Eur. Phys. J. B (2013) 86: 491DOI: 10.1140/epjb/e2013-40562-8

Regular Article

THE EUROPEANPHYSICAL JOURNAL B

Magnetic annealing of the ion-beam sputtered IrMn/CoFeBbilayers – positive exchange bias and coercivity behaviour

M. Rajua, Sujeet Chaudhary, and D.K. Pandya

Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, 110 016 New Delhi, India

Received 11 June 2013 / Received in final form 9 September 2013Published online 2 December 2013 – c© EDP Sciences, Societa Italiana di Fisica, Springer-Verlag 2013

Abstract. The effect of optimum dilution of antiferromagnetic (AF)/ferromagnetic (FM) interface nec-essary for observance of positive exchange bias in ion-beam sputtered Si/Ir22Mn78 (tAF = 12, 18,24 nm)/Co20Fe60B20(tFM = 6, 9, 15 nm) exchange coupled bilayers is investigated by magnetic anneal-ing at 380, 420 and 460 ◦C for 1 h at 5 × 10−6 Torr in presence of 500 Oe magnetic field. While thecoercivity of the exchange coupled FM layer decreases with the increase in annealing temperature irre-spective of the value of tAF or tFM, the hysteresis loops however shift by ≈+10 Oe whenever the coercivitydrops in the 10–15 Oe range. This is consistent with the phase diagram of exchange bias field and coerciv-ity derived from Meiklejohn and Bean model. The X-ray diffraction and X-ray reflectivity measurementsconfirmed that the texture, grain size and interface roughness of IrMn/CoFeB bilayers are thickness de-pendent and are correlated to the observed magnetic response of the bilayers. The results establish thatoptimum dilution of the IrMn/CoFeB interface by thermally diffused Mn-spins is necessary in inducingthe effective coupling between the IrMn domains and diluted CoFeB layer. It is further shown that theannealing temperature required for the optimum dilution of the CoFeB interface critically depends on thethickness of the layers.

1 Introduction

Exchange bias (EB) effect is known to be the manifesta-tion of magnetic ordering at the interface between a fer-romagnetic (FM) and antiferromagnetic (AF) layers. TheEB effect discovered by Meiklejohn and Bean [1,2] contin-ues revealing its newer aspects owing of its complex inter-connection with various parameters like material system,growth, post growth processing parameters and difficultyin characterizing the interfaces at atomic scale [3–6]. Theeffect is seen as a shift in magnetic hysteresis (M-H) loopof FM layer coupled to AF layer as well as increase incoercivity (HC) of FM layer [7,8].

Depending on the nature of the coupling between theAF and FM spins at the interface, the M-H loop shiftstowards or opposite to the inducing field direction (i.e.,direction along which field was applied during growthand/or subsequent annealing). Whereas the ferromagneticcoupling of the spins at the AF/FM interface is knownto result in negative exchange bias (NEB), the antifer-romagnetic coupling results in a positive exchange bias(PEB) [9–11]. It is also found that the interface rough-ness between the AF and FM layers and strength ofthe inducing field can also influence the sign of the loopshift. As first reported in e-beam evaporated Fe/FeF2

and Fe/MnF2 systems, the lower interface roughness andhigher cooling fields result in PEB, whereas, the higherinterface roughness and low inducing fields result in

a e-mail: rajuhcu519@gmail.com

NEB [9,10]. PEB was also reported for variety of AF/FMsystems like magnetron sputtered GdFe/NiCoO [12],CoNi/Gd/CoNi trilayers and CoNi/Gd bilayers [13],Ni81Fe19/Ir20Mn80 [14], and IrMn/CoFeB [15,16]. It maybe pointed out here that the understanding of EB phe-nomenon and its dependency on nature of interface inmaterial systems like IrMn/CoFeB, IrMn/NiFe, etc. arequite interesting and important for both the fundamen-tal understanding of physics and the technological ap-plications, e.g., magnetic recording read heads, MRAM,and exchange biased magnetic tunnel junctions (MTJ’s),etc. In particular, IrMn/CoFeB is an ideal system for ex-change biased MTJs as high thermal stability and highblocking temperature of IrMn makes it more favorablecompared to other AF materials like FeMn and NiMn.Furthermore, the texture and grain size of IrMn can becontrolled by using suitable under layers [17–19]. As oftoday, the IrMn/CoFeB has emerged as one of the im-portant material system in spintronic devices [20–22]. Inthe present work, we have used the Fe rich compositionof the CoFeB (Co-20at%, Fe-60at%, B-20at%) as it isreported to yield highest TMR in MTJs [23]. The asso-ciated lower cobalt concentration (20at%) will avoid ten-dency for reduced nucleation rate and non-uniform crys-tallization. Significantly higher interfacial energy betweenCo and MgO compared to that between Fe and MgO isshown to result in non-uniform crystallization of CoFeBin contact with MgO adversely affecting the magnitude ofTMR of the MTJs [24]. Similarly, use of higher B content

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(>20at%) in CoFeB hampers its crystallization, as higherB content is known to stabilize the amorphous phase ofCoFeB [25]. For the AF layer of IrMn, both the extentof AF ordering and its anisotropy strength (and henceEB) depend critically on the composition of IrMn. In thepresent work, we have employed the IrMn target having22at% Ir and 78at% Mn in view of the well-establishedresult that at this composition IrMn induces maximumEB in the neighboring FM layer [26,27]. Recently, wehave reported some of these process-property correla-tions in several ion-beam sputtered bilayer and trilayersystems. These include exchange bias, associated train-ing effects and reversal asymmetry in IrMn/CoFe [28],IrMn/CoFeB [15] and IrMn/NiFe [29], process-dependentinterface microstructure in MgO/CoFeB and CoFeB/MgObilayers [30,31], and interlayer exchange coupling inCoFe/MgO(t)/NiFe trilayers [32].

It may be interesting to note that the commonly re-ported increase in HC is understood as the result of EBeffect in these AF/FM bilayers. Contrary to this generalincrease in HC in the exchange biased bilayers, Spray andNowak [33], have shown in their theoretical study thatfor bilayers with mixed interfaces low interface roughnesscauses reduction in HC . More recently, this aspect wastheoretically revisited in detail by Usadel and Stamps [34]who predicted that a reduced concentration of magneticions at the AF/FM interface could lead to reduced HC

and increased exchange bias field. It was also shown thateither HC or loop shift (HEB) could be altered with-out changing the other one [35–37]. In this communica-tion, we provide the experimental support for the simul-taneous occurrence of exchange bias and reduced HC intwo different sets of ion beam sputtered bilayers Si/IrMn(18 nm)/CoFeB (6, 9 and 15 nm), and Si/IrMn (12, 18and 24 nm)/CoFeB (9 nm). The bilayers exhibited a pos-itive loop shift of ∼10 Oe in as-deposited and annealedconditions whenever the HC is in the 10–15 Oe range.While these results provide support to the theoretical pre-dictions made in previous reports [33,34], they naturallyprovide experimental correlation of the thermally inducedmicrostructural changes across the interface and depen-dence on thicknesses of AF or FM layers conducive forobservance of EB effect.

2 Experimental details

The Si/Ir22Mn78 (IrMn)/Co20Fe60B20 (CoFeB) bilayersare deposited using NORDIKO 3450 ion beam sputter-ing system (base pressure ∼1 × 10−6 Torr) in a work-ing pressure of 1.3 × 10−4 Torr. The n-type Si (100)substrates are thoroughly cleaned using acetone andpropanol in an ultrasonic bath and then the substratesare loaded for deposition without removing the native ox-ide (SiO2). The targets are pre-sputtered for 20 min be-fore deposition. Two series of bilayers were deposited (a)Si/IrMn (tAF = 18 nm)/CoFeB (tFM = 6, 9, 15 nm), re-ferred henceforth as SI18C6, SI18C9 and SI18C15, and(b) Si/IrMn (tAF = 12, 18, 24 nm)/CoFeB (tFM =9 nm), referred henceforth as SI12C9, SI18C9 and SI24C9.

A dc-magnetic field of 1.5 kOe is applied parallel to thesubstrate surface during the growth of the film to set theexchange bias direction in the bilayers. All the layers aredeposited at room temperature (RT). Magnetic annealingof these bilayers is carried out in a separate vacuum cham-ber in 5×10−6 Torr at 380, 420 and 460 ◦C for 1 h. Theseannealing temperatures are chosen keeping in mind the re-ported values of crystallization temperature of amorphousCoFeB. Annealing is an essential thermal treatment forexchange biased MTJs with CoFeB layers [23,25]. The an-nealing temperature was controlled using a PID controller.Magnetic field of 0.5 kOe is applied during annealing andsubsequent cooling of the bilayers to RT in the same di-rection as the magnetic field applied during deposition.The M-H measurements are carried out using a home-builtlongitudinal magneto optical Kerr effect (MOKE) set-upbased on photoelastic modulation. A solenoid is used togenerate the magnetic field in steps of 0.3 Oe by sweepingthe current using a programmable constant current source(Mode 2440 Keithley Source meter). The thickness of theindividual layers is determined using specular X-ray re-flectivity (XRR) measurements carried out using X-pertPanalytical X-ray diffractometer. To determine the layerparameters (thickness, density and interface width) theXRR data is simulated using X-Pert reflectivity software(WinGixa Version 1.102). The software employs absolutesquare root difference scheme for minimization of differ-ence between experimental and simulated profiles. Herethe interface width measured in the specular reflectivityis the combination of root mean square roughness and thegrading width due to density variation across the interface.

3 Results and discussion

Figure 1 shows MOKE M-H measurements on as-deposited and annealed bilayers of SI18C6, SI18C9 andSI18C15. The HC of as-deposited bilayers is found to varyfrom 10 to 55 Oe with the increase in the thickness of theCoFeB layer. In addition, irrespective of the thickness ofthe CoFeB layer, HC is found to decrease gradually withincrease in the annealing temperature. As is emphasizedin Figure 1 by a horizontal arrow, the bilayers with the 6,9 and 15 nm CoFeB coupled to 18 nm IrMn exhibitedpositive loop shift of ∼10 Oe upon annealing at 380, 420,and 460 ◦C, respectively. It is interesting to note that theannealing temperature necessary for inducing loop shiftis higher for thicker FM layer. On further increase in an-nealing temperature the loop shift is found to disappear.Extra care was taken to verify the direction of the ob-served shift of M-H loops with respect to inducing fielddirection and it was found that the M-H loop shifts inthe same direction as inducing field leading to PEB. It isimportant to stress here that repeated measurements re-produced the observed loop shift and HC indicating theabsence of training effect in these bilayer samples.

Figure 2 shows the MOKE M-H measurements on as-deposited and annealed SI12C9, SI18C9 and SI24C9 bi-layers in which the thickness of the IrMn layer has been

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Fig. 1. MOKE M-H loops of the as-deposited and magneticallyannealed SI18C6, SI18C9 and SI18C15 bilayers.

Fig. 2. MOKE M-H loops of the as-deposited and magneticallyannealed SI12C9, SI18C9 and SI24C9 bilayers.

varied keeping CoFeB layer thickness same at 9 nm. In-terestingly, the bilayer with thinnest AF layer of 12 nm(sample SI12C9) shows the PEB in as-deposited condi-tion; whereas, SI18C9 and SI24C9 exhibited PEB afterannealing at 420 ◦C. The HC behavior in these bilayers isfound to be same as observed for the other set of bilayersi.e., SI18C (tFM) (Fig. 1).

As mentioned in introduction, the sign of the exchangebias is governed by the nature of coupling of spins acrossthe AF/FM interface [11]. It is reported that while Fespins couple antiferromagnetically with Mn spins, the Cospins couple ferromagnetically with Mn spins [38–40]. Ac-cordingly, the anti-ferromagnetic coupling between Mn-Fespins leads to PEB while, the ferromagnetic coupling be-tween Mn-Co leads to negative exchange bias (NEB). Asthe composition of CoFeB target in the present work isCo20Fe60B20, it is more likely that the IrMn/CoFeB inter-face is rich in Fe compared to Co which understandably

Fig. 3. MOKE M-H loops of as-deposited CoFeB thin filmshaving different thicknesses (6, 9 and 15 nm).

Fig. 4. Dependence of coercivity HC on the thickness (t) ofCoFeB layer and IrMn layer. The shown variations are forCoFeB (t) single layers, IrMn (18 nm)/CoFeB (t) bilayers andIrMn (t)/CoFeB (9 nm) bilayers.

results in the presence of dominant Mn-Fe spin couplingacross the interface leading to the observed PEB. We be-lieve that for observance of PEB in our samples, the highercontent of Fe in CoFeB and higher content of Mn in IrMnis critical.

Figure 3 shows the MOKE M-H plots of single layers ofCoFeB (6, 9 and 15 nm), and Figure 4 shows the variationof HC in as-deposited IrMn (18)/CoFeB (6, 9 and 15 nm),IrMn (12, 18 and 24 nm)/CoFeB (9) bilayers as comparedto single layers of CoFeB (6, 9 and 15 nm). It is clear thatthe HC of the exchange biased CoFeB is influenced bythickness of both CoFeB and IrMn layers. This increasein HC of exchange biased CoFeB layer with the thicknessof IrMn and CoFeB could result from either the varia-tion in exchange coupling across the interface or the vari-ation in the microstructure of IrMn and CoFeB layers. Inreality, the interfacial exchange coupling is influenced byseveral factors such as interface roughness, interdiffusion,thickness of the layers, grain size, anisotropy strength of

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Table 1. Grain size and relative intensity of IrMn (111) and CoFeB (110) in as-deposited and annealed (at 460 ◦C) Si/IrMn(12, 18 and 24)/CoFeB (9) bilayers measured from XRD.

SampleRelative intensity of the XRD-peak Grain size (nm)IrMn (111) CoFeB (110) IrMn (111) CoFeB (110)

As-deposited

SI12C9 56 Amorphous 6.43 ± 0.26 AmorphousSI18C9 99 Amorphous 8.68 ± 0.46 AmorphousSI24C9 100 31 8.21 ± 0.41 5.60 ± 0.19

Annealed at 460 ◦CSI12C9 53 100 9.23 ± 0.52 8.33 ± 0.41SI18C9 100 52 11.36 ± 0.70 7.89 ± 0.38SI24C9 94 44 7.39 ± 0.33 9.38 ± 0.52

Fig. 5. Variation in coercivity HC of Si/IrMn (18 nm)/CoFeB(6, 9 and 15 nm) bilayers with annealing temperature. Arrowsindicate the occurrence of M-H shift (PEB) in various samples.

the AF and FM layers, so the HC is a result of compli-cated combination of all these parameters. For example,the coercivity of an exchange biased FM can be given byHC ∝ HC (0) + α

tFM; where HC(0) is free layer coercivity,

α is a constant parameter used to represent HC enhance-ment and tFM is the thickness of ferromagnetic layer. Itmay be pointed out that the second term on right side,which is inversely proportional to tFM, is actually the re-sult of exchange coupling across the interface [41]. Also,according to Meiklejohn and Bean’s model [42], the depen-dence of HC on the AF layer thickness (tAF), anisotropyconstant (KAF) and interface exchange energy (JEB) isgiven by HC ∝ KAFtAF

JEB; with increase in the tAF, the

HC increases upto a critical thickness of AF layer whereloop shift appears and decreases on further increase intAF [43]. It may be noted that the marginal increase inHC from ∼6 Oe in single CoFeB (9) layer to ∼10 Oe inthe as-deposited Si/IrMn (12)/CoFeB (9) (see Fig. 1) isaccompanied by the presence of small PEB.

Figure 5 summarizes the HC variation and loop shiftwith annealing temperature (TA) as observed in Fig-ure 1. Irrespective of the CoFeB thickness, a systematicand gradual decrease of HC is observed with increasein TA. This is a clear indication of the reduction in thestrength of exchange interaction across the IrMn/CoFeBinterface possibly due to enhancement in the thermally

induced atomic inter-diffusion. It should be pointed outthat the HC of a single CoFeB layer is expected to in-crease as it gets crystallized on annealing at temperaturesof 360–420 ◦C [44,45]. The present results showing a de-crease of HC , therefore naturally suggest that the observedreduction of HC in these bilayers on annealing is signifi-cantly influenced by thermally induced changes across theinterface and within the layers rather than the crystalliza-tion of CoFeB. As the annealing temperature is increased,relatively more number of magnetic atoms at the interfacewould be replaced by the diffused Mn atoms leading tothe magnetic dilution at the CoFeB/IrMn interface. Con-sistent with the theoretical predictions [34], this reductionof concentration of magnetic ions at the interface wouldlead to the reduced HC and can result in an increase ofthe exchange bias field due to better adjustment of thedomains in IrMn layer to the diluted CoFeB layer com-pared to undiluted CoFeB. From the foregoing discussion,we are of the view that the dominance of Mn-diffusionacross the IrMn/CoFeB interface over the crystallizationof CoFeB layer is responsible for reduced HC , and hencethe simultaneous observance of reduced HC and shift ofM-H loop in our work.

To understand the possible structural origin of theHC behavior, we carried out the XRD and XRR mea-surements on some of the as-deposited and annealed bi-layers. Figure 6 shows the expanded view of XRD pat-terns in the 2θ range of 37.5–50◦ as recorded on Si/IrMn(12, 18 and 24)/CoFeB (9) bilayers, both in as-depositedand 460 ◦C annealed conditions. Presence of two diffrac-tion peaks due to IrMn, namely the (111) at ∼41◦ and(110) at ∼36◦ (the later not shown for clarity) clearlysuggests that IrMn is polycrystalline in nature. Table 1shows the variation of relative intensity and grain size ofIrMn (111) and CoFeB (110) peaks in as-deposited andannealed bilayers. It is evident from the variation of in-tensity of the diffraction peak due to IrMn (111) that thefilms of higher IrMn thickness i.e. 18 and 24 nm are (111)textured and their grain sizes are higher than the 12 nmcase. Compared to bilayers having thinner IrMn layer (12and 18 nm), the 24 nm bilayer SI24C9 shows the presenceof large sized IrMn (111) grains which seem to be favor-able for the crystallization of the CoFeB, as is evidentin the form of a discernable hump (FWHM = 1.54◦) ataround 2θ ∼ 45◦ corresponding to bcc-CoFeB (110). Thus,the correlation in the texture/grain size of the IrMn under-layer and the changes in the microstructure of CoFeB are

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(a) (b)

Fig. 6. XRD patterns recorded on (a) as-deposited and (b) annealed SI12C9, SI18C9 and SI24C9 bilayers.

(a) (b)

Fig. 7. (a) Specular XRR measurements on as-deposited Si/IrMn (12, 18 and 24)/CoFeB (9). (b) Comparison of diffuse XRRmeasurements on Si/IrMn (12)/CoFeB (9) and Si/IrMn (24)/CoFeB (9).

established. This correlation is also reflected in the ob-served increase in HC of the CoFeB layer (Fig. 4) withincrease in thickness of IrMn layer. Thus, the magnetic re-sponse of CoFeB is clearly influenced by the (111) textureand the large sized IrMn grains having (111) orientation.Here we emphasize that the (111) orientation of IrMn isknown to be crucial for increasing exchange bias as thisparticular atomic plane possesses the largest areal densityof spins [46].

The broad hump at 2θ ≈ 45◦ seen in as-deposited bi-layers corresponds to CoFeB and it develops to a sharperpeak corresponding to CoFeB (110) on annealing as seenin Figure 6. It can be clearly seen that while the CoFeB isamorphous like in as-deposited condition, it is crystallineafter annealing in all the three bilayers. The intensity ofCoFeB (110) peak in annealed bilayers, however, exhibitsa monotonic decrease with increase in the thickness of

underlying IrMn layer. One of the possible reasons couldbe the disordering of the CoFeB as a result of Mn diffu-sion into this layer which is limited by the supply of Mn(depending on thickness of the IrMn layer) rather thandiffusion-kinetics (since annealing temperatures are iden-tical). As will be argued later in the paper, this conjectureis further supported by XRR analysis of SI18C9.

Figure 7a shows the specular XRR profiles (togetherwith the simulated curves) recorded on as-depositedSi/IrMn (12, 18 and 24)/CoFeB (9) bilayers. The simu-lation results summarized in Table 2 show that the in-terface width between IrMn and CoFeB is dependent onIrMn thickness and is higher at lower thickness of IrMn.In our opinion, the higher interface width could have twoorigins, i.e., the higher rms roughness at the interface andthe grading at the interface due to the compositional vari-ation. To resolve the exact origin of increased interface

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Table 2. Fitting results of specular XRR of as-deposited Si/IrMn (12, 18 and 24)/CoFeB (9) bilayers.

SampleIrMn CoFeB

Thickness Density Interface width Thickness Density Interface width

(nm) (g/cc) (nm) (nm) (g/cc) (nm)

±0.01 ±0.05 ±0.05 ±0.01 ±0.05 ±0.05

SI12C9 12.77 5.47 0.52 9.36 6.97 0.24

SI18C9 17.37 6.22 0.46 9.67 6.98 0.29

SI24C9 21.52 6.79 0.43 9.31 7.38 0.37

Table 3. Fitting results of specular XRR on as-deposited and magnetically annealed (MA) Si/IrMn (18)/CoFeB (9) bilayer.

Sample detailsIrMn CoFeB

Thickness Density Interface width Thickness Density Interface width

(nm) (g/cc) (nm) (nm) (g/cc) (nm)

±0.01 ±0.05 ±0.05 ±0.01 ±0.05 ±0.05

SI18C9

As dep 17.37 6.22 0.45 9.67 6.98 0.29

MA380 15.93 7.22 0.46 7.68 6.99 0.12

MA420 15.92 6.51 0.44 7.75 6.29 0.50

MA460 15.76 6.41 0.59 7.31 6.44 0.46

Fig. 8. Specular XRR measurements on Si/IrMn (18)/CoFeB(9) bilayers annealed at 380, 420 and 460 ◦C.

width, we performed the diffuse XRR measurements. Fig-ure 7b shows the comparison of diffuse XRR profiles ofSI12C9 and SI24C9 bilayers. The sizable occurrence of thereflected X-ray intensity in the off-specular directions (in-dicated by arrows) for SI12C9 bilayer clearly establishesthe presence of higher rms-roughness, in sharp contrast toSI24C9 bilayer. The higher rms-roughness and observedshift in MOKE M-H loop in the as-deposited SI12C9 seemsto be correlated.

Figure 8 shows the specular XRR measurements onSi/IrMn(18)/CoFeB(9) bilayer annealed at different tem-peratures. The simulation results presented in Table 3,show that the interface width between IrMn and CoFeBremains unchanged till 420 ◦C. But these results also

reveal a clear increase in the interface width on anneal-ing at 460 ◦C. This trend of increase in interface widthis consistent with the observation of thermally enhancedMn diffusion reported by Hayakawa et al. [21], where theauthors have seen a significant Mn diffusion only after an-nealing above 425 ◦C. As a result of the atomic diffusionof Mn across the interface causing increase in interfacewidth, the strength of exchange coupling between IrMnand CoFeB would get understandably affected leading toreduced HC , as is observed in all the annealed bilayers(shown in Fig. 5).

It is interesting to see that the loop-shift in all thebilayers is seen when the HC of the bilayer reaches toabout ∼10–15 Oe and this is achieved at different anneal-ing temperatures for different thicknesses of the CoFeBlayer with fixed IrMn thickness. However in case of IrMn(12)/CoFeB (9), the HC of the bilayer falls in the abovementioned range right in as-deposited conditions leadingto shifted M-H loop. In general, the EB is not expectedin the as-deposited bottom pinned AF/FM systems. How-ever, for sputtered polycrystalline AF layers, the surfaceis not expected to be atomically flat and therefore it maylead to presence of uncompensated spins at the surface. Asproposed by Geshev et al. [47] in bottom pinned AF/FMinterface, right in the as-deposited condition one expectsa population of uncompensated spins at the interface be-tween the already grown AF layer and subsequently grownFM layer in the presence of applied field. Further, theseuncompensated spin clusters are expected to possess ananisotropy lesser than the bulk of AF layer and henceare expected to be influenced by the direction of alignedFM spins which are easily influenced by the presence ofthe dc-magnetic field and/or the stray field present in thedeposition chamber. The preliminary diffuse and specu-lar XRR measurements and their simulations on these as-deposited and magnetic annealed bilayers confirm that the

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intermixing in as-deposited IrMn (12)/CoFeB (9) bilayeris intrinsically caused due to presence of spatial rough-ness. We believe that the high spatial roughness in thisparticular bilayer leads to the formation of clusters ofuncompensated spins at the AF/FM interface. These spinsare responsible for the observed loop shift along the induc-ing field direction in the as-grown bottom pinned IrMn(12)/CoFeB (9) bilayer, as suggested in reference [47].

Furthermore, in a physical vapor deposition, such assputtering, the energy carried by the ad-atoms plays a cru-cial role and can lead to different kinds of growth modesranging from strictly two dimensional to three dimensionalone. It is well-known that compared to rf-magnetron sput-tering, the ion-beam sputtering technique involves moreenergetic landing of sputtered species at the surface ofgrowing film or substrate. In our opinion, while on oneextreme there could be severe damage of the growing filmextending to few under layers, on the other extreme withjust right energy of the sputtered species, this could befavorable for mobilizing the adatoms so much so that anatomically sharp interface could be realized in case of mul-tilayers and/or epitaxial growth at even at lower substratetemperatures. It is therefore quite likely that in the presentcase of IrMn (12)/CoFeB (9) bilayers, the optimum di-lution of CoFeB with Mn-spin diffusion occurred in as-deposited condition itself. Further studies are needed toassert this opinion.

Returning back to occurrence of observed M-H loopshift at different temperatures for different thicknesses ofCoFeB layer, we feel that although the exact microscopicorigin of the correlation between CoFeB layer thicknessand annealing temperature appropriate to loop shift isunclear, but the observation of loop shift when the HC ofbilayers reaches to ∼10–15 Oe indicates that the thermallyinduced microstructural changes play a critical role andthese changes as suitable for loop shift are achieved atdifferent temperatures depending on the thickness of theCoFeB layer.

4 Conclusions

In summary, we have studied the effect of thickness of thelayers and magnetic annealing in exchange biased bilayersof IrMn/CoFeB deposited by ion beam sputtering. Irre-spective of the thickness of the layers, the bilayers haveshown gradual decrease in HC with increasing annealingtemperature, which is attributed to the magnetic dilutionof the CoFeB interface by thermally activated diffusionof Mn across the interface. The bilayers have shown M-Hloop shift of ∼10 Oe whenever the HC of the bilayersdrops to ∼10–15 Oe indicating the optimum dilution ofthe CoFeB interface for effective coupling of the IrMn do-mains to the diluted CoFeB layer. The fact that the loopshift is achieved at different annealing temperatures de-pending on the thickness of the CoFeB layer is the sig-nificant evidence in favor of optimum dilution. Contraryto the general understanding of increased HC in exchangebiased layers, the experimental observations here indicatethat the thermally induced optimum microstructure of theinterface can lead to stronger exchange coupling strength

across IrMn/CoFeB resulting in shifted M-H loop withouthaving increase in HC . This study points to the need ofdetailed interface and microstructural analyses of the bi-layers to understand the microscopic origin of the observedPEB.

One of the authors (MR) acknowledges the CSIR-India for theSRF fellowship. The financial Grant (1(14)/2007-M&C) fromDepartment of Information Technology, Govt. of India is alsothankfully acknowledged.

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