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Shape-anisotropy-controlled magnetoresistive response in magnetic tunneljunctions

Yu Lu,a) R. A. Altman, A. Marley,b) S. A. Rishton, P. L. Trouilloud, Gang Xiao,a)

W. J. Gallagher,c) and S. S. P. Parkinb)IBM Research Division, T. J. Watson Research Center, Yorktown Heights, New York 10698

~Received 27 January 1997; accepted for publication 17 March 1997!

We show that shape anisotropy can be used to control the response characteristics of magnetictunnel junctions. By varying the junction shape, the resistance versus field curve was made to varyfrom a nonhysteretic linear curve with a high-field sensitivity~0.3%/Oe! to a hysteretic responsecurve with high squareness. ©1997 American Institute of Physics.@S0003-6951~97!04019-9#

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Magnetoresistance~MR! in tunnel junctions with ferro-magnetic metal electrodes on both sides was first demstrated by Julliere1 more than twenty years ago. Followintwo reports in 1995 of large~18%! MR effects at room tem-perature in magnetic tunnel junctions~MTJs!,2,3 there hasbeen renewed interest in these devices.4 For instance, werecently showed that deep submicron MTJs can be mwith large MRs~.20%! and reasonable resistance levels5,6

In this letter, we demonstrate control of the magneticsponse through shape anisotropy and discuss the magcoupling between the electrode layers.

The magnetoresistance effect in tunnel junctions wnonmagnetic barriers arises from the spin imbalance inelectron density of states~DOS! in ferromagnetic metals. Inthe absence of spin–flip scattering, the tunneling currentbe separated into spin up and spin down currents, defiwith respect to the magnetization direction of one of telectrodes~assuming, for ease of discussion, uniformly manetized electrodes!. When the magnetic moments of the eletrodes are aligned parallel to each other, there is an optimmatch of filled majority spin states in one electrode wempty majority spin states in the other electrode, and thetunneling current is a maximum. When the moments aretiparallel to each other, a DOS mismatch of spin up and sdown components results in a minimum of total tunnelicurrent. Slonczewski7 showed that tunneling conductanceproportional to the cosine of the angle between the magnmoment vectors in the two electrodes. Of course, actual Mdevices will not, in general, have electrodes that are uformly magnetized and their magnetoresistive responsebe complicated by the detailed magnetic reversal procewithin the electrodes.

Microlithographic techniques are required to fabricasmall devices of accurately defined shape and size. Detaiour fabrication process have been described elsewhere.5,6 Inwhat follows, we briefly review the procedure for making tdevices used here. First, a multilayered film containingjunction structure is sputter deposited onto a Si wafer.the devices used in this work, in particular, the layerquence is Si~100!/50Ta/200Pt/40Py/100FeMn/60Py/20CAl–O/150Py/200Ta, where the numbers are layer thicknin angstroms and Py refers to permalloy Ni81Fe19. The 40Py

a!Also at: Physics Department, Brown University, Providence, RI 02912b!IBM Research Division, Almaden Research Center, Almaden, CA 951c!Electronic mail: glgr@watson.ibm.com

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layer, the 60Py/20Co bilayer, and the 150Py layer areferred to as the seed layer, pinned layer~bottom electrode!,and free layer~top electrode!, respectively. The bottom junction electrode is pinned by exchange bias from the 100Felayer. The purpose of the Co in the bilayer used for tpinned layer is to increase the spin polarization at the barinterface. The aluminum oxide~Al–O! barrier layer wasformed by depositing 9.5 angstroms of Al and then plasoxidizing it for 30 s. A magnetic field of about 100 Oeapplied in the plane of the film during deposition to induceuniaxial anisotropy, and thereby, define the exchangening direction and the easy-axis direction of the free layThe magnetic properties of blanket film multilayer structurare characterized using a vibrating sample magnetom~VSM!.

A self-aligned lithographic process is used to patterndevice structures.4,5 For the devices reported here, opticlithography was used with a minimum feature size of aboumm. First, the base electrode and the junction top electrarea are defined by a sequence of two ion milling procesThe second milling step is timed to stop just below the tsurface of the bottom electrode, some overmilling beingevitable. This overmilling has important effects on the manetic coupling of the two electrodes. Next, the junctionscoated with a SiO2 insulating layer, which is lifted off toexpose self-aligned contact holes to the junction top etrode. Finally, contact pads and wiring to the top electroare patterned out of a subsequent metallization layer. Fig1 shows schematic drawings of the finished junction strture ~a! in a cross-sectional view and~b! in a top view show-ing some typical junction shapes~b!. Our mask patterns in-clude 200 junctions on a quarter of a 1 in. silicon wafer withvarious junction areas (1–104 mm2) and shapes~mostlyrectangular!.

The MTJ devices on the chip are measured electricusing a probe station in ambient environment. A magnefield along the easy axis of the film is applied to changemagnetic states of the devices. The field uniformity is bethan 1% over the area of the chip. The dynamic tunnelresistance (dV/dI) is measured using an ac method wizero dc bias. Voltage amplitude on the junction is kesmaller than 10 mVp-p to avoid the voltage dependent deradation of the MR effect.1,3

Figure 2~a! shows the magnetizationM hysteresis curveof the sheet multilayer film~one quarter of a 1 in. diamwafer! measured using the VSM. There are three we

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separated subloops corresponding to the magnetic reversthe seed layer, the pinned layer, and the free layer, in oof increasing net magnetizations~due to the layers’ differenthicknesses!. Figure 2~b! shows the tunneling resistance of2.5mm312.5mm device as a function of the magnetic fieapplied along the easy axis of the film, which is also the loaxis of the junction. The two hysteresis loops seen in F2~b! are due to the corresponding loops of the pinned la

FIG. 1. Schematic drawing showing MTJ device structure:~a! cross-sectional view, and~b! top view of typical device geometries.

FIG. 2. ~a! Magnetic hysteresis loop of sheet film with multilayer junctiostructures.~b! Tunneling resistanceR and magnetoresistance ratioDR/Rp ,vs magnetic field along the easy axis of a MTJ with a rectangular 2.5312.5mm2 top electrode.

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and the free layer. The exchange bias shown in the tunneresistance data is much stronger than that of the sheetThis can be explained by the reduced pinned-layer thickndue to overmilling in the area surrounding the junctiowhich acts to stabilize the magnetization of the junction belectrode~which is not reduced in thickness!.

The size of the MR effect can be understood within tsimple model proposed by Julliere.1 The MR ratio for thedevices described above should be given by

DR/Rp5~Rap2Rp!/Rp52PCoPPy/~12PCoPPy!, ~1!

whereRp and Rap are the resistances for the parallel aantiparallel M configurations, respectively, andPCo andPPy are the spin polarization of the Co (PCo'0.35) and Py(PPy'0.30) electrodes.9 For our MTJ devices, we expecMR5DR/Rp524.5%, which is in agreement with the valuewe observe. This is consistent with tunneling being the mconduction mechanism and with the barrier being freemagnetic impurities and other spin–flip scattering center

Figures 3~a! and 3~b! show the resistance versumagnetic-field curves of two series~hereafter, called series Aand B, respectively! of devices on the chip, normalized ttheir saturation resistanceRp . The magnetic-field sweeprange is limited so that only the free-layer magnetizatreverses. Devices in series A all have the same normal abut the shape of the rectangle is evolving from a needlerectangle perpendicular to the easy axis of the film~which isalso the magnetic-field direction! at the left end of the plot toa squarish shape in the middle of the series to a thin nealong the easy axis at the right end. Devices in series B hvarying size but the same shape, in this case, rectanglesa 5:1 aspect ratio along the easy-axis direction.

The changing response characteristic of the curvesFig. 3~a! demonstrates that the shape anisotropy in ourvices is more important than the intrinsic anisotropy inducduring film deposition. Thus, the MR response of the Mdevices can be ‘‘engineered’’ by simply changing the shaof the free-layer electrode. At the left end of the plot whe

FIG. 3. ~a! Junction resistance versus magnetic field for series A deviwhich have the same nominal junction area but different shapes. Thenumber in the label is the dimension of the junction top electrode alonghard axis inmm, while the second number is the dimension along the eaxis. ~b! Junction resistance versus magnetic field for series B devices,different areas but the same nominal 1–5 aspect ratio for the junctionelectrode. The unusually large MR in the largest junction 283140 (mm2) isdue to a nonuniform current distribution effect as discussed in Ref. 8.

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the electrodes are shaped as thin rectangles normal toeasy axis, the demagnetization field will break the magnezation into a buckled domain pattern. The magnetic reversin this case, will most likely be a combination of rotation andomain-wall motion. The MR curve reflects the averageresponses of these domains and is linear and nearly nonhteretic. The field sensitivity reaches up to 0.3%/Oe~in the6434 mm2 junction!. At the right end of Fig. 3~a!, it is mostlikely that reversal domains nucleate at the ends andaround edge roughness, and sweep through the whole dewhen the domain pinning force is exceeded, giving rise tosquare hysteresis loop with discontinuous jumps in restance. In Kerr images taken of some of our devices, we haseen evidence for this reversal mechanism. Figure 3~b!shows, however, that this simple picture gets more compcated with junction areas below 100mm2. In this case, webelieve edge roughness and other defects play an increingly important role.

Figures 4~a! and 4~b! show the coercive forceHc and theeffective coupling fieldHeff ~center of the hysteresis loops!in series A and B, as a function of aspect ratio~hard-axis-side length to easy-axis-side length! and nominal junctionarea, respectively. The coercive force drops with increasiaspect ratio, as expected from the increased demagnetizafactor, although the dependence is weaker than simplepectations from a single-domain picture. The slight dropHc with larger junction size can possibly be explained by thincreased probability of finding a nucleation site with smallecoercive force in the top electrode.

The effective coupling field has two dominant sourceNeel ‘‘orange-peel’’ coupling~ferromagnetic! due to corre-lated interface roughness10 and stray field coupling~antifer-romagnetic! due to poles in the overmilled pinned layer. ThNeel coupling depends only on the sheet film paramete~free-layer thickness, barrier thickness, and roughness! andshould be independent of device geometry. The stray fiecoupling depends on the amount of overmilling as well athe actual shape of the junction electrodes, and increawith decreasing length along the easy-axis direction. Tsum of these two effects determines the overall couplinstrength, which is reflected in the quantityHeff . The data inFigs. 4~a! and 4~b! show thatHeff is ferromagnetic. The sizeof Heff increases significantly with decreasing aspect rati

FIG. 4. ~a! Coercive forceHc and effective coupling fieldHeff as a functionof the nominal junction aspect ratio for series A devices. The error bindicates the variation of these quantities with multiple cycles of the manetic field. ~b! Hc andHeff as a function of the junction area for series Bdevices, which all have a 1–5 nominal aspect ratio.

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reflecting the strong shape dependence of the stray fieldpling. For fixed aspect ratio, Fig. 4~b! shows that there is noa strong size dependence ofHeff , indicating that the strayfield coupling is not strongly size dependent over the ranof sizes studied.

The above discussion neglects exchange coupthrough the barrier. Compared to spin valves with metaspacer layers, the exchange coupling in tunnel junctishould be smaller due to the insulating nature of the tunbarrier, less than 1023 Oe according to a estimate madfollowing Ref. 7. On the other hand, the extreme thinnessthe oxide tunnel barrier leads to larger Ne´el orange peel cou-pling.

In conclusion, we have demonstrated that the magnetsistance effect in magnetic tunnel junctions can be controby the shape anisotropy of the top electrode. Linearsponses with high-field sensitivity~0.3%/Oe! and hystereticresponses with high squareness can be achieved on thechip. In all cases, the size of the MR effect at room tempeture is consistent with estimates made using Julliere’s spolarized tunneling model. The magnetic coupling betwethe electrodes in the tunnel junction is dominated by a cobination of orange-peel coupling and stray magnetostfield coupling.

The authors wish to thank J. C. Slonczewski, X. W. LK. P. Roche, S. L. Brown, J. J. Connolly, A. Gupta, G.Gong, and R. Laibowitz for stimulating discussions and heThe work at Brown University was partially supported bNational Science Foundation Grant Nos. DMR-9414160 aDMR-9258306 and at IBM partially by DARPA and by thOffice of Naval Research.

1M. Julliere, Phys. Lett. A54A, 225 ~1975!.2T. Miyazaki and N. Tezuka, J. Magn. Magn. Mater.139, L231 ~1995!.3J. S. Moodera, L. R. Kinder, T. M. Wong, and R. Meservey, Phys. RLett. 74, 3273~1995!.

4J. S. Moodera and L. R. Kinder, J. Appl. Phys.79, 4724 ~1996!; C. L.Platt, B. Dieney, and A. E. Berkowitz, Appl. Phys. Lett.69, 2291~1996!;K. Matsuyama, H. Asada, H. Miyoshi, and K. Taniguchi, IEEE TranMagn. 31, 3176 ~1995!; Yu Lu, X. W. Li, G. Q. Gong, A. Gupta, P.Lecoeur, J. Z. Sun, Y. Y. Wang, and V. P. Dravid, Phys. Rev. B54, 8357~1996!; J. Z. Sun, W. J. Gallagher, P. R. Duncombe, L. Krusin-Elbaum,A. Altman, A. Gupta, Yu Lu, G. Q. Gong, and G. Xiao, Appl. Phys. Le69, 3266~1996!.

5W. J. Gallagher, S. S. P. Parkin, Yu Lu, X. P. Bian, A. Marley, R.Altman, S. A. Rishton, K. P. Roche, C. Jahnes, T. M. Shaw, and G. XJ. Appl. Phys.~in press!.

6S. Rishton, Y. Lu, R. A. Altman, A. C. Marley, X. P. Bian, C. Jahnes,Xiao, W. J. Gallagher, and S. S. P. Parkin, Magnetic Tunnel JunctiFabricated at Tenth-Micron Dimensions by Electron Beam LithograpMicroelectronic Engineering Conference Proceedings, 1996~in press!.

7J. C. Slonczewski, IBM Tech. Discl. Bull.19, 2328 ~1976!; J. C. Slonc-zewski, Phys. Rev. B39, 6995~1989!.

8J. S. Moodera, L. R. Kinder, J. Nowak, P. LeClair, and R. MeservAppl. Phys. Lett.69, 708 ~1996!.

9For a review of spin-polarized tunneling studies, see, e.g., R. Meseand P. M. Tedrow, Phys. Rep.239, 174 ~1994!.

10L. Neel, C. R. Acad. Sci.255, 1676~1962!.

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