Detection of the magnetite by giant magnetoimpedance sensor Tao Wang n , Lei Guo, Chong Lei n , Yong Zhou Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Dongchuan Road 800, Shanghai 200240, China article info Article history: Received 6 April 2015 Received in revised form 27 May 2015 Accepted 30 May 2015 Available online 2 June 2015 Keywords: Detection Magnetoimpedance Magnetite Sensor Exploration abstract Meander-line NiFe/Cu/NiFe films-based giant magnetoimpedance (GMI) sensor was employed to detect raw magnetite aiming to develop a sensitive magnetometer for the exploration of magnetite. The GMI effect was greatly decreased after placing the magnetite on the GMI sensing elements, which was probably related to the transverse magnetization process becoming strongly inhibited by the stray magnetic field. In addition, we observed an interesting phenomenon: the GMI curve on frequency de- pendency has shifted to a higher frequency when the magnetite was magnetized at the edge of the sensing elements. Significant changes in impedance can also be found when the magnetite was placed at the edge of GMI sensor without using the external magnetic field, demonstrating the possibility of using the GMI sensor for preliminary exploration of magnetic minerals. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Magnetic minerals are the important raw materials of industry. Raw magnetite, as a typical magnetic mineral, has been used in a wide variety of applications for many years, including iron and steel industry, microelectronics, nuclear physics, communication and sensing technology, and biomedical applications, etc. There- fore, the high-quality exploration of magnetite is very significant for the development of industry. Magnetite always exhibits ferri- magnetism due to the non-zero spontaneous magnetic moment in internal magnetic domain, the constituents of which are Fe 3 O 4 and some impurities (Si and C, etc.). The spontaneous magnetization in magnetite can arise a stray magnetic field that can be detected by appropriate magnetic sensors. Over the last few decades, the magnetite explorations were performed by many conventional magnetic sensors such as fluxgate magnetometer [1], proton- precession magnetometer [2], superconducting magnetometer [3], etc. However, there are still many serious shortcomings in these conventional magnetic sensors when used for detecting magnetic minerals, for example, fluxgate sensor possesses a large size, the proton-precession magnetometer can be only available for multi- points measurements, and the superconducting magnetometer requires high consumption. In recent years, magnetic sensors that are based upon the giant magnetoimpedance (GMI) effect have attracted much attention because of their ultrahigh field-sensi- tivity over the conventional magnetic sensors. The GMI effect refers to a large change in alternating current (AC) impedance of a soft magnetic conductor when subjected to an external magnetic field [4–18]. It was found that the sandwich films [19–21] could have significant GMI effect even at relatively low frequencies ( o10 MHz), in particular, the meander-line sandwich films are highly sensitive to the external magnetic field [22,23], and are very suitable for magnetic sensing applications. While most previous studies mainly focused on exploring the GMI sensor for detection of magnetic particles [24–28] and bio- sensing applications [29–31], there have been no reports on de- tection of the raw magnetite by GMI sensor so far. In this paper, two simple methods based on the GMI sensor were used to detect the magnetite. Experimental results indicated that the GMI sensor was highly sensitive to the stray magnetic fields of the magnetite. 2. Experimental details The fabrication of the meander-line GMI sensor has been re- ported elsewhere [32], in order to avoid the direct contact be- tween the magnetite and the sensing elements, a 10 mm positive photoresist was coated on the soft magnetic thin films for use as an insulating layer. The magnetite for testing was the raw ore purchased from Haoyu Stone Company, and the weight of the magnetite is about 3 g. The magnetite was characterized by Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS), and the results are shown in Fig. 1. It can be inferred from Fig. 1(a) that the magnetite possesses the irregular surface topography. EDS characterizations [Fig. 1(b) and Table 1] Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2015.05.151 0167-577X/& 2015 Elsevier B.V. All rights reserved. n Corresponding authors. Fax: þ86 21 34204843. E-mail addresses: skins@sjtu.edu.cn (T. Wang), leiqhd@sjtu.edu.cn (C. Lei). Materials Letters 158 (2015) 155–158