Journal of Alloys and Compounds 574 (2013) 191–195

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Journal of Alloys and Compounds

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Annealing induced compositional changes in SmCo5/Fe/SmCo5 exchangespring trilayers and its impact on magnetic properties

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.04.019

⇑ Corresponding authors. Address: Department of Physics, National TaiwanUniversity, Taipei 106, Taiwan (P. Saravanan). Tel.: +886 2 33665162; fax: +886 233665892.

E-mail addresses: psdrdo@gmail.com (P. Saravanan), jhhsu@phys.ntu.edu.tw(J.-H. Hsu).

P. Saravanan a,b,⇑, Jen-Hwa Hsu a,⇑, G.L.N. Reddy c, Sanjiv Kumar c, S.V. Kamat b

a Department of Physics, National Taiwan University, Taipei 106, Taiwanb Defence Metallurgical Research Laboratory, Hyderabad 500 058, Indiac National Centre for Compositional Characterization of Materials, Bhabha Atomic Research Centre, ECIL Post, Hyderabad 500 062, India

a r t i c l e i n f o

Article history:Received 2 April 2013Accepted 4 April 2013Available online 13 April 2013

Keywords:Nanocomposite magnetsExchange couplingSmCo5/Fe/SmCo5

TrilayersSputteringMagnetic thin films

a b s t r a c t

SmCo5/Fe/SmCo5 sandwich configurations have gained renewed interest when compared to the conven-tional SmCo5/Fe bilayers, as they exhibit enormous scope to impart high energy product values withstrong magnetization reversal behavior. The compositional changes that occur in the intermediate softlayer upon annealing play a critical role in determining the phase chemistry, as well as magnetic perfor-mance of the trilayers and herein we address such a study. SmCo5/Fe/SmCo5 sandwich films with fixedlayer thicknesses of SmCo5 (25 nm) and Fe (8 nm) were grown by sputtering on Cr buffered Si (100) sub-strates and subsequently, annealed at different temperatures: 673, 773 and 873 K. The effect of post-deposition annealing on the structure, composition and magnetic properties of the trilayer architecturewas investigated by X-ray diffraction (XRD), Rutherford back-scattering (RBS) and super-conductingquantum interference device (SQUID), respectively. The XRD studies showed significant decrease in thelattice parameter values with increasing annealing temperature, which suggests an increase in the dif-fused Co-content in the Fe-intermediate layer. The XRD results were further validated by RBS measure-ments, which confirmed that both composition and thickness of the Fe-intermediate layer were stronglyaffected by the annealing temperature. The SQUID measurements demonstrated the existence of in-planeanisotropy and strong exchange coupling between the hard and soft layers for all the annealed sandwichfilms. The trilayer stack annealed at 773 K showed the best magnetic performance such as high coercivity(814 kA/m), remanence (944 kA/m) and energy product (243 kJ/m3).

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

‘Exchange-spring’ magnets, composed of suitably nanodi-spersed hard (SmCo5) and soft (Fe) magnetic phases that are ex-change-coupled at their interfaces, hold great potential as highenergy product permanent magnets [1–3]. The physics of interfa-cial conditions for the adjacent ferromagnetic phases with appro-priate exchange stiffnesses and their effect on the interphaseexchange coupling – have continuously been the subject of interest[4]. A strong interphase exchange coupling between the hard andsoft phases manifests as a single-phase demagnetization curve inthe nanocomposite magnets and this has been demonstrated inbulk Sm–Co/Fe nanocomposite magnets such as those made usingmechanical milling [5–7] and rapid solidification techniques [8,9].However, it is often difficult to probe the crystallography and con-

dition of each interface in these bulk nanostructured materials. Assuggested by Skomski and Coey [3], layered structures provide asimplified architecture for systematically studying the magneticproperties of exchange coupled nanocomposite materials. A simplemodel system for the study of interphase exchange coupling istherefore a bilayer system in which the interfacial condition is var-ied systematically. In this context, fabrication of exchange coupledSmCo5/Fe bilayers has gained considerable attention and, of course,substantial improvement in the energy product has been achievedin the SmCo5/Fe nanocomposites [10–12]. In such bilayers, apartfrom the soft layer thickness, one of the factors which determinethe extent of exchange coupling between the hard and soft layersis the formation of Fe–Co intermixed phase at the interface[13,14]. It should be noted that in the SmCo5/Fe bilayers, the inter-diffusion of some amount of Co from SmCo5 into Fe-phase is quiterapid at the coupling interface even at moderate annealing temper-atures [15]. By properly tuning the deposition and/or annealingtemperatures, the interfacial mixing of Co with Fe can be con-trolled to achieve optimal phase chemistry and composition forthe soft layer and thereby, the performance of SmCo5/Fe bilayerscan be improved [12–16].

(a)

(b)

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192 P. Saravanan et al. / Journal of Alloys and Compounds 574 (2013) 191–195

Along these lines, it is worth mentioning that the sandwich con-figurations of SmCo5/Fe/SmCo5 films have gained considerableattention, owing to their ability to produce high energy productvalues with strong interphase exchange coupling [17–19]. For a tri-layer system with identical SmCo5 layers, the geometry is symmet-ric with respect to the central Fe layer [20]. When compared to thetypical SmCo5/Fe bilayers, the trilayer architecture has the abilityfor rigid coupling of a large volume fraction of high moment softphase to the hard magnetic SmCo5 phase [19]. Thus, the investiga-tions on controlling the interphase chemistry and phase fraction ofintermediate layer in such sandwich films assume greater impor-tance. Zhang et al. showed the formation of Fe–Co intermixedphase (bcc-Fe65Co35) in Sm2Co7/Fe/Sm2Co7 trilayers annealed at723 K, through transmission electron microscopy (TEM), energydispersive X-ray analysis (EDX) and parallel electron energy lossspectroscopy [15]. Besides, Sawatzki et al. [19], exploited the po-tential of Rutherford backscattering (RBS) in studying the inter-phase chemistry of epitaxially grown SmCo5/Fe/SmCo5 films at773 K, as a function of Fe-layer thickness (tFe). The increase in ac-tive soft layer thickness and the compositional changes due tothe interfacial mixing were estimated by RBS and were substanti-ated with the TEM/EDX studies. By this means, they elaborated ameaningful correlation for the remanence enhancement and highenergy product (310 kJ/m3) achieved in these trilayers whentFe = 12 nm. At this juncture, it should be noted that besides tuningthe tFe for optimal interphase coupling; a good phase chemistryand phase fraction for the intermediate soft layer and hence en-hanced magnetic properties can be obtained by choosing appropri-ate annealing temperature with fixed tFe. Accordingly, we hereinpresent our results to illustrate annealing induced compositionalchanges that occur at the intermediate layer of SmCo5 (25 nm)/Fe/SmCo5 (25 nm) sandwich films through RBS and XRD withtFe = 8 nm.

(d)

2. Experimental

A sequence of Si (100)/Cr (90 nm)/SmCo5 (25 nm)/Fe (8 nm)/SmCo5 (25 nm)/Cr(10 nm) layered structures was grown by a multiple-gun sputtering system. Therole of 90 nm Cr layer deposited on Si substrate was to act as a buffer; whereasthe role of the 10 nm Cr cap layer was to prevent oxidation of the highly reactiveSmCo5. The sputtering was carried out at a chamber base pressure <6.6 � 10�5 Paand high purity argon (99.9999%) was used as sputter gas. The powder compactof SmCo5 alloy sintered at 1473 K was used to make SmCo5 target. Both the sputter-ing pressure and power were adjusted to obtain the stoichiometric composition of1:5 for the SmCo5 hard layer. Fe and Cr targets were commercially obtained andwere of 99.9 at.% purity. The layer thicknesses of Cr, SmCo5 and Fe were estimatedfrom the sputtering time. For the preparation of SmCo5/Fe/SmCo5 trilayer thin film,the individual layers were deposited immediately one after another without break-ing the vacuum and the substrate was maintained at room temperature. After depo-sition, the samples were subjected to annealing at different temperatures: 673, 773and 873 K for 0.5 h. The phase composition of the trilayer stacks was identified byXRD. Rutherford backscattering spectrometry with 6.8 MeV 12C was used to verifythe layered architecture of the stacks. Magnetic hysteresis loops of the trilayerswere obtained using superconducting quantum interference device (SQUID) witha maximum applied field of 7 T. The magnetization values (M) obtained with theSQUID measurements were converted into B, using the equation B = l0 (H + M);where l0 is the permeability of free space, B is the flux density and H is the appliedfield. From the B–H curves, the maximum BH product, (BH)max – corresponding tothe largest rectangular area drawn within the second quadrant of B–H curve wasestimated.

Fig. 1. XRD patterns of SmCo5/Fe/SmCo5 trilayers: (a) as-grown, (b, c and d)correspond to the films annealed at 673, 773 and 873 K, respectively.

3. Results and discussion

The XRD patterns for the trilayer samples annealed at differenttemperatures are shown in Fig. 1. Though, all the XRD patterns canbe indexed to SmCo5 as a major constituent; the evolution of othersecondary phases such as Sm(Co,Fe)7 and Sm2Co7 are unavoidableespecially at higher annealing temperatures. The formation of suchsecondary phases either during deposition or annealing of Sm–Cofilms are generally observed due to the combined effects of atomic

diffusion and local variation of Sm and Co ratio [21,22]. The peaklocated at �44.8� is assigned to the (110) plane of the bcc-Fe(Co) phase rather than Fe. This is because, this Bragg diffractionpeak is slightly shifted in relation to that of the pure Fe and thiscould possibly be attributed to the diffusion of Co from SmCo5 into

Table 1Thickness and composition of Fe-intermediate layer in the SmCo5/Fe/SmCo5 sandwich films as a function of annealing temperature.

Annealing temperature (K) RBS XRD

Thickness (nm) Co-content (at.%) Lattice parameter (Å) Co-contenta (at.%)

673 8.5 18 2.860 22773 9.2 34 2.855 36873 10.1 54 2.848 49

a Approximate Co-contents were taken from Ref. [24] for the calculated lattice parameter values.

P. Saravanan et al. / Journal of Alloys and Compounds 574 (2013) 191–195 193

Fe-phase during annealing [23]. Accordingly, the compositionchanges that have occurred in the Fe layer were estimated basedon the variations in the lattice parameter values of bcc-Fe (Co)phase [24] and the results are given in Table 1. It can be observedthat as the annealing temperature increases, the lattice parametervalues tend to decrease and this can be attributed to the propor-tionate increase in the Co-content of Fe-layer. Besides, uponincreasing the annealing temperature from 673 to 873 K, the inten-sity of Fe (Co) peak was found to increase – indicating that the softlayer thickness might have increased.

The experimental and the simulated RBS spectra for SmCo5/Fe/SmCo5 trilayer films with nominal interlayer thickness of 8 nm Feare shown in Fig. 2. In these spectra, the solid curves represent thesimulated spectra, while the open circles show the experimentaldata. The simulation was performed using the SIMNRA software[25], which allows the determination of thickness and compositionof each individual layer. Furthermore, careful analysis shows that

Fig. 2. RBS spectra of SmCo5/Fe/SmCo5 trilayers annealed at different temperatures:(a) 673, (b) 773 and (c) 873 K.

assumption of a sharp interface between the three magnetic layerswould not lead to a satisfactory description of RBS data, as well astheir fitting. To resolve this discrepancy, the diffusion of Co into theintermediate Fe layer should be accounted, as described previously[26]. The thickness values for the bottom and top hard layers(SmCo5) were estimated by RBS and were found to be less thantheir initial nominal layer thicknesses. It should be noted thatthough one can not expect any obvious reduction in the top andbottom layer of SmCo5, as a cause of rapid diffusion of Co fromSmCo5 into Fe phase; the thickness difference can be accountedto the formation Fe-Co intermixed layer at the interface and prob-able diffusion of Fe into the SmCo5 phase [15]. This makes our esti-mation of SmCo5-layer thickness slightly less than their nominalthickness values; however the overall thickness of the trilayersstacks did not change. Similar findings on the estimation of re-duced thicknesses for the top and bottom layers have been previ-ously reported in the SmCo5/Fe/SmCo5 sandwich films [17]. Theeffective soft layer thickness in the present study was estimatedby assuming a layer of Fe (Co) solid solution (intermixed layer)formed at the interface of hard and soft layers. The calculatedFe-interlayer thickness and diffused Co-content in the Fe layerare given in Table 1. It can be seen that the increase in annealingtemperature results in gradual increase in the Co-content accom-panied by moderate increase in the soft layer thickness. Theestimated Co-content values in the Fe-layer are consistent withthe calculations based on the XRD analysis.

Fig. 3 shows both in-plane (||) and perpendicular plane (\) hys-teresis loops obtained for the trilayer stacks annealed at differenttemperatures. It can be seen that the in-plane hysteresis curvesdemonstrate a typical hard magnetic behavior with distinctivesquareness – reflecting the existence of strong-exchange couplingamong the SmCo5 (bottom), Fe (middle) and SmCo5 (top) layers.On comparing the in-plane hysteresis curves with those in the per-pendicular plane, a significant reduction in the remanence (Mr\)and coercivity (Hc\) values is evident – indicating that the trilayersexhibit in-plane anisotropic characteristics. The Hc||, Mr|| and(BH)max|| values were estimated for the trilayers annealed at differ-ent temperatures and are given in Table 2. Relatively, high valuesof Hc|| (814 kA/m), Mr|| (944 kA/m) and (BH)max|| (243 kJ/m3) wereachieved for the sample annealed at 773 K. The significant increasein Mr|| and the associated (BH)max|| values can be ascribed to the for-mation of a high moment soft Fe phase with diffused Co-content of�34 at.% [27] and a metastable hard magnetic phase: Sm(Co,Fe)7

(Fig. 1c). It should be noted that the TbCu7-type SmCo7-phase is of-ten stabilized by doping third elements such as Ti, Zr, Cu and Sn[28,29]. The occurrence of Sm(Co,Fe)7-phase in the trilayers isprobably due to the intermixing of Fe with the SmCo5-phase. Whilesome amount of Co from the SmCo5-phase diffuses into the Felayer to form Fe(Co) phase, it can be expected that a significantamount of Fe simultaneously intermixes with the SmCo5 layer dur-ing annealing. Thus, in our case, Fe acts as third element in stabi-lizing the Sm(Co,Fe)7-phase without altering the interfacestoichiometry [30]. Among the Sm–Co hard magnetic phases; itis known that SmCo7 possesses the merits of both SmCo5 and Sm2-

Co17-phases [31,32]. Besides the high moment Fe (Co) phase, the

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1000M

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Fig. 3. In-plane and out-of-plane hysteresis loops of SmCo5/Fe/SmCo5 trilayers: (a)as-grown, (b, c and d) correspond to the films annealed at 673, 773 and 873 K,respectively.

Table 2Hc, Mr and (BH)max values obtained for the SmCo5/Fe/SmCo5 sandwich films atdifferent annealing temperatures.

Annealingtemperature (K)

In-plane (||) Out-of-plane (\)

Hc||

(kA/m)Mr||

(kA/m)(BH)max||

(kJ/m3)Hc\

(kA/m)Mr\

(kA/m)

673 660 752 160 165 176773 814 944 243 103 138873 605 845 197 89 113

194 P. Saravanan et al. / Journal of Alloys and Compounds 574 (2013) 191–195

occurrence of SmCo7 at 773 K is therefore considered as a signifi-cant factor in improving the Mr|| without sacrificing the Hc|| value.In contrast, the trilayers annealed at 873 K, showed poor perfor-mance in terms of lower Hc|| and (BH)max||, attributed to the pres-

ence of Sm2Co7-phase (Fig. 1d), which has relatively lowmagnetocrystalline anisotropy as compared to SmCo5 [33]. TheMr|| value obtained at this temperature is also lower when com-pared to the sample annealed at 773 K, probably due to the largerdiffused Co-content (54 at.%) in the Fe-layer suppressed the mag-netization [27]. The proportionate increase in the soft phase con-tent of SmCo5/Fe/SmCo5 trilayers with increase in annealingtemperature observed with the XRD and RBS studies can be furthercorrelated to the gradual decrease in the Mr\ values at the expenseof Hc\ values (Table 2). Thus, the above results indicate that theresultant magnetic properties of the trilayers were not only af-fected by the composition changes occurring at the Fe intermediatelayer; but also due to the hard magnetic phases such as Sm(Co,Fe)7

and Sm2Co7 formed due to annealing. Recently, trilayers of SmCo5

(25 nm)/Fe/SmCo5 (25 nm) were epitaxially grown at 773 K withtFe = 0, 3, 6, 9, 12 15, 18 and 21 nm by Sawatzki et al. [18]. In theirstudy, the deposition temperature was fixed at 773 K for all the tri-layers and the effect of varying tFe was studied towards exchangecoupling and remanence enhancement. A maximum value of(BH)max (310 kJ/m3) was achieved when tFe = 12 nm. Along theselines, it should be mentioned that the (BH)max values obtained inthe present study can be further improved by varying tFe with fixedSmCo5 layer thickness and subjecting them to suitable annealingtemperature. Of course, such a study would be more useful in de-sign and processing of SmCo5/Fe/SmCo5 films with tailor-made(BH)max values and these investigations are currently underway.

4. Summary

Annealing induced compositional changes that occur in the Fe-intermediate layer of SmCo5/Fe/SmCo5 sandwich films were inves-tigated using XRD, RBS and SQUID. A significant variation in thelattice parameter values of bcc-Fe (Co) was evident from the XRDanalysis. The diffused Co-content in the Fe-intermediate layerwas estimated both by XRD and RBS, which explains the unex-pected increase in Mr|| due to the formation of high moment softmagnetic phase. Maximum values of Hc|| (814 kA/m), Mr||

(944 kA/m) and (BH)max|| (243 kJ/m3) were obtained for the trilayerannealed at 773 K. Through this study, it is demonstrated that aoptimal interphase coupling and good phase chemistry for theFe-intermediate layer and hence, enhanced magnetic propertiesfor the trilayers can be achieved by choosing appropriate annealingtemperature.

Acknowledgments

The financial support for the present study was extended by De-fence Research and Development Organization (DRDO), Govern-ment of India. The keen interest shown by the Director, DMRL inthis work is gratefully acknowledged. One of the authors PS thanksNational Science Council of Taiwan for providing postdoctoral fel-lowship under contract No. NSC-99-2112-M-002-020-MY3.

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