M. Ipatov

Thin Magnetically Soft Wires for Magnetic Microsensors

Recent advances in technology involving magnetic materials require development of novel advanced magnetic materials with improved magnetic and magneto-transport properties and with reduced dimensionality. Therefore magnetic materials with outstanding magnetic characteristics and reduced dimensionality have recently gained much attention. Among these magnetic materials a family of thin wires with reduced geometrical dimensions (of order of 1–30 μm in diameter) have gained importance within the last few years. These thin wires combine excellent soft magnetic properties (with coercivities up to 4 A/m) with attractive magneto-transport properties (Giant Magneto-impedance effect, GMI, Giant Magneto-resistance effect, GMR) and an unusual re-magnetization process in positive magnetostriction compositions exhibiting quite fast domain wall propagation. In this paper we overview the magnetic and magneto-transport properties of these microwires that make them suitable for microsensor applications.

Stress tunable properties of ferromagnetic microwires and their multifunctional composites

We report the results of a systematic study on stress tunable absorption of glass-coated amorphous Co68.7Fe4Ni1B13Si11Mo2.3 microwires and their composites. The magnetic microwires possess good stress-impedance properties and yield a stress dependence of absorption at gigahertz frequencies. The stress compensates the reverse effect of magnetic field on absorption. There exist strong stress dependences of the effective permittivity and transmission parameters. Composite failure due to the wire damage results in a dramatic change of the sign and magnitude of effective permittivity. The double peak is identified in the stress dependence of field tunability, in contrast to the single peak for the magnetic field tunability. All these results indicate that the present composites are very promising for detecting the ambient stress levels and interrogating the structural integrity.

Manipulation of domain wall dynamics in amorphous microwires through the magnetoelastic anisotropy

We studied the effect of magnetoelastic anisotropy on domain wall (DW) dynamics and remagnetization process of magnetically bistable Fe-Co-rich microwires with metallic nucleus diameters (from 1.4 to 22 μm). We manipulated the magnetoelastic anisotropy applying the tensile stresses and changing the magnetostriction constant and strength of the internal stresses. Microwires of the same composition of metallic nucleus but with different geometries exhibit different magnetic field dependence of DW velocity with different slopes. Application of stresses resulted in decrease of the DW velocity, v, and DW mobility, S. Quite fast DW propagation (v until 2,500 m/s at H about 30 A/m) has been observed in low magnetostrictive magnetically bistable Co56Fe8Ni10Si10B16 microwires. Consequently, we observed certain correlation between the magnetoelastic energy and DW dynamics in microwires: decreasing the magnetoelastic energy, Kme, DW velocity increases.

Mechanisms of the ultrafast magnetization switching in bistable amorphous microwires

Two magnetization reversal regimes were found in magnetically bistable Fe-rich microwires. The first one, exhibiting an almost linear dependence of the domain wall velocity v on magnetic field H reaching 1.7 km/s, is related to single DW propagation. The second essentially nonlinear regime is observed when H exceeds some critical magnetic field, HN, determined by the microwire inhomogeneities. At H>HN, new reverse domains can be nucleated, and consequently a tandem remagnetization mechanism can be realized. Ultrafast magnetization switching through additional nucleation centers created artificially can be applied in spintronic devices for enhancing their performance.

Magnetoimpedance sensitive to dc bias current in amorphous microwires

We have investigated the impedance dependence of magnetically soft microwire on the internal circumferential magnetic field HB created by the dc bias current IB and theoretically and experimentally demonstrated that in a conductor with helical magnetic anisotropy, the high frequency impedance depends on the dc bias current IB (or the corresponding bias field HB) and this dependence is hysteretic. We have experimentally observed a change of impedance more than 35% upon changing the bias current. The possible applications of the dc current-driven magnetoimpedance effect are discussed.

Exceptional electromagnetic interference shielding properties of ferromagnetic microwires enabled polymer composites

We present systematic studies of the electromagnetic interference (EMI) shielding and microwave properties of a new class of shielding material, i.e., the ferromagnetic microwires-embedded polymer composites. We show that at 1–2 GHz the shielding effectiveness (SE) of the continuous-wire composite reaches a high value of 18 dB (98.4% attenuation) for a very low filler loading of 0.024% and a thickness of 0.64 mm. The normalized SE of this new composite is about 70 times higher than that of the bucky paper-based composite and is two to four orders of magnitude higher than those of other shielding candidate materials. Complex permeability, permittivity, and impedance experiments reveal that the absorption of electromagnetic radiation is a dominant mechanism for EMI shielding of the studied composites. The advantages of high shielding efficiency, good physical integrity, low fabrication costs, and multifunctionalities make them an attractive candidate material for a variety of technological applications.

Tuneable Metacomposites Based on Functional Fillers

Metamaterials, traditionally in the form of artificial structures with surprising electromagnetic properties, have triggered unprecedented opportunities to achieve those fascinating applications that previously only exist in science-fiction works, for example, Harry Potter’s cloak. Nevertheless, their massive manufacturing costs incurred by their complicated structures restrict the scale-up and mass production. The ultimate properties are primarily (if not solely) determined by the intrinsic structures of metamaterials that make them merely ‘meta-structures’. In response to these issues, it is desirable to have a genuine engineering composite yet with metamaterial characteristics. Thus, ‘metacomposite’ has been proposed to account for a real piece of composite material. This has subsequently become a nascent area where metamaterial properties are attained under wider operating frequencies with certain tunability towards external magnetic fields or mechanical stresses. In this chapter, we start with an overview of metacomposites containing various dielectric and/or magnetic fillers following the fillers’ dimensions from 0D, 1D to 2D. We then critically discussed some progresses in metacomposites containing ferromagnetic microwires together with unparalleled advantages in microwave sensing and cloaking areas. Finally, the chapter is closed with an outlook of strategies for improving existing metacomposites and some future perspectives.

Effect of transverse magnetic field on domain wall propagation in magnetically bistable glass-coated amorphous microwires

We experimentally studied domain wall (DW) propagation in amorphous Fe69Si10B15C6 and Co56Fe8Ni10Si11B16 microwires. We found that, in some cases, application of transverse magnetic field increases DW velocity in studied microwires. This effect is explained considering effect of transverse magnetic anisotropy on DW propagation. Considerable increase of DW velocity has been observed at enhanced longitudinal magnetic field, H. Such abrupt increasing of DW velocity can be related with defects contribution.

Magnetic field effects in artificial dielectrics with arrays of magnetic wires at microwaves

A magnetic field tunable electromagnetic response in periodic lattices of conducting magnetic wires is demonstrated. The wire medium having a negative permittivity in the lower frequency band is customarily investigated as an important component of so-called double negative metamaterials. Here we are interested in a strong dispersion of the permittivity in these structures and a possibility to alter it by changing the losses in magnetic wires with an external magnetic field. The theoretical approach is based on calculating the relaxation parameter depending on the wire surface impedance, and hence, on the wire magnetic properties. Thus, in arrays of Co-based amorphous wires the application of a moderate magnetic field (of about 1–2 kA/m) which causes the magnetization reorientation is capable of few fold permittivity change in the frequency range of 1–2 GHz. Such efficient tuning for certain structural and magnetic parameters was confirmed experimentally by measuring the transmission and reflection spectr...

Correlation of surface domain structure and magneto-impedance in amorphous microwires

The correlation between surface domain structure (SDS) and high frequency magneto-impedance (MI) in amorphous microwires has been systematically studied. First, we applied the magneto-optical polarizing microscopy to determine the SDS and found that it is considerably different in unstressed microwire and in microwires to which tensile and torsional stress were applied. Then, we measured the longitudinal and off-diagonal MI in these microwires and also observed quite different MI dependencies. We analyzed the experimental MI curves and their dependence on the SDS with the help of a simple model that nevertheless yields good qualitative agreement with experiment. We have demonstrated that the analysis of the MI dependencies, especially the off-diagonal one, can reveal the SDS in the microwires. The obtained results can also be useful for magnetic and stress sensing applications.