Comparative study of magnetic, microwave properties and giant magnetoimpedance of FeNi-based multilayers with different structure

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Highlights

  • FeNi/Cu/FeNi and [FeNi/Cu]4/FeNi/Cu/[FeNi/Cu]4/FeNi multilayers were studied.

  • The presence of Cu sub-layers increases the total thickness.

  • The presence of Cu sub-layers results in the mixed interfaces formation.

  • The presence of Cu sub-layers insures stress relaxation and magnetic flux closing.

  • Despite the magnetic inhomogeneities increase MI is adequate for applications.

Abstract

FeNi(100 nm)/Cu(500 nm)/FeNi(100 nm) and [FeNi(100 nm)/Cu(3 nm)]4/FeNi/Cu(500 nm)/[FeNi(100 nm)/Cu(3 nm)]4/FeNi(100 nm) multilayers in the shape of elongated stripes were prepared by rf-sputtering. Magnetization curves were measured by vibrating sample magnetometry. Magnetic domains were studied by the Bitter technique. Magnetoimpedance (MI) was studied as a function of the external magnetic field (H) for a frequency range of 1–300 MHz. Ferromagnetic resonance was measured as a function of the applied field for a 8.85 GHz frequency. Maximum MI sensitivities for total impedance at 75 MHz of 1.1 Ω/Oe (at 3.0 Oe) for [FeNi/Cu]4/FeNi/Cu/[FeNi/Cu]4/FeNi and 0.8 Ω/Oe (at 6 Oe) for FeNi/Cu/FeNi were obtained. Despite the increase of the magnetic inhomogeneities related to small contribution of mixed interfaces in the [FeNi/Cu]4/FeNi/Cu/[FeNi/Cu]4/FeNi structure, its MI response is more adequate for applications.

Introduction

Magnetic, microwave properties and giant magnetoimpedance (MI) of magnetic multilayers are hot topics of basic research and technological applications [1]. High frequency characterization like ferromagnetic resonance (FMR) and MI play a more and more important role when complete characterization is requested [2], [3]. It is also important to mention that precise MI measurements of thin films and multilayered structures request a rather complex procedure. At the same time, rapid characterization of MI materials would be a great advantage. As both FMR and MI are magnetic phenomena in which a dynamic magnetic permeability is involved, a number of attempts were made to use conventional FMR measurements for MI materials characterization [3], [4]. Despite years of experimental and theoretical efforts [5], [6] for MI multilayers, the maximum theoretical value of MI is still much higher than the obtained experimental results [7]. In recent years visible progress was made in the enhancement of both MI value and MI sensitivity of thin film based structures. The strategy of employing multilayers instead of thin films was successful [8], [9]. There were many comparative studies of MI structures with open and closed magnetic flux [6], [10]. In the case of the MI sandwich with opened magnetic flux central conductor has the same width as the width of the magnetic layers. Keeping in mind that magnetic properties of permalloy films depend on the film thickness due to the existence of a transition into a “transcritical” state [9], magnetically soft FeNi-based multilayers were designed with the total thickness of magnetic layers being much higher than the critical thickness of the transition into “transcritical” state [8], [10]. At the same time the direct comparison of the magnetic properties and MI for ferromagnet/conductor/ferromagnet (F/C/F) structures in the case when the ferromagnetic layers are either a thin film or a multilayer is still absent.

In this work magnetic, microwave properties, magnetic domain structure, MI and FMR were comparatively analyzed for F/C/F MI sandwiches of the same geometry with opened magnetic flux (where F is either a single FeNi layer, or a [FeNi/Cu]4/FeNi multilayer, both prepared in the same conditions.

Section snippets

Experimental

F/C/F MI sandwiches were deposited onto glass substrates by ion-plasma sputtering in an Ar atmosphere at a working pressure of 103 Torr, providing FeNi films of the highest quality [10]. The preliminary vacuum was 2·106 Torr. Ferromagnetic layers were either FeNi(130 nm) film (S-I) or [FeNi(100 nm)/Cu(3 nm)]4/FeNi(100 nm) multilayer (S-II). The thickness of permalloy for FeNi/Cu/FeNi sandwich was on purpose slightly higher than the FeNi layer thickness of 100 nm in [FeNi/Cu]4/FeNi multilayer, but

Results and discussion

Fig. 1 shows the VSM hysteresis loops for both multilayered structures. The shape of the loops confirms the formation of the in-plane transverse magnetic anisotropy induced during the deposition. The values of the coercive field, Hc, and the anisotropy field, Ha, are collected in Table 1. One can see that the coercivity is much lower for the S-II sample and the anisotropy field is much higher for the S-I case.

Fig. 2 shows field dependences of total impedance of S-I and S-II samples for selected

Conclusions

Magnetic, microwave properties, magnetic domain structure and MI were comparatively analyzed for FeNi/Cu/FeNi and [FeNi/Cu]4/FeNi/Cu/[FeNi/Cu]4/FeNi multilayers of the same geometry. FeNi/Cu/FeNi multilayer has higher coercivity and anisotropy field. Maximum MI sensitivities for total impedance at 75 MHz of 1.1 Ω/Oe for [FeNi/Cu]4/FeNi/Cu/[FeNi/Cu]4/FeNi and 0.8 Ω/Oe for FeNi/Cu/FeNi were obtained. Despite the increase of the magnetic inhomogeneities related to small contribution of mixed

Acknowledgments

This work was performed under MAT2011-27573-C04, project no. 215 “Magnetodynamics of High-Permeability Nanostructured Media” at the Ural Federal University and ETORTEK-ACTIMAT grants. Collaboration with Prof. S.M. Bhagat (University of Maryland) is highly appreciated. Selected measurements were made by I. Orue at UPV-EHU SGIker service.

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