- NANO EXPRESS
- Open Access
Magnetic and Magnetoresistive Properties of 3D Interconnected NiCo Nanowire Networks
© The Author(s) 2016
- Received: 25 June 2016
- Accepted: 7 October 2016
- Published: 19 October 2016
Track-etched polymer membranes with crossed nanochannels have been revealed to be most suitable as templates to produce large surface area and mechanically stable 3D interconnected nanowire (NW) networks by electrodeposition. Geometrically controlled NW superstructures made of NiCo ferromagnetic alloys exhibit appealing magnetoresistive properties. The combination of exact alloy compositions with the spatial arrangement of NWs in the 3D network is decisive to obtain specific magnetic and magneto-transport behavior. A proposed simple model based on topological aspects of the 3D NW networks is used to accurately determine the anisotropic magnetoresistance ratios. Despite of their complex topology, the microstructure of Co-rich NiCo NW networks display mixed fcc-hcp phases with the c-axis of the hcp phase oriented perpendicular to their axis. These interconnected NW networks have high potential as reliable and stable magnetic field sensors.
- Anisotropic magnetoresistance
- 3D nanowire network
- Ferromagnetic alloys
- Nanochannel-confined electrodeposition
The particular architectures and high degree of nanowire (NW) interconnectivity of three-dimensional (3D) NW networks make them attractive nanodevice components for a wide range of applications in energy harvesting/storage systems [1–3], electronic sensing devices and actuators [4–6], catalysts , electrochromic elements , solar cells , biosensors , and bio-analytical devices [11, 12]. Magnetic NW networks are also expected to play an important role in the development of next-generation multifunctional devices like 3D superstructures with controlled anisotropy and microwave absorption properties  and for the storage and logic operation of information carried and processed by domain walls flowing along them . Template-assisted synthesis has proven to be a versatile bottom-up approach for low-cost, reliable, and large-scale fabrication of 3D NW networks with controlled size, geometry, composition, and surface morphology. Typically, these 3D NW networks are obtained by simple electrochemical deposition within the hierarchical nanopores of a suitable template. Among the various 3D nanoporous templates used for this purpose, track-etched polymeric membranes [7, 13] is the most promising as dense networks of crossed cylindrical nanopores can be obtained through sequential polycarbonate (PC) film irradiation with energetic heavy ions at different incidence angles, followed by selective chemical etching of the ion tracks within the polymer film . This template-assisted synthesis enables excellent control over the geometry, chemical composition, and nanoarchitectures that can be the framework for nanoscale devices and systems. The 3D nanoarchitecture also facilitates the ability to perform electron transport measurements through the interconnected, crossed nanowires (CNWs). In this work, we demonstrate the suitability and reliability of using CNWs of different magnetic materials to obtain tunable magnetoresistive behavior. Interconnected NW networks made of electrodeposited NiCo alloys were chosen for the present study because of the interest in these alloys for a wide variety of applications, including magnetic storage systems [16, 17], magnetic and microresonator sensors [18, 19], fuel cells , microelectromechanical systems (MEMS) , hydrogen storage , and materials as catalysts . It is shown that magnetic alloy CNWs of controlled composition can be easily obtained through careful control of the deposition potential and using different electrolytic solutions. The subtle interplay between magnetic and magnetoresistive properties, and structural features is found to be crucial to tailor the magnetic and transport properties of such CNWs. Finally, in order to precisely determine the anisotropic magnetoresistance ratio of NiCo CNW networks from simple magneto-transport measurements, we propose a model that considers the spatial arrangement of NWs in the 3D network.
The 20- μm thick crossed nanoporous templates have been prepared by performing a sequential multi-step exposure of energetic heavy ions, at various angles with respect to the normal of the PC film surface. For the present study, a PC film was subjected to a first irradiation step over a wide angular range from −45° to +45° with respect to the normal axis of the PC surface. Next, for the second irradiation step the film was rotated in the plane by 90° and re-exposed to the same angular variable irradiation flux to form finally a complex 3D nanochannel network. The angular standard deviation in both irradiation steps was ±5° maximum around the target maximum angle. Both irradiation steps with quasi-continuous angular variation are a key requirement to obtain highly interconnected porous networks. The latent tracks were chemically etched in 0.5 M NaOH aqueous solution at 70 °C to form 40-nm diameter nanopores, following a previously reported protocol . The as-prepared polymer membrane containing dense networks of 3D interconnected cylindrical nanopores [7, 13] was designed with volumetric porosity of approximately 20 %. In a second stage, the PC templates were coated on one side using an e-beam evaporator with a metallic Cr/Cu bilayer to serve as cathode during the electrochemical deposition. The thickness of the thin adhesion layer of Cr was 10 nm, while for a uniform and consistent nanopore coverage withstanding the electrodeposition process, the Cu film thickness was set to 150 nm.
It should be noted that the results obtained by four- and two-probe measurements were the same, as the typical resistance values of the prepared specimens (in the range of few tens of Ω) were usually much larger than the ones attributed to the corresponding leads and contacts to the sample. The measured samples were about 1 cm long, and the electrical contacts were directly made by Ag paint. For each sample, the input power was kept below 0.1 μW to avoid self-heating, and the resistance was measured within its ohmic resistance range with a resolution of one part in 105.
Figure 1 a shows the tilted view SEM micrograph of the self-standing 40-nm diameter Ni CNW network after the complete dissolution of the PC membrane. As seen, the CNWs network exhibits a complex interconnected structure providing a high degree of electrical connectivity and good mechanical stability, as the entire 3D-CNW networks (with an area of ∼1 cm2) are self-supported and can be easily handled by tweezers, as seen in the inset to Fig. 1 a. The topological structure of the porous membranes is a key feature in giving rise to robust network architectures made up of self-standing magnetic CNW networks. The top-view SEM micrograph of Fig. 1 b shows a homogeneous orientation of the cylindrical NWs along the angular range of ±45°. This figure also displays a detailed view of the complex structure of the interconnected NWs.
where ρ av =(1/3)ρ ||+(2/3)ρ ⊥ is the average magnetoresistance in 3D systems.
In this work, we demonstrate the ability to perform reliable magneto-transport measurements through CNW networks fabricated from pure magnetic metals and alloys by electrodeposition into the crossed and interconnected nanochannels of track-etched polymer membranes. The magnetic properties and magnetoresistive response as a function of the angle between the magnetic field and the plane of the nanowire network films made of NiCo alloys can be controlled by a suitable choice of the composition of the ferromagnetic alloy. Both structural and magnetic characterization reveal the presence of mixed fcc-hcp phases as the Co content increases, with the c-axis of the hcp phase oriented perpendicular to the nanowire axis. The AMR values were accurately determined using a model that considers the spatial arrangement of NWs in the 3D network. The measured electrical resistances were very stable with time, so these CNW structures can be used to obtain reliable and stable magnetic field sensors. Finally, the present work opens up the possibility for a controlled template-assisted synthesis of complex nanowire-based architectures with excellent control over geometrical features and chemical composition, leading to tunable magnetic and magneto-transport properties.
This work was partly supported by the Fédération Wallonie-Bruxelles (ARC 13/18-052, Supracryst), the CONACYT project CB-177896, and the 2015 UNAM-DGAPA-PAPIIT Program project IA102915. The authors are thankful to Dr. Etienne Ferain and it4ip Company for supplying the polycarbonate membranes. We also thank Mrs. Anne Iserentant (Earth and Life Institute, UCL) for the XRD measurements.
TCSCG performed most experiments and analyzed the data. JTM analyzed the data and contributed to the writing of the manuscript. ML contributed to the synthesis and to the structural and physical characterization of the nanowire networks. LP contributed to the initial ideas, analyzed the data, and contributed to the writing of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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