Giant magnetoresistance in hybrid magnetic nanostructures

Giant magnetoresistance in hybrid magnetic nanostructures

ELSEVIER L.B. Steren a~*, R. Morel at A. BartlGlimy a, F. Petroff ‘, A. Fert a, R. Loioee ‘, ?.A. Schroeder b a Laboratoire de Physiqae des Solides, ...

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ELSEVIER

L.B. Steren a~*, R. Morel at A. BartlGlimy a, F. Petroff ‘, A. Fert a, R. Loioee ‘, ?.A. Schroeder b a Laboratoire de Physiqae des Solides, Universiti Paris-Sd 9140s Omy, Frame bDeparttnent of Physics, Michigan State University, East Lansing, Michigan 48824, USA

Abstract We report on magnetization and magnetoresistance measurements in hybrid structures, composed of soft m of permalloy and hard magnetic clustered-layers of cobalt separated by silver. We also present a study of t the giant magnetoresistance effect with the angle between the magnetizations of successive magnetic layers.

Since the discovery of GMR in Fe/C; mu&layers [l], the GMR effect has been obtained in a variety of multilayered and clustered nanostructures. More recently such GMR effects have been observed in intermediate structures obtained by annealing magnetic multilayers so that flat magnetic clusters are formed [2]. Another way to obtain discontinuous layers is to prepare multilayers with very thin magnetic layers, as already done in Co/Ag [3]. In this work we present magnetoresistance and magnetization results in hybrid structures [4] composed of conrinuous N&Fe, layers and &feted ultrathin Co layers separated by Ag layers. Samples of (Co 4 e/Agt,/EFe t~i~~/AgtAB) X 15 with fNiFc = 20 or 40 A and fAg varying from 10.5 to 40 A, were deposited by sputtering on Si(100) substrates. In Fig. I(a) we show the magnetization curve for (Co4 $Ag40 i/NiFe40 i/Ag40 h,,, Two well defined steps are seen:Tsharp one - at low field - which is due to the abrupt reversal of the soft NiFe layers magnetiza;ions (saturation field Hs = 5 Oe) and< broader one ( H, = 2500 Oe) due to the progressive reversal of the magnetizations of the Co dusters. The magnetoresistance curve shown on Fig. l(b) confirms what can be expected from the magnetization curve. Indeed, an abrupt increase of the resistivity is observed at very low field, when the NiFc magnetization turns from a parallel to an antiparallelrrangement with respect to the Co magnetization. The resistance remains nearly constant for fields up !o 400 Oe and then decreases slowly due to the progressive alignment of the Co magnetization with the field. The total magn=toresistance ratio is about 30%, but 21% of the resistance change occurs in 9 Oe leading to a slope of 2.3%/Oe. This slope is not

uniform and values of 6S%/Oe are reached in part. Similar results were obtained in most of gated samples. Fig. 2 shows the variation of the MR ratio with thickness. The MR ratio increase? rapidly as the Ag Iayer thickness varies from 10.5 Ag layers, the MR is reduced by the presence of pi or other types of ferromagnetic c,oupling throu magnetic layer. For tAg .sm

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* Corresponding author. Fax: 33 1 [email protected] 0304-8853/95/$09.50 El 1995 Elsevier SSDZ 0304-8853(94)01529-S

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Fig+ 1. (a) Magnetk$on vers,u~ in-9 MR curve for (Co4 A/[email protected] A/Fe

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L.B. Stertw et ul. jJour)tal

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Fig. 2: Magnetoresistance dependence on the Ag thickness for (Co4 A/Agt,, @Fe20 A/Agr,g),,, at 4.2 K. reaches a maximum0 of 43% and then decreases slowly to 30% for tAg = 40 A. We have performed minor magne-

toresistance loops measurements (as shown in Fig. 31, which confirm the presence of weak ferromagnetic couplings. The coupling is given by the difference between the switching fields, H, and H,, measured on the major and minor loops respectively. The magnetization of the Co is positive at H, and negative at Hz, so rhat for the relative orientation of NiFe and Co, reversing the NiFe to negative orientation means going from parallel to azparallel at H,, and from antiparallel to parallel at H,: HI = -HCNiFc - Hcoupling 9 Hz = -4~s~ - + HcoupIingr where H,.i, is the coercive field of the NiFe layers. This imp!& Hcoupling =(Hz -H,)/2. In %e sample with 35 A of Ag and 20 A of NiFe, a ferromagnetic coupling field of 17 Oe is found, ascan be seen in Fig. 3. In these hybrid samples, where it is possible to freeze the magnetization of the Co clusters in a saturated state and to rotate only the magnetization of the soft NiFe iayers, we have studied the dependence of the GMRGith the angle between the magnetizations of Co and NiFe. We have saturated the magnetization of the Co layers, in a field of -5000

Oe, at an angle of 45” relative

to the

current. The magnetization of the NiFe layers is then Fversed in a, small fitld (160 Oe forthe (Co4 A/AgSS A/NiFe20 A/Ag35 A),, sample), and the resistivity is measured as the magnetization of the NiFe layers is rotated with respect to that of the Co by sweeping the field angle, both clockwise (CW) and counterclockwise (CCW). By symmetry arguments, we obtain the angular variation of

Fig. 4. Resistivity (9) and conductivity (A) as a function of sin’(y/Z) for (Co4 A/Ag35 [email protected] A/Ag35 &.

the AMR of the NiFe layers by substracting, and the angular dependenceof the GMR b, summing, the CW and CCW experimental curves. Fig. 4 shows the variation of the normalized resistivity ( P/Pantiparallcl) and conductivity versus sin’(y/2), where y is the angle be(~/~prallel) tween Co and NiFe magnetizations [5]. The angular dependence of the fiR of the NiFe layers shows the expected variation and has a small~mplitude (0.8%). As seen in Fig. 4, the conductivity is almost linear in sin’(y/2). We have observed such a linear or quasi-linear dependence for several samples. Such variation of the GMR with sin2(y/2) has already been observed in (NiFe/Cu/NiFe/FeMn) [6] and (Co/Cu/NiFe) [7] multilayers. Since significant departures from sin2(y/2) are expected by the theory [8] when interface potential steps are taken into account, our results seems to show that the potential steps do not play a significant role in the GMR of Co/Ag/NiFe structures. Achowledgemertrs: This research was partly supported by the US-Franc? cooperation program grant INT-92-16909 and Companion grant AIO693, the ESPRIT Basic Research Project No. 6146 SMMMS. L.B.S. thanks the Consejo National de Investigaciones Cientificas y Tecnicas de la RepGblica Argentina. References 111 MN. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Fricderich and J. Chazelas, Phys. Rev. Let!. 61 (1988) 2472. [2] B. Rodmacq, G. Palumbo and P. Gerard, J. Magn. Magn. Mater. 118 Lll (1993); T.L. Hylton, K.R. Coffrey, M.A. Parker and J.K+ Howard, Science 261 (1993) 1021.

[3] R. Loloee, P.A. Schroeder, W.P. Pratt, J. Bass and A. Ferf, Physica B 204 W!W 274. [4] P. Holody, L.B. Steren, R. Morel, A. Fert, R. Loloee and P.A. Schroeder, Phys. Rev. 3, IO bc published. (51 The angle y hehveen the magnetizations of NiFe and Co layers is the angle between the field and the Co &us a small correction taking into account the interlayer coupling, see LB. Steren, R. Morel, A. Barth6lBmy and A. Fert, Phys. Rev. B, to

he published. [6l B. Dieny, VS. Speriosu, S.S.P. Parkin, A. Gurney, D,R. Wilhoit and D. Mauri, Phys. Rev. B 43 (1991) 1297. Fig. 3. MR loops for (Co4 A/Ag35 A/NiFe20 A/Ag35 ,& at 4.2 K. The reversal of the NiFe magnetization from positive to negative occurs at H, and%, for the major and respectively.

[7] T. Okuyama, H. Yamamoto and T. Shinjo, J. Magn. Magn. Mater. 113 (1992) 79. [SiA. Vedyayev, B. Dieny, N. Ryzhanova, J.B. Genin and C. Cowache, Europhys. L&t. 25 (6) (1994) 465.