Journal of Magnetism and Magnetic Materials 240 (2002) 212–214

Hard magnetic CoCr layer in ferromagnetic tunnel junctions

M. Justus*, H. Br .uckl, G. Reiss

Department of Physics, University of Bielefeld, Nano Device Group, P.O. Box 100131, 33501 Bielefeld, Germany

Abstract

In magnetic tunnel junctions a highly spin-polarizing layer is usually exchange biased by an antiferromagnetic layer,an artificial antiferromagnetic layer system or a combination of both, while the magnetically soft layer is free to rotate.

The use of a single layer of a hard magnetic material is rarely investigated up to now. In this paper, we present theelectric and magnetic properties of tunnel junctions with a hard magnetic Co83Cr17 layer. The soft magnetic electrodeconsists of either a single Co layer or a Co=Ni80Fe20 bilayer. The magnetic anisotropy and coercive fieldHC of the CoCr

layer depend on its thickness and the kind of the bottom layer (Cu or Ta) and can vary from HC ¼ 502700Oe. It isfound that a thin Co cap layer also influences the hysteretic behavior. Furthermore, only small changes after annealingup to 4501C promise a high thermal stability for the application in magnetic tunnel junctions. Measurements of the

tunnel magnetoresistance on large area junctions, however, show a strong magnetic coupling of the hard and softelectrodes. r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Coercivity – thickness dependence; Tunneling; Magnetoresistance; Annealing

1. Introduction

Up to now, the use of hard magnetic materials inferromagnetic tunnel junctions was rarely investigated

[1,2]. In these elements, a highly spin-polarizing layer isusually pinned by an antiferromagnetic layer [3], anartificial antiferromagnetic layer system [4], or a

combination of both, whereas the magnetically softlayer is free to rotate. A single hard magnetic layer (e.g.CoCr alloy), however, should be a simpler and perhapsmore stable (e.g. against annealing) alternative to the

established stack sequences.It has been reported that CoCr thin films contain

ferromagnetic regions with low Cr content which are

compositionally isolated by a paramagnetic host matrixof larger Cr concentration, predominantly in the grainboundaries [5]. This compositional separation is the

main reason for the reduction of intergranular interac-tion, which causes the coercivity to increase. In thispaper, we present studies on Co83Cr17 alloy thin films as

hard magnetic pinning layer in ferromagnetic tunnel

junctions. We discuss the dependence of magnetic andelectric properties on thickness and seed layer material,and the evolution of the coercivity upon annealing.

2. Experimental

Two different types of samples were prepared: for the

investigation of the magnetic properties, Co83Cr17 filmswith varying thicknesses of tCoCr=0–60 nm on differentunderlayers (Si-oxide, Ta½5 nm�; Ta½5 nm�=Cu½45 nm�) were

covered by a 2 nm Al protection layer. For electricalinvestigations, complete tunnel stacks were patter-ned from (Ta½5 nm�=Cu½45 nm�=CoCr½tCoCr�=Al½1:4 nmþoxide�=Co½2 nm�=Ni80Fe20½15 nm�=Cu½45 nm�). The films were depos-

ited by magnetron sputtering at a pressure of1� 10�3 mbar Ar onto oxidized Si substrates in aUHV DC sputtering system with a base pressure of

1� 10�7 mbar. The aluminum oxide was prepared byplasma oxidation of aluminum in a separate chamber.The samples were investigated by alternating gradient

magnetometer (AGM), magneto optical Kerr effect(MOKE) and scanning Auger microscopy (SAM).

*Corresponding author. Fax: +49-521-106-6046.

E-mail address: justus@physik.uni-bielefeld.de URL:

www.spinelectronics.de (M. Justus).

0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 7 6 2 - 4

The isochronal annealing was done in a separatevacuum system with a base pressure better than

o5� 10�5 mbar.

3. Results and discussion

The magnetic properties of the Co83Cr17 filmsstrongly depend on the kind of underlayer, i.e. glass orTa and Ta/Cu seed layers. Furthermore, a simple Co cap

layer has an enormous influence on the hysteresis shape.Single sputtered Co83Cr17 films on oxidized Si show

typical hysteresis loops as illustrated in Fig. 1: themagnetizations shows a HC of E280Oe. At a field of

E60Oe, however, the curve shows an additional kink.Such stepped magnetization loops were found for allthicknesses in the range from 10–60 nm. This behavior

hints to the presence of two different magnetic phases,which spatially coexist in the film plane. Future MFMinvestigations should clarify the involved domain

structure. Whereas the shape of the hysteresis loopremains unchanged, the coercivity increases with thick-ness (Fig. 2). Since such a two-step behavior in the loops

is not desirable in the reference electrodes of magnetictunnel junctions, Cu and Ta seed layers were tested inorder to improve the magnetic properties.In comparison to the single layer on bare Si-oxide, the

thickness-dependent coercivity of CoCr on Ta½3:3 nm� andTa½3:3 nm�=Cu½45 nm� are shown in Fig. 2. In all cases,thicker films show larger coercivities. The films on

Ta½3:3 nm�; however, show a strong increase of thecoercivity beyond 20 nm from 460 to around 650Oe.Simultaneously, the squareness of the loop changes

(Fig. 3). The remanent magnetization MR is reduced toabout the half of the saturation value MS: The originalsquareness can be regained if the CoCr films are cappedby a thin Co layer. Both CoCr½tCoCr�=Co½E2 nm� double

layers and Co½E2 nm�=CoCr½tCoCr�=Co½E2 nm� triple layers

show a much larger MR again, while the coercivity isonly slightly reduced (Fig. 3).

The effects of annealing on the magnetization loopsfor stacks with and without a Co cap are shown in Fig. 4for tCoCr ¼ 23 nm. The samples were annealed from2501C to 4501C in steps of 501C for 60min and the

magnetization was measured in between the steps atroom temperature. While the coercivity of the uncappedCoCr starts to decrease at 3001C; the HC of the Co-

capped films increases between 3001C and 4501C from330 to around 420Oe. Depth profile of the uncappedCoCr in a SAM investigations show no differences

comparing the annealed with the as-prepared samples.Thus, we can rule out diffusion of Cr perpendicular tothe film.Tunnel magnetoresistance (TMR) measurements mir-

ror the magnetic behavior described above. For

-400 -200 0 200 400

-1.0

-0.5

0.0

0.5

1.0

M [

a.u

.]

H [Oe]

Fig. 1. AGM measurement of in-plane magnetization of a

Co83Cr17½50 nm�Al½2 nm� film on oxidized Si shows two separated

ferromagnetic phases with HCE60Oe and 270Oe.

0 10 20 30 40 50 600

100

200

300

400

500

600

700

800 seed layer: Ta5

seed layer: Ta5Cu45

no seed layer

Hc

[Oe]

CoCr thickness [nm]

Fig. 2. The in-plane coercivity of Co83Cr17 films on Cu and Ta

increases with film thickness. The Ta-seed layer leads to larger

coercivity than the Ta/Cu-seed layer.

-1000 -500 0 500 1000-1.25

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

without Co-cap layerwith Co-cap layer

mag

net

izat

ion

[a.

u.]

magnetic field [Oe]

Fig. 3. MOKE measurement of the in-plane magnetization

of Ta½3:3 nm�=CoCr½23 nm�=Al½2 nm� and Ta½3:3 nm�=CoCr½23 nm�=Co½2:4 nm�=Al½2 nm�: The first stack is not yet completely

saturated.

M. Justus et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 212–214 213

patterned tunnel stacks with a Co-capped CoCr layer,

we found a much higher TMR ratio (approximately afactor of 2) than for ‘uncapped’ stacks. Both the largerMR and the higher spin polarization of Co with respectto CoCr support these findings. The TMR series of

Fig. 5 show the coercivity increase with CoCr thicknessfor stacks with Co-capped CoCr layers. Additionally,the switching field of the soft Co/Py detection layer is

increased due to orange-peel coupling or the stray fieldsof e.g. domain walls [6,7]. Minor loop measurementsshow an increase of the orange-peel coupling field from

4Oe for tCoCr ¼ 17 nm to more than 20Oe for tCoCr ¼23 nm; while the squareness as well as the TMR isreduced. To clarify the details, MFM investigations ofdomain formation are in progress. It seems, however, to

be clear that a strong orange-peel coupling will lead to areduction of the TMR.

4. Conclusion

Electric and magnetic properties of Co83Cr17 thinfilms were investigated. Compared to single layers on Sioxide or Cu, the coercivity is largely increased if the

Co83Cr17 is deposited on a Ta seed layer. Furthermore,the squareness of the hysteresis loops was considerablyimproved by adding a thin Co cap layer on CoCr. Forthis system, the remanent magnetization reaches nearly

the saturation value. In view of a possible use as a hardreference electrode in TMR elements, coercivity andthermal stability are quite promising for the Co capped

CoCr films on Ta. The coercivity can be adjusted in a

wide range by the film thickness. The crosstalk of thehard and soft electrodes by orange-peel coupling,however, has to be reduced. Further experiments for

the realization of smoother films are therefore inprogress.

Acknowledgements

The authors like to thank J. Schmalhorst for SAM

investigations. This work is supported by the GermanyMinistry of Research and Education (BMBF) underGrant no. 13N7989.

References

[1] S.S.P. Parkin, et al., Appl. Phys. Lett. 75 (4) (1999) 543.

[2] J.F. Bobo, et al., J. Appl. Phys. 83 (11) (1998) 6685.

[3] S. Gider, B.-U. Runge, A.C. Marley, S.S.P. Parkin, Science

281 (1998) 797.

[4] J. Schmalhorst, H. Br .uckl, G. Reiss, R. Kinder, G. Gieres,

J. Wecker, Appl. Phys. Lett. 77 (2000) 3456.

[5] J.E. Snyder, M.H. Kryder, J. Appl. Phys. 73 (10) (1993)

5551.

[6] J.L. Prieto, et al., J. Magn. Magn. Mater. 177–181 (1998)

215.

[7] M. Labrune, J. Miltat, J. Magn. Magn. Mater. 151 (1995)

231.

0 100 200 300 400200

300

400

500

Ta3.3nm

CoCr23nm

Al2nm

Ta3.3nm

CoCr23nm

Co2.4nm

Al2nm

Hc

[O

e]

temperature [˚C]

Fig. 4. The dependence of HC on annealing for 1 h at different

temperatures with and without Co-cap layer.

02468

10

tCoCr

=23nm

TM

R [

%]

-250 -200 -150 -100 -50 0 50 100 150 200 250

02468

101214

tCoCr

=17nm

H [Oe]

0

2

4

6

8t

CoCr=29nm

Fig. 5. TMR measurements of Ta½5 nm�=Cu½45 nm�=CoCr½tCoCr �=Co½2 nm�=Al½1:4 nmþoxide�=Co½2 nm�=Py½15 nm�=Cu½45 nm�:

M. Justus et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 212–214214