Solid State Communications 150 (2010) 45–48

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Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Tuning exchange coupling by replacing CoFe with amorphous CoFeB in theCoFe/Ru/CoFe synthetic antiferromagnetic structureZ.B. Guo a,∗, B.Y. Zong a, J.J. Qiu a, P. Luo a, L.H. An a, H. Meng a, G.C. Han a, H.K. Hui ba Data Storage Institute, Agency for Science, Technology and Research (A*STAR), DSI Building, 5 Engineering Drive 1, Singapore 117608, Singaporeb Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 3 Research Link, Singapore 117602, Singapore

a r t i c l e i n f o

Article history:Received 8 September 2009Accepted 6 October 2009by A.H. MacDonaldAvailable online 15 October 2009

PACS:75.50.Kj75.70.-i75.70.Cn

Keywords:A. Magnetic films and multilayersC. Transmission electron microscopy

a b s t r a c t

We studied exchange coupling in the CoFe/Ru/CoFe synthetic antiferromagnetic structure withsystematical replacement of the crystalline CoFe with amorphous CoFeB. Antiferromagnetic exchangecoupling intensity decreases with an increase in the replacement in the bottom magnetic layer, whichindicates that exchange coupling intensity could be tuned by the replacement. The origin of weakeningantiferromagnetic exchange coupling is attributed to the amorphous CoFeB replacement inducingincomplete crystallization and disordered orientation in the Ru layer.

© 2009 Elsevier Ltd. All rights reserved.

Recently, magnetic tunnel junctions (MTJs) and magnetic in-terlayer coupling comprising the amorphous CoFeB layer haveattracted intensive interest due to their intrinsic physics and im-portant applications [1–5]. For example, in the AlO-based MTJswith amorphous CoFeB electrodes, large tunneling magnetore-sistance ∼70% has been observed, which is much larger thanthe magnetoresistance ∼47% in MTJs with crystalline CoFe elec-trodes [6–8]. According to Juliere’s model, the higher magnetore-sistance should be attributed to the higher spin polarization inthe ferromagnetic (FM) electrodes [9]. Recently, Huang et al. havefound the spin polarization in the amorphous CoFeB electrodescould be as high as 65%, much higher than the value of 37% forCoFe, which means that the magnetoresistance for an AlO basedMTJ with amorphous CoFeB electrodes could be as high as 146%[10,11]. Amorphous CoFeB has also been extensively employedin a MgO-based MTJ with a typical structure of an antiferromag-net/CoFe/Ru/CoFeB/MgO/CoFeB, in which the MgO layer grownon an amorphous CoFeB has a highly (001)-oriented polycrys-talline structure, which acts a template to crystallize CoFeB in abody centered-cubic structure with (001) orientation during hightemperature annealing [1,12,13]. Large magnetoresistance∼604%has been observed due to the nature of coherent spin-dependenttunneling [13,14]. In the structure, the antiferromagnetic (AFM)

∗ Corresponding author.E-mail address: guo_zaibing@dsi.a-star.edu.sg (Z.B. Guo).

0038-1098/$ – see front matter© 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2009.10.009

layer, such as PtMn and IrMn, is exchange coupled to the CoFelayer, which provides unidirectional anisotropy. The CoFe layer isstrongly AFM coupledwith the amorphous CoFeB layer via the thinRu interlayer, which is called a synthetic antiferromagnetic (SAF)structure.The coupling strength of the SAF CoFe/Ru/CoFeB trilayers is

comparable to that of CoFe/Ru/CoFe [15–17]. For both of them,the bottom FM layer is CoFe. However, when replacing the bot-tom CoFe layer with an amorphous CoFeB layer, the couplingstrength becomes very small, about one to three orders of mag-nitude smaller than the formers’ [3–5]. To investigate the ori-gin of the coupling strength dependence on the structure of theFM layers, in this paper, we conduct a systematical study ofthe exchange coupling in three series of samples, such as SiO2substrate/NiFe(2nm)/IrMn(8nm)/CoFe(3−xnm)/CoFeB(xnm)/Ru(0.8 nm)/CoFeB(3.3 − y nm)/CoFe(y nm). This is an exchangebiased SAF structure, which is commonly incorporated inmagneticrecording heads. The exchange bias, which provides a pinning fieldon the bottom FM layer, enables one to distinguish the magnetiza-tion reversal of the FM layers besides the Ru layer when the cou-pling is weak AFM or FM. We found that the coupling strength isvery strongly dependent on the structure of the bottom FM layeradjacent to the Ru layer, and the method employed here could beused to tune AFM interlayer coupling strength for practical appli-cations.The structures of three series of samples are as follows, the unit

of the film thickness is in nanometers.

46 Z.B. Guo et al. / Solid State Communications 150 (2010) 45–48

a b

c

Fig. 1. M–H loops of the samples of (a) series A with the structure of IrMn(8)/CoFe(3 − x)/CoFeB(x)/Ru(0.8)/CoFeB(3.3), (b) series B with the structure ofIrMn(8)/CoFeB(x)/CoFe(3 − x)/Ru(0.8)/CoFeB(3.3) and (c) series C with the structure of IrMn(8)/CoFeB(3)/Ru(0.8)/CoFe(3.3 − y)/CoFeB(y). The insets in the figuresare enlarged view of the curves around the zero field.

Series A: Ta(5)/NiFe(2)/IrMn(8)/CoFe(3 − x)/CoFeB(x)/Ru(0.8)/CoFeB(3.3)/Ta(10).Series B: Ta(5)/NiFe(2)/IrMn(8)/CoFeB(x)/CoFe(3 − x)/Ru(0.8)/CoFeB(3.3)/Ta(10).Series C: Ta(5)/NiFe(2)/IrMn(8)/CoFeB(3)/Ru(0.8)/CoFe(3.3− y)/CoFeB(y)/Ta(10).

All of the samples were deposited at room temperature by mag-netron sputtering on thermally oxidized Si(100) substrates undera field of 100 Oe. The base pressure of the system is 5×10−10 Torr.The CoFe and CoFeB layers were sputtered from Co90Fe10 andCo60Fe20B20 (at.%) alloy targets, respectively. The NiFe(2 nm) layeracts as a seed layer to promote IrMn(111) orientation and the topTa(10 nm) layer acts as a protection layer. After deposition, to setthe pinning direction, the samples were annealed at 235 ◦C undera field of 1 T along the field direction applied during film deposi-tion. The hysteresis loops were measured by using a Superconduc-tor Quantum Interference Detector (SQUID). The structures werecharacterized by using high resolution transmission electron mi-croscopy (HRTEM) and X-ray diffraction (XRD).It is well known, for a SAF structure of FM(t1)/spacer/FM(t2),

the interlayer coupling energy J is given by

J = −µoHsatM1t1M2t2/(M1t1 +M2t2) (1)

where, Hsat is the saturation field. M1 and t1 are the magne-tization and thickness of the bottom magnetic layer, respec-tively. M2 and t2 are the magnetization and thickness of the topmagnetic layer, respectively [18]. For our synthetic structure ofCoFe(t1CF )/CoFeB(t1CFB)/Ru(0.8)/CoFeB(t2CFB)/CoFe(t2CF ) with the

composite FM layers, the coupling energy can be expressed by

J = −µoHsat(MCF t1CF +MCFBt1CFB)(MCF t2CF +MCFBt2CFB)/(MCF t1CF +MCFBt1CFB +MCF t2CF +MCFBt2CFB) (2)

whereMCF andMCFB are the saturation magnetization of CoFe andCoFeB, respectively. For our samples, MCF is ∼1320 emu/cm3 andMCFB is∼1060 emu/cm3.M–H loops of the samples are shown in Fig. 1. Shown in Fig. 1a

are the M–H loops of the samples of series A with the structureof IrMn(8)/CoFe(3 − x)/CoFeB(x)/Ru(0.8)/CoFeB(3.3). With in-creasing x, the bottom CoFe layer at the interface adjacent to Rulayer is gradually replaced by the CoFeB layer. For x = 0, Hsat ,defined as the field at the positive field branch where the mag-netization is saturated, is about 3300 Oe, corresponding to J ∼−0.68 J/m2. For x = 1, i.e., a 1 nm thick CoFe layer was replaced bya 1 nm thick CoFeB layer, Hsat drops from 3300 to 120 Oe, resultingin J ∼ −0.025 J/m2, which is about ∼27 times weaker than thatof x = 0. From the inset in Fig. 1a, it can be found Hsat is reducedfurther to 20 Oe for x = 2.Fig. 1b shows the M–H loops of the samples of series B with the

structure of IrMn(8)/CoFeB(x)/CoFe(3 − x)/Ru(0.8)/CoFeB(3.3).With increasing x, the bottom CoFe layer is gradually replaced bya CoFeB layer. It is evident that the antiferromagnetic interlayercoupling decreases with an increase in the replacement. Forexample, when a 2 nm thick CoFe layer was replaced by a 2 nmthick CoFeB layer, the Hsat decreases from 810 to 220 Oe.The strength of antiferromagnetic interlayer coupling in the SAF

structures shown in Fig. 1a and b decreases with an increase inthe replacement of CoFe with CoFeB. In such a way, the interlayer

Z.B. Guo et al. / Solid State Communications 150 (2010) 45–48 47

a b

Fig. 2. XRD patterns of (a) Ta(3)/CoFeB(15)/Ta(1) and Ta(3)/CoFe(15)/Ta(1), (b) Ta(3)/CoFeB(3)/Ru(15)/Ta(3) and Ta(3)/CoFe(3)/Ru(15)/Ta(3).

a b

c d

Fig. 3. (a) Lowmagnification and (b) high magnification cross-sectional TEM images of Ta(3)/CoFe(3)/Ru(15)/Ta(3). (c) Lowmagnification and (d) high magnification cross-sectional TEM images of Ta(3)/CoFeB(3)/Ru(15)/Ta(3).

coupling can be easily tuned, which could be used for magneticrandom access memory cell design [19,20].Fig. 1c shows the M–H loops of the samples of series C with the

structure of IrMn(8)/CoFeB(3)/Ru(0.8)/CoFe(3.3 − y)/CoFeB(y),for these samples, the bottom FM layer is kept as CoFeB, while thetop CoFeB layer is gradually replaced by the CoFe layer at the in-terface. With increasing y, Hsat is 90, 30, and 10 Oe for y = 0, 2,and 3.3, corresponding to J = 0.018, 0.006, 0.002 J/m2, respec-tively. The exchange coupling intensity of all the samples is verysmall as compared with that of CoFe/Ru/CoFeB (Fig. 1a), indicatinga weak dependence of the top FM layer on the structure. On thecontrary, for the SAF structures shown in Fig. 1a and b, the inter-layer coupling exhibits a strong dependence on the bottom mag-netic layer adjacent to the Ru layer, large interlayer coupling in thesamples with crystalline CoFe, but small interlayer coupling withamorphous CoFeB. Therefore, it could be deduced that, comparedto the topmagnetic layer, the bottom FM layer adjacent to Ru layerplays a critical role in the amplitude of the coupling intensity in thepresent samples.XRDpatterns of Ta(3)/CoFeB(15)/Ta(1) andTa(3)/CoFe(15)/Ta(1)

films after annealing at 235 ◦C for 2 h are shown in Fig. 2a. As hasbeen expected, the CoFeB film is amorphous while the CoFe film ispolycrystalline with a strong (111) orientated texture [2].To examine the crystalline structures of the Ru layer on the

amorphous CoFeB and crystalline CoFe underlayers, samples of

Ta(3)/CoFeB(3)/Ru(15)/Ta(3) and Ta(3)/CoFe(3)/Ru(15)/Ta(3) havebeen fabricated and followed by annealing at 235 ◦C for 2 h. XRDpatterns are shown in Fig. 2b. The peak around 42◦ correspondsto the (00.2) plane of the Ru hcp phase, the intensity for Ruwith the CoFe underlayer is about one order larger than thatwith the amorphous CoFeB underlayer, showing the preferentialorientation for the Ru film on polycrystalline CoFe, whichdemonstrates that the underlayer has an important effect on thestructure of the Ru film. The cross-sectionalHRTEM images of thesesamples are shown in Fig. 3. The Ru film with the CoFe underlayerexhibits a columnar grain structure and the Ru film is completelycrystallized with a preferential orientation (Fig. 3a and b). On thecontrary, for the Ru layer with an amorphous CoFeB underlayer, itcan be seen clearly that the Ru layer is partially crystallized and theorientation of the crystallized grains is disordered. (Fig. 3c and d)The HRTEM images are consistent with the XRD results in Fig. 2b.The origin of interlayer coupling between two FM layers sepa-

rated by a non-magnetic spacer layer has been understood on thebasis of the well-known RKKY theory [11–23]. The band structureof the spacer layer, which is directly correlated to its crystallinestructure, plays a key role on the coupling strength [24]. Weak in-terlayer coupling has been observed in magnetic multilayers withan amorphous spacer due to the structural disorder in the spacerlayer [25,26]. Compared with the large exchange coupling in theSAF structure with crystalline CoFe as the bottom layer, the weak

48 Z.B. Guo et al. / Solid State Communications 150 (2010) 45–48

exchange coupling in the SAF structure with an amorphous CoFeBbottom layer could be attributed to the incomplete crystallizationand disordered orientation in the Ru layer.In conclusion, the present study indicates that the exchange

coupling intensity in the SAF structure is greatly dependent on thecrystalline structure of the bottom FM layer, due to the fact that the(111) orientation of the CoFe underlayer promotes a well orientedRu(00.2) texture, compared to the incomplete crystallization anddisordered orientation of the grains in the Ru layer with theamorphous CoFeB underlayer, which induces the easy tuning ofthe interlayer coupling intensity by replacing the crystalline CoFein the bottom FM layer with the amorphous CoFeB.

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