Giant magnetoimpedance materials: Fundamentals and applications

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Abstract

Since the discovery of the magnetoimpedance (MI) effect just over a decade ago, international research interest into the giant magnetoimpedance (GMI) effect has been growing. This article aims to provide a comprehensive summary of the GMI topic, encompassing fundamental understanding of the GMI phenomena, the processing and properties of GMI materials and the design and application of GMI-based magnetic sensors. The paper starts with the definition of GMI and an assessment of the current theoretical understanding of the frequency dependence of GMI. Then a detailed description of processing methods for the production of amorphous and nanocrystalline GMI materials in the form of wires, ribbons and thin films is given, with an examination of the advantages and disadvantages of each technique. Properties of existing GMI materials including magnetic, mechanical, electrical and chemical properties are described, and a correlation between domain structures and magnetic properties is established. The influences of measuring and processing parameters on the GMI effect are systematically analysed and the underlying physical origins of hysteretic and asymmetric phenomena of GMI are explained. This enables the selection of optimal conditions to design high-performance GMI-based sensors. After discussing the material selection criteria, a range of candidate materials are evaluated and nominated for the design of GMI-based sensors. Finally, a variety of potential applications of GMI-based magnetic sensors are presented with an outlook of future research development in this field.

Introduction

Magnetic sensors play an essential role in modern technology. They are widely used in nearly all engineering and industrial sectors, such as high-density magnetic recording, navigation, military and security, target detection and tracking, antitheft systems, non-destructive testing, magnetic marking and labelling, geomagnetic measurements, space research, measurements of magnetic fields onboard spacecraft and biomagnetic measurements in the human body [1], [2], [3].

A wide range of magnetic sensors, such as induction sensors, fluxgate sensors, Hall-effect magnetic sensors, magneto-optical sensors, giant magnetoresistive (GMR) sensors, resonance magnetometers, and superconducting quantum interference device (SQUID) gradiometers, are now available [3]. A magnetic sensor directly converts the magnetic field into a voltage or resistance with, at most, a dc current supply, and the field sensitivity of a magnetic sensor plays a key role in determining its operating regime and potential applications. For instance, SQUID gradiometers with a high sensitivity of 10−10–10−4 Oe have been used for measuring field gradients or differences due to permanent dipole magnets in major applications of brain function mapping and magnetic anomaly detection. Induction, fluxgate and GMR sensors with a medium sensitivity of 10−6–102 Oe have been used for measuring perturbations in the magnitudes and/or direction of Earth’s field due to induced or permanent dipoles in major applications of magnetic compasses, munitions fuzing and mineral prospecting. Hall-effect sensors with a low sensitivity of 1–106 Oe have been used for applications of non-contact switching, magnetic memory readout and current measurements. In addition to the sensitivity requirement, other factors affecting the practical uses of magnetic sensors include the processing cost and power consumption. When comparing the processing cost and power consumption of existing magnetic sensors, the GMR sensor shows the lowest cost and power consumption. However, the field sensitivity of the GMR sensor is rather low (∼1%/Oe).

Recently, the development of high-performance magnetic sensors has benefited from the discovery of a new magnetic phenomenon, giant magnetoimpedance (GMI) (i.e., a large change in the ac impedance of a magnetic conductor with an ac current when subjected to an applied dc magnetic field), in metal-based amorphous alloys [4], [5]. It has been demonstrated that magnetic sensors based upon the giant magnetoimpedance (GMI) effect offer several advantages over conventional magnetic sensors. The decisive factor is the ultra-high sensitivity of GMI sensors. When compared with a GMR sensor that has a sensitivity of ∼1%/Oe, the field sensitivity of a typical GMI sensor can reach a value as high as 500%/Oe [3]. Though the development of GMI sensors is still at an early stage, it is likely that their low prices and high flexibility will warrant wide-ranging application in the near future.

Historically, GMI has attracted particular interest in the scientific community only since Panina and Mohri for the first time announced their discovery of the so-called GMI effect in Co-based amorphous wires in 1994 [4]. In actual ferromagnetic materials, the maximum value of GMI effect experimentally obtained to date is much smaller than the theoretically predicted value [3]. Consequently, the research in this field has been focused mainly on special thermal treatments and/or on the development of new materials for properties improvement [6], [7], [8], [9], [10], [11], [12]. In order to design and produce novel GMI sensors, a thorough understanding of the GMI phenomena and the properties of GMI materials with an emphasis on how a magnetic sensor utilising the GMI effect can be best designed for technological applications is indispensable.

The present paper serves such a purpose and aims to provide a systematic and comprehensive analysis of the fundamental aspects of GMI and its potential applications. Section 1 provides a definition of GMI before the theoretical models developed for explaining the frequency dependence of GMI are examined in Section 2. Sections 3 Processing techniques for the production of GMI materials, 4 Domain structure and properties of GMI materials review the processing techniques, properties and domain structures of GMI materials, and a correlation between the domain structures and magnetic properties is established. Analyses of the effects of measuring and processing parameters on GMI are provided in Sections 5 Influence of measuring parameters on GMI, 6 Influence of processing parameters on GMI, and the underlying physical origins of hysteretic and asymmetric GMI phenomena are examined in Section 7. The materials selection criteria for the design of GMI sensors are discussed in Section 8. Finally, the authors summarise GMI sensors and their applications with an outlook of future research and development in this field.

Section snippets

What is “giant magnetoimpedance – GMI”?

When a soft ferromagnetic conductor is subjected to a small alternating current (ac), a large change in the ac complex impedance of the conductor can be achieved upon applying a magnetic field (see Fig. 1a). This is known as the giant magnetoimpedance (GMI) effect. A typical example of the GMI effect is displayed in Fig. 1b.

The relative change of the impedance (Z) with applied field (H), which is defined as the giant magneto-impedance (GMI) effect, is expressed byΔZ/Z(%)=100%×Z(H)-Z(Hmax)Z(Hmax)

Melt spinning

Amorphous metallic alloys can be produced by a variety of rapid solidification processing techniques, including splat quenching, melt spinning, gas atomisation and condensation from the gas phase. Among the existing techniques, the melt spinning technique has been most widely used to produce amorphous metallic alloys at cooling rates of 104–106 K/s [42]. Metallic amorphous wires are also prepared by the melt spinning method that has been used to yield amorphous ribbons [43], [44]. Diameters of

Domain structure

Different domain structures are observed in different types of materials. The domain structure of a rapidly quenched material is often determined by coupling between magnetostriction and internal stresses frozen in during the fabrication process. Such knowledge of the domain structure of an actual material is extremely important in controlling and tailoring the magnetic properties of the material. This section is devoted to describing the formation of the domain structures in materials (e.g.,

Alternating current

It is generally accepted that the circular (or transverse) magnetisation process takes place due to the ac circular magnetic field created by an alternating current (ac). Meanwhile, the change in impedance is directly related to this magnetisation process [4], [5], [6], [7], [8], [9], [10]. Therefore, a dependence of the impedance on the ac exists, according to Eq. (2.2). The higher the amplitude of the ac (Im), the larger the impedance (Z) obtained. It has been theoretically predicted that for

Effect of glass coating on GMI

The influence of glass coating on the domain structure and magnetic properties of amorphous wires have been discussed in Refs. [6], [7], [10], [60]. This section will focus on describing the influence of glass coating on the GMI profile in amorphous and nanocrystalline wires. The effects of removing the glass coating on the GMI profile are also discussed.

Hysteresis in GMI

A typical example involving the hysteretic feature in GMI profiles with respect to increasing and decreasing applied dc magnetic field is displayed in Fig. 35. A two-peak behavior with a dip near zero field was observed at frequencies f  1 MHz. With increasing frequency, the dip η(%) and hysteresis in the GMI profiles first increased up to f = 5 MHz and then slightly decreased at higher frequencies (see the inset of Fig. 35). The increase in the dip reflects an increase of anisotropy in the

Criteria for selecting GMI materials

For a GMI material to be employed for GMI sensor applications, two main requirements should be met, namely, a high GMI ratio (or a large GMI effect) and a high sensitivity to the applied field (or a high magnetic response). In view of the theoretical analyses and experimental results, it is concluded that a large GMI effect should exist in magnetic materials having:

  • low resistivity, ρ,

  • high magnetic permeability, μ,

  • high saturation magnetisation, Ms, and

  • small ferromagnetic relaxation parameter (or

Types of GMI sensors

Since GMI changes as a function of external dc magnetic field or applied dc/ac current, it is possible to design GMI-based sensors that can measure either magnetic fields or dc/ac currents. GMI is also sensitive to applied stress, and this provides a new opportunity for developing stress sensors. These sensors will be briefly described and evaluated below.

Concluding remarks and future perspectives

The present paper provides the state-of-the-art of the processing-domain structure-magnetic properties of GMI materials and the development of high-performance GMI sensors. The concluding remarks are as follow:

  • (i)

    Existing theoretical models of GMI are developed for specific materials and/or certain frequency ranges. None of them can explain all GMI features in a wide frequency range (from a few kHz up to GHz). It is believed that, to assess the underlying mechanisms of GMI for a variety of

Acknowledgements

The authors wish to acknowledge their collaborators: Prof. Seong-Cho Yu (Chungbuk National University, South Korea), Prof. Cheol Gi Kim (Research Center for Advanced Magnetic Materials, South Korea), Prof. Heebok Lee and Dr. Le Anh Tuan (Kongju National University, South Korea), Prof. Nguyen Chau and Mr. Nguyen Duc Tho (Center for Materials Science, Vietnam) Prof. Nguyen Hoang Nghi (Hanoi University of Technology) and Prof. Manuel Vázquez (Institute of Materials Science, Spain). The authors

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