In this review, devoted to the 30th anniversary since discovery the giant magnetoimpedance (MI) we analyzed and generalized results of experimental, theoretical studies and perspective of practical applications in this research area. As the MI field in the course of 30 years of intensive studies had created huge number of referenced sources, we made special effort to provide representative bibliography, which can be useful for the future reading. The high sensitivity of MI with respect to applied magnetic field allows us to anticipate that more types of MI and MI biosensors may be developed up to commercial level shortly. However, there are still fundamental and technical problems to understand and solve.

The effect of giant magnetoimpedance in magnetic structure with inhomogeneous electric current distribution is studied. A special design reduces the magnitude of the intrinsic magnetic field outside the structure. The magnetoimpedance of such a planar structure made of magnetically soft material with radial current distribution has been investigated both theoretically and experimentally. The theoretical evaluations show a strong variation of the impedance of the structure with the frequency and the permeability value of the material. The results of calculations are qualitatively confirmed by experimental data for model samples.

It was observed recently that the giant magnetoimpedance (GMI) effect in Fe-rich glass-coated amorphous microwires with positive magnetostriction can be improved significantly by means of post-annealing. The increase in the GMI is attributed to the induced helical magnetic anisotropy in the surface layer of the microwire, which appears after the annealing. The application of external stresses to the microwire may result in changes in its magnetic structure and affect the GMI response. In this work, we study theoretically the influence of the tensile and torsional stresses on the off-diagonal magnetoimpedance in annealed amorphous microwires with positive magnetostriction. The static magnetization distribution is analyzed in terms of the core–shell magnetic structure. The surface impedance tensor is obtained taking into account the magnetoelastic anisotropy induced by the external stresses. It is shown that the off-diagonal magnetoimpedance response exhibits strong sensitivity to the magnitude of the applied stress. The obtained results may be useful for sensor applications of amorphous microwires.

Planar ensembles of multilayer periodic square microelements of the type [Cu(6 nm)/FeNi(100 nm)] 5 were obtained by ion-plasma sputtering on glass substrates using mesh copper masks. The surface features of trace elements were analyzed using a stylus profilometer, optical and scanning electron microscopy. Static magnetic properties and features of the magnetic domain structure were studied using the magneto-optical Kerr effect. The resulting planar ensembles can be used as an integral part of the film elements of weak magnetic field detectors operating on the basis of magnetic impedance (MI) to increase the sensitivity of the MI response to an external magnetic field. Keywords: multilayer film structures, ensembles of trace elements, periodic structures, giant magnetoimpedance effect. Keywords: multilayer film structures, ensembles of trace elements, periodic structures, giant magnetoimpedance effect.

Thin FeNi films 100 nm in thickness and arrays of periodic square microelements (50, 70, and 300 µm in size of individual microelements) located in the same plane have been developed, synthesized, and experimentally studied. The structure, static magnetic properties, and ferromagnetic resonance features of film arrays created by ion-plasma sputtering with the use of masks have been comparatively analyzed. It is implied that a similar magnetic structure could be incorporated into multilayer sensor elements with a high giant magnetoimpedance to increase their sensitivity to magnetic field.

Планарные ансамбли многослойных периодических квадратных микроэлементов типа [Cu(6 nm)/FeNi(100 nm)]5 были получены методом ионно-плазменного распыления на стеклянные подложки с использованием сетчатых медных масок. Особенности поверхности микроэлементов анализировались с помощью стилусного профилометра, оптической и сканирующей электронной микроскопии. Статические магнитные свойства и особенности магнитной доменной структуры исследовались с использованием магнитооптического Керр-эффекта. Полученные планарные ансамбли могут использоваться как составная часть пленочных элементов детекторов слабых магнитных полей, работающих на основе магнитного импеданса (МИ), для повышения чувствительности МИ отклика к внешнему магнитному полю. Ключевые слова: многослойные пленочные структуры, ансамбли микроэлементов, периодические структуры, гигантский магнитоимпедансный эффект.

A new type of multilayer film structures with a giant magnetic impedance (GMI) is proposed and theoretically studied. The multilayer structure consists of a highly conductive central layer and two outer ferromagnetic layers located under and on the conductive layer. The upper layer is a periodic structure consisting of N multilayer elements and N + 1 regions without multilayer elements (the upper layer is profiled). An electrodynamic model has been developed that allows one to find the transverse permeabilities μ1 and μ2 for the upper and lower layers of the GMI structure using the standard procedure for solving the linearized Landau–Lifshitz equation and the equation for equilibrium magnetization angles. It is shown that, for a profiled structure, with a decrease in the angle of deviation of the effective magnetic anisotropy axis from the transverse direction, the permeability of the upper layer increases, which leads to the enhancement of the skin and GMI effects.

The paper presents a theoretical study of the effect of torsional stresses on the giant magnetoimpedance of a Fe-rich amorphous microwire with helicoidal anisotropy induced in the surface layer as a result of annealing. It is shown that torsional stresses lead to a change in the shape of the dependence of the impedance of the microwire on an external field at relatively low frequencies of the excitation current. The change in impedance is maximal near the stress value at which an effective circular anisotropy in the surface region of the microwire appears. The results can be used to create stress and weak magnetic field sensors.

The effect of the number and thickness of permalloy layers and the thickness and material of interlayers on the value of magnetic impedance (MI) effect of multilayer film structures is theoretically analyzed in this work. The distributions of electromagnetic fields over the thickness of the film structure and MI are obtained on the basis of the joint solution of the Maxwell equations and the Landau–Lifshitz equation. The performed analysis shows that the MI increases with decreasing number of permalloy layers and corresponding increase in their thickness. It is established that the values of conductivities of the central nonmagnetic layer and interlayers influence the MI value differently. It is predicted that a further increase in MI may be achieved by using interlayers made of ferromagnetic materials.

Polyelectrolyte gels and ferrogels (FG) are attracting special interest in biomedicine. Here we describe our experience developing functional magnetic ferrogels for regenerative medicine and magnetoimpedance biosensing for measuring stray fields of nanoparticles in FG with a multilayered sensitive element. We discuss the possibility of developing a new generation of drug delivery systems for magnetic field assisted delivery, positioning and biosensing.

The magnetoimpedance effect in a stress-annealed Fe-rich amorphous microwire with induced magnetic anisotropy in a surface region is studied theoretically. The static magnetization distribution within the microwire is described in the framework of the core–shell magnetic structure. It is assumed that the microwire has an axial magnetic anisotropy in the inner core and a helical anisotropy in the external shell appearing due to the stress-annealing. Both the diagonal and off-diagonal magnetoimpedance responses are found in terms of the surface impedance tensor. The influence of the bias current on the magnetoimpedance is analyzed. The obtained theoretical dependences are shown to be in a qualitative agreement with the results of recent experimental studies of the magnetoimpedance effect in stress-annealed Fe-rich glass–coated amorphous microwires.

One of the fundamental characteristics of a magnetic field detector is sensitivity to the external magnetic field. Of all the known magnetic effects, the giant magnetoimpedance (MI) has the highest sensitivity with respect to the external magnetic field. Here, we describe our experience in designing, fabrication, experimental and theoretical characterization of [FeNi(50 nm)/Ti(6 nm)] 6 /Cu(500 nm)/[Ti(50 nm)/FeNi(6 nm)] 6 multilayered structures for biometric detector. The designed device operates at room temperature, with a maximum sensitivity of the order of 0.4 Ohm/Oe for the total impedance and its real part and 0.1 Ohm/Oe for the imaginary part of the total impedance. An automatic system based on a ZVA-67 (Rohde & Schwarz) vector network analyzer was built for one-scan microwave absorption studies of both magnetoimpedance and ferromagnetic resonance of multilayered sensitive elements. Measurements were made with the coplanar line type holder in the increasing and decreasing fields. The obtained experimental and theoretical results for MI range were in a satisfactory agreement with each other. They could be useful for optimization of the MI multilayered elements for practical applications, including applications in different types of magnetic biosensors.

A prototype biosensor operating on the basis of the giant magnetoimpedance effect (GMI) with a [Cu/FeNi] 5 /Cu/[FeNi/Cu] 5 multilayer sensing element is developed to study ferrogels. The GMI is measured in the initial state and in the presence of ferrogels with different concentrations of superparamagnetic nanoparticles, allowing the stray fields of ensembles of magnetic nanoparticles in ferrogels to be characterized. The description produced using the proposed electrodynamic model agrees satisfactorily with the GMI experimental data obtained for a ferrogel-coated film element.

The magnetoimpedance effect in nanostructured multilayers is studied theoretically. The multi-layered film structure consists of highly conductive non-magnetic central layer and two external multilayers containing soft magnetic layers of the same thickness separated either by magnetic spacers of different kind or by non-magnetic spacers. In order to describe the magnetoimpedance in the multilayer an electrodynamic model is proposed. The influence of geometric parameters and physical properties of the layers on the magnetoimpedance response is analyzed. The results obtained could be useful both for better understanding of high-frequency behavior of nanostructured multilayers and for optimization of multilayer parameters aiming to enhance the magnetoimpedance effect for practical applications.

Asymmetric giant magnetoimpedance (AGMI) effect has been investigated in as-prepared and current annealed amorphous (Co0.9Fe0.05Ni0.05)(75)Si15B10 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. (C) 2018 Elsevier B.V. All rights reserved.

In-tissue embedded magnetic nanoparticle (MNPs) detection is one of the most interesting cases for cancer research. In order to understand the origin, the limits and the way of improvement of magnetic biosensor sensitivity for the detection of 3D mezoscopic distributions of MNPs, we have developed a magnetoimpedance biosensor prototype with a [Cu (3 nm)/FeNi(100 nm)]5/Cu(500 nm)/[FeNi(100 nm)/Cu(3 nm)]5 rectangular sensitive element. Magnetoimpedance (MI) responses were measured with and without polyacrylamide ferrogel layer mimicking natural tissue in order to evaluate stray fields of embedded MNPs of γ-Fe2O3 iron oxide. A model for MI response based on a solution of Maxwell equations with Landau-Lifshitz equation was developed in order to understand the origin of the prototype sensitivity which reached 1.3% of ΔZ/Z per 1% of MNPs concentration by weight. To make this promising technique useful for magnetically labeled tissue detection, a synthesis of composite gels with MNPs agglomerates compactly located inside pure gel and their MI testing are still necessary.