Figure 1. Schematic picture of surface helical domain structure.

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1 Advances in Science and Technology Vol. 93 (2014) pp Submitted: (2014) Trans Tech Publications, Switzerland Accepted: doi: / Influence of Magnetic Field of Super High Frequency on Hysteretic Properties of Soft Magnetic Microwires Alexander Chizhik 1, a *, Julian Gonzalez 1,b, Arcady Zhukov 1,2,c, Andrzej Stupakiewicz 3,d, Andrzej Maziewski 3,e 1 Universidad del Pais Vasco, UPV/EHU, San Sebastian, Spain 2 IKERBASQUE, Bilbao, Spain 3 Laboratory of Magnetism, University of Bialystok, Bialystok, Poland a oleksandr.chyzhyk@ehu.es, b julianmaria.gonzalez@ehu.es, c arkadi.joukov@ehu.es, d and@uwb.edu.pl, e magnet@uwb.edu.pl Keywords: Amorphous magnetic wire, Hysteresis, Magnetooptic Kerr effect Abstract. The influence of super high frequency (SHF) circular magnetic field on magnetization reversal in the Co-rich glass covered microwire has been investigated. The study has been performed by magneto-optical Kerr effect (MOKE) technique. It was found that the presence of the SHF field causes the change of the re-magnetization mechanism the rotation of the magnetization is observed instead of domain walls motion. Also the hysteresis loop has an asymmetric shape that confirms the co-existence of the stable and meta-stable helical magnetic states in the surface of microwires. Introduction The optimization of main parameters of sensors based on giant magnetoimpedance (GMI) effect [1] is an actual task of modern electronics. Glass covered microwires is the basic element of this type of magnetic sensors and during last years we directed our efforts to elucidation of fundamental mechanisms of magnetization reversal and domain structure formation in these microwires [2]. Now we understand that the magnetic structure consist of helically magnetized domains (Fig. 1). There are two possible types of this structure: serpentine or elliptic. The present paper is devoted to the study of the magnetization reversal in the presence of electric current of high and super-high frequency. The investigations have been performed using the magneto-optical Kerr effect technique which demonstrated very high efficiency in the study of the surface magnetic properties of cylindrically shaped samples. Experimental Details Figure 1. Schematic picture of surface helical domain structure. The longitudinal impedance was measured with a vector network analyzer (VNA) through the reflection coefficient [3]. The microwire was soldered in microstrip cell (Fig. 2). One wire end was connected to the inner conductor of a coaxial line through a matched microstrip line while the other was connected to the ground plane. The sample holder was placed inside a sufficiently long solenoid that creates a homogeneous magnetic field in the sample. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /10/14,15:17:34)

2 204 6th Forum on New Materials - Part A Figure. 2 Sample cell (a). Reflection of the light from the microeire (b). The external magnetic field was the sum of axial magnetic field (H AX ) (up to 200 Oe) and super high frequency circular magnetic field (H CIR ) (up to 20 ma). To obtain the surface hysteresis loops MOKE polarizing microscopy was used. First, surface domain imaging was performed by means of MOKE microscope. Magnetic domains are observed because of different in-plane components of the surface magnetization. This difference transforms to black-white contrast when the polarized light reflects from the top of the cylindrically shaped surface of the microwire. Second, the histeresis loops were obtained from the magneto-optic intensity as a result of the MOKE images processing. We studied glass-coated microwire with the nominal composition Co 67 Fe 3.85 Ni 1.45 B 11.5 Si 14.5 Mo 1.7 (metallic nucleus radius is 11.2 µm) supplied by Tamag Ibérica. Experimental Results and Discussion Figure 3. GMI curves.

3 Advances in Science and Technology Vol Fig. 3 demonstrates the dependences of the real part impedance on external magnetic field. According to the classical interpretation, at moderate frequency of 100 MHz the maximum of GMI curve is related to anisotropy filed H k of the sample that can be estimated as 210 A/m. Below H k the impedance is determined by domain walls motion and the rotation of the magnetization. Above H k the surface layer of microwire is saturated and the impedance decreases as the transverse susceptibility decreases. At higher frequencies (>1GHz), the impedance becomes dominated by ferromagnetic resonance with maxima displacing to the high field with increasing frequencies. Figure 4. MOKE hysteresis sloops obtained in the presence of 1MHz electric current. Fig. 4 shows the hysteresis loops obtained in the axial magnetic field and in the presence HF circular magnetic field of 1 MHz. The HF circular magnetic field causes the transformation of the hysteresis loop. It means the strong transformation of the magnetization reversal process. The hysteresis consists of the rotation of the magnetization and the surface domain structure transformation when the HF electric current is small (2mA). Figure 5. Hystereis loops obtained in the presence of electric current of 2 GHz frequency and amplitude (a) 2 ma; (b) 20 ma.

4 206 6th Forum on New Materials - Part A The asymmetric transformation of hysteresis loop takes place for higher amplitude of HF electric current (10 ma). The following increase of the amplitude of electric current (20 ma) induces the change of the mechanism of magnetization reversal. The pure rotation of the magnetization dominates. The HF circular field induces the vibration of the surface magnetization difference from DC circular field. In the presence of axial magnetic field this vibration causes the successive inclination of magnetization towards the circular direction. In such a way the HF field stabilizes the inclined meta-stable helical structure. The degree of the inclination depends on the value of the frequency. Figure 6. The MOKE hysteresis loops obtained in the presence of SHF electric current of 20 ma amplitude and frequency band of 0.1 GHz 3 GHz. Fig. 5 presents the MOKE hysteresis loops obtained in presence of electric current of 2 GHz frequency range. The observed local laydown could be related to the effect of the local domain disappearance. The helical magnetic states are very sensitive to SHF circular field. The increase of circular field results in competition between meta-stable helical structures. Also SHF circular field makes preferable the DW propagation or domain nucleation. The reason of the observed effects is determined mainly by the co-existence of different helical states in the microwire. This co-existence is realized in the domain structure in which the angle of the DW changes the direction in the length of the wire. The DWs which divides different helical states have different inclination angles. Fig. 6 presents the MOKE hysteresis loops obtained in the presence of SHF electric current of 20 ma amplitude and frequency band of 0.1 GHz 3 GHz. Fig. 6(a) shows the hysteresis loops for 0.1 GHz and 1 GHz. For such high value of electric current the shape of the curves is interpreted in the following way: one of the surface circular domains is in an advantageous state in the presence of SHF circular magnetic field. The curve demonstrates the rotation of the magnetization in this advantageous domain. The circular field of 1 GHz induces higher inclination of the circular magnetization toward the circular direction. It is the reason of the observed acceleration of the rotation of the magnetization. For the frequency of 2 and 3 GHz (Fig. 6(b)) the pure rotation of the magnetization is also observed, but the direction of the rotation differs from the case of 1 GHz: now the other circular domain is the advantageous one. This effect could be explained in supposition of the existence of small initial helicality of surface magnetic structure. The increase of the amplitude of the vibration of the magnetization is induced by SHF field. When the vibrating circular magnetization overcomes

5 Advances in Science and Technology Vol the axial direction of the microwire the meta-stable advantageous domain change the sign to the opposite one. Generally the effect observed in this frequency band could be interpreted in the frame of low field absorption and splitting between giant magnetoimpedance and ferromagnetic resonance. Conclusions The process of surface magnetization reversal was studied in glass covered microwires in the presence of electric current of MHz-GHz range. Now it is clear that the circular magnetic field affects the magnetization reversal even at SHF range. The real mechanism of the magnetization reversal depends on the amplitude of the SHF electric current. The circular magnetic field induces the meta-stable inclined helical state. The frequency and the amplitude of the SHF field determine the sign and the degree of the helicality of the meta-stable states. References [1] A. Zhukov, V. Zhukova, Magnetic Properties and Applications of Ferromagnetic Microwires with Amorphous and Nanocrystalline Structure, Nova Science Publishers, New York, [2] A. Chizhik, J. Gonzalez, Magnetic Microwires. A Magneto-Optical Study, Pan Stanford Publishing, Singapore, [3] M. Ipatov, A. Chizhik, V. Zhukova, J. Gonzalez, A. Zhukov, Correlation of surface domain structure and magneto-impedance in amorphous microwires, Journal of Applied Physics 109 (2011)