Journal of Magnetism and Magnetic Materials 324 (2012) 3933–3936

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Journal of Magnetism and Magnetic Materials

0304-88

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

Influence of ferromagnetic layer on the exchange couplingof antiferromagnetic NiO-based films

S. Guo, W. Liu n, X.H. Liu, W.J. Gong, J.N. Feng, Z.D. Zhang

Shenyang National Laboratory for Materials Science and International Centre for Materials Physics, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road,

Shenyang 110016, PR China

a r t i c l e i n f o

Article history:

Received 20 March 2012

Received in revised form

12 June 2012Available online 4 July 2012

Keywords:

Exchange bias

Blocking temperature

Thickness

Interface

Interlayer

53/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jmmm.2012.06.044

esponding author. Tel.: þ86 24 83978856; f

ail address: wliu@imr.ac.cn (W. Liu).

a b s t r a c t

Strong effects of ferromagnetic layer (FMQCo, and Ni80Fe20) on the magnitude and blocking

temperature of exchange coupling are observed in antiferromagnetic NiO-based films NiO (5 nm)/

FM1 (t nm)/FM2 (6-t nm). The existence of interfacial spins configuration in glass-like state and FM

anisotropy are proposed to interpret a minimum shown in thermal magnetization curves for films with

strong exchange coupling effect. The microstructural change of FM layer and the long-range interaction

of exchange bias are taken into account to explain a strong dependence of exchange coupling energy

density on the thickness tF of FM layer when tFo5 nm.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

The exchange bias (EB) effect was first discovered by Meiklejohnand Bean in oxide-coated cobalt particles [1]. Exchange couplingacross the interface between an antiferromagnetic (AFM) and aferromagnetic (FM) layer results in unidirectional anisotropy and/or coercivity enhancement [2]. Up to now, EB effect has remainedat the forefront of research in thin-film magnetism because of itselusive mechanism and significant technological importance inspin-valve elements and magnetic tunneling junctions. Numeroustheoretical studies have focused on interfacial spin configurationor AFM domains to understand the EB’s mechanism. For example,the random exchange field model proposed by Malozemoff [3],the AFM domain-wall mechanism proposed by Mauri et al. [4],and the ‘spin–flop’ perpendicular interfacial mechanism [5] com-bined with interface defects proposed by Schulthess and Butler[6] all base on the existence of net uncompensated magnetiza-tions at FM/AFM interface. On the contrary, Nowak et al. [7]considered the entire volume of the AFM as a crucial element forthe emergence of EB. However, it is also important and necessaryto investigate the effect of FM layer on the exchange couplingeffect of FM/AFM couples. Gokemeijer et al. [8] reported thatlong-range coupling between FM and AFM materials across a non-magnetic spacer layer can extend to as much as 5 nm, in contrastto the prevailing assumption that EB is due to a short-range

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interaction at the FM/AFM interface. It was also reported thatlong-range coupling had an oscillatory character, which can beused to enhance the strength of the FM–AFM exchange coupling[9,10]. These interesting results encourage us to study therelationship between exchange interaction and the FM layerthickness. In the present work, we design two series of NiO-basedmagnetic films to study the effect of different FM layers on theinterfacial exchange coupling. The blocking temperature of theNiO-based FM/AFM couples is changed by different FM layerseven if all deposition conditions of AFM (NiO) films are fixed.Moreover, the exchange interaction energy can be changedstrongly by varying the thickness of FM films when tFo5 nm.

2. Samples and experimental

Two series of NiO/FM1/FM2 films of the type Si (100)/Ta(10 nm)/NiO (5 nm)/FM1 (t nm)/FM2 (6-t nm)/Ta (5 nm) with(1) FM1¼Co, FM2¼Ni80Fe20, and (2) FM1¼Ni80Fe20, FM2¼Co,were prepared by DC and RF magnetron sputtering at roomtemperature. The films were grown in a high-vacuum chamberequipped with multi-sputtering guns. The base pressure of thechamber was better than 2�10�7 Torr and Ar gas was kept at apressure of 4�10�3 Torr during sputtering. Commercial Ta, Co,Ni80Fe20, and NiO targets with 99.99% purity were used. X-raydiffraction (XRD) of the films were conducted using Cu–Ka

radiation. The magnetic properties were measured in a super-conducting quantum interface device (SQUID). The EB field (HE)was defined as HE¼(H1þH2)/2, where H1 and H2 are fields at

S. Guo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3933–39363934

which the magnetization is zero in the hysteresis loop. We definethe coercivity as HC¼(H1�H2)/2.

Fig. 2. X-ray diffraction patterns of samples (a) NiO (5 nm)/Co (6 nm) and (b) NiO

(5 nm)/Ni80Fe20 (6 nm).

3. Results and discussion

The value of HE are measured at 10 K after field cooling (FC) ina field of 1 kOe for both series. For NiO/Ni80Fe20 (t nm)/Co(6-t nm) samples, the strength of the exchange couplingdecreases rapidly, although the thickness of the inserted Ni80Fe20

layer is only about 1–2 nm. This decay indicates that the interfacialspin configuration at the FM/AFM interface is very important for EB.When we insert a Co layer at the NiO/Ni80Fe20 interface, HE increasesslowly when tCor3 nm. However, at an inserted Co layer thicknessof 4 nm, HE increases rapidly. We can obtain the value of Jex (erg/cm2), using the relation Jex¼HEMFtF, where Jex is the exchangeenergy across the FM/AFM interface per unit area, MF and tF arethe saturation magnetization and the thickness of the ferromagneticlayer, respectively [11]. Fig. 1 shows the dependencies of Jex on thethickness of inserted layer (a) Co and (b) Ni80Fe20 in the two series ofNiO-based magnetic films, respectively.

In order to investigate the structure characterization of theseNiO-based films, X-rays diffraction (XRD) patterns are used. It isdifficult to get precise XRD patterns when the films are too thin.However, we confirm carefully that the Co layers are fcc (111)textured, and the NiFe layers are also (111) textured. Fig. 2represents the X-ray diffraction patterns of samples (a) Ta(10 nm)/NiO (5 nm)/Co (6 nm)/Ta (5 nm) and (b) Ta (10 nm)/NiO (5 nm)/Ni80Fe20 (6 nm)/Ta (5 nm).

Fig. 3 represents magnetic hysteresis loops of the samples(a) NiO/Ni80Fe20 (5 nm)/Co (1 nm), (b) NiO/Ni80Fe20 (1 nm)/Co(5 nm), (c) NiO/Co (1 nm)/Ni80Fe20 (5 nm), and (d) NiO/Co (5 nm)/Ni80Fe20 (1 nm), measured at 10 K after FC in a applied field of1 kOe from 300 K. For NiO/Ni80Fe20 (5 nm)/Co (1 nm) and NiO/Co(1 nm)/Ni80Fe20 (5 nm), the exchange-coupling effect is relativelyweak. Large values for HE of about 2.7 kOe and for HC of about3.4 kOe are observed for NiO/Co (5 nm)/Ni80Fe20 (1 nm) inFig. 2(d), while large values for HE of about 1.1 kOe and for HC

of about 2.1 kOe are found also for NiO/Ni80Fe20 (1 nm)/Co (5 nm)in Fig. 2(b). According to classic theory of exchange bias [11,12], itis energetically more favorable that the spins in the FM and theAFM rotate together in the hysteresis loop, which will enhancethe coercivity of FM, if the magnetic anisotropy of some AFMgrains is lower than interface exchange coupling constant (Jexb

KAFMtAFM). The interfacial exchange energy density (Jex) of Co/NiOis stronger than that of Ni80Fe20/NiO in our investigated types of

Fig. 1. Dependence of the interfacial unidirectional energy density (Jex) of samples

(a) NiO (5 nm)/Co (t nm)/Ni80Fe20 (6-t nm) and (b) NiO (5 nm)/Ni80Fe20 (t nm)/Co

(6-t nm) on the thickness of the inserted (a) Co layer and (b) Ni80Fe20 layer,

respectively, measured at 10 K. The lines are drawn as guides to the eye.

Fig. 3. Hysteresis loops at 10 K of the samples (a) NiO/Ni80Fe20 (5 nm)/Co (1 nm),

(b) NiO/Ni80Fe20 (1 nm)/Co (5 nm), (c) NiO/Co (1 nm)/Ni80Fe20 (5 nm), and (d) NiO/Co

(5 nm)/Ni80Fe20 (1 nm) measured after FC from 300 K in an applied field of 1 kOe.

trilayers. Moreover, the magneto-crystalline anisotropy of Co isstronger than that of Ni80Fe20. So, it is reasonable to interpret thatthe coercivity of Co/NiO couple is larger than that of Ni80Fe20/NiOcouple.

Fig. 4 illustrates the temperature dependencies of the magne-tization of the sample NiO/Ni80Fe20 (6 nm) measured whilewarming up in an applied field of 0.2 kOe after FC from 300 K in(a) absence of a field and at (b) 1 kOe and (c) 30 kOe. The ZFCcurve exhibits a typical blocking progress with the blockingtemperature at TBE160 K, which agrees well with the tempera-ture dependencies of the EB field measured after FC in a appliedfield of 1 kOe from 300 K.

Fig. 4. Temperature dependencies of the magnetization of the sample NiO/

Ni80Fe20 (6 nm), measured while warming up in an applied measuring field of

0.2 kOe after field cooling in a field of (a) 0 kOe, (b) 1 kOe, and (c) 30 kOe. Inset of

Fig. 4: Temperature dependence of HE of sample NiO/Ni80Fe20 (6 nm) after field

cooling in 1 kOe from 300 K.

Fig. 5. Temperature dependencies of the magnetization of the sample NiO/Co

(6 nm), measured while warming up in an applied measuring field of 0.2 kOe after

field cooling in a field of (a) 0 kOe, (b) 1 kOe, and (c) 30 kOe. Inset of Fig. 5:

Temperature dependence of HE of sample NiO/Co (6 nm) after field cooling in

1 kOe from 300 K.

S. Guo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3933–3936 3935

Fig. 5 presents the temperature dependencies of the magneti-zation of the sample NiO/Co (6 nm) measured while warming upin an applied field of 0.2 kOe after FC from 300 K in a field of(a) 0 kOe, (b) 1 kOe and (c) 30 kOe. For sample NiO/Co (6 nm)shown in Fig. 5, the FC magnetization curves reveal a rapidincrease in magnetization at low temperature, in contrast to thegradual increase in magnetization for sample NiO/Ni80Fe20 (6 nm)in Fig. 4. There are stronger temperature dependencies of mag-netization, as shown in the FC M–T curves for NiO/Co (6 nm) withstrong exchange coupling, especially upon a lager cooling fieldwhich can be helpful to enhance the exchange coupling strength[12]. Moreover, the ZFC magnetization curve exhibits a typicalblocking progress with the blocking temperature but atTBE200 K, which can also be consistent with the temperaturedependencies of the EB field measured after FC in a applied fieldof 1 kOe from 300 K. It is well accepted that the blockingtemperature of the FM/AFM couples has a strong dependenceon the layer-thickness, grain size, Neel temperature and uniaxialanisotropy of AFM layer [13,14]. Clearly, the effect of FM layercovering on AFM layer should be considered to interpret suchdifferent blocking temperatures, because all deposition conditionsof AFM (NiO) layers are fixed for our FM/AFM films. Regan et al.[15] found that a metal layer (Fe, Co or Ni) adjacent to an oxide

(NiO or CoO) was partially oxidized, using high-resolution L-edgeX-ray absorption spectroscopy which was element specific andsensitive to chemical environment and spin orientation. Not onlythe degree of oxidation may be different for diverse metals,but also the magnetic character of the interface oxide maybe different, which will strongly affect the exchange couplingstrength at the interface of FM/AFM couples. Furthermore, theratio between the FM and AFM anisotropies has a critical effecton exchange coupling strength, originating from the interplaybetween interfacial frustration and intrinsic anisotropies of FMand AFM [16]. It is reasonable to propose that the interfacialexchange coupling as another important factor may influence thethermal activation states of AFM layer, leading the changes ofblocking temperature of FM/AFM couples.

It is clear from Fig. 5 that the magnetization graduallyincreases with increasing temperature above about 200 K for FCcurves. According to the model of Meiklejohn and Bean [1], theanisotropy of the AFM interface layer varies from Kint¼0 next tothe FM layer to Kint¼KAF next to the AFM layer, where KAF is theanisotropy constant of an AFM with presumed uniaxial aniso-tropy. Wu et al. [17] reported that rotatable and frozen AFM spinsare uniformly distributed and 80% of the AFM spins can be frozenin the AFM film when the thickness of the AFM layer is larger than3 nm. Zheng et al. [18] reported that the EB training effect ing-Fe2O3 coated Fe nanoparticles can be well interpreted within amodified model, in which the g-Fe2O3 shells show a frozen state.Recently, Baltz et al. [19] discussed that the existence of FM/AFMinterfacial disordered spins which exhibit spins-glass-like beha-vior. All these findings suggest the existence of interfacial spinswith low anisotropy Kint, in which the frozen state leads to EB atlow temperature. For the Co/NiO couples with strong exchangecoupling, the interfacial disordered uncompensated frozen spinsare more than those of Ni80Fe20/NiO couples with weak exchangecoupling. When the interfacial spins in the spin-glass-like stateare relaxed at a critical temperature, these spins may be graduallyaligned to the direction of applied field leading to the increase oftotal magnetization. Moreover, the axial magneto-crystallineanisotropy and AFM-induced axial anisotropy of Co are stronglydependent on the temperature. When the magnitude of anapplied field is not big enough to saturate all the magnetizationto its direction, further increasing the temperature will makemore magnetic moments align to the direction of the appliedfield. For the Ni80Fe20/NiO systems with weak exchange coupling,the interfacial frozen disordered spins are limited, and the axialmagneto-crystalline anisotropy and AFM-induced axial aniso-tropy of Ni80Fe20 are very weak. So, the thermal destabilizationof magnetization is still a dominating factor to lead a downwardtrend of the magnetization with increasing the temperature, asshown in Fig. 4.

Fig. 6 shows the inverse of the FM thickness (1/tF) dependen-cies of EB field (HE) measured at 10 K after FC from 300 K in anapplied field of 1 kOe for (a) NiO (6 nm)/Co (t nm) and (b) NiO(6 nm)/Ni80Fe20 (t nm). According to the classical theory [11] ofEB, assuming that it is an interface effect, the interfacial exchangeenergy density Jex (erg/cm2) can be defined by

Jex ¼HEMFtF ð1Þ

From our results shown in Fig. 6, it is concluded that EB isroughly inversely proportional to the thickness of the FM layers(Co or Ni80Fe20) when tFZ5 nm. In other words, if the thickness ofFM layer is larger than 5 nm, the exchange coupling energydensity is a constant value. Our experimental date are perfectlyin agreement with Eq. (1) for tFZ5 nm, which can be furtherused to support that EB is an interface effect as assumed by theclassical theory. However, Eq. (1) does not describe the measuredHE when tFr4 nm. Using the relation Jex¼HEMFtF, we can obtain

Fig. 6. The magnitude of the exchange bias field (HE) of samples (a) NiO (5 nm)/Co

(t nm) and (b) NiO (5 nm)/Ni80Fe20 (t nm) vs the inverse of the thickness of FM

layer (1/tF), respectively, measured at 10 K after field cooling in 1 kOe from 300 K.

The lines are drawn as guides to the eye.

S. Guo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3933–39363936

that the value of Jex almost gradually increases with increasing thethickness of ferromagnetic film when tFo5 nm. Previously, muchattention has been paid to the microstructure and growth of theFM layer, suggesting that the FM layer may become discontinuousif the FM layer is too thin [20]. Yet, Gokemeijer et al. [8] reportedthat EB is due to a long-range interaction extending to as muchas 4–5 nm. It means that it is also useful to increase the totalexchange coupling energy with increasing the thickness of FMlayer due to the long-range interaction between interfacial AFMand FM atoms away from the FM/AFM interface. So, the long-range interaction for EB should also be taken into accountto interpret a maximum of exchange coupling energy value attFZ5 nm.

4. Conclusion

In summary, it has been found that ferromagnetic layer in NiO-based films NiO (5 nm)/FM1 (t nm)/FM2 (6-t nm) have stronginfluence on the magnitude and the blocking temperature ofexchange coupling. The thermal activated relaxation of interfacialfrozen spins and anisotropy of FM (Co) are proposed to explaina minimum found in the temperature dependence of the FCmagnetization of the films with strong exchange coupling effect.Due to the long-range interaction for EB and the microstructural

change of ferromagnetic layer, the exchange coupling energyincreases with increasing the thickness of ferromagnetic layerup to 5 nm. Then, the exchange coupling energy stabilizes at aconstant value, which further indicates that EB is an interfacialeffect in our FM/AFM films.

Acknowledgments

This work has been supported by the National Basic ResearchProgram (No.2010CB934603) of China, the National High Tech-nology Research and Development Program of China (863Program; No. 2011AA03A402), the Ministry of Science and Tech-nology of China and the National Nature Science Foundation ofChina under projects 50931006, and 50971123.

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