Cu multilayered nanowires

Journal of Magnetism and Magnetic Materials 156 (1996) 317-320

Invited paper

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Perpendicular magnetoresistance in Co/Cu multilayered nanowires L. Piraux a,* S. Dubois

a

C. Marchal a J.M. Beuken a L. Filipozzi a J.F. Despres a K. Ounadjela b, A. Fert c

a Unit~ de Physico-Chimie et de Physique des Mat£riaux, Universit£ Catholique de b)u~ain, Place Croix du Sud 1, B-1348 Loul~ain-la-Neuue, Belgium h lnstitut de Physique et Chimie des Mat£riaux de Strasbourg, F-67037 Strasbourg, France c UnitJ Mixte de Recherche du Centre National de la Recherche Scientifique et de Thomson, Laboratoire Central de Recherches Thomson, 91404 Orsay, France et Universitg Paris-Sud, Bat. 510. F-91405 Orsay. Franee

Abstract We have carded out magnetoresistance measurements in the CPP (current perpendicular to the planes) geometry of electrodeposited C o / C u multilayered nanowires with Cu and Co layer thicknesses varying over wide ranges. The data are compared with the results of the Valet-Fert model for perpendicular transport. The observed behaviour agrees with the corresponding predictions in various limits of the layers thicknesses. The interface and bulk spin-dependent scattering parameters as well as the spin diffusion lengths in the nonmagnetic and ferromagnetic layers are extracted from this analysis.

Magnetic multilayers have attracted a great deal of interest in recent years because of their unusual giant magnetotransport properties [1,2]. Recently, GMR was reported in a new type of multilayered structure consisting of electrodeposited magnetic and nonmagnetic layers into the cylindrical nanopores of a template polymer membrane [3-5]. This system has the special interest of allowing MR measurements in the CPP geometry, whereas most experimental observations of GMR in usual multilayers refer to measurements with the current in the planes of the layers (CIP geometry). The main difference between both geometries, as described by the Valet-Fert (VF) model [6,7] is that the scaling length of the CPP-GMR is the relatively long spin diffusion length (SDL), whereas the scaling length of the CIP (current in the planes) problem is the much shorter electron mean free path. As a consequence, the GMR phenomenon is no more restricted to multilayer structures with layer thicknesses in the nanometre range as it is the case in the CIP geometry. Till now, experimental investigations of CPP-MR in multilayers [8-14] were almost exclusively restricted to the limit where the thicknesses of the nonmagnetic and magnetic layers, t N and t F, respectively, are much shorter than the spin diffusion lengths t~N) ~SF and I~F) ~SF" In this regime, using simple predictions of the VF model, the interface and bulk spin-depen-

Corresponding author. Email: [email protected]; fax: + 32-10-473452. *

dent scattering parameters have been determined tbr various magnetic multilayers. It is only recently that determination of spin diffusion lengths in the nonmagnetic layers was made by adding impurities with strong spin-orbit coupling or paramagnetic impurities in these layers [15]. Our nanowire system has a particular interest for the CPP problem. Indeed, given the thickness of the nanoporous host membrane, multilayered structures can be produced with a large number of repeats and layer thicknesses ranging between a few nm to a few/xm. It is thus possible to investigate the CPP-GMR with layer thicknesses in the range of the spin diffusion lengths and to determine these scaling lengths. Electrodeposited C o / C u multilayered filaments (of about 90 nm in diameter) were made from a single sulphate bath using a pulse deposition technique. GMR values close to 50% were obtained at 77 K for a Co(4 nm)/Cu(10 nm) multilayer sample [16]. The dependences of GMR and magnetization as a function of layer thicknesses have been investigated over wide ranges (i.e. from the nanometre range to the micrometre range). Experimental details and a comprehensive presentation of the results are given elsewhere [16,17]. In the present paper, we report and discuss data obtained on two series of samples: Co(8 and 25 nm)/Cu(ll) < tcu < 350 nm), which we call series 1, and Co(60 nm
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 8 8 2 - 9

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Fig. 1. Magnetoresistance and magnetization vs. applied field at room temperature for Co(8 nm)/Cu(80 nm) multilayered nanowires (a,b) and Co(80 nm)/Cu(8 nm) multilayered nanowires (c,d). The external field is parallel (---) or perpendicular ( ) to the wire axis.

Fig. 1. When thin Co layers are separated by thick Cu layers (Figs. la and lb), both magnetoresistance and magnetization curves indicate that the easy direction is in the plane of the layers. All the samples of series 1 show this common magnetic behaviour. In contrast, in all the sampies of series 2, the easy axis lies along the wire axis as long Co rods are separated by thin Cu layers. These contrasting behaviours originate from demagnetizing fields that are governed by the shape of the multilayered structure. It should also be noted that the resistance may be slightly larger in the virgin state (before field application) than at the peak recorded during the field sweep (see for example Fig. lb). This means that the randomness of the magnetizations in the virgin state and at the coercive field are not exactly the same. However, the difference never exceeds ~ 10% of the total magnetoresistance, so that we have neglected this difference and systematically identified the resistance of the random arrangement to that of the peak. We finally point out that we have systematically performed the measurements in both field directions, parallel and perpendicular to the current, which allows us to correct the data for the small contribution from anisotropic magnetoresistance. A plot showing the inverse of the square root of the magnetoresistance at T = 77 K for Co(8 nm)/Cu(10 < tcu < 350 nm) multilayered nanowires (series 1) is given in Fig. 2. Data obtained on samples with t c o = 25 nm are also reported in the inset of Fig. 2. For both Co thick-

nesses, a linear variation is observed in our experimental results for tc. smaller than about 150 nm. In addition, the slope of this linear variation decreases as tco increases. Such features are consistent with the long spin diffusion length limit of the VF model. Indeed, in the limit where tN, tF <( l!N), "sf I(F), the following simple expression can be easily derived from expressions (44)-(46) of Ref. [6]. (A~)

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Fig. 2. Plot of ( A R / R A P ) -I/2 vs. tCu at T = 77 K for Co(8 nm)/Cu(tcu) multilayered nanowires. ( - - - ) Fit of the results obtained by using the full expressions of the VF model (expressions (40)-(42) of Ref. [6]). The inset shows also the data obtained on Co(25 nm)/Cu(tcu) multilayered nanowires (O).

L. Piraux et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 317-320 with AR = R AP - R P. We use the same notations as in Ref. [10], i.e. p ~ ( $ ) = 2 0 ~ in Cu, p ] ' ( $ ) = 2 p ~ [ l ( + ) / 3 ] in Co and r T ( $ ) = 2 r b [ 1 - - ( + ) 7 ] at the interfaces where /3 and 7 are the bulk and interfacial spin asymmetry coefficients, respectively. From the above expression, we may also assert that the behaviour observed in Fig. 2 is characteristic of 7 >/3- Indeed, for 7 / 3 , the straight lines corresponding to the two values of tco are expected to pass through a single point having t c u = Z ( y - / 3 ) r c o / c J / 3 p ~ and /3 i for coordinates. As tco increases, the GMR ratio decreases for tcu < t ~ and increases for tcu > tc~. The ordinate of the crossing point in Fig. 2 allows a direct determination of /3 (/3 = 0.36 + 0.04). The determination of all the other parameters, PN, P~, r b and y involved in Eq. (1) can be achieved in the following way. First, for copper layers much thicker than the cobalt ones, say for tc~ = 300 nm with tco=, 8 nm; the total measured resistance, Rexp, is almost completely due to copper, By measuring Rexp at two temperatures, say 77 and 300 K, we derive p~(300K)/p~(77 K ) = 1.47. On the other hand, we know that [ pc~(300 K ) - p~(77 K)] equals 1.45 × 10 - s ~ m , so that we straightforwardly derive Pcu(77 K) = 3.1 × 10 - s ~ m . Then, by identifying the equations of the straight lines in Fig. 2 for two different thicknesses of Co with Eq. (1), we derive:

pc,, = 1 8 _ + 2 × 10 s [~m, rc~o/cu=3_+O.5×lO-161)m 2,

/3=0.36_+0.04; "/=0.85_+0.1.

(We had already found the same value of /3 from the intercept of the two lines in Fig. 2.) For tc~ > 150 nm, a clear departure from the linear dependence is observed (see Fig. 2). In the framework of the VF model, such behaviour corresponds to the onset of the exponential decrease of the GMR ratio as exp(tc~/l~,cu)) for tc~ >> l~cu). Using the full expressions (40)(42) of Ref. [6] and the above parameters determined in the long SDL limit, a fit of our data in the range 10 < tcu < 350 nm was done with o~ft tc") as the only free parameter (see the dashed curve in Fig. 2). This gives I~c~) = 140 _+ 10 nm at T = 77 K. From these values, we also deduced x(c~) ~Lel ~ 21 nm and "~rxtc")~. 5600 nm, the elastic and spin-flip scattering mean free paths, respectively. We now discuss the results obtained on C o / C u multilayered samples with a constant thickness of Cu (tc~ = 8 nm) and tco varying between 60 and 950 nm (series 2). In the limit where t F >> 1~ ) and t N << 1~ ), we deduce the following expression from Eqs. (40)-(42) of the VF model

[6]: AR R p = ~(l

2/321~ ) -- /32)f

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319

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tco (nm) Fig. 3. Linear variation of the inverse magnetoresistance vs. tco for Co(tc,,)/Cu(8 nm) multilayered nanowires at T = 77 K

where ~ is 1 for a strict antiparallel arrangement between the ferromagnetic layers in the zero-magnetized state; takes a value around 0.5 for a randomly demagnetized state. As expected, linear behaviour of R P / A R as a function of tco is demonstrated by our experimental results in Fig. 3. At T = 77 K, by introducing ]3 = 0.36 in Eq. (2), we derive l~c°) = 45 nm assuming ~ = 0.5. In summary, we have measured the CPP-GMR of C o / C u multilayered nanowires with layer thicknesses varying between the nanometre and micrometre ranges. Our results at T = 77 K can be accounted for by the VF model and we have derived the spin asymmetry coefficients in the cobalt layers (/3 = 0.36 _+ 0.04) and at the interfaces ( 7 = 0.85 -+0.1), as well as the spin diffusion lengths, l~fcu) = 140 nm and %;f t¢co) ___45 nm. Acknowledgements: We thank Whatman s.a. (Belgium) for providing the polycarbonate membrane used in this study. Collaboration between PCPM and LPS is supported by the 'Human Capital and Mobility Network CHRXCT93-0139'. L.P. is a Research Associate of the National Fund for Scientific Research (Belgium). The work performed in Louvain-la-Neuve was carried out under the financial support of the programme 'Action de Recherche Concert~e' sponsored by the 'DGESR de la Communaut6 Franqaise de Belgique'. References [l] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van 1)au, F. Petroff, P. Etienne, G. Creuzet, A. Friederich and J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [2] G. Binasch, P. Griinberg, F. Saurenbach and W. Zinn, Phys, Rev. B 39 (1988) 4828. [3] L. Piraux, J.M. George, J.F. Despres, C. Leroy, E. Ferain, R. Legras, K. Ounadjela and A. Fert, Appl. Phys. Lett. 65 (1994) 2484. [4] A. Blondel, J.P. Meier, B. Doudin and J.P. Ansermet, Appl. Phys. Lett. 65 (1994) 3019. [5] K. Liu, K. Nagodawithana, P.C. Searson and C.L. Chien, Phys. Rev. B 51 (1995) 7381. [6] T. Valet and A. Fert, Phys. Rev. B 48 (1993) 7099.

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