DSpace at VNU: Magnetoimpedance-Based Probe of Various Concentrations of Corrosive Chemicals

IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 4004404 Magnetoimpedance-Based Probe of Various Concentrations of Corrosive Chemicals Jagannath Devkota1, Ngo Thu Huong2, Hariharan Srikanth1 , and Manh-Huong Phan1 Department Faculty of Physics, University of South Florida, Tampa, FL 33620 USA of Physics, Hanoi University of Science, Hanoi 10000, Vietnam We report on the possibility of exploiting the giant magnetoimpedance (GMI) effect of a soft ferromagnetic amorphous ribbon as a magnetic label-free probe of corrosive strengths of chemicals Scanning electron microscopy, dc magnetization, and GMI measurements reveal that the presence of nitric acid (HNO3 ) significantly alters the surface morphology, the magnetic permeability, and consequently the GMI response of a Co65 Fe4 Ni2 Si15 B14 amorphous ribbon With an increase in the exposure time, the GMI ratio sharply decreases in the first few minutes and then remains almost unchanged The relative change in the GMI ratio depends strongly on the corrosive strength of the chemical used (i.e., the concentration of HNO3 ) These findings are of practical importance in developing GMI-based detectors of corrosive chemicals Index Terms— Amorphous ribbons, corrosive chemicals, giant magnetoimpedance (GMI), label-free chemical detection I I NTRODUCTION T HE GIANT magnetoimpedance (GMI) effect has been a subject of extensive research for its technological importance in magnetic sensor applications [1]–[3] GMI is a large change in the ac impedance of a soft ferromagnetic material subject to an external dc magnetic field The impedance of a ribbon-shaped material is expressed as Z = RDC ikt coth (ikt) (1) where Rdc is the dc resistance and t is half of the thickness of the ribbon i is the imaginary unit and k = (1 + i )/δ is a parameter that relates the impedance (Z ) to the skin depth (δ), which is calculated by [1] δ= ρ 2π f μT (2) Here, ρ and μT are the resistivity and transverse permeability of the ribbon and f is the frequency of an ac current In the high frequency range ( f > ∼1 MHz), the skin effect is strong enough to confine the ac current to a sheath close to the surface of the ribbon (δ t) Thus, GMI is very sensitive to changes in near-surface magnetic signals [4], [5] Based on this phenomenon, the GMI effect of a soft ferromagnetic amorphous ribbon has recently been exploited in detecting weak magnetic fields arising in magnetic nanoparticle based biomarkers [6]–[8] This biodetection technique uses the perturbation caused by the fringe field of the magnetic biomarkers to the effective permeability of the ribbon to sense their presence while retaining the surface of the ribbon [3] A variety of techniques, such as chemical corrosion [9]–[11], coatings [4], [5], [12], and annealing [13], have been reported to tailor the ribbon surface and hence their GMI response The modification of the magnetoimpedance is a consequence Manuscript received November 20, 2013; accepted February 6, 2014 Date of current version June 6, 2014 Corresponding author: M.-H Phan (e-mail: phanm@usf.edu) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2014.2305910 of changes in several parameters that determine the magnetic permeability including flux closure, magnetic anisotropy, and the geometry of the ribbon Since GMI materials usually suffer degradation when exposed to corrosive chemicals, Kurlyandskaya et al [9]–[11] proposed that changes in the GMI signal of a ribbon due to corrosive chemicals could be used to develop a new class of reliable and disposable sensors for magnetic label-free detection of corrosive chemicals of various strengths However, the research in this field is still in its infancy and a deeper understanding of the correlation between the surface morphology, magnetic softness, and GMI response of a magnetic ribbon exposed to corrosive chemicals is needed to explore this technique fully In this paper, we report the detection of various concentrations of nitric acid using a label-free method based on the GMI effect of a Co65 Fe4 Ni2 Si15 B14 amorphous ribbon We found that when the ribbon was etched with nitric acid of various concentrations, the surface irregularities and the pore sizes were significantly altered Such surface modifications had a considerable impact on the magnetic permeability and hence the GMI response of the ribbon II E XPERIMENT Co65 Fe4 Ni2 Si15 B14 amorphous ribbons (Metglas 2714A) with a thickness of 15 μm were used as the GMI sensing elements Nitric acid (HNO3 ) with an original concentration of 70% by volume was used as a corrosive chemical Scanning electron microscopy (SEM) was used to characterize the surface morphology of ribbons before and after acid treatment Magnetic measurements were performed using a vibrating sample magnetometer These measurements were performed at room temperature on identical ribbon pieces, which were etched by μl of nitric acid of three different concentrations (4%, 8%, and 17% by volume) for 30 min, and then washed with water and ethanol Magnetoimpedance measurements were conducted along the length of the ribbon with dimensions 10 mm × mm by the four point measurement technique using a HP4192A 0018-9464 © 2014 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information 4004404 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 Fig Room temperature M-H curves of the PR and ribbons treated with various concentrations of nitric acid (ER-I, ER-II, and ER-III) Fig SEM images of (a) PR and ribbons etched with (b) %/vol., (c) 8%/vol., and (d) 17%/vol nitric acid impedance analyzer, with an axial ac current of mA over the frequency range 0.1–13 MHz in the presence of axial dc fields up to ±120 Oe The etching effect on GMI was measured by drop-casting an equal volume (5 μl) of various concentrations of nitric acid in a central area of the ribbon surface The GMI ratio for a given measurement frequency was calculated as Z (H ) − Z (Hmax) Z = × 100% Z Z (Hmax ) (3) where Z (H ) is the impedance measured in the presence of an applied field H The sensor sensitivity (η) was defined as the difference in the maximum GMI ratio [ Z /Z ]max (or [MI]max ) between a plain ribbon (PR) and an etched ribbon (ER) as η = [MI]max,PR − [MI]max,ER (4) III R ESULTS AND D ISCUSSION Fig shows the SEM images of the surface morphology of (a) a PR and (b)–(d) ribbons etched with nitric acid of concentration ∼4% (ER-I), ∼8% (ER-II), and ∼17% (ER-III), respectively It can be observed that the surface morphology of the ribbon was significantly modified when etched with nitric acid The roughness of the ribbon increased with increasing the concentration of the acid Nano/microsized pores are also observed on the ERs The density and sizes of these pores were found to increase with increasing acid concentration The average pore sizes were estimated to be about 500 nm, μm, and μm for ER-I, ER-II, and ER-III, respectively The increases in the surface roughness (Rq ), the density and size of pores in the ERs are shown below to significantly affect the magnetic properties and hence the GMI response of the ribbon Fig shows the magnetic hysteresis (M–H ) curves of the plain and etched ribbon samples taken at room temperature Fig (a) 3-D GMI profile of a PR and (b) frequency dependence of the maximum GMI ratio ([ Z /Z ]max ) for plain and ∼8% acid-treated ribbons Inset of (b): enlarged portion of the [ Z /Z ]max versus f data As one can see clearly in this figure, the M–H curves show negligible hysteresis (coercive field, Hc ∼1 Oe), and the shape of these curves is almost identical for all the plain and ERs, indicating no obvious effect of the chemical etching on the DEVKOTA et al.: MAGNETOIMPEDANCE-BASED PROBE OF VARIOUS CONCENTRATIONS OF CORROSIVE CHEMICALS 4004404 Fig (a)–(c) Magnetic field dependence of the GMI ratio ( Z /Z ) for as-cast and acid-treated ribbons of various concentrations at MHz (d) Relative change in the maximum GMI ratio of an acid-treated ribbon with reference to its as-cast counterpart as a function of exposure time for different concentrations magnetic coercivity and anisotropy However, a remarkable decrease in the saturation magnetization (Ms ) was observed in the ERs relative to their plain counterparts, and the Ms was found to decrease considerably as the concentration of the nitric acid was increased (Fig 2) The values of Ms were determined to be about 53.31, 48.37, 37.26, and 26.52 emu/g for PR, ER-I, ER-II, and ER-III, respectively This can be understood by considering the fact that when a ribbon was treated with a stronger acid, a larger amount of magnetic mass was removed during the etching process, thus increasing the surface irregularity (Fig 1) and decreasing the Ms of the ribbon (Fig 2) Since the GMI behavior of a ribbon depends primarily on the magnetic softness and surface conditions [14], the corresponding increase and decrease in Rq and Ms are expected to decrease the GMI ratio in ERs Fig 3(a) shows the 3-D magnetic field and frequency dependences of GMI ratio ( Z /Z ) for a PR sample The GMI curves show a single-peak behavior at low frequencies ( f < MHz) and a double-peak behavior at high frequencies ( f > MHz) The single- and double-peak behaviors of the GMI profile at low and high frequencies are attributed to the presence of longitudinal and transverse anisotropies, respectively [1] The maximum GMI ratio ([ Z /Z ]max ) was also observed to increase with increasing frequency, reach a maximum at f ∼ MHz, and then decrease slowly for higher frequencies This dependence can be understood by considering the relative contributions of inductance, domain wall motion, and spin rotation to the impedance in different frequency ranges [1], [8] It has been reported that inductive voltage has a dominant contribution to the impedance at very low frequencies (