IrMn junctions

Applied Surface Science 257 (2010) 1484–1486

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Tunneling magnetoresistance in exchange-biased CoFeB/AlOx /Co/IrMn junctions Yuan-Tsung Chen a,∗ , Jiun-Yi Tseng b , S.U. Jen b , T.L. Tsai c , Y.D. Yao d a

Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan, ROC Institute of Physics, Academia Sinica, Taipei 11529, Taiwan, ROC c Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, ROC d Graduate Institute of Applied Science and Engineering, Fu Jen Catholic University, Taipei 242, Taiwan ROC b

a r t i c l e

i n f o

Article history: Received 17 June 2010 Received in revised form 16 August 2010 Accepted 20 August 2010 Available online 25 September 2010 PACS: 75.47.−m 85.75.−d 75.30.Et Keywords: Magnetic tunneling junction Tunneling magnetoresistance Tunneling resistance Interfacial boundaries Anisotropic phenomena

a b s t r a c t A series of exchange-biased magnetic tunneling junctions (MTJs) were made in an in-plane ˚ ˚ deposition field (h) = 500 Oe. The deposition sequence was Si(1 0 0)/Ta(30 A)/CoFeB(75 A)/ ˚ ˚ ˚ ˚ where d was varied from 12 A˚ to 30 A. ˚ The MTJ was formed by A)/IrMn(90 A)/Ta(100 A), AlOx (d A)/Co(75 the cross-strip method with a junction area of 0.0225 mm2 . The tunneling magnetoresistance (R/R) of each MTJ was measured. The high-resolution cross-sectional transmission electron microscopic (HR X-TEM) image shows the very smooth interface and clear microstructure. X-ray diffraction (XRD) demonstrates that the IrMn layer of the MTJ exhibits a (1 1 1) texture. From the results (R/R) increases ˚ The tunneling resistance (Ro ) of these junctions ranges from 17% to 50%, as d increases from 12 A˚ to 30 A. from 150  to 250 . The exchange-biasing field (Hex ) of the MTJ is 50–95 Oe. Finally, the saturation resistance (Rs ) was measured as a function of the angle (˛) of rotation, where ˛ is the angle between h and the in-plane saturation field (Hs ) = 1.1 kOe. The following figure presents the dependence of Rs on ˛, instead of originally expected independence, the curve actually varies with a period of . © 2010 Elsevier B.V. All rights reserved.

1. Introduction

2. Experiments

Magnetic tunneling junction (MTJ) is an important issue in the nonvolatile magnetoresistive random access memory (MRAM) industry, magnetic read heads, and gauge sensor applications, because it has a high tunneling magnetoresistance (TMR) ratio at room temperature [1–5]. The MTJ is a sandwiched structure, which is composed of ferromagnetic (FM) layer/insulating tunneling layer/ferromagnetic (FM) layer. Widely used FM and tunneling layers include CoFeB, CoFe, Fe and crystalline MgO and amorphous AlOx , respectively [6–8]. Both of the interfaces between the tunneling layers have a key role in spin-tunneling, which is determine by surface roughness and structure of the texture [9,10]. The TMR ratio is generally defined as R/R = (RAP − RP )/RP , where RP and RAP are the tunneling resistances when the magnetization of the two electrodes are align in parallel and antiparallel, respectively. This study elucidates various tunneling barriers of the form AlOx in the MTJ and the effect of the ratio of surface roughness to the TMR. The Co/IrMn top configuration exchange-biasing phenomenon was also observed.

The multilayered structure was deposited onto a Si(1 0 0) substrate using a dc magnetron with in-plane external field (h) = 500 Oe. The typical base chamber pressure was 1.5 × 10−7 Torr and the Ar working chamber pressure was ˚ ˚ A)/ 5 × 10−3 Torr. MTJ had of the form Si(1 0 0)/Ta(30 A)/CoFeB(75 ˚ ˚ ˚ ˚ for which the values of AlOx (d A)/Co(75 A)/IrMn(90 A)/Ta(100 A), ˚ 17 A, ˚ 22 A, ˚ 26 A, ˚ and 30 A. ˚ To the tunneling thickness d were 12 A, form AlOx barrier, Al was first deposited on the bottom electrode by flowing O2 native-oxidation (Al-oxidized) process; then AlOx was formed by reactive sputtering in the Ar/O2 mixture during the deposition of AlOx . The ratio Ar/O2 was 9:16 and the plasma oxidation time of the differential AlOx layers was between 30 s and 70 s. The cap Ta layer was used to protect the IrMn against oxidation. MTJs were fabricated using the cross-strip method, each with an area of 0.0225 mm2 . Moreover, the target compositions of CoFeB and IrMn alloys were 60 at.% Co, 20 at.% Fe, 20 at.% B and 20 at.% Ir, 80 at.% Mn, respectively. To investigate the TMR ratio, the conventional four-point approach was used. The microstructure of each layer was characterized by high-resolution cross-sectional transmission electron microscopy (HR X-TEM) picture. The structure of the MTJ was characterized by the X-ray diffraction (XRD) method using a CuK␣1 line.

∗ Corresponding author. Tel.: +886 7 657 7711x3119; fax: +886 7 657 8444. E-mail address: [email protected] (Y.-T. Chen). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.08.077

Y.-T. Chen et al. / Applied Surface Science 257 (2010) 1484–1486

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˚ Fig. 1. Magnetoresistance versus measured field H in Si(1 0 0)/Ta(30 A)/ ˚ ˚ ˚ ˚ sample. Inset demonstrates ˚ A)/IrMn(90 A)/Ta(100 A) CoFeB(75 A)/AlO x (12 A)/Co(75 boundary configuration of cross-strip MTJ.

The exchange-biasing field (Hex ) was measured using a LakeShore Model 7300 vibrating sample magnetometer (VSM) and the interfacial surface morphology and roughness (Sq ) between the AlOx layers was determined using an atomic force microscope (AFM). 3. Results and discussion Fig. 1 plots the dependence of the magnetoresistance (R) on the measured field (H). The nominal terms RAP , RP , Ro , and Rs are defined below. RAP refers to the high MR state of antiparallel spin. RP refers to a low state of parallel spin. Ro is the tunneling resistance when H is zero. Rs represents the saturation resistance when H reaches the saturated field (Hs ). The inset presents the boundary configuration of the cross-strip MTJ. The numbers from 1 to 4 in Fig. 1 mean the direction of applied magnetic field during measurements, the corresponding spin states of numbers from 1 to 4 in this figure show in Fig. 2. As the measured sample cannot be maintained in parallel ៝ of MTJ exhibits a relationship between ៝ the deposited field (h) H, ៝ This figure reveals the abnormal trend in Rs . the angle () and H. It indicates that has a larger magnetoresistance than the RP state. First, the point 2 state is considered. The initial state of point 2 spin flip is the RAP high state. From the zero field of the Hs state, the point 2 of the spin state is the Ro state. Then, as Hs increases slightly, the point 2 spin state becomes the RP state. When Hs increases to saturation, the point 2 spin state is the Rs state. The point 2 spin state in Fig. 2 is responsible for the increase in magnetoresistance when Hs is saturated. This strange phenomenon is caused by the dependence of Hs on these spin states.

Fig. 2. Dependence of saturation resistance (Rs ) on angle (˛) of rotation where ˛ is the angle between h and the in-plane saturation field (Hs ) = 1.1 kOe.

Fig. 3. (a) High-resolution cross-sectional transmission electron microscopic (HR ˚ ˚ ˚ ˚ ˚ A)/IrMn(90 A)/ X-TEM) image of Si(1 0 0)/Ta(30 A)/CoFeB(75 A)/AlO x (17 A)/Co(75 ˚ sample. (b) X-ray diffraction pattern of Si(1 0 0)/Ta(30 A)/CoFeB(75 ˚ ˚ Ta(100 A) A)/ ˚ ˚ ˚ ˚ sample. A)/IrMn(90 A)/Ta(100 A) AlOx (17 A)/Co(75

Fig. 2 plots Rs versus rotation angle (˛) where ˛ equals /2 −  in Fig. 1. Based on Julliere’s model [11], Rs should be a straight line that is independent of ˛. However, the results herein show that Rs is an anisotropic trait to ˛. Points 1–4 of spin states reveal the boundary configurations of the MTJ spin state in Fig. 1. The spin flip period is . Fig. 3(a) displays a typical high-resolution cross-sectional transmission electron microscopic (HR X-TEM) image, which shows the very smooth interface and clear microstructure. The tunneling AlOx layer has a closely packed structure, such that the MTJ has no pin hole or crack. The corresponding XRD pattern of Fig. 3(a) is shown in Fig. 3(b). According to XRD result, it exhibits the clear diffraction peaks of the texture of both the IrMn (2 = 41.3◦ ) and the Co (2 = 44.7◦ ) layers. No apparent diffraction peaks can be observed about CoFeB and AlOx layers. It suggests that the CoFeB and AlOx layers are amorphous state. Fig. 4 presents the TMR ratio as a function of d for various buffer layers. The thickness d increases with MR, and the ratio increases ˚ from 17% to 50% as d increases from 12 A˚ to 30 A. Fig. 5 plots the tunneling resistance Ro as a function of d with different buffer AlOx layers. Based on the tunneling model [12,13], Ro is well known to increase monotonically with AlOx thicknesses. The results herein are almost entirely consistent this model, except ˚ The anomalous Ro is very in that the thickness of AlOx is 12 A. large because of over oxidation at the CoFeB/AlOx interface [14]. Elsewhere Ro varies linearly. However, thickness of AlOx 12 A˚ is inconsistent with the exponential shape, because the interfacial roughness in the CoFeB/AlOx and AlOx /Co interfaces influences Ro ,

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Table 1 presents the roughness (Sq ) and exchange-biasing field (Hex ). From this table, the roughness of a CoFeB/AlOx interface is ˚ and that of the AlOx surface varies with d. As d approaches 2.48 A, ˚ a larger Sq causes a larger Ro . Then, Sq declines as the thickness 12 A, d increases further. Additionally, Hex of the top Co/IrMn configuration varies from 50 Oe to 95 Oe. 4. Conclusions

Fig. 4. TMR ratio of MTJ as function of tunneling thickness d when MTJs are grown on various buffer layers.

˚ ˚ ˚ The series Si(1 0 0)/Ta(30 A)/CoFeB(75 A)/AlO x (d A)/Co ˚ ˚ ˚ was fabricated, with a tunneling (75 A)/IrMn(90 A)/Ta(100 A) ˚ Rs is larger than RAP because of the thickness d from 12 A˚ to 30 A. deposition field (h) and measured field (H) are at an angle () from each other, and Hs depends on the spin states. The tunneling resistance (Ro ) varies from 150  to 250 . Ro mostly fits the tunneling ˚ because of model, except at the anomalous thickness of d = 12 A, over oxidation during AlOx fabrication and the Orange-peel effect. The TMR of MTJ increases from 17% to 50%, as d increases from ˚ The saturation resistance (Rs ) as a function of rotation 12 A˚ to 30 A. angle (˛) is not based on Julliere’s model, because the spin flip at every point is responsible for feature of the curve. The curve is periodic and has a period of . Acknowledgments This work was supported by the National Science Council and I-Shou University, under Grant Nos. NSC97-2112-M214-001-MY3 and ISU98-S-02, respectively. References

Fig. 5. Tunneling resistances at zero field (Ro ) against tunneling thickness d when MTJs are grown on different buffer layers. Table 1 Surface roughness (Sq ) of CoFeB/AlOx interface and AlOx surface measured by AFM. Exchange-biasing field (Hex ) of top Co/IrMn configuration with VSM is shown. ˚ d (A)

˚ Sq (A)

Hex (Oe)

0 12 17 22 26 30

2.48 4.93 3.07 3.49 2.56 2.23

– 62 50 68 52 95

as presented in Table 1. When considering the Table 1 result, the AlOx with the thickness of 12 A˚ has very roughness of about surface ˚ This large surface roughness may give rise of roughness of 4.93 A. Orange-peel effect during R–H measurement, which also induces anomalous larger Ro .

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