IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005 2703

Co-Existence of Biquadratic and UnidirectionalAnisotropy in IrMn/CoFe/FeOx/CoFe Films

Chih-Huang Lai1, Member, IEEE, Chih-Ta Shen1, Po Hsiang Huang1, H. J. Lin2, and C. T. Chen2

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, R.O.C.National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu 300, Taiwan, R.O.C.

We observed the coexistence of biquadratic and unidirectional anisotropy in the CoFe layer by inserting the nano-oxide-layer FeOx inthe IrMn/CoFe bilayers. Analyses of X-ray magnetic circular dichroism suggested that the FeOx possessed ferromagnetic components.Mixed valence states of Fe, Fe2+ and Fe3+ were observed in the FeOx layer, which implied that both antiferromagnetic and ferro-magnetic components might co-exist in the FeOx layer. The uniaxial anisotropy in CoFe at 90 may originate from the interaction ofCoFe with the antiferromagnetic components of the FeOx layer in the presence of interfacial magnetic roughness. The unidirectionalanisotropy at 0 may result from the exchange biasing from the IrMn layer through the ferromagnetic components in the FeOxlayer.

Index Terms—Biquadratic coupling, exchange biasing, FeOx, interlayer coupling, nano-oxide-layer.

I. INTRODUCTION

DURING the past years, interlayer coupling between theferromagnetic layers separated by a spacer attracted much

attention. Most of the interlayer coupling was bi-linear, resultingin either ferromagnetic (FM) or antiferromagnetic (AFM) cou-pling. However, 90 coupling was observed in FM/AFM/FMtrilayers with AFM spacers of NiO [1], [2] or FeMn [3]. 90coupling between two FM layers separated by NiO AFM spaceroriginated from magnetic roughness at the AFM/FM interfaces[1], [2]. For the FeMn spacer, the angle between two FM layerswas determined by the thickness of the FeMn, which formed aspiraling spin structure [3].

To enhance magnetoresistance (MR) ratio in spin valves,nano-oxide layers (NOLs) were inserted in free or/and pinnedlayers [4], [5]. Various couplings were observed between ad-jacent FMs through NOLs. Ferromagnetic coupling occurredthrough most NOLs so the pinning AFM layer can also exert anexchange biasing on the FM layers on the NOL. However, theinsertion of nano-Co Fe O layers, formed by ion assistedoxidation (IAO) under high oxygen exposure, exhibited thetwist coupling between adjacent CoFe layers [6]. In addition, abiquadratic coupling between the top- and bottom-pinned CoFelayers separated by a NiFeO layer was also observed [7].Insertion of the CoFe O layer also induced the biquadraticcoupling [8]. In this work, we inserted the FeO layers in thepinned layer of CoFe. Not only the uniaxial biquadratic cou-pling but the uni-directional exchange coupling was observedbetween the pinned CoFe layers separated by the FeO layer.The mechanism of the interlayer coupling through FeO -NOLwas discussed.

II. EXPERIMENT

Spin valves with the structure of Ta 3 nm /Cu 1.2 nm / IrMn8 nm /CoFe (FM1) 2 nm /FeO 2.2 nm /CoFe (FM2) 2 nm/Cu 2.3 nm /CoFe (F) 4 nm /Ta 3 nm were deposited on Si

Digital Object Identifier 10.1109/TMAG.2005.855225

(100) wafers by using a sputtering system at a base pressureof 5 10 Torr. FeO layers were formed by exposing 1.5 nmFe films to oxygen plasma for 10 s at an oxygen partial pressureof 3 mTorr. To set the exchange biasing direction (0 ), all filmswere postannealed in vacuum at 200 C for 15 min at a pres-ence of 1 kOe. To investigate the interlayer coupling betweenthe top (FM2) and bottom (FM1) pinned layer directly, the sam-ples without free (F) and spacer (Cu) layers were also depositedunder the same condition. A vibrating- sample magnetometer(VSM) was used to measure hysteresis loops. MR ratios weremeasured using a four-point probe at room temperature. To fur-ther understand the properties of the FeO layer, we preparedthe sample with the structure of IrMn 8 nm /Co 2 nm /FeO2.2 nm /Co 2 nm /Ta 3 nm. In this structure, only the NOL layercontained Fe. The characteristics of the FeO layer were inves-tigated by using X-ray magnetic circular dichroism (XMCD),an element-specific magnetization measurement. XMCD mea-surements were made at RT at the Beamline 11A at the NationalSynchrotron Radiation Research Center in Taiwan. The energyresolution of incident light was set to be 0.4 eV and the XMCDspectra were obtained in a total electron yield mode with an 80probing depth. An x-ray photoelectron spectrometer (XPS) wasemployed to determine the oxidation state of the FeO layer.

III. RESULTS AND DISCUSSION

MR curves of a spin valve with a FeO layer exhibited anunusual behavior, shown in Fig. 1(a) and (b), measured parallel(0 ) and perpendicular (90 ) to the field- annealing axis. The90 curve showed a pseudo-spin-valve-like behavior [Fig. 1(b)],different from the typical spin valves. This behavior was also ob-served in spin valves with the insertion of Co Fe O or NiFeONOLs [6], [7], in which the existence of uniaxial anisotropy at90 was reported. However, unlike previous reports [6], [7], ashifted complicated loop was observed in 0 , shown in Fig. 1(a).

A series of samples were made to further understand theorigin of the uncommon loops. First, we verified that thebottom pinned layer (FM1) was pinned quite well by IrMn,shown in Fig. 2(a-1), with an exchange field Oe;second, we realized that part of the FeO was ferromagnetic

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2704 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

Fig. 1. R-H loops of spin valves measured at (a) 0 and (b) 90 with respectto the annealing direction.

Fig. 2. M-H loops of Ta/Cu/IrMn/CoFe/FeO /CoFe/Cu/Ta samples measuredin (a) 0 and (b) 90 with respect to the annealing direction. The insetFigs. (a-1) and (a-2) were the M-H loops of Ta/Cu/IrMn/CoFe/Ta andTa/Cu/IrMn/CoFe/FeO /Ta, respectively.

because the H was reduced to 610 Oe, shown in Fig. 2(a-2),and the magnetization value was increased by depositing theFeO layer on the top of the IrMn/CoFe (FM1) bilayers.Moreover, to investigate the interlayer coupling betweenthe top and the bottom pinned layer directly, the sample ofIrMn/CoFe (FM1)/FeO /CoFe (FM2)/Cu/Ta was prepared.Hysteresis loops measured at 0 and 90 were shown inFigs. 2(a) and (b), respectively. Although a clear unidirectionalanisotropy existed in FM1 and FeO due to the exchange biasbetween IrMn and CoFe/FeO , an observed complicated loopof IrMn/FM1/FeO /FM2, shown in Fig. 2(a), indicated thatthe FeO may not only deliver the ferromagnetic couplingbetween FM1 and FM2, but may introduce extra anisotropy

Fig. 3. R-H loop of spin valves measured at 15 . The inset showed a schematicdiagram of measuring angles.

in FM2. Fig. 2(b) displayed a combination of a sharp-tran-sition easy-axis-like loop and a sheared hard-axis-like loop.Since FM1 and FeO were pinned at 0 by IrMn, shown inFigs. 2(a-1) and (a-2), the sheared loop was mainly contributedby FM1 and FeO . The sharp-transition loop may imply that auniaxial anisotropy may exist in the orientation perpendicularto the field-annealing direction, which was consistent with theobserved pseudo-spin-valve-like MR curves measured in 90[Fig. 1(b)].

Since M-H curves were contributed from all layers, it was dif-ficult to distinguish the magnetization orientations in differentlayers. On the other hand, because MR curves mainly reflectedthe relative orientation between the free layer and FM2, MRcurves could be used as an indicator to trace the magnetizationorientations and anisotropy in FM2. In Fig. 1(a), a clear shiftedMR loop revealed the existence of the unidirectional anisotropyin FM2 but the strength was reduced due to the presence ofFeO . Furthermore, the presence of the uniaxial anisotropy inFM2 at 90 might induce a local energy minimum at 90 . Sincethe exchange field was decreased, the magnetization of FM2might not be hold steadily when the field was reduced from thenegative saturation field and magnetization of FM2 might begradually rotated to 90 (local minimum) before the magnetiza-tion reversal of the free layer took place; therefore, the resistanceonly reached a half maximum value at the zero field.

The effects of co-existence of unidirectional anisotropy at0 and local energy minimum at 90 in FM2 were more pro-nounced in the R-H loops measured in the orientations otherthan 0 . Fig. 3 and the inset diagram show the R-H loop mea-sured at 15 . The co-existence of two anisotropies results in anunusual loop, in which an extra MR minimum (marked as po-sition 2 in Fig. 3) near the zero field appeared. When the fieldwas reduced from the negative saturation field, the magnetiza-tion of the FM2 layer first gradually rotated to the local en-ergy minimum (90 ), leading to a 0.37 (position 1 inFig. 3). When the field approached to zero, magnetizations ofFM2 and the free layer were close to 90 , resulting in an extraMR minimum. With increasing a positive field, the magnetiza-tion of the free layer was switched to the applied field direc-tion while that of FM2 was kept near 90 , accompanying anincrease of MR (position 3). With further increasing positivefields, both the magnetizations of the FM2 and free layers were

LAI et al.: CO-EXISTENCE OF BIQUADRATIC AND UNIDIRECTIONAL ANISOTROPY IN IrMn/CoFe/FeO /CoFe FILMS 2705

Fig. 4. XMCD spectra at the Fe L edges measured at RT. The XMCDspectra were taken with the projection of the spin of incident photons parallel(� , solid curve) and antiparallel (� , dashed curve) to the spin of the Fe 3dmajority electrons.

Fig. 5. XPS results of the FeO layer.

gradually aligned in the parallel direction so MR reached a min-imum again.

The unusual behavior occurred when the FeO layers insertedinto the exchange biased films; therefore, the characteristics ofthe FeO layers played important roles. XMCD analyses wereperformed on the FeO layers by gathering the Fe signals toclarify their magnetic characteristics. A significant differencewas observed by using right-hand and left-hand polarized light(Fig. 4), and a clear MCD signal was observed, which stronglysuggested that the FeO possessed ferromagnetic (or ferrimag-netic ) components. Moreover, the XPS results revealed thatmixed valance states of Fe, Fe and Fe co-existed in theFeO layer, as shown in Fig. 5. The un-oxidized Fe and/or theoxidized Fe-compounds ( -Fe O , Fe O ) in the FeO layerscould form the magnetic channels so that the uni-directionalanisotropy from IrMn can be exerted through the FeO layer

on the FM2 layer. In addition, FeO, –Fe O , or Fe O mightexist in the FeO layer, which possessed antiferromagnetic orferrimagnetic properties. The biquadratic coupling in CoFe(FM1)/FeO /CoFe (FM2) might be explained by the existenceof the magnetic roughness at the FM2/FeO interface, asreported in NiO [2] and CoFe O systems [8].

IV. CONCLUSION

Based on observed results, we proposed that mixture ofantiferromagnetic (ferrimagnetic) and ferromagnetic clusterswas present in the FeO layer. The interaction between theCoFe (FM2) layer and different components of the FeOlayer resulted in the observed complicated loops. The uni-axial anisotropy in CoFe (FM2) at 90 may originate fromthe interaction with AFM components in the FeO layer, andunidirectional anisotropy at 0 may result from the couplingto IrMn/CoFe (FM1) through the FM components in the FeOlayer.

ACKNOWLEDGMENT

This work was supported in part by the National ScienceCouncil of R.O.C. under Grant NSC 93-2112-M- 007-033 andGrant 94-2120-M-007-001.

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Manuscript received February 1, 2005.