Interface coupling-induced enhancement of magnetoimpedance effect in heterogeneous nanobrush by adjusting textures of Co nanowires
© Zhang et al.; licensee Springer. 2013
Received: 25 September 2013
Accepted: 27 October 2013
Published: 9 November 2013
Interface coupling-induced and interface coupling-enhanced magnetoimpedance (MI) effect in heterogeneous nanobrush has been investigated. The nanobrush is composed of Fe25Ni75 nanofilm and textured hexagonal close-packed cobalt nanowire array, respectively fabricated by RF magnetron sputtering and electrochemical deposition. The design of this structure is based on the vortex distribution of magnetic moments in thin film, which can be induced by the exchange coupling effect at the interfaces of the nanobrush. The texture of nanowires plays an important role in the MI effect of the nanobrush, which is regulated by controlling the pH values and temperatures of the deposition process. The ‘parallel’ and ‘perpendicular’ coupling models were used to explain the different MI results of the nanobrush with cobalt nanowires, which have (100) and (002) textures, respectively. The optimized MI effect of the nanobrush brought by (100) nanowires can be magnified by 300% with more than 80%/Oe magnetic sensitivity at a low frequency, which has great application potentials in low-frequency MI sensors.
KeywordsNanobrush Magnetoimpedance Nanowire texture Micromagnetic simulation
In recent years, low-dimensional nanomaterials have attracted considerable attention due to their potential application in many areas. One-dimensional nanowires with large shape anisotropy and surface area have attracted much attention, which will be useful in a wealth of applications that include catalysis, magnetic recording, and some physical fundamental researches[2, 3]. Two-dimensional magnetic nanofilm is widely used for various kinds of magnetic sensors, planar inductors, and so on[4, 5]. Great efforts have been made to combine different structures for three-dimensional multifunction materials. For instance, Qin et al. fabricated a microfiber-nanowire hybrid structure for energy scavenging, and Yan et al. fabricated three-dimensional metal-graphene nanotube multifunctional hybrid materials[6, 7]. As a typical hybrid nanostructure, nanobrush has been under extensive studies as one of the nanodevices for its special characters[8, 9]. In a magnetic composite material, the exchange coupling effect at the interface is significant[10, 11]. In order to investigate its influence on nanobrush, a heterogeneous nanobrush with magnetic film and different textured cobalt nanowires is dwelt on in detail in this paper. Different coupling models at the interface induced by different cobalt crystal textures have been investigated. The structure shows great performance as far as the magnetoimpedance effect is concerned.
The magnetoimpedance (MI) effect has been considered as a potential physical effect with higher field sensitivity and better signal intensity for magnetic sensors than the giant magnetoresistance effect. Since MI changes with the external direct current (dc) magnetic field or applied dc/alternating current (ac) current, it is possible to design MI sensors used to measure magnetic fields or dc/ac currents. Several kinds of industrial and engineering applications of MI sensors have been proposed and realized to date, such as in the field of traffic controls, automobile uses, and biomedical sensors[13–16]. Amorphous wires, ribbons, and composited soft magnetic wires are traditional MI materials[12, 17, 18]. Normally, the diameter of amorphous wires and the thickness of ribbons are up to micrometer scale. With the rapid development of nanomaterials, the size of magnetic sensors is projected to reach nanoscale. The traditional MI materials cannot satisfy the desired size, and multilayer film MI materials have increasingly become the hot spot. However, the multilayer films may come into being only when an obvious MI ratio reaches gigahertz[19, 20], and it is not good for the application of MI sensors. Therefore, finding new kinds of nanomaterials, which can have both an obvious MI effect and a rapid magnetic response at low frequency, is a great challenge.
The MI effect is normally attributed to a combination of skin effect and high sensitivity of transverse permeability to the external applied field. In a magnetic medium, the skin depth is dependent on the transverse magnetic permeability (μt) through, where σ and μt, respectively, are the electrical conductivity and the transverse permeability of the ferromagnetic material. For amorphous ribbons and wires, many ways have been tried to improve the MI ratio, which include annealing, ion irradiation, glass coating, and patterning[21–23]. Essentially, all the above approaches to enhance the MI ratio are based on the changes of magnetic domain and induced transverse distribution of magnetic moments. For films, the sandwich structure is an effective approach to depress the skin effect and improve the MI ratio, but a low MI ratio and high working frequency pose major negative factors for applications. Obviously, it is urgent to solve the problem of how to induce transverse moment distribution and enhance the MI ratio in the nanomaterial.
The structure of heterogeneous nanobrush with strong interface coupling may provide new ideas for these challenges. As our former works turn out, the giant MI (GMI) ratio has been enlarged than the single FeNi film on an anodized aluminum oxide (AAO) template, and the exchange coupling effect between nanowires and film has been supposed to be the main reason of the enhanced MI ratio. However, how the exchange coupling effect acting on MI results is unclear. In this paper, a kind of magnetic nanobrush, which combines Fe75Ni25 film and cobalt nanowire arrays with different textures, is prepared. The obvious diversity of MI curves has been apparently observed in (100)- and (002)-textured nanobrushes. Micromagnetic simulation is used to analyze the phenomenon.
X-ray diffraction (XRD) confirmed the composition of the nanowire arrays. The surface topography and nanostructure were observed via scanning electron microscopy (SEM). The magneto-optic Kerr effect (MOKE) was used to obtain the surface magnetic properties of the composite material. Micromagnetic simulations were performed with the three-dimensional (3D) object-oriented micromagnetic framework (OOMMF) method. The exchange constants of the film and wires, respectively, were 1.3 × 10-11 and 1.75 × 10-11 J/m. The damping parameter α was 0.5, the mesh size was 5 × 5 × 5 nm3, and the saturation magnetization of the permalloy film and Co nanowires, respectively, were 8.6 × 105 and 1.42 × 106 A/m. Prior to MI measurement, the samples were tailored into small pieces with a length of 20 mm and width of 3 mm. An impedance analyzer (Agilent 4294A, Agilent Technologies, Inc., Santa Clara, CA, USA) was used in the four-terminal contact mode to measure the impedance (Z). The magnitude of the driving voltage is 500 mV. All the electronic instruments were controlled using LabVIEW (National Instruments, Austin, TX, USA).
Results and discussion
It should be emphasized that not only the MI ratio but also the magnetic response is important for high-performance sensor application. The inset of Figure 6 shows the magnetic response to the different textures of 20-nm nanowires. The sensitivity (S) of the MI is defined as follows: S (%/Oe) = (ΔZ/Z)/ΔH, where ΔH is the change of the magnetic field. At a very small external applied field, the field sensitivities of the MI effect of the 20-nm nanobrush are 80% and 25%. Afterwards, it begins to decrease and approach a value which is approximately equal to zero. The MI ratio and sensitivity of the nanobrush with FeNi film and 20-nm (100)-textured Co nanowires are higher than some typical MI results of single film and multilayer film[31, 32].
The MI effect of the nanobrush with FeNi film and texture-controllable cobalt nanowires has been investigated. Cobalt nanowires with (100), (002), and mixed structures have been fabricated by different pH values and deposition temperatures. The optimized results of the (100)-textured nanobrush are 320% and 350% with 20- and 50-nm diameters, respectively. The phenomenon can be explained by the different distributions of transverse magnetic moments, induced by the exchange coupling effect between the interface of nanowires and film. Micromagnetic simulation shows the magnetic moment distribution when the nanowires act on the film. The parallel and perpendicular exchange coupling models are supposed to be the main reason of the different MI performances.
JBW and QFL are professors at the Institute of Applied Magnetics, Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University. YZ is a Ph.D. student.
This work is supported by the National Basic Research Program of China (2012CB933101), the National Science Fund of China (11074101, 51171075), and the Fundamental Research Funds for the Central Universities (lzujbky-2012-209, lzujbky-2013-32, and 2022013zrct01).
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