elsevier.com/locate/intermet

Structural and Mossbauer studies on mechanical milledSmCo5/a-Fe nanocomposite magnetic powders

P. Saravanan, M. Manivel Raja, R. Gopalan*, N.V. Rama Rao,K. Suresh, D.V. Sridhara Rao, V. Chandrasekaran

Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad 500 058, India

Received 7 November 2007; received in revised form 9 January 2008; accepted 15 January 2008

Available online 14 March 2008

Abstract

The SmCo5/a-Fe nanocomposite powders were prepared by high energy ball milling and the inter-diffusion reaction between the SmCo5 anda-Fe magnetic phases were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry(VSM) and 57Fe Mossbauer spectroscopy. While structural and magnetic measurements could reveal only the presence of SmCo5 and a-Fephases, Mossbauer studies could clearly specify the extent of alloying between Fe and Co atoms in terms of evolution of a-Fe(Co) phase asa function of milling time. It has been found that the fractional volume of a-Fe(Co) solid solution tends to increase at the expense of the initiala-Fe phase upon progressive milling.� 2008 Elsevier Ltd. All rights reserved.

PACS: 75.50.ww; 75.50.Tt; 76.80.þy; 81.20.Ev

Keywords: A. Rare-earth intermetallics; B. Magnetic properties; C. Mechanical alloying and milling; D. Microstructure; F. Spectroscopic methods, various

1. Introduction

Efforts are underway to improve the energy product[(BH )max.] and Curie temperature (Tc) of rare-earth permanentmagnets through nanocomposite approach. The concept whichlies behind the evolution of these novel magnetic materialsarises from the possibility of spring exchange coupling be-tween the hard and soft magnetic phases when their size is re-duced to nanoscale. The combination of large saturationmagnetization of the soft phase and the large coercivity ofhard phase is predicted to produce nanocomposite with supe-rior magnetic properties. Among the rare-earth permanentmagnets, SmCo5 has large magneto-crystalline anisotropy field(>240 kA/m) and also high Tc (w1000 K). However, there areprocess limitations in realizing the energy product values ofSmCo5 magnets [(BH )max. (theoretical)¼ 1/4(m0Ms)

2¼ 265 kJ/m3,

* Corresponding author. Tel./fax: þ91 40 2434 0884.

E-mail address: rg_gopy@yahoo.com (R. Gopalan).

0966-9795/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.intermet.2008.01.011

where m0 is permeability in free space and Ms is saturationmagnetization]. In order to overcome this barrier, two-phasenanocomposite magnet comprising of SmCo5 (hard) andFeCo (soft) magnetic phases seems to be more promising[1e3]. Recently, it has been demonstrated that enhancedmagnetization can be obtained in mechanically milled andspark plasma sintered SmCo5þ 10e15 wt% Fe nanocompo-site magnet [4e6]. Despite the fact that Fe has very limitedsolubility in SmCo5 phase [7], nearly a single-phase mag-netic behavior is seen for the SmCo5/a-Fe nanocompositemagnetsdimplying a strong magnetic coupling between thesoft and hard phases [6,8]. It is presumed that during me-chanical milling of SmCo5 and Fe powders, some amountof Fe could be probably dissolved in the SmCo5 crystal lat-tice. However, according to the SmeCoeFe phase equilibria,Fe has almost nil solubility in SmCo5 phase [7] and hence itis important to understand the extent of the interaction thattakes place between these two phases during milling. In gen-eral, structural/phase formation during mechanical alloyingof SmCo5 and Fe powders is studied mainly by X-ray

Fig. 1. X-ray diffraction patterns of as-milled SmCo5/a-Fe nanocomposite

powders for various milling times. The Miller indices of the main diffraction

peaks are indicated in the figure.

637P. Saravanan et al. / Intermetallics 16 (2008) 636e641

diffraction and transmission electron microscopy. Each ofthese techniques has its own merits and demerits to exactlyfinger-print the formation of the FeeCo phase, as the crystalstructure of a-Fe and a-Fe(Co) are same. On the other hand,the Mossbauer spectroscopy is a powerful tool to investigatethe local interactions between iron nuclei and their nearestneighbours through discrimination of the magnetic contribu-tions of Fe, thus providing useful information regarding for-mation of the Fe(Co) phase. Recently, Blazquez et al. [9]have investigated the microstructure and local arrangementsof Fe atoms that continuously evolve during ball milling ofnanocrystalline FeeNbe(B,Ge) alloys using Mossbauer ef-fect measurements. In the present work, we have investigatedthe effect of Fe addition on local structural changes thatevolve during mechanical milling of SmCo5 and a-Fe pow-ders using Mossbauer spectroscopy and the results are dis-cussed in complement with X-ray diffraction (XRD),transmission electron microscopy (TEM) and magnetizationstudies. Such a study is considered to be scientifically andtechnologically important for the development of SmCo5/Fenanocomposite magnets e as the occurrence of various mag-netic phases such as a-Fe, SmCo5 and Sm(Co,Fe)5 duringmilling can significantly influence the magnetic propertiesof bulk magnets.

2. Experiment

The precursor SmCo5 alloy was prepared by melting of el-emental Sm and Co in high purity argon atmosphere. The alloyingot was crushed into powder size of w300 mm and wasmixed with commercially available a-Fe powders (Alfa Aesar;purity: 99.5%) having particle size less than 10 mm. The a-Fepowder of about 10 wt% was selected for mixing with SmCo5

powder for ball milling, as this fraction of Fe has been re-ported to yield relatively high coercivity and magnetizationin isotropic nanocomposite magnets [4,5]. Milling was per-formed in a planetary ball mill (FRITSCH pulverisette) withthe milling vial and balls made of tungsten carbide. Duringball milling, blocks of Nd2Fe14B permanent magnets werefixed around the vial, to create a radial magnetic field insidethe vial to synthesize anisotropic powders during milling.The milling time was varied typically from 2 to 30 h at a con-stant speed of 200 rpm with ball to powder ratio of 10:1. Theprogress of phase formation at various milling time was stud-ied by X-ray diffraction (XRD) using a Philips diffractometerusing Cu Ka radiation. High resolution transmission electronmicroscopy (model: FEI-TECNAI 20 UT) was used for micro-structural studies of as-milled powders. Specimens for TEMstudies were prepared by dimpling and ion milling in Gatanmake Dual Ion Mill (model 600) at 5 kV. TEM specimens ofpowder samples were prepared by mixing in a high-tempera-ture conducting polymer epoxy subsequently dried at 130 �Cand a 3 mm disk was finally cut for ion milling. Magneticproperties of the powder samples were evaluated using a vi-brating sample magnetometer (VSM) (ADE make, modelEV9) up to a maximum field of 2 T. Mossbauer spectra wererecorded using FAST comtec (German make) spectrometer

at room temperature in a transmission geometry witha 25 mCi 57Co source in Rhodium matrix. Prior to the exper-iments, the spectrometer was calibrated using a standard a-Fefoil of thickness 25 mm. The Mossbauer spectra of each milledpowder sample were recorded for duration of about 5 dayswith total background counts up to 8e10� 105. The Moss-bauer spectra were fitted with the PCMOS-II least-square fitprogram [10].

3. Results and discussion

The XRD patterns of SmCo5/a-Fe powder mixtures milledfor different milling times are shown in Fig. 1. The milledpowders essentially consist of two phases, viz. SmCo5 (mainphase) and a-Fe (minor phase) and there is no evidence forthe presence of any oxide impurities. Upon increasing themilling time, broadening of XRD peaks and reduction in theirintensities are observed owing to the grain refinement andstrain accumulation induced during the milling process. TheXRD peaks of the SmCo5 and the Fe phases have overlappedafter 10 h milling due to the broadening effect. Further, it is

Fig. 3. Hysteresis loops for SmCo5/a-Fe nanocomposite powders processed at

different milling times. The inset shows the plot of saturation magnetization

vs. milling time.

638 P. Saravanan et al. / Intermetallics 16 (2008) 636e641

evident from the XRD patterns that as the milling time in-creases, the angular position of (110) peak corresponding toa-Fe phase slightly shifted in relation to that of the pure a-Fe phase present in the starting powder. This angular shift ofthe (110) a-Fe peak could be possibly due to the dissolutionof Co from SmCo5 phase into Fe lattice during milling.

We investigated the microstructure of the milled powdersusing high resolution transmission electron microscopy(HRTEM) and the typical HRTEM micrograph of 15 h milledSmCo5/a-Fe powder is shown in Fig. 2. The SAED ring pat-tern shown in the inset of Fig. 2 indicates the formation ofnanocrystallites. From fast Fourier transformation (FFT) anal-ysis of the HRTEM images and also by lattice fringe distancemeasurements the crystallites were indexed to SmCo5 and a-Fe phases. Although it cannot be ruled out the formation ofa-Fe(Co) phase in the milled powder, it may be difficult to dis-tinguish a-Fe and a-Fe(Co) phases from HRTEM analysis.The average grain of the magnetic phases estimated fromHRTEM was found to be in the range of 5e10 nm.

The hysteresis loops measured for the SmCo5/a-Fe pow-ders processed at various milling time are shown in Fig. 3and the dependence of magnetization on milling time is shownin the inset. The saturation magnetization value of the milledpowders is strongly affected by the processing time; the satu-ration magnetization value increased from 46.3 (2 h) to70.2 emu/g (10 h) and almost attained a constant value(72 emu/g) up to 30 h of milling. The low value of saturationmagnetization obtained for the 2 h milling time can be attrib-uted to the disordering at the surfaces caused by the particlesize reduction. Further increase in saturation magnetizationupon progressive milling may be attributed to the dissolutionof Co into Fe lattice and therefore to the formation of a-Fe(Co) phase [11]. It can be seen from inset of Fig. 3 that thereis noticeable constancy in the saturation magnetization valuesafter 15 h of milling e this can be attributed to the saturationof Fe dissolution in SmCo5 lattice, making the phase more

Fig. 2. High resolution transmission electron micrograph of the 15 h milled

SmCo5/Fe nanocomposite powder. Inset shows the corresponding SAED

pattern.

homogeneous with the increase of milling time [4,12]. Itshould be mentioned that the increase in magnetization valueswith the increase of milling time alone cannot be taken as anindication for the evolution of a-Fe(Co) phase, as the mea-sured magnetization values may have their components origi-nating from other magnetic phases such as a-Fe, SmCo5 andSm(Co,Fe)5 present in the milled powder.

While the existence of a-Fe(Co) phase formation in themilled SmCo5/a-Fe powders was not clearly substantiated bythe magnetic measurements, Mossbauer studies allowed usto track the process of a-Fe(Co) solid solution formationand its influence over the bulk magnetic properties at variousmilling times. Fig. 4 shows Mossbauer spectra obtained fromthe powder samples processed for different milling times rang-ing from 2 to 30 h and the results of fitting of the Mossbauerspectra are summarized in Table 1. During the first 2 h of mill-ing, the Mossbauer spectra are consistent with a sextet havingan average hyperfine magnetic field of about 33 T correspond-ing to a-Fe. The spectra which were obtained between 6 and15 h of milling time can be fitted well with two subspectra:a sextet and a broad sextet. These subspectra can be associatedto a a-Fe phase, with a hyperfine magnetic field of 33 T and toa a-Fe(Co) disordered ferromagnetic phase having hyperfinemagnetic fields in the range of 33e35.6 T. As the hyperfinefield of the latter phase is higher than that of the a-Fe phase,it clearly suggests that some of the Co atoms from the SmCo5

hard phase are alloyed with Fe, thus forming a-Fe(Co) solidsolution during milling. It should be recalled that the presenceof one Co atom in the vicinity of the Fe atoms can increase thehyperfine magnetic field by about 0.8 T [13]. The Mossbauerspectra which were obtained at longer milling duration(>15 h) showed broadening of the Mossbauer lines associatedwith a shift towards higher hyperfine fields (>33 T). The for-mer fact was a result of reduction in the grain size accompa-nied by high lattice strain in ball milled powders and the

Fig. 4. Room temperature Mossbauer spectra obtained for the SmCo5/a-Fe nanocomposite powders processed at different milling times.

639P. Saravanan et al. / Intermetallics 16 (2008) 636e641

latter may be attributed to the alloying process of Fe and Coand consequently, to the presence of Co atoms as first and sec-ond nearest neighbours of Fe atoms. In addition to the a-Feand a-Fe(Co) contributions, it can be expected that the diffu-sion of Fe into SmCo5 would definitely occur leading to theformation of Sm(Co,Fe)5 regions [14]. However, in the presentstudy, no attempt has been made to fit this magnetic contribu-tion due to the low percentage transmission. It should be notedthat even after 20 and 30 h of milling, the magnetic contribu-tion of Sm(Co,Fe)5 corresponding to hyperfine field values of26e33 T was not resolved in the Mossbauer spectra and this

Table 1

Mossbauer parameters for as-milled SmCo5/Fe nanocomposite powders for

various milling times

Milling

time (h)

Assigned

phase(s)

I.S.� 0.005

(mm/s)

Bhf� 0.2

(T)

L.W.� 0.02

(mm/s)

INT.

(%)

2 a-Fe �0.002 33.2 0.28 100.0

a-Fe(Co) e e e e

6 a-Fe 0.002 33.2 0.32 89.5

a-Fe(Co) �0.043 33.7 0.38 10.5

10 a-Fe �0.002 33.2 0.36 83.4

a-Fe(Co) �0.027 35.3 0.38 16.6

15 a-Fe 0.017 33.1 0.30 70.5

a-Fe(Co) �0.051 35.6 0.38 29.5

20 a-Fe �0.045 32.9 0.40 25.7

a-Fe(Co) �0.025 33.9 0.40 74.3

30 a-Fe �0.036 32.7 0.40 24.8

a-Fe(Co) �0.029 33.8 0.40 75.2

I.S. e isomer shift (values are given relative to a-Fe at room temperature);

Bhf e hyperfine field; L.W. e line width; INT. e relative intensity.

may be due to the fact that the powders possibly attainedamorphous/disordered nature with longer duration of milling.

The variation in the relative intensities of the a-Fe and a-Fe(Co) magnetic contribution corresponding to the Mossbauerspectra of SmCo5/a-Fe nanocomposite powders as a functionof milling time is shown in Fig. 5a. It can be seen that withthe increase of milling time, the fractional volume of the dis-ordered ferromagnetic phase tends to increase at the expenseof the initial a-Fe phase. The relative intensity (proportionalto the Fe content) of the disordered ferromagnetic phase (a-Fe(Co)) increased from 0 to 75.2% with the increase of millingtime from 2 to 30 h. Meanwhile the relative intensity corre-sponding to the a-Fe subspectrum decreases in a very similaramount from 100 to 24.8% as the processing time increasesfrom 2 to 30 h. In Fig. 5b, we show the evolution of averagehyperfine magnetic fields of a-Fe(Co) phase as a function ofmilling time, which depicts the progress of inter-diffusion re-action between Fe and Co atoms. A rapid increase in hyperfinemagnetic field between 6 and 15 h of milling and subsequentdecrease in hyperfine field at longer milling time can be no-ticed. The formation of a-Fe(Co) solid solution during themilling of SmCo5 and the a-Fe can be related to the rapid in-crease in hyperfine field, while the reduced grain size and theamorphous nature of the powders could be responsible for thedecrease in hyperfine field. Significantly, a maximum hyper-fine magnetic field (35.6 T) was obtained for the 15 h milledpowder, corresponding to the formation of a-Fe(Co) solid so-lution and the estimated proportion of Co in a-Fe, correspond-ing to these hyperfine fields, is ranging between 22 and 32% ofCo atoms [15]. Further, the presence of a magnetic contribu-tion close to a hyperfine field of 33 T (corresponding to purea-Fe) with a relative intensity w25% could be noticed even

Fig. 5. (a) Mossbauer relative intensities of the two magnetic contributions to

the Mossbauer spectra of SmCo5þ 10 wt% Fe nanocomposite powders and (b)

evolution of average hyperfine magnetic fields of a-Fe(Co) phase as a function

of milling time.

640 P. Saravanan et al. / Intermetallics 16 (2008) 636e641

after 30 h of milling (see Table 1) e suggesting that some re-sidual amount of the initially added a-Fe (10 wt%) is stillpresent in the milled powders.

With the above results of Mossbauer experiments, hereinwe try to correlate with those obtained using the VSM usingSlaterePauling curve [16]. It suggests that the saturation mag-netization increases to a maximum in the case of Fe alloyswith 30 at% of Co and it decreases on further increase of Cocontent. In a similar manner, the observed hyperfine magneticfield variations of SmCo5/a-Fe powders against milling timecan be accounted for the evolution of the inter-diffusion reac-tion between Fe and Co and also for the increasing proportionof Co content in the a-Fe phase. However, the trend of varia-tion in the saturation magnetization values against milling timedoes not follow the hyperfine field variation; the saturationmagnetization values nearly appear to be constant after 15 hof milling, in contrast to the observed decrease in hyperfine

field. It should be recalled that the VSM measurements alwaysimpart the magnetization values e which are cumulative of allthe magnetic phases present in the powder samples; on theother hand, the Mossbauer experiments can resolve the indi-vidual magnetic contribution of Fe atoms and their local inter-action with their nearest neighbours. Although it is quitedifficult to estimate the actual composition of Fe(Co) phase,we believe that the possibility of obtaining a-Fe(Co) phasewith more (>30 at%) Co content could be favored with longermilling duration based on the previous studies [5,6]. TheMossbauer results obtained in the present study can be consid-ered as a direct evidence to confirm the formation of thea-Fe(Co) phase during milling and interestingly the soft mag-netic phase will have a large saturation magnetization whichcan further enhance the ultimate magnetic properties of the ex-change coupled nanocomposite magnets.

4. Conclusions

The alloying of Fe and Co in the formation of a-Fe(Co)during mechanical milling of SmCo5/a-Fe nanocompositepowders has been investigated by XRD, TEM and 57Fe Moss-bauer spectroscopy. XRD and HRTEM studies of the milledpowders could only show the presence of SmCo5 and a-Fephases; on the other hand, Mossbauer results indicate the evo-lution of a-Fe(Co) phase in terms of hyperfine field variationas a function of milling time. An increase in hyperfine fieldvalue up to 35.6 T was observed at 15 h of milling e attributedto the increasing proportion of Co content in the a-Fe phase.The Mossbauer analysis also suggests that the fractional vol-ume of a-Fe(Co) phase tends to increase at the expense ofthe initial a-Fe phase upon progressive milling.

Acknowledgments

The authors are grateful to Defence Research and Develop-ment Organization, Government of India for the financial sup-port to carry out this work. The keen interest shown by theDirector, DMRL in this work is gratefully acknowledged.

References

[1] Chu S, Majetich SA, Huang M, Fingers RT. J Appl Phys 2003;93:

8146e8.

[2] Jiang JS, Pearson JE, Liu ZY, Kabius B, Trasobares S, Miller DJ, et al.

Appl Phys Lett 2004;85:5293e5.

[3] Rama Rao NV, Gopalan R, Manivel Raja M, Chandrasekaran V,

Chakravarty D, Sundaresan R, et al. J Magn Magn Mater

2007;312:252e7.

[4] Pop V, Isnard O, Chicinas I, Givord D. J Magn Magn Mater

2007;310:2489e90.

[5] Zhang J, Zhang S-y, Zhang H-w, Shen B-g. J Appl Phys 2001;89:

5601e5.

[6] Ito M, Majima K, Umemoto T, Katsuyama S, Nagai H. J Alloys Compd

2001;329:272e7.

[7] Bushchow KHJ. J Less-Common Met 1973;31:359e64.

[8] Tung I-C, Zhang H-w, Yao S-Y, Shih J-C, Shen B-g, Chin T-S. J Phys D

Appl Phys 1999;32:1587e90.

641P. Saravanan et al. / Intermetallics 16 (2008) 636e641

[9] Blazquez JS, Franco V, Conde CF, Conde A. Intermetallics

2007;15:1351e60.

[10] Cranshaw TE. J Phys E 1974;7:122e4.

[11] Landolt-Bornstein. In: Mehrer H, editor. Diffusion in solid metals and al-

loys, new series group III, vol. 26. Berlin: Springer; 1990. p. 127.

[12] Li H-F, Ramanujan RV. J Electron Mater 2004;33:1289e97.

[13] Vincze I, Campbell IA, Meyer AJ. Solid State Commun 1974;15:1495e9.

[14] Pop V, Isnard O, Chicinas I, Givord D, Le Breton JM. J Optoelectron

Adv Mater 2006;8:494e500.

[15] Fultz B, Hamdeh H, Okamoto J. Hyperfine Interact 1999;54:799e803.

[16] Cullity BD. Introduction to magnetic materials. Reading, Massachusetts:

Addison-Wesley; 1972.