Enhanced giant magnetoimpedance in heterogeneous nanobrush
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A highly sensitive and large working range giant magnetoimpedance (GMI) effect is found in the novel nanostructure: nanobrush. The nanostructure is composed of a soft magnetic nanofilm and a nanowire array, respectively fabricated by RF magnetron sputtering and electrochemical deposition. The optimal GMI ratio of nanobrush is promoted to more than 250%, higher than the pure FeNi film and some sandwich structures at low frequency. The design of this structure is based on the vortex distribution of magnetic moments in thin film, and it can be induced by the exchange coupling effect between the interfaces of nanobrush.
Keywordsnanobrush giant magnetoimpedance micromagnetic simulation FMR
Giant magnetoimpedance (GMI) effect has been considered as a potential physical effect that may take the place of giant magnetoresistance (GMR) because of its higher field sensitivity and better signal intensity . Since GMI changes as the function of external dc (direct current) magnetic field or applied dc/ac (alternating current) currents, it is possible to design GMI-based sensors that can measure either magnetic fields or dc/ac currents [2, 3]. Based on the applied stress dependence of the GMI effect, new kinds of stress sensors have been proposed [4, 5, 6]. Many industrial and engineering applications of GMI sensors have been proposed and realized to date, including traffic controls, automobile uses, biomedical sensors, and so forth [7, 8, 9]. Typical materials which may give rise to GMI effect are amorphous wires, amorphous ribbon, multilayer films, and other soft magnetic materials [10, 11, 12, 13, 14, 15]. Normally, the diameter of amorphous wires and the thickness of ribbons are up to micrometer scale, and the multilayer films with a high-GMI ratio work in gigahertz. In recent years, with the rapid development of nanomaterials, the size of magnetic sensors is projected to reach nanoscale. However, traditional GMI materials do not satisfy the desired size, and it is a challenge to find new kinds of nanomaterials, which can have both an obvious GMI effect and a rapid magnetic response at a low frequency.
GMI effect is normally attributed to a combination of skin effect and high sensitivity of transverse permeability to the external dc field [16, 17]. In a magnetic medium, the skin depth is dependent on the transverse magnetic permeability (μt) through , where σ and μt are the electrical conductivity and the transverse permeability of the ferromagnetic material, respectively. For amorphous ribbons and wires, many approaches have been tried to improve the GMI ratio, such as annealing, ion irradiation, glass coating, and patterning [18, 19, 20]. Essentially, all the above approaches to enhance GMI ratio are based on the changes of magnetic domain and induced transverse distribution of magnetic moments . For the films, the sandwich structure is the traditional approach to depress the skin effect and improve the GMI ratio, but the low GMI ratio and high working frequency constitute major negative factors for applications. Obviously, it is urgent to solve the problem of how to induce the transverse moment distribution and enhanced GMI ratio in the nanomaterial.
As a typical nanostructure, nanobrush, has been studied as one of the nanodevices for its special characters [21, 22, 23]. However, it is rare as far as the magnetic sensor applications are concerned. In this paper, a kind of magnetic nanobrush, consisted of nanofilm and nanowire array, is prepared. Considering that the origin of GMI is the change of transverse permeability under an external applied field, the design of the structure is based on the different vortex distributions of magnetic moments in thin film, which could be induced by the exchange coupling effect in the interface of heterogeneous nanobrush. To our relief, the magnetic heterogeneous nanobrush shows an excellent GMI ratio and high magnetic response at a low frequency.
Self-ordered anodic aluminum oxide (AAO) templates were prepared using the two-step anodization process in an oxalic acid solution (0.3 mol/L). The anodization potential was 40 V (20 V for sulfuric acid, 80 V for phosphoric acid). The process consisted of two distinct steps: in the first one, the high purity (99.9995%) aluminum foil was subjected to the first anodization for 40 min; after the selective removal of the alumina film formed on the surface of the Al anodized foil with an aqueous solution of 1.8% H2CrO4 and 6% H3PO4, the Al foil was subjected to the second anodization step for 6 h. Fe25Ni75 thin films were prepared by RF magnetron sputtering onto the surface of nanowire arrays with common base pressure below 3 × 10−5 Pa and processing Ar pressure of 0.4 Pa. The RF power was 140 W, and the time of deposition was 30 min. Moreover, the FeNi film would have to cover the top of AAO template, and the surface of the sample was conductive.
X-ray diffraction confirmed the composition of the nanowires array. Magneto-optic Kerr effect (MOKE) was used to obtain the surface magnetic properties of the composite structure. The surface topography and nanostructure were observed by scanning electron microscope (SEM). Micromagnetic simulations were performed with the three-dimensional object-oriented micromagnetic framework method. The exchange constants of the film and wires were 1.3 × 10−11 J/m and 1.75 × 10−11 J/m, respectively. The damping parameter α was 0.5, the mesh size was 5 × 5 × 5 nm3, and the saturation magnetization of permalloy film and Fe nanowires were 8.6 × 105 and 1.71 × 105 A/m, respectively. The ferromagnetic resonance was performed using an X-band spectrometer (JES-FA300; f = 9 GHz; JEOL Ltd., Tokyo, Japan). Before magnetoimpedance (MI) measurement, the samples were tailored into small pieces with length of 20 mm and width of 3 mm. An impedance analyzer (Agilent 4294A; Agilent Technologies Inc., CA, USA) was used in the four-terminal contact mode to measure the impedance (Z). All the electronic instruments were controlled using LabVIEW.
Results and discussion
It should be emphasized that not only the GMI ratio but also the magnetic response is important for high-performance sensor application. Figure 3b shows the magnetic response to the different compositions of nanowires. The sensitivity (S) of the GMI was defined as follows: S (%/Oe) = (ΔZ / Z) / ΔH, where ΔH is the change of the magnetic field. At very small external applied field, the field sensitivities of GMI effect of the nanobrush are 25%/Oe, 20%/Oe, 45%/Oe. Afterwards, it begins to decrease and display a value which is approximately equal to zero. The inset table shows the GMI ratios and sensitivities of several typical materials. The optimal structure and size is the FeNi film with 50-nm Co nanowire arrays which show a higher GMI ratio and more sensitive response than these amorphous wires, ribbon, and NiFe/Au/NiFe sandwich structure [27, 28, 29, 30, 31].
A new kind of exchange coupling effect is used to enhance the transverse moment distribution. A surprising GMI ratio and magnetic respond of heterogeneous nanobrush are found in the experiment. The diameter of the nanowires has obvious influence on the GMI characters of the nanobrush. The sample combined with 50 nm Co nanowires and Fe25Ni75 film shows the best GMI ratio and sensitivity, which could reach 270%/Oe and 45%/Oe, respectively. The phenomenon can be explained by the obvious distribution of transverse magnetic moments, which is induced by the exchange coupling effect between the interface of nanowires and film, and the exchange coupling effect can be regulated by the content of Co element of nanowire arrays. Micromagnetic stimulation shows the distribution of magnetic moments when the nanowire array act on the soft magnetic film. Out-of-plane FMR result confirms the different coupling effects of the different nanobrushes, which corresponds well with the GMI results of the nanobrushes.
JW and QL are professors of the Institute of Applied Magnetics, Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University. YZ and CM are Ph.D. students.
Sincere thanks goes to the National Basic Research Program of China (2012CB933101), National Science Fund of China (11074101, 51171075), the Fundamental Research Funds for the Central Universities (lzujbky-2012-k29 and lzujbky-2012-209), and the Program for New Century Excellent Talents in University.
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