Research articles Asymmetric giant magnetoimpedance effect created by micro magnets S. Atalay a,⇑ , T. Izgi a , N.A. Buznikov b , V.S. Kolat a a Inonu University, Science Faculty, Physics Department, Malatya, Turkey b Scientific and Research Institute of Natural Gases and Gas Technologies – Gazprom VNIIGAZ, Razvilka, Leninsky District, Moscow Region 142717, Russia article info Article history: Received 24 November 2017 Received in revised form 12 January 2018 Accepted 14 January 2018 Keywords: Asymmetry Magnetoimpedance Amorphous ribbon abstract Asymmetric giant magnetoimpedance (AGMI) effect has been investigated in as-prepared and current annealed amorphous (Co 0.9 Fe 0.05 Ni 0.05 ) 75 Si 15 B 10 ribbons. Asymmetry was created by micro magnets. Different numbers of magnets were used and it was found that increasing number of magnet, the shift in AGMI curves increases. When two micro magnets were placed 1 cm away from the ends of ribbon, a distortion in two peak shape of the GMI curve was observed. At high frequency range, a linear change in the AGMI was observed for the current annealed sample. Ó 2018 Elsevier B.V. All rights reserved. 1. Introduction When a magnetic conductor carrying a high-frequency alternat- ing current is subjected to an external magnetic field, the impe- dance of the material changes. The phenomenon has been discovered many years ago for iron-nickel wires [1]. The effect has been attracted much attention when advanced technologies of the production of conducting soft magnetic materials with a high permeability have appeared [2–4]. Due to large change in the impedance in soft magnetic conductors this effect has been referred to as the giant magnetoimpedance (GMI) [3–5]. GMI sen- sors can be used in many applications such as magnetic field, stress, torque, biosensor, current, force [6–14]. GMI properties of samples with various compositions have been investigated in the ribbon, wire, film, magnetic tube and multilayer film forms (see, for example, [5,15] and references therein). The GMI can be explained by the skin effect, which is related to the permeability of the magnetic material. The skin depth, d, for a ferromagnetic conductor can be written as d =(q/pfmm 0 ) 1/2 [16], where q is the electric resistivity, f is current frequency, m 0 is the permeability of free space and l is the permeability. In magnetic materials, l, so d depend on the frequency and amplitude of ac cur- rent and the external magnetic field. The strong dependence of l on the external magnetic field in soft magnetic materials leads to the GMI effect. Different measurement and annealing methods have been applied to ferromagnetic materials with different composition and shapes to improve the magnitude of GMI effect. One of the main problems in the GMI effect is that magnetic materials show generally same dependence on positive and negative magnetic fields values and the impedance changes gradually around zero magnetic fields. In practise, a linear and sharp change in output as a function of magnetic field is desired. Asymmetric GMI effect could be one of the possible solutions to design the linear GMI sen- sors. Basically, the asymmetric giant magnetoimpedance effect (AGMI) can simply be defined as the distortions of symmetry in GMI curves. There are several types of the AGMI [5]. The first one has been observed when Co-based amorphous wire was twisted and the DC bias and AC driving currents were applied to the wire together [17]. It has been shown that without bias current a symmetric double-peak GMI curve can be observed. When the DC bias current increases, one peak enhances and the other diminishes, depending on the DC bias current orientation [18]. The origin of this type of the asymmetry has been attributed to the combination of helical magnetic anisotropy with circumferential DC field produced by the bias current. Effects of bias current and torsional stresses on the AGMI have been reported in many other studies [19–24]. Another method of producing the AGMI consists in applying an axial AC bias field to a sample [25]. In this case, the asymmetry results from the mixing of the diagonal and off-diagonal compo- nents of the impedance tensor due to the AC cross-magnetization process. However, large power consumption is required in these methods [5]. In this connection, other methods to obtain the AGMI have been studied. The third type of the AGMI has been observed in Co-based amorphous ribbons annealed in the presence of a weak magnetic field [26,27]. It has been observed that the GMI peak in the region where the applied field is antiparallel to the annealing field https://doi.org/10.1016/j.jmmm.2018.01.034 0304-8853/Ó 2018 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: selcuk.atalay@inonu.edu.tr (S. Atalay). Journal of Magnetism and Magnetic Materials 453 (2018) 163–167 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm