Multicore Off-Diagonal Magnetoimpedance Sensors Utilising Amorphous Wires

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Physics Procedia Volume 75, 2015, Pages 1419 1426 20th International Conference on Magnetism Multicore Off-Diagonal Magnetoimpedance Sensors Utilising Amorphous Wires Nikolay A.

Makhnovskiy 3, Arkady Zhukov 4 1 National University of Science and Technology (MISIS), Moscow, Russia 2 Institute for Design Problems in Microelectronics RAS, Moscow, Russia 3 The Advanced

1 Physics Procedia Volume 75, 2015, Pages th International Conference on Magnetism Multicore Off-Diagonal Magnetoimpedance Sensors Utilising Amorphous Wires Nikolay A. Yudanov 1, Alexander A. Rudenok 1, Larissa V. Panina 1,2, Alexander T. Morchenko 1, Dmitry P. Makhnovskiy 3, Arkady Zhukov 4 1 National University of Science and Technology (MISIS), Moscow, Russia 2 Institute for Design Problems in Microelectronics RAS, Moscow, Russia 3 The Advanced Composites Manufacturing Centre, Plymouth University, UK 4 Dpto. Física de Materiales, Facultad de Química, UPV/EHU, 1072, 20080, San Sebastián, Spain lpanina@plymouth.ac.uk, drlpanina@gmail.com Abstract Magnetoimpedance (MI) sensors are attractive for realising a localised detection of weak magnetic fields owing to micro-size and high sensitivity of the MI elements based on Co-rich amorphous wires of diameter microns. The MI sensors in off-diagonal configuration when the signal is taken from the coil mounted on the wire have a number of advantages including linearity, enhanced output voltage, and resonance detection circuitry. In this work, the off-diagonal impedance of a number of Co-rich glass-coated amorphous wires placed in the same coil is investigated. The wires are connected in parallel and excited by a harmonic current with optimised dc bias which is used to remove the domain structure. The best sensitivity of about 350mV/Oe is obtained for two closely spaced wires at the excitation frequency of 20 MHz which corresponds to the resonance conditions for the detection circuit having only 55 turns. The full scale and linearity range are also increased almost twice in comparison with those when only one wire is used. The response drops if the distance between the wires is increased which is due to a strong effect of the magnetostatic interaction between the wires. We conclude that the off-diagonal MI sensors with multiple wires can provide higher transfer function per one turn and could be advantageous for localised field detection. Keywords: Off-diagonal magnetoimpedance, magnetoimpedance sensor, linear sensor, amorphous magnetic wire, bias field 1 Introduction Magnetic sensors for detecting localised weak magnetic fields find a wide range of applications. There is a constant demand in enhanced sensitivity, improved signal-to-noise ratio, and smaller size. Selection and peer-review under responsibility of the Scientific Programme Committee of ICM 2015 c The Authors. Published by Elsevier B.V. doi: /j.phpro

2 [1, 2]. Magnetoimpedance (MI) sensors based on very large change in ac impedance in soft magnetic conductors may satisfy many of these requirements [3, 4]. Thus, the sensitivity of the relative impedance change in Co-based amorphous wires of micron in diameter is increased up to 600% in characteristic fields of few Oersted at MHz frequencies [5]. Two main strategies have been adopted for the development of MI sensors for particular applications. The first one is associated with tailoring the materials properties by various technological treatments to design the required MI characteristics [6,7]. In the other approach, the sensor construction is considered [8-11]. In this paper, multiwire offdiagonal MI sensor is proposed with the aim to realize a miniature sensing head with improved full scale of operation, sensitivity and signal-to-noise ratio. In general, off-diagonal MI sensors [8,12,13] when the MI wire is excited by high frequency current and the signal is taken from the coil mounted on the wire have a number of advantages. In this configuration the MI sensor is vectorial having a linear region of the output signal with respect to the sensed magnetic field. It is also possible to realise a resonance or pulsed excitation conditions with greatly enhanced sensitivity. It has been recently reported that MI sensor utilising pulse- driven Cobased amorphous wire with a detection coil of 600 turns has a resolution of 3 pt/hz 0.5 [14]. However, a many-turn detection coil may be a problem considering the complexity of sensor construction, sensor-head size, and increased electronic noise. A relatively novel approach to tailoring the MI response of magnetic wires is based on the dipole interaction between them. Placing two or more magnetic wires close to each other will change their static and dynamic magnetization due to the stray fields generated by neighboring wires. It was reported that high frequency MI and its field sensitivity could be greatly improved in a parallel array of glass-coated microwires [15, 16]. This multi-wire system was also demonstrated to have a potential for sensitive detection of biomolecules [17]. In this work, the off-diagonal impedance of a number of Co-rich glass-coated amorphous wires placed in the same coil is investigated. The wires are connected in parallel and excited by harmonic or pulsed current. The best sensitivity of about 300mV/Oe is obtained for two wires excited by a sinusoidal current and realising the resonance conditions for the detection circuit, which has only turns. The full scale and linearity range are also increased more than twice in comparison with those when only one wire is used. We conclude that off-diagonal MI sensors with multiple wires can provide higher transfer function per one turn and could be advantageous for localised field detection. Our approach is different from that developed in [18] where MI head consisted of 4 wires in individual coils connected in series, which somehow helped to reduce the noise. However, the sensor head size also increases which may be a problem for localised detection. To a certain extent, the off-diagonal MI sensor configuration is similar to that for perpendicular fluxgate [19-21]. However, the excitation and detection principles of fluxgates are different and are based on a non-linear magnetisation and generation of higher harmonics (typically a deep saturation into both polarities and the second harmonic mode are utilised). In the case of MI sensors, the response signal is generated by linear magnetisation dynamics, preferably, in a saturated state. The offset signal which influences the second harmonic mode is not a problem in this case. Multiwire core orthogonal fluxgates have been also proposed to increase the sensitivity [22, 23]. When the wires were closely packed, a nonlinear increase in sensitivity was observed, however, this was related with realising a close to resonance excitation conditions. 2 Principles of Off-diagonal MI Sensor When an ac current is flowing along a magnetic wire which has a permeability of a tensor form, an inductive voltage is generated along the wire ( ) and also in the coil ( as shown in Fig. 1. The coil voltage is induced due to the cross-magnetization processes: the circular magnetic field gives rise to both the circular magnetization (responsible for the wire voltage) and axial magnetization 1420

3 (responsible for the coil voltage). At certain conditions which include a strong skin effect and a transverse magnetic structure, both of these voltages are very sensitive to the external magnetic field applied along the wire [24-26]. This is referred to as a magnetoimpedance (MI) effect. The ratios and are known as diagonal and off-diagonal impedances, respectively. If the wire is excited by ac current only, the voltages and are defined by a diagonal component and an off-diagonal component of the surface impedance tensor, respectfully. At a mixed excitation by an ac current and ac axial magnetic field, the both components contribute to each voltage. Here we consider the current excitation excluding mixing the impedance components in the voltage response. The magnetic wire can be also excited by a high-frequency magnetic field produced by the coil mounted on the wire and the voltage signal is taken across the wire. In this case, the wire voltage is defined by the off-diagonal component of the surface impedance tensor. However, this off-diagonal configuration may not be preferable since the generated magnetic field is restricted by the coil self-inductance at high frequencies. Typically, the excitation scheme of Fig. 1b is used in off-diagonal MI sensors. In wires with a circumferential anisotropy and excited by the current the voltages and show symmetric and anti-symmetric behavior with respect to, respectfully. Therefore, the off-diagonal impedance can be used for measuring not only amplitude but also the field direction. Moreover, ( ) has almost a linear behavior in low field region which is a very important sensing characteristic. If the wire has an ideal circular domain structure, the off-diagonal MI response from domains with opposite circumferential magnetizations have opposite signs, the averaged response over domains is null. A dc bias current eliminating the domain structure should be used to enhance. A pulse current excitation of the wire supplied by C-MOS IC multivibrator or a microcontroller is typically used in off-diagonal MI sensors [27-29] which provides both high frequency and dc currents to the wire. However, a harmonic excitation with a dc current can have advantages since the resonance conditions of the detection circuit can be realized and the amplitude of the bias current can be optimized with respect to magnetic hardness in the circumferential direction [30, 31]. Figure: 1 Methods of excitation and detection for MI sensing element. A high frequency current is provided to the MI element and the output voltage is measured across the wire (a) or from a coil mounted on it (b). 3 Experimental Amorphous glass-coated wires of composition Co 66.94Fe 3.83Ni 1.44B 11.57Si 14.59Mo 1.69 were used in off-diagonal MI sensors. The metal core diameter was 40 microns and the total diameter was 46 microns. The detection coil had 55 turns of 40 micron diameter Cu wire mounted directly on the MI wire as shown in Fig. 2. The wire length was 8 mm. The wire magnetic properties were investigated by measuring the hysteresis curves by a standard inductive method. The diagonal MI in these wires was found from measuring -parameter (forward reflection) with the help of vector network analyser. For these measurements, the wire of the same length was soldered in a specially designed high frequency cell. The details of the measurement setup can be found in [8]. 1421

Conducting pads MI wires Substrate Detecting coils Figure 2: Schematics of off-diagonal MI element with multiple wires The off-diagonal response was measured with the setup shown in Fig. 3.

4 Conducting pads MI wires Substrate Detecting coils Figure 2: Schematics of off-diagonal MI element with multiple wires The off-diagonal response was measured with the setup shown in Fig. 3. The functional generator may supply to the wire a harmonic waveform of various frequencies up to 50 MHz and with various dc bias current. The detection coil with the wires is placed between Helmholtz coils which produce a magnetic field in the range of ± 60 Oe. The signal from the pickup coil is captured by digital oscilloscope. The detection coil with MI wires constitutes a resonance LC-circuit with the resonance frequency of about 20 MHz for the given parameters. The output signal has a sharp maximum at this frequency (see Fig. 4) so the resonance frequency was used in other experiments. Figure 3: Schematics of the experimental setup for off-diagonal MI 1422

5 Figure 4: Frequency characteristic of the output signal with a magnetic field of 1 Oe and a dc bias current of 7.7 ma. 4 Experimental Results and Discussion The magnetization curves in a quasi dc magnetic field applied along the wire for a single wire and two wires placed close together are shown in Fig. 5. The magnetization slope is slightly sharper for a single wire with the circumferential anisotropy of about 80 A/m (~1 Oe). The decrease in the magnetization slope and some increase in the effective anisotropy are caused by the magnetostatic interaction between the wires which depends on the applied field and is more pronounced for higher fields. The MI characteristics for the diagonal impedance are shown in Fig. 6. Both the real and imaginary parts of the impedance display two symmetrical peaks with a small hysteresis which is typical of a wire with a circumferential anisotropy. The observed hysteresis corresponds to the behavior of the M/M S H, A/m Figure 5: Hysteresis loops of single (1, black curve) and two (2, red curve) glass-coated Co66.94Fe3.83Ni1.44B11.57Si14.59Mo1.69 microwires. 1423

6 Figure 6: Magnetoimpedance characteristics for real and imaginary parts of the diagonal impedance for a frequency of 20 MHz. static magnetization shown in Fig. 5. The magnetic field of the real part peak is very close to the anisotropy value of about 1.2 Oe and it is about 2 Oe for the imaginary part peak. Therefore, the chosen magnetic wire has a magnetic anisotropy with the easy axis very close to the circumferential direction and a small value of the effective anisotropy field. These are the conditions to realize very large and sensitive magnetoimpedance with the sensitivity of more than 50%/Oe at 20 MHz in a low field region before the magnetization is set by the field along the wire. A small hysteresis seen in Figs. 5, 6 can be removed by applying a dc current generating a circular bias magnetic field. An optimal bias field was created by a dc current of 7.7 ma. This was used for offdiagonal sensor excitation. Figure 7 shows the off-diagonal voltage response for the MI element with a single wire and with two wires placed close to each other. The total excitation current in two-wire MI element is the same with that used for a single wire. The sensitivity to the field is just slightly lower for the two-wire sensor, however, the maximum output voltage in the linear regime is higher by about 45% and the field of this maximum is also increased to 1.8 Oe. The increase in the characteristic field of the voltage maximum is consistent with the magnetization loops shown in Fig. 5. The hysteresis in the output voltage behavior is not noticeable since the wire has no the domain structure due to the bias current. Figure 7: Output signal amplitude vs magnetic field of the off-diagonal MI elements with single wire and two-wire cores. The excitation voltage included a dc component of 2 V which generated a dc current in the wire of 7.7 ma. 1424

When each wire is excited by the same current and wires are far away from each other so they do not interact, the sensitivity increases linearly with the number of wires but the characteristic field

7 When each wire is excited by the same current and wires are far away from each other so they do not interact, the sensitivity increases linearly with the number of wires but the characteristic field does not change. If the interaction between the wires is essential, the sensitivity considerably drops if more than two wires are used. Therefore, in terms of power consumption, sensitivity and full scale the MI element consisting of two closely spaced wires seems to be optimal. Figure 8 compares the response of the two-wire MI core when no dc bias is applied and in the presence of dc bias (positive and negative). It is seen that without a bias and in the low- field region the sensor response is almost zero. However, at higher fields the sensitivity increases and is almost the same as that obtained with the help of the dc bias. This behavior can be explained by the influence of the magnetostatic interaction between the wires which increases with the field. The effect of positive and negative bias is almost the same as it should be in the case of the circumferential anisotropy. Figure 8: Output signal amplitude vs magnetic field of the two-wire core MI sensor when no dc bias is applied (green curve), positive bias (+Idc, blue curve) and negative bias (-Idc, red curve). Conclusion The off-diagonal impedance from two-wire core utilising Co-based glass-coated amorphous wires with large magnetoimpedance effect could be advantageous in comparison with the response from a single wire core. Two wires connected in parallel and excited by a harmonic current with a frequency corresponding to the resonance frequency of the detection circuit give an increase in the output voltage by 45% and the saturation field increases almost twice improving the linearity range in comparison with the characteristics of a single wire MI. The response drops if the distance between the wires is increased and the magnetostatic interaction is not essential. The maximum sensitivity for two wire sensor is about 380mV/Oe within the field interval ± 1.8 Oe. Acknowledgements This work was supported by the Russian Federation State contract for organizing a scientific work and partially supported by the Russian Foundation for Basic Rresearch, grant The authors are very thankful to Dr. V. Larin, MFTI Ltd for donating the wire samples. References [1] D. Robbes, C. Dolabdjian, S. Saez, Y. Monfort, G. Kaiser, P. Ciureanu, (2001) IEEE Transaction on Applied Superconductivity, 11,

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The Development of a High Sensitive Micro Size Magnetic Sensor Named as GSR Sensor Excited by GHz Pulse Current

1 The Development of a High Sensitive Micro Size Magnetic Sensor Named as GSR Sensor Excited by GHz Pulse Current Y. Honkura 1 and S. Honkura 2 1 Magnedesign Corporation, Nagoya, Japan 2 Nanocoil Incorporation,