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~ H Journalof magnetism and magnetic ~ I ~ materials
Journal of Magnetism and Magnetic Materials 175 (1997) 127 136
Arrays of nanowires of magnetic metals and multilayers: Perpendicular GMR and magnetic properties L. P i r a u x a'*, S. D u b o i s a, J.L. D u v a i P , K . O u n a d j e l a b, A. F e r t c UnitO de Physico-Chimie et de Physique des MatOriaux, UniversitO Catholique de Louvain, B-1348 Louvain-la-Neut~e, Belgium b lnstitut de Physique et Chimie des Mat~riaux de Strasbourg, F-67037 Strasbourg, Franee c Unit~; Mixte de Recherche du Centre National de la Recherche Scient!fique et de Thomson, Laboratoire Central de Recherches Thomson, 91404 Orsay, France et Universit~ Paris-Sud, Bat. 510, 91405 Orsay, France
Abstract
The template strategy combined with electrodeposition techniques have been used to fabricate arrays of nanowires of magnetic metals and multilayers in the cylindrical pores of track-etched polymer membranes. The giant magnetoresistance effects have been investigated in two different types of multilayered nanowires systems: Co/Cu and NisoFe2o/Cu. In addition, a comparative study of the magnetic properties of sub-micron Ni, Co, Fe and NisoFe2o wires is made by means of anisotropic magnetoresistance and magnetization experiments. PACS: 72.15.Gd; 72.10.Fk; 75.50.Rr Keywords: Nanowires; Giant magnetoresistance; Multilayers; Magnetoresistance
1. Introduction
Since the pioneering work of Pratt et al. [1], measurements of the giant magnetoresistance (GMR) of magnetic multilayers in the CPP (current perpendicular to the planes) geometry have become increasingly attractive. The growth of magnetic multilayered nanowires in the cylindrical pores of track-etched polymer membranes enables to investigate G M R effects in the perpendicular geometry
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perpendicular
and provides a valuable testing ground for theories describing the C P P - G M R in various limits [2-6]. In this paper, a review is given of recent G M R data in two types of multilayered nanowires systems: Co/Cu and NisoFe2o/Cu. The variation of the C P P - G M R of Co/Cu multilayered nanowires has been explored with the thicknesses of the Co and Cu layers varying over very wide ranges (between the nanometer and the micrometer ranges). Combining the data obtained in the low-temperature range (where spin-mixing effects are negligible) with theoretical predictions of the Valet Fert (VF) model [7] led to an estimation of the spin diffusion lengths (SDL) in the nonmagnetic and
0304-8853/97/$17.00 ~C 1997 Elsevier Science B.V. All rights reserved P I I S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 1 5 7- 1
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L. Piraux et al. /Journal of Magnetism and Magnetic Materials 175 (1997) 127 136
ferromagnetic layers. For applications, large resistance changes at low fields for temperatures at or above room temperature are required. Thus, multilayers containing magnetically soft NisoFe20 layers are of particular interest. Two types of structures have been studied: conventional NisoFe2o/Cu multilayers and multilayers composed of NisoFe2o/ Cu/NisoFe2o trilayers magnetically isolated by long Cu rods. Finally, electrodeposited nanowires are of great interest because they provide a relatively simple and inexpensive way to study the magnetic properties of nanoscale objects [8]. Due to the shape anisotropy, those elements are predominantly magnetized along their length. However, there are still open questions about the mechanisms responsible for the magnetization reversal. We have recently shown that the Ni and Co-based systems exhibit different magnetic and magnetoresistive behaviors due to competing crystal anisotropy in the Co-based system [9, 10]. In this paper, we also report on preliminary data obtained on arrays of Fe and NisoFe2o sub-micron wires. In Section 2, we briefly report on the preparation of arrays of nanowires of magnetic metals and multilayers. The GMR properties of these multilayered nanowires are discussed in Section 3. Finally, a comparative study of anisotropic magnetoresistance and magnetic properties in arrays of sub-micron magnetic wires is made in Section 4.
2. Growth of arrays of electrodeposited nanowires of magnetic metals and multilayers
layered nanowires were made from a single bath using a pulsed deposition technique. The noble element (Cu) is kept in dilute concentration so that the rate of reduction of Cu is slow and limited by diffusion. The electrodeposition process is controlled by a computer which continuously integrates the charge during each layer deposition. The potential is switched when the deposition charges for the nonmagnetic and the magnetic layers, QNMand QM, respectively, reach the set value. Such a procedure is required to give uniform layer thicknesses all along the filament. The electrodeposition process is stopped when the wires emerge from the surface (as evidenced by a sudden increase of the plating current). Dividing the membrane thickness (20 gm) by the number of cycles gives the average period for the wires involved in the GMR experiment. Using XEDS (X-rays Energy Dispersive Spectroscopy) microanalysis, the average chemical composition was determined. We can alternatively use another method based on the relationship between the layer thicknesses dNM and dM and the electric charges QNM and QM transferred at the cathode during the corresponding pulses. Fig. 1 shows the linear variation of the bilayer thickness dbilayer as a function of
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Track-etched polycarbonate membranes were used as nanoporous host material for the growth of multilayered nanowires [11]. A gold film serving as cathode was first evaporated on one side of the membrane. The membrane sample is then placed in a home-made teflon cell and a 0.1 cm 2 area is exposed to the electrolyte. Electrodeposition is performed using an EG&G Princeton Applied Research Model 283 potentiostat/galvanostat under quiescent conditions at T ~ 25°C. Ferromagnetic Co, Ni, Fe and NisoFe2o nanowires were electrodeposited in nanoporous membranes with diameters ranging from 30 to 500 nm using a sulphate bath. Electrodeposited Co/Cu and NiFe/Cu multi-
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Qcn ( m e ) Fig. 1. Relation between the mean thickness of the bilayer in Co/Cu multilayered nanowires dbilayer and the electric charge Oc, transferred at the cathode during Cu deposition at - 0.3 V. For this series, Oco is kept constant at 0.1 m C corresponding to a Co thickness of 24 nm. dbilayer is calculated using the ratio between the thickness of the membrane (20 gm) and the number of cycles required for the filling of the pores.
L. Piraux et al. / Journal of Magnetism and Magnetic Materials 175 (1997) 127 136
Qcu for the Co/Cu system. In this series, Qco is kept constant at a value of 0.1 mC and Qcu varies up to I mC; dbilayer was determined experimentally from electrodeposition process as explained above. Using the Faraday's law, the following relationship can be derived: dbilayer
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where A is the cathode surface area (in mm 2) which depends on the pore density and diameter; ~/cu and qco are the cathode current efficiencies for pulsed electrodeposition of Cu and Co, respectively; the two charges Qcu and Qco in Eq. (1) are expressed in inC. From the slope and intercept of the straight line of Fig. 1, we obtain qCo/r/cu = 0.72 and dco ~ 24 nm. Assuming a 100% current efficiency for Cu deposition, this leads to r/Co= 0.72. The microstructure of the single metal and multilayered
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nanowires has also been studied in detail, using X-ray diffraction and analytical transmission electron microscopy [6, 9, 12]. Fig. 2a shows a TEM image of a Co nanowire of diameter around 70 nm. The diffraction pattern is consistent with the HCP structure of cobalt with a c-axis oriented perpendicular to the wire axis. Fig. 2b shows a TEM image of a Nis0Fe20/Cu multilayer grown on a pure permalloy [6, 12]. It was shown that, in spite of the relative simplicity of growth by electrolysis, these multilayers exhibit single crystal structures, with single crystal grains including several tens of layers. The structure is FCC and the (1 1 0) axis is parallel to the growth direction. The layers look flat in the single crystal regions, while they can be quite distorded in the polycrystalline regions. The period of the superlattice shown in Fig. 2b is about 5 nm. The combination of TEM and EDX results tell us that the Cu layers are 1 nm thick and the permalloy ones are about 4 nm. It is noted that for a 20 gm
Fig. 2. (a) Transmission electron microscopy of a cobalt wire, in (1 0 0) projection. Dark field image and corresponding diffraction pattern. (b) TEM micrograph of a NisoFezo(4 nm)/Cu(l nm) multilayered nanowire grown on top of pure permalloy. The shadow on the left comes from another nanowire in the field of view. These micrographs are extracted from Ref. [6].
L. Piraux et al. / Journal o['Magnetism and Magnetic Materials 175 (1997) 127 136
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thick membrane, multilayered nanowires composed of more than 3000 bilayers can be prepared using this method.
difference of less than 10% between the two magnetoresistance ratios was also observed on Co/Cu multilayered nanowires I-4, 5]. At room temperature, the MR is reduced by a factor of 3. The saturation and coercive fields extracted from the magnetization curves are similar to those observed in GMR measurements. The large saturation fields observed for magnetic fields parallel to the layers are due to the demagnetizing fields arising from the multilayered nanowire structure. In order to reduce the saturation fields, we have recently proposed another structure composed of Nis0Fe2o(3 nm)/Cu(10 nm)/NisoFe20(3 nm) trilayers magnetically isolated by thick layers of Cu (100nm or more). A typical magnetoresistance curve for in-plane field is shown in Fig. 3c. For such structure, the expected magnetic behavior is that of isolated trilayers, essentially governed by their in-plane shape anisotropy. Our magnetization
3. CPP-GMR in Ni80Fe20/Cu and Co/Cu multilayered nanowires Magnetoresistance and magnetization curves obtained with magnetic fields parallel to the layers at 4.2 K on NisoFe2o(12 nm)/Cu(4 rim) multilayered nanowires are shown in Fig. 3a and Fig. 3b [-6]. The GMR ratio in the virgin state approaches 80% at low temperature which is at least a factor 20 larger than the values reported for Nis0Fezo/Cu multilayers in the CIP (current in the planes) geometry for comparable Cu layer thicknesses 1-13, 14]. The MR at the peak (73%) is somewhat smaller than in the virgin state. A similar relative
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