Journal of Magnetism and Magnetic Materials 156 (1996) 377-378 , i ~ JOlUUl of

-Z-" magnetism and

~ a magnetic inatorlals

ELSEVIER

Giant magnetoresistance of dissymmetrical Co/Au multilayers

E. Kolb a,*, M.J. Walker b, E. V61u a M.A. Howson b p. Veillet a D. Greig b

J.P. Renard a, C. Dupas a

a Institut d'Electronique Fondamentale, Uniuersit~ Paris Sud, Bat. 220, Orsay, France b Department of Physics, University of Leeds, Leeds LS2 9JT, UK

Abstract Results are presented for the magnetoresistance (MR) of s a p p h i r e / N b 3 / C u 3 / A u s / ( C o / A u 8 ) , dissymmetrical multilay-

ers built by alternating a 0.3 nm discontinuous Co layer with a 0.7 nm continuous one. The observed enhanced MR is related to a higher spin scattering asymmetry for the granular Co layers.

Ultrathin C o / A u multilayers grown by evaporation or MBE are model systems for magnetoresistance (MR), displaying perpendicular magnetization below 8 atomic layers (A.L.) of cobalt and very flat interfaces. In a previous paper [1], we had studied A u ( 1 1 1 ) / C o / A u sim- ple trilayers in the very low Co thickness limit. The deposition of less than 2 A.L. of cobalt leads to discontinu- ous magnetic layer and the structure can be considered as a two-dimensional granular system, where the magnetic do- mains are limited by the Co islands' lateral sizes. At a temperature of 1.4 K, the existence of a large coercivity deduced from MR measurement in perpendicular applied field shows the magnetization remains perpendicular to the film plane, even for a Co thickness as low as 0.2 A.L.

In order to increase the signal from both magnetization and MR measurements, granular multilayers with many periods were grown at Leeds University by MBE. The multilayers were grown on (11.0) sapphire with a 3 nm bcc (110) Nb buffer layer grown at 950°C followed by 3 nm of an fcc (l 1 l) Cu layer grown at 375°C. We have studied two series of s a p p h i r e / N b 3 / C u 3 / A u s / ( C o / A u s ) n dis- symmetrical multilayers built by altemating a 0.3 nm discontinuous Co layer with a 0.7 nm continuous one. The total number of Co layers n varies between 1 and 4 for the first series and between 4 and 7 for the second one. The first sample n = 1 corresponds to s apph i r e /Nh3 /Cu3 / Aus /Coo .a /Au s, where all the thicknesses are indicated in rim. It was grown for comparison with the simple sand- wich float g l a s s /Au2s /Co /Au25 previously described [1]. The thick 8 nm gold interlayer was chosen to avoid coupling between the Co in the subsequent multilayers. An

" Corresponding author. Fax: +33-1-6019-2593; email: kolb @ief-paris-sud.fr.

antiparallel configuration of the successive Co layers' magnetization directions is achieved by means of the different coercive fields of the 0.3 nm and 0.7 nm layers. MR measurements were performed between 1.4 and 300 K with the magnetic field perpendicular to the film plane.

In the s a p p h i r e / N b 3 / C u a / A u s / C o o . a / A u s sample the Co layer is discontinuous. This is shown by the rapid fall in the coercive field, seen in the magnetoresistance, with temperature, disappearing above 200 K. This has also been seen in the (C00.3//Au8)7 multilayers reported by us else- where in this conference for which ZFC, FC and remanent magnetization measurements show a blocking temperature of about 200 K [2]. The existence of this temperature indicates the presence of superparamagnetic clusters of cobalt.

The giant magnetoresistance in the samples with n >_ 2 results from the different coercive fields of the 0.3 and 0.7 nm Co layers allowing antiparallel alignment of the mag- netic moments in each layer. Below 100 K the coercive field of the 0.3 nm layer is greater than that of the 0.7 nm Co. But between 100 and 180 K, as the coercive field of the 0.3 nm Co is rapidly falling, there is a crossover with the coercive field of the 0.7 nm layer. At this crossover temperature, the MR curve no longer exhibits the usual plateau shape, but consists in a narrow and sharp peak located at the common coercive fields. This leads to a reduction of the MR [3]. Finally at 300 K the MR curve is characterized by a large tail at high fields up to 10 T. This absence of saturation is due to the super-paramagnetic Co clusters which remain unblocked at this temperature.

The magnitude of the MR at 1.4 K as a function of the number of Co layers n is plotted in Fig. 1 for the two investigated series. As expected, the magnetoresistance increases as each new Co layer is added. But surprisingly, the increase is larger when the resulting multilayer has an

0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)00906-X

378 E. Kolb et al . / Journal of Magnetism and Magnetic Materials 156 (1996) 377-378

odd number of layers. In each sample the first deposited Co layer is a 0.3 nm one, thus leading to a multilayer with (p ) 0.7 nm layers and ( p + 1) 0.3 nm layers when n = 2p + 1 is odd.

This behaviour of the MR with n can be explained by a Camley-Barnas model calculation [4] if we assume a larger scattering asymmetry c~ for the 0.3 nm layers. The other parameters used for the fit have been determined by comparing the n = 1 sample with the A u / C o / A u grown on float glass. The observed reduced MR for the sample on sapphire is attributed to a poorer electron reflection coeffi- cient R b at the interface with the buffer layer. By taking R b = 0 and R~ = 0.5 for the outer surface, we can leave unchanged the mean free path at 1.4 K, A o = 2000 ,~, and the transmission probability T 2 for an electron with spin antiparallel to the Co majority spin direction, T 2 = 0.1, as in the C o / A u multilayers on float glass. Then the only adjustable parameters are the transmission probabilities T l for spin parallel for the two different layers. The MR versus n curve can be well fitted by using T~(0.3 n m ) = 0.85 and Tl(0.7 n m ) = 0.6, corresponding to c~(0.3 nm) = (1 - ~/T2)/(I - ~/T l) = 8.7 and a(0.7 nm) = 3.0 as defined in the Camley-Barnas model.

The difference in the values of T~ for 0.3 and 0.7 nm js not clearly understood since the electrons mean free path in Co is likely much larger than the film thickness. A confirmation of this effect has been obtained by comparing the dissymmetrical samples described above with the cor- responding granular multilayers (Coo.3/Auo.8) n. In the latter, all the Co layers are identical and n is varied between 2 and 5. The resulting MR curve displays a maximum of resistance at the 0.3 nm Co layer coercive

2 0 I

1 8

1 6

. ~ ' 1 4

tr '12 / "Q

I o ' "0- Exp. :!sem,~ ' : - ?- Exp.._2~ series

JL MR Fit

6 ~ , - ~ I I I 2 3 4 5 6

n

Fig. 1. Magnetoresistance at 1.4 K versus the total number of Co layers for the dissymmetrical multilayers.

2 0 I I I I

Granular n=2 [] Granular n=5

L - ~ Dissymetrical n=2 1 5

o~ 1 0

=E

5

0 I I I I I I 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0

T (K)

Fig, 2. Magnetoresistance versus temperature for dissymmetrical and granular multilayers.

field (peaked shape) whatever the temperature. The tem- perature dependence of the MR is plotted in Fig. 2 for n = 2 and n = 5 in the two cases.

At the crossover temperature defined above for the dissymmetrical samples, each magnetic layer is divided at the common coercive field into an equal number of do- mains with magnetizations either parallel or antiparallel to the field. This magnetic situation is statistically equivalent to the one for the granular multilayers at the 0.3 nm coercive field, thus leading in principle to the same MR value if the a values are the same for the 0.3 and 0.7 nm layers. However at 180 K, the granular multitayer clearly gives a larger magnitude of MR.

One possible explanation is to introduce again a larger spin scattering asymmetry coefficient for the 0.3 nm layer, which agrees with our first results. It should however be mentioned that a distribution of the 0.3 nm coercivity allowing local antiparallel alignment of the magnetizations between successive layers should also explain the en- hanced MR observed in these granular multilayers.

R e f e r e n c e s

[1] E. Kolb, M. Mulloy, C. Dupas, M. Galtier, D. Renard, J.P. Renard, F. Trigui, E. V~lu, ICMFS 94 Proceedings, J, Magn. Magn. Mater. 148 (1995) 315.

[2] J. Xu, M.A. Howson, B.J. Hickey, D. Greig, P. Veillet, E. Kolb, J. Magn. Magn. Mater. 156 (1996) 379 (these Proceed- ings).

[3] E. Vrlu, C. Dupas, W. Nabhan, F. Trigui, J.P. Renard, D. Renard, J. Appl. Phys. 71 (1992) 503.

[4] J. Barnas, A. Fuss, R.E. Camley, P. Grtinberg, W. Zinn, Phys. Rev. B 42 (1990) 8110.