The Effect of Interface Roughness and Spin-Depo- larization Due to the Proximity of a Buffer Layer

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IT/ Nano • IFF Scientific Report 2006 130 I 131

Current-in-Plane Giant Magnetoresistance:

The Effect of Interface Roughness and Spin-Depo- larization Due to the Proximity of a Buffer Layer

M. Breidbach, D. E. Bürgler, and P. Grünberg

CNI – Center of Nanoelectronic Systems for Information Technology (IFF-9)

We report on proximity effects of a Au buffer layer on the current-in-plane giant magnetore- sistance effect (CIP-GMR) in high-quality, epitax- ial Fe/Cr/Fe(001) trilayers. The lower Fe layer is grown as a wedge and allows simultane- ous preparation of 24 GMR stripes with differ- ent lower Fe thickness, dFe. The layer-by-layer growth gives rise to (i) well controlled rough- ness changes from stripe to stripe as confirmed by reflection high-energy electron diffraction and (ii) to a varying influence of the underlying Au buffer. The oscillatory roughness variation yields an oscillatory GMR behaviour as a function of dFeand confirms our previous result that slightly increased interface roughness causes a higher GMR ratio. The proximity of the Au buffer to the GMR trilayer results in a decrease of the GMR ra- tio with decreasing dFe. The latter effect is ex- plained by spin-depolarization at the Fe/Au inter- face and in the bulk of the Au buffer. These re- sults reveal limits for further decreasing the mag- netic layer thickness in CIP-GMR structures.

Giant magnetoresistance in layered magnetic structures with the current flowing in the sample plane (CIP-GMR) is interpreted as due to spin-dependent electron scattering at the interfaces or in the bulk of the ferromagnetic films [1]. Hence, lattice defects, interface roughness, intermixing at interfaces, etc. should have an important effect on the GMR ratio.

We present a comparison of the GMR in Fe/Cr/Fe samples, which differ only with respect to interface roughness and thickness of the lower Fe layer (dFe), but are otherwise as much alike as possible [2].

We exploit the fact that in layer-by-layer growth, ob- tained by MBE, the growth front produces alternately smooth and rough surfaces as revealed by intensity oscillations in reflection high-energy electron diffraction (RHEED). We show the relation between interface roughness and GMR in a whole range ofdFe. Furthermore, a decrease of GMR with decreasing thickness of the Fe layer adjacent to the Au buffer is observed and attributed to the attenuation of GMR due to spin-depolarization induced by the proximity of the Au buffer.

A sketch of our sample structure is shown in Fig.

1. Details on the growth have been described in [3]. The Au buffer layer is necessary to obtain sufficiently good sample quality to enable the observation

of RHEED oscillations for all layers. The lower Fe film is prepared in the form of a wedge. The sample is lat- erally structured by optical lithography and ion-beam etching into 24 stripes, each 8 mm long and only 50 µm wide, as indicated in Fig. 1. They run parallel to a magnetic hard axis of Fe(001), while the external field is applied along an easy axis. Therefore, con- tributions due to anisotropic magnetoresistance drop out of the difference∆Rmax=RB=0−Rsatbecause the magnetizations are aligned along an easy axis and thus under an angle of 45^◦to the current direc- tion at zero field as well as in saturation.

FIG. 1: Layer sequence and schematic layout of the 24 conducting stripes for GMR measurements. The lower Fe film has an increasing thicknessdF efrom left to right (red lines indicate the wedge shape).

The lateral variation of the surface roughness is char- acterized by monitoring the RHEED intensity while the sample is moved under the electron beam. The RHEED intensity oscillates along the Fe wedge before and after depositing the Cr spacer. In both cases, the oscillations reflect the thickness variation by 1.5 ML [red curve in Fig. 2b] and confirm that the roughness of both Fe/Cr and Cr/Fe interfaces varies along the wedge in phase [2]. In Fig. 2a, we dis- play the dependence of the GMR ratio∆Rmax/Rsat

ondF e. There is a continuous increase of the GMR ratio withdF e. This is opposite to the effect of shunting, which decreases the GMR ratio for increasing dF e. The observed decrease of GMR for decreas- ingdF ecan be explained by the proximity of the Au buffer layer, which gives rise to spin-depolarization of the current. The mechanism leading to GMR is still thought to be mainly spin-dependent scattering at the Fe/Cr interfaces. But apart from the number

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IFF Scientific Report 2006 • IT/ Nano

130 I131 of such scattering events there has to be sufficiently

high spin polarization for the GMR effect to occur. If a spin depolarizer such as non-magnetic Au is present within the mean free path for spin-flip scattering (λ), then this will have a destructive effect on the GMR effect. This basic idea is sketched in Fig. 3, where a cross-section of a wedge sample is shown. In CIP geometry, only electrons drifting in the sample plane within a distance of 1-2 timesλfrom the Cr spacer layer contribute to GMR, because they have a finite probability to move due to the Fermi velocity distri- bution from one Fe layer across the Cr spacer to the other Fe layer. This region centred at the Cr spacer is indicated in Fig. 3 by the two red lines. For ferromagnetic materialsλis typically only a few nm and thus exceeds the thickness of the lower Fe layer (left part) with two consequences: (i) The Fe/Au interface is within the reach of the electrons that contribute to GMR, and (ii) these electrons cover a part of their drift pathway inside the Au buffer. Spin-flip scattering events at the Fe/Au interface and in the bulk of Au depolarize the current, and the spin polarizationP at the Fe/Cr interfaces is less than the Fe bulk value Pbulk. Of course, the spin-flip length in Au is larger thanλ, but the travel distance along the interface (8 mm in our case) exceeds the spin-flip length by or- ders of magnitudes. With increasingdF e, these effects become less important andP is expected to behave like P(dF e) ∼ Pbulk[1−exp(−dF e/λ)] as shown by the green curve in Fig. 3. For smalldF e

the GMR ratio is expected to exhibit a similar dependence asPwith saturation before it again decreases due to shunting. The dashed lines in Fig. 3 indicate the Fe thickness range of our wedges, which is limited by the need for layer-by-layer growth. The ex- ponential curve is rather linear in this part explaining the linear increase of the GMR ratio withdF ein Fig.

2a.

stripe #

12 14 16 18 20 22 24 (a)

(b)

4 6 8 10

1.20 1.24 1.28 1.32

∆Rmax/Rsat(‰)

-0.004 -0.002 0.000 0.002 0.004

RHEED intensity (a.u.)

18.8 19.0 19.2 19.4 19.6 19.8 thickness of lower Fe layer d_Fe(Å)

∆Rmax/Rsat(‰)

FIG. 2: (a)∆Rmax/Rsatas a function of d_{F e}. The green line is a linear fit. (b) The∆Rmax/Rsatdata of (a) after subtracting the linear background. The blue line is a guide to the eyes. The RHEED intensity curve taken along the surface of the Fe wedge is superimposed in red.

Figure 2b shows again the data of Fig. 2a but now after subtraction of the linear background. The blue line is a guide to the eye and indicates an oscillation. We superimpose in red the RHEED intensity curve taken along the Fe wedge of the same sample and find a clear correlation between interface roughness and GMR: The rougher the Fe/Cr and Cr/Fe interfaces (minimum RHEED intensity), the larger the GMR ratio. This very sensitive dependence confirms our previous findings [3] and can be understood by assum- ing that interface roughness increases the number of spin-dependent scattering sites, which are the origin of CIP-GMR.

FIG. 3: Schematic drawing of the model for spin- depolarization due to the proximity of the Au buffer. The red lines mark the region around the Cr spacer within which electrons contributing to CIP-GMR are drifting.

In summary, it has been shown that in epitaxial Fe/Cr/Fe structures interface scattering at the Fe/Cr interfaces is the main source of CIP-GMR. The GMR ratio can be enhanced by moderately increasing the interface roughness. For optimizing CIP-GMR, in addition to the normal shunting effect in the bulk of the Fe layers and the underlying buffer, spin- depolarization effects due to the buffer and its interface towards the GMR trilayer have to be taken into account. This spin-depolarization poses a lower limit for the thickness of the ferromagnetic layers in CIP- GMR structures.

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Zinn, J. Magn. Magn. Mater. 140-144, 1 (1995) [2] M. Breidbach, D.E. B ¨urgler, P.A. Gr ¨unberg, J.

Magn. Magn. Mater. 307, L1 (2006)

[3] D. Olligs, D.E. B ürgler, Y.G. Wang, E. Kentzinger, U. R ücker, R. Schreiber, Th. Br ückel, P.

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