GMR in multilayered nanowires electrodeposited in

ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 308 (2007) 35–39 www.elsevier.com/locate/jmmm

GMR in multilayered nanowires electrodeposited in track-etched polyester and polycarbonate membranes F. Nasirpouria,b,, P. Southerna, M. Ghorbanib, A. Iraji zadb, W. Schwarzachera a

H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK Departments of Materials Science & Engineering and Physics, Sharif University of Technology, Tehran, Iran

b

Received 28 July 2005; received in revised form 3 December 2005 Available online 2 June 2006

Abstract Commercially available track-etched polyester membranes were used as templates to electrodeposit Co–Ni–Cu/Cu multilayered nanowires, giving room-temperature current perpendicular to plane (CPP) giant magnetoresistance (GMR) values of up to 12%. In contrast to similar nanowires electrodeposited in track-etched polycarbonate membranes, the GMR obtained in multilayered nanowires electrodeposited in the polyester membranes increased with decreasing Cu-layer thickness tCu, for tCu in the 2–7 nm range, indicating a lack of ferromagnetic coupling through pinholes, etc. Transmission electron micrographs showed clear evidence for smooth, parallel layer interfaces in the nanowires. r 2006 Elsevier B.V. All rights reserved. PACS: 75.47.De; 75.70.Cn; 81.15.Pq Keywords: Magnetoresistance; Nanowires; Electrodeposition; PETE; Co–Ni–Cu

1. Introduction Following the discovery of giant magnetoresistance (GMR) in electrodeposited superlattices [1], a great deal of attention has been devoted to the magnetic and magnetoresistance properties of multilayered or superlattice nanowires obtained by template electrodeposition [2,3]. Wires consisting of alternating layers of a ferromagnetic and a non-magnetic metal with diameters down to a few tens of nanometre and lengths of several micrometre or greater have been grown successfully in polycarbonate track-etched (PCTE) or anodic aluminium oxide (AAO) membranes [4,5]. The current perpendicular to plane (CPPGMR) measured for such nanowires can be significantly Corresponding author. Present address: Department of Materials Engineering, Sahand University of Technology, Tabriz, Iran. Tel./fax: +98 412 3444333. E-mail addresses: [email protected], [email protected] (F. Nasirpouri).

0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.04.035

larger than the current-in-plane (CIP-) GMR measured for conventional planar multilayers. Another type of nanoporous polymer membrane that can be produced like PCTE by etching tracks formed through heavy ion bombardment is the polyester track etched membrane (PETE) [6]. Although much less commonly used as a template for electrodeposition than PCTE, this membrane has the major advantage over PCTE of being naturally hydrophilic. There is therefore no need to add a surfactant as is done during the manufacture of PCTE membranes. Furthermore, PETE is very resistant to chemical attack and is both durable and stable. In this paper, we demonstrate that like multilayered nanowires electrodeposited in PCTE membranes, Co–Ni–Cu/Cu superlattice nanowire arrays grown in PETE membranes show significant CPP-GMR (up to 12% at room temperature). We present detailed results for the magnetic, magnetotransport and microstructural properties of Co–Ni–Cu/Cu nanowires with a range of layer thicknesses electrodeposited in PETE

ARTICLE IN PRESS 36

F. Nasirpouri et al. / Journal of Magnetism and Magnetic Materials 308 (2007) 35–39

membranes and for similar nanowires electrodeposited in PCTE.

3. Results Fig. 1 (a) is a low magnification TEM Co–Ni–Cu (3.3 nm)/Cu (3.3 nm)-superlattice electrodeposited in a polyester membrane, dissolution of the latter. Some of the imaged have lengths of 4–5 mm, which approaches the

2. Experiments Commercially available track-etched polyester and polycarbonate membranes with nominal pore diameters 100 and 30 nm, respectively, (GE-Osmonics) were used as templates for the electrodeposition of Co–Ni–Cu/Cu multilayers. Further details of the templates are given in Table 1. In each case an Au film was evaporated onto one face of the membrane to act as a contact. The multilayers were electrodeposited from an electrolyte containing 2.3 mol/L H2O Ni sulphamate, 0.4 mol/L H2O Co sulphate, 0.05 mol/L H2O Cu sulphate and 30 g/L H2O boric acid in a standard three-electrode cell and the deposition potentials used were 0.2 and 1.8 V relative to a saturatedcalomel reference electrode for Cu and Co–Ni–Cu alloy, respectively. Deposition was carried out under computer control, and the current during deposition was recorded. Two-point CPP magnetotransport measurements were made at room temperature with applied fields up to 3000 Oe in the plane of the membrane (i.e. perpendicular to the nanowire axis) using mechanical contacts to the top of the membrane where the multilayered nanowires had grown out of the pores, and to the Au rear contact. Magnetization curves were measured at 20 and 300 K using a Quantum Design SQUID magnetometer with the applied field parallel and perpendicular to the plane of the membranes. The membrane surfaces were examined at different stages of the nanowires’ growth by high-resolution scanning electron microscopy in order to study the process of pore-filling and overgrowth. Transmission electron microscopy (TEM) was used to study the length, diameter, and layer structure of the wires after removal from the membranes. Crystalline structure was studied by means of TEM diffraction patterns. In order to prepare samples for TEM, polyester membranes were dissolved in concentrated potassium hydroxide+ethanol at 60–70 1C [6]. An ultrasonic bath was always used to aid the dissolution process. In each case, a droplet of solvent containing the dispersed wires was placed on a carbon-coated TEM grid.

image of nanowires following nanowires maximum

Fig. 1. Low-magnification (a) and high-magnification (b) TEM images of Co–Ni–Cu (3.3 nm)/Cu (3.3 nm)-superlattice nanowires electrodeposited in a polyester membrane.

Table 1 Properties of track-etched polyester and polycarbonate nanoporous templates Membrane type

Nominal pore diameter (nm)

Measured wire diameter (nm)

Pore density (pore/cm2)

Membrane thickness (mm)

Hydrophilic

Polyester (PETE) Polycarbonate (PCTE)

100 30

125 (7 20) 74 (78)

4 108 6 108

6 6

Yes (intrinsic) Yes (surfactant added)

ARTICLE IN PRESS F. Nasirpouri et al. / Journal of Magnetism and Magnetic Materials 308 (2007) 35–39

Co-Ni-Cu (3.3 nm) / Cu (2 nm) Co-Ni-Cu (3.3 nm) / Cu (3.3 nm) Co-Ni-Cu (3.3 nm) / Cu (6.6 nm)

% MR

15

10

5

0 -3000

-2000

-1000

0

1000

2000

3000

Applied field (Oe)

(a)

M / MS

1

0

-1 -5000

0

5000

Applied field (Oe)

(b)

Fig. 2. (a) Measured %MR for series of Co–Ni–Cu/Cu-superlattice nanowires with fixed nominal Co–Ni–Cu layer thickness and varying Cu layer thickness electrodeposited in track-etched polyester membranes; (b) magnetization curve measured at 300 K with the applied field in the plane of the membrane for the Co–Ni–Cu (3.3 nm)/Cu (2 nm) sample.

Co-Ni-Cu (6 nm) / Cu (6 nm) Co-Ni-Cu (6 nm) / Cu (3.5 nm)

15

% MR

expected for a membrane thickness of 6 mm, but shorter fragments are also seen, perhaps resulting from wire breakages during the dissolution of the template. This procedure is much more difficult than for polycarbonate membranes, which are easily dissolved in chloroform at room temperature, and the sample of Fig. 1(a) clearly still includes a significant amount of residual polymer. Images like this were used to determine the measured wire diameters quoted in Table 1. Fig. 1(b) is a higher magnification image of two Co–Ni–Cu (3.3 nm)/Cu (3.3 nm)-superlattice nanowires. All quoted layer thicknesses are nominal thicknesses, calculated from the charge passed during the deposition of that layer and assuming a current efficiency of 100%, i.e. that all the current is associated with metal deposition, which in practice will not be the case. To calculate the layer thickness from the charge passed, it is also necessary to know the deposition area, and this was calculated from the manufacturer’s quoted pore density and the measured wire diameter quoted in Table 1. The estimated error in the nominal layer thicknesses is730% for nanowires in the polyester membranes and720% for nanowires in the polycarbonate membranes. Although the residual polymer detracts from the quality of the image, fine layering corresponding to the superlattice repeat distance is clearly seen in both wires. The repeat distance estimated from this image is 5.8 (70.7) nm, which is in good agreement with the nominal value of 6.6 (72) nm. Although successive interfaces are parallel, the layer interfaces are not flat. Surface tension considerations suggest that the evaporated Au contact at the start of a pore is unlikely to be flat. As this contact forms the initial growth surface for the nanowire, the layer interfaces are likely to be curved [7]. Fig. 2(a) shows the measured percentage magnetoresistance (%MR) for series of Co–Ni–Cu/Cu superlattice nanowires with fixed nominal Co–Ni–Cu layer thickness tCo–Ni–Cu and varying Cu-layer thickness tCu. The %MR is defined as %MR ¼ RðHÞ RðH max Þ=RðH max Þ 100, where R is the resistance, H is the applied field and Hmax the maximum applied field that could be applied in our MR apparatus (3 kOe). In each case, the %MR curve is broad, with changes in the MR continuing to be recorded even for H42 kOe. There is also hysteresis (as well as some drift) in the MR data. For comparison, Fig. 2(b) shows the magnetization curve measured at 300 K with the applied field in the plane of the membranes, as for the MR measurements, for the Co–Ni–Cu (3.3 nm)/Cu (2 nm) sample. Comparison of Figs. 2(a) and (b) shows that the maximum % MR occurs close to H ¼ Hc (the coercive field). Note that from Fig. 2(a) the maximum measured %MR increases with decreasing tCu as is expected according to the Valet–Fert (VF) model for CPP-GMR, because decreasing tCu decreases the component of the resistance that is independent of magnetization [8,9]. This simple model will only apply, however, if decreasing tCu does not influence, e.g. the effective tCo–Ni–Cu. For example, if interlayer roughness or pinholes lead to strong ferromagnetic

37

10

5

0 -3000

-2000

-1000

0

1000

2000

3000

Applied field (Oe) Fig. 3. Measured %MR for a series of Co–Ni–Cu/Cu-superlattice nanowires with fixed nominal Co–Ni–Cu layer thickness and varying Cu layer thickness electrodeposited in track-etched polycarbonate membranes.

coupling between the Co–Ni–Cu layers when tCu is reduced, the VF model will break down. Hence the observation of increasing %MR with decreasing tCu is an


M / MS

1

0

in plane out of plane

-1

-5000 (a)

0 Applied field (Oe)

5000

M / MS

1

0

-5000

0 Applied field (Oe)

(4)

-0.06

in plane out of plane

-1

(b)

nanowires’ long axis (out of the plane of the membrane) a hard axis, ferromagnetic coupling between the layers would make this direction an easy axis. Fig. 4 shows hysteresis loops measured with the applied field parallel and perpendicular to the plane of the membrane. For Co–Ni–Cu (6 nm)/Cu (6 nm)-superlattice nanowires electrodeposited in polycarbonate membranes there is clearly an easy axis parallel to the nanowires’ long axis, suggesting ferromagnetic coupling, whereas in the case of Co–Ni–Cu (3.3 nm)/Cu (3.3 nm)-superlattice nanowires electrodeposited in polyester membranes there is no obvious hard or easy direction. Fig. 5 shows the Co–Ni–Cu current as a function of time during deposition of (a) Co–Ni–Cu (6 nm)/Cu (6 nm)superlattice nanowires in polycarbonate membranes and (b) Co–Ni–Cu (3.3 nm)/Cu (3.3 nm)-superlattice nanowires in polyester membranes. The Co–Ni–Cu current was recorded at the end of the deposition of each Co–Ni–Cu layer, just before switching to the potential for Cu deposition. As usual for nanowire electrodeposition, the current transients show four distinct growth stages [10].

(3) 5000

Fig. 4. (a) Hysteresis loops measured at 300 K with the applied field in and out of the plane of the membrane for Co–Ni–Cu (3.3 nm)/Cu (3.3 nm)superlattice nanowires electrodeposited in track-etched polyester; (b) hysteresis loops measured at 300 K with the applied field in and out of the plane of the membrane for Co–Ni–Cu (6 nm)/Cu (6 nm)-superlattice nanowires electrodeposited in track-etched polycarbonate.

Cathodic Current(A)

38

(2-2) -0.04

(2-1)

-0.02 (1)

0

400 Time(sec)

600

800

-0.060 (3) Cathodic Current(A)

important criterion by which the structural quality of superlattice nanowires may be judged. Fig. 3 shows that the VF model does not apply to the Co–Ni–Cu/Cu-superlattice nanowires that we electrodeposited in polycarbonate membranes. For these nanowires, like those electrodeposited in polyester membranes, the real layer thicknesses appear to be close to the nominal layer thicknesses, since TEM measurements of Co–Ni–Cu (6 nm)/Cu (6 nm)-superlattice nanowires gave an estimated repeat distance of 9.0 (70.4) nm, which is in reasonable agreement with the nominal value of 12 (72.5) nm. Although the general shape of the MR curves is similar to Fig. 2(a), the maximum measured %MR decreases between tCu ¼ 6 and 3.5 nm. This suggests that, at least by this criterion, the structural quality of Co–Ni–Cu/Cu multilayer nanowires electrodeposited in polyester membranes is superior to that of similar nanowires electrodeposited in polycarbonate membranes. Although the shape anisotropy of the individual layers would make the direction parallel to the superlattice

200

(a)

(4)

(2-2) -0.055 (2-1) (1) -0.050

-0.045 0 (b)

200

400 Time(sec)

600

800

Fig. 5. Co–Ni–Cu current as a function of time during deposition of (a) Co–Ni–Cu (6 nm)/Cu (6 nm)-superlattice nanowires in polycarbonate and (b) Co–Ni–Cu (3.3 nm)/Cu (3.3 nm)-superlattice nanowires in polyester membranes.


These are: (1) initial nucleation and growth of the nanowires at the ends of the pores, (2) growth of wires within the pores, (3) pore filling and (4) overgrowth. Stage (2) may be subdivided into a time during which the current increases (2–1) and a time during which the current changes much more slowly (2–2). A probable explanation of the increasing current during stage (2–1) is that poor wetting leads to a delay in nucleation in some pores, so that the number of pores in which growth takes place and therefore also the current rise gradually [11]. The increase in current during stage (2–1) and its duration are significantly greater for the polycarbonate membranes than for the polyester ones, consistent with poorer wetting in the former case. According to Fokkink et al. [11], poor pore-wetting also leads to a smearing out of the transition to bulk growth and it is noticeable that stage (3) is much longer for the polycarbonate than for the polyester membranes. Steady state current transients also show that the ratio of the steady state ratio of Cu to the Co–Ni–Cu current is larger for the polyester than the polycarbonate membranes. This ratio determines the Cu content of the Co–Ni–Cu layers, which is therefore likely to be greater for the nanowires grown in polyester membranes. This could be one explanation for the observation that the maximum measured %MR for the polycarbonate membranes (14% for tCu ¼ 6 nm) is greater than that measured for polyester membranes (12% for tCu ¼ 2 nm). In conclusion, we have shown that track-etched polyester membranes are an appropriate template for growing superlattice nanowires that exhibit CPP-GMR. TEM images of such nanowires show clear evidence of a welldeveloped layer structure. Though not conclusive, evidence

39

from current transients suggests that pore wetting is better for the polyester than for track-etched polycarbonate membranes. In contrast to nanowires grown in polycarbonate membranes, nanowires electrodeposited in polyester membranes exhibit %MR increasing with decreasing tCu, which is one criterion for high structural quality. However, the fact that the maximum measured %MR for the polycarbonate membranes (14% for tCu ¼ 6 nm) is greater than that measured for polyester membranes (12% for tCu ¼ 2 nm) suggests that other considerations are also important.

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