Ag multilayers with discontinuous magnetic layers

Ag multilayers with discontinuous magnetic layers

Journal of Magnetism and Magnetic Materials 165 (1997) 316-319 ~ i ~ Journalof magnetism and magnetic materials ELSEVIER Giant magnetoresistance in...

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Journal of Magnetism and Magnetic Materials 165 (1997) 316-319

~ i ~ Journalof magnetism and magnetic materials

ELSEVIER

Giant magnetoresistance in Cog0Fel0/Ag multilayers with discontinuous magnetic layers F. Fettar a,b,*, L.B. Steren a,b,l, A. Barth616my a,b, R. Morel a,b, A. Fert a,b, J.A. Barnard c J.D. Jarratt c UnitJ Mixte de Recherche CNRS / Thomson, Domaine de Corbeville, 91404 Orsay, France b Universit£ Paris-Sud, Bat. 510, 91405 Orsay, France c University of Alabama, Tuscaloosa, AL 35487-0202, USA

Abstract We have studied the change in transport and magnetic properties of C090Fe~o/Ag multilayers with the magnetic layer and spacer thickness. By annealing or increasing the magnetic layer thickness, the character of the C090Fe~o layers changes from a clustered-like to a continuous one. Keywords: Giant magnetoresistance; Magnetic nanostructures

1. Introduction Few years after the discovery of the giant magnetoresistance effect (GMR) in F e / C r multilayers [1], its study has been extended to granular alloys [2]. One of the main differences with this geometry is its field dependence. With the aim of using GMR devices as magnetic sensors, intermediate structures between multilayers and granular alloys have been first elaborated by Hylton et al. [3]. By annealing N i F e / A g multilayers, they obtained columns of stacked, flat NiFe clusters separated by silver. Due to a weak dipolar coupling, the columnar order of the magnetic clusters along the multilayer favors an anti-parallel alignment of adjacent magnetic layers. As a result, good low field sensitivity of 0 . 8 % / O e have been obtained. Another way to grow pancake-like clusters, which has been developed at Michigan State University [4], is to fabricate multilayers with very thin magnetic layers ( 2 - 6 ,~) in such a way that, below percolation, they are discontinuous. In this paper, we present a study of the change in magnetotransport and magnetic properties of sputtered Co90Fel0/Ag nanostructures, when going from a flat cluster-based structure to a multilayered one. The CoFe alloy was chosen because it has been calculated that antiferromagnetic coupling in multilayers of CoFe alloys in this 9:1

* Corresponding author. Fax: +33-1-6933-0740; email: fettar @lcr.thomson.fr. 1 Present address: Centro At6mico Bariloche, 8400 S.C. de Bariloche, Argentina.

ratio should give higher MR values than pure Co [5]. For this purpose, three series of samples with different magnetic layer thickness - tCoFe = 5, 7.5, and 10 ,~ - have been sputtered on glass substrates. In each series, the silver spacer thickness have been varied from tA~ = 7.5 ,~ tO tAg = 55 ,~. These samples are labeled Ta 50 A/(CogoFelo tCoFe/Ag tAg) X 2 0 / T a 120 ,~. Annealing was performed under flowing argon gas between 300 and 400°C for 15 min. Additional preparation details and structural characterization, as well as preliminary magnetic and transport measurement results at room temperature, have been given elsewhere [5]. 2. Experimental results To characterize the magnetic behavior we have measured the temperature dependence of the magnetization in 100 Oe, in zero field-cooled (ZFC) and field-cooled (FC) samples, and the field dependence of the magnetization at different temperatures. The magnetoresistance (MR) has been measured using a four-point probe with the magnetic field applied in the plane of the sample and parallel to the current. The MR ratio is defined as MR = {R(Hmax) R ( H = O ) } / R ( H = 0), where Hmax is the maximum field

(7 W). In the first series - tcove = 5 ,~ - we observe that the magnetization changes from a superparamagnetic to a blocked state as the temperature decreases. This can be seen from Fig. la where there is a bifurcation in the ZFC and FC curves below 30 K while, at high temperature, the magnetization is reversible and follows a superparamag-

0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-8 85 3(96)00540-9

F. Fettar et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 316-319 200

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TEI'IPERATU RE (K) Fig. 1. FC and ZFC temperature dependence of the magnetization at 100 Oe for (a) (CoFe 5 A/Ag 55 4 ) × 2 0 and (b) (CoFe 7.5 A/Ag 55 ,~)×20. (-O) As-deposited; (11) annealed at 3-~°C for 15 min.

netic Curie law. The average cluster size and the CurieWeiss temperature ((9) have been deduced from this high temperature behavior. The average cluster size decreases rapidly as tag increases up to tag = 20 A, while above this thickness it remains nearly constant at 15 ,~ diameter (in this case, because the in-plane and out-of-plane magnetization are isotropic, the clusters were assumed spherical). This behavior can be attributed to a progressive decoupling of successive magnetic layers, which is almost completed for 20 ~, of silver. For thin values of Ag, @ is positive, suggesting ferromagnetic interactions between clusters. (9 decreases as the Ag thickness increases, being another indicative of the progressive decoupling. Magnetoresistance results also support this interpretation. As shown in Fig. 2a, the MR ratio is maximum at 20 A, with 20% GMR, and decreases for thinner silver layers to finally vanish at about 7 A. This rapid decrease with thin Ag layers is likely due to magnetic bridging by pinholes. However, it can be noted that, even through some coupling is present with thin silver layers, the measured GMR is much higher with these clustered multilayers than what is found in continuous multilayers [5]. At tag = 20 A, the GMR maximum reflects an optimum between the size of the clusters and the relative concentration of CoFe and Ag (with CoFe = 20 vol%). For thicker silver layers, the GMR decreases smoothly due mainly to the progressive dilution of the clusters. With thin CoFe layers, annealing does not produce significant changes, neither in magnetic nor in transport properties - see Fig. la and Fig. 2a. In the second series, where tcoFe = 7.5 A, there is an increase in the blocking temperature compared to the previous series. The samples seem to be made mostly of large, ferromagnetic islands, with a possible contribution

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tA~(A) Fig. 2. Magnetoresistance at 4.2 K, as a function of silver thickness, for (a) (CoFe 5 A/Ag tag)×20 and (b) (CoFe 7.5 ,~/Ag tag)X 20 serie--s. (©) As-deposited and (B) anne--aledat 300°C for 15 min.

from smaller superparamagnetic clusters. However, we cannot estimate the contribution of small single-domain particles, if any, from F C - Z F C measurements. On the other hand, magnetization loops measurements at low temperature are hindered by the paramagnetic signal of impurities from the substrate. For this reason, they were measured at 100 K (at this temperature, the substrate contribution is negligible in almost all the samples). We found that the remanence ratio, M J M ~ , oscillates with the silver thickness, suggesting the existence of an oscillating coupling between magnetic layers (Fig. 3). Fig. 2b shows the GMR as a function of silver thickness. Like the remanence, it oscillates with the spacer thickness with a first maximum, at 22%, centered at tag = 20 A, and a second one at 33 ,~. Giant magnetoresistance maxima coincide with remanence minima. In annealed samples the magnetization decreases (Fig. lb), showing in many cases a superparamagnetic component. Also, a diminution in the squareness of the hysteresis loops is observed. Both observations lead to the conclusion that the large ferromagnetic units of

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F. Fettar et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 316-319

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as-deposited samples are broken into smaller parts during the annealing process. The GMR increases a little, but oscillations are now less noticeable. This last result may indicates that GMR oscillations are linked with the layered character of these structures, while annealing induces clustering of the CoFe and blum~ng of the oscillations in GMR. Finally, for tCoFe= l0 A, the temperature dependence of magnetization is that of a bulk ferromagnetic system, up to 20 A of silver. As silver thickness increases, differences between ZFC and FC curves are observed. Hysteresis loops at 5 K and at room temperature are also ferromagnetic like, with small coercivity ( ~ 100 Oe at 5 K) and high remanence. The GMR effect is significant only for silver thickness definitely l~ger than 20 A, and reaches a maximum at 13% near 35 A (Fig. 4), which is typical of continuous magnetic layers coupling by pinholes. Annealing increases the GMR ratio and shifts the threshold for the appearance of GMR and its maximum to smaller thickness. These results are similar to those obtained at room temperature [5]. 3. Discussion The magnetic and GMR measurements reveal that the nature of the CoFe layers are completely different in the three series. In the series with tcoF~= 5 A, the magnetic behavior is that of superparamagnetic clusters, with size near 15 .~. For silver thickness above 20 .~, the GMR peaks at 20% and then decreases slowly, due to dilution effect. More, the GMR does not show oscillations with the silver spacer thickness, like those found in multilayers with continuous layers. This is due to the fact that the RKKY coupling, which underlies these oscillations, strongly relies on the presence of successive planes of interacting magnetic moments. In our case, as we have layers of magnetic clusters, the inter-plane distances are ill-defined and the indirect exchange interactions average to zero. It has to be noted that Loloee et al. found MR oscillations in C o / A g multilayers with tco= 4 and 6 A [4]. Oscillations with thinner magnetic layers could be due to smoother layering of their Co compared with our CoFe. With tCoFe= 7.5 A, the structure of the layer is more o

complex, in the sense that it shows evidences of both ferro and superparamagnetic contributions. It is thus more difficult to characterize. However, even if the layers are still discontinuous the layered character is now better defined and oscillating indirect exchange coupling leads to oscillations in the GMR and remanence. But at the same time, these layers are less affected by pinholes or other structural defects than strictly continuous CoAg multilayers in which magnetic bridging deteriorates the GMR. At this thickness, the behavior of the layers is intermediate between clustered systems and continuous multilayers with both superparamagnetic islands and coupling oscillations. Our results agree with those presented by van Halphen et al. in Ref. [6]. In that work the authors concluded from NMR measurements, that tco= 7 .~ is the thickness of passage from discontinuous character in C o / A g multilayers. This detrimental direct coupling is clearly evident in the third series, where the CoFe layers are completely ferromagnetic - and continuous - and where the GMR is low and only present for thicker silver layers. With thin silver layers, the magnetic layers are coupled irrespective to their structure. This reflects in the fact that, for the three series, the GMR rapidly falls to zero when the silver thickness is below 20 A. Also, in samples with 5 .~ of CoFe, the apparent increase in the size of the magnetic clusters in films with thin silver layers can be attributed to the direct coupling of magnetic clusters through silver. However, the effect of the direct coupling between magnetic layers is more evident in the GMR. On the other hand, CoFe clusters layers are less sensitive to that direct coupling than the complete, continuous layers. This is simply due to the fact that, with clusters, the in-plane magnetization correlations are reduced, so that local bridges between adjacent layers only couple part of the layers Annealing does not lead to important changes in multilayers with tcoF~ = 5 .~, where clusters are already present. However, in the other two series, the same annealing process breaks the ferromagnetic units of the multilayers into smaller parts that are less sensible to direct coupling. As a result of the clustering of the CoFe layers, the oscillation in GMR is reduced. It has to be noted that in such multilayers, interface diffusions may involve the whole magnetic layer, and thus growth conditions are critical in determining their structural, magnetic and transport properties. In conclusion, we have shown that the magnetic and GMR properties of Co9oFelo/Ag multilayers are very sensitive to the nature of the magnetic units. This is mainly due to the fact that the interactions between successive layers, or within layers, are different according to whether the CoFe forms clusters or layers. Also, it seems that the best compromise, in terms of GMR amplitude, is found for CoFe thickness such that it forms incomplete layers of flat clusters. The origin of the oscillating magnetic coupling in this regime is not well understood yet, and further study is under way.

F. Fettar et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 316-319

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[3] T. Hylton, K. Coffey, M. Parker and J. Howard, Science 261 (1993) 1020. [4] R. Loloee, P.A. Schroeder, W.P. Pratt, Jr., J. Bass and A. Fert, Physica B 204 (1995) 274. [5] J.D. Jarratt and J. Barnard, IEEE Trans. Magn. 31 (1995) 3952; J.D. Jarratt and J. Barnard, J. Appl. Phys., in press. [6] E.A.M. van Alphen and W.J.M. de Jonge, Phys. Rev. B 51 (1995) 8182.