G. V. Kurlyandskaya

Very large magnetoimpedance effect in FeCoNi ferromagnetic tubes with high order magnetic anisotropy

The extraordinarily high (up to 800% magnetoimpedance ratio) and sensitive magnetoimpedance effect has been found and studied in FeCoNi magnetic tubes electroplated onto CuBe nonmagnetic wire at frequency of about 1 MHz order. Special annealing was done in order to induce magnetic anisotropy. The high harmonic response has been studied, and the harmonics show larger variations with the external magnetic field than the fundamental. This huge sensitivity of the harmonics (up to an order of the tens of thousands %/Oe) is promising in regard to the increase of the sensitivity of giant magnetoimpedance sensors. To explain the experiment results, we calculated the high frequency transverse susceptibility taking into account the magnetic anisotropy of first and second orders. The susceptibility is extremely high at the points of orientational phase transitions in the magnetic layer which gives rise to strong nonlinear effects.

Magnetoimpedance biosensor for Fe3O4 nanoparticle intracellular uptake evaluation

Iron oxide (Fe3O4) nonspecific nanoparticles of 30nm are embedded inside human embryonic kidney (HEK 293) cells by intracellular uptake with a concentration of ∼105 particles/cell. An amorphous ribbon of Co64.5Fe2.5Cr3Si15B15 exhibiting large magnetoimpedance (MI) serves as the sensing element. The presence of fringing fields of the nanoparticles changes the superposition of the constant applied field and the alternating field created by a current flowing through the ribbon that can be detected as a change in MI. This response is clearly dependent on the presence of the magnetic nanoparticles inside the cells and on the value of the external field.

Iron oxide nanoparticles fabricated by electric explosion of wire: focus on magnetic nanofluids

Nanoparticles of iron oxides (MNPs) were prepared using the electric explosion of wire technique (EEW). The main focus was on the fabrication of de-aggregated spherical nanoparticles with a narrow size distribution. According to XRD the major crystalline phase was magnetite with an average diameter of MNPs, depending on the fraction. Further separation of air-dry EEW nanoparticles was performed in aqueous suspensions. In order to provide the stability of magnetite suspension in water, we found the optimum concentration of the electrostatic stabilizer (sodium citrate and optimum pH level) based on zeta-potential measurements. The stable suspensions still contained a substantial fraction of aggregates which were disintegrated by the excessive ultrasound treatment. The separation of the large particles out of the suspension was performed by centrifuging. The structural features, magnetic properties and microwave absorption of MNPs and their aqueous solutions confirm that we were able to obtain an ensemble in...

Giant magnetoimpedance biosensor for ferrogel detection: Model system to evaluate properties of natural tissue

Thin-film based magnetoimpedance (MI) sensors were used for quantitative determination of the concentration of magnetic nanoparticles (MNPs) in ferrogels. Ferrogels (model systems for biological tissue) were synthesized by radical polymerization of acrylamide in a stable aqueous suspension of γ-Fe2O3 MNPs fabricated by laser target evaporation. MI [FeNi/Ti]3/Cu/[Ti/FeNi]3/Ti sensitive elements were prepared by sputtering. Thorough structural and magnetic studies of MNPs, ferrogels, and multilayered sensitive element insure the complete characterization of biosensor prototype. The MI response of the sensitive element was carefully evaluated in initial state and in the presence of ferrogels with different concentration of iron oxide MNPs from 0 to 2.44 wt. %, which produced systematic changes of the MI in a frequency span of 300 kHz to 400 MHz.

The influence of field- and stress-induced magnetic anisotropy on the magnetoimpedance in nanocrystalline FeCuNbSiB alloys

The correlation between magnetic anisotropy and magnetoimpedance has been studied in field- and stress-annealed Fe73.5Cu1Nb3Si13.5B9 ribbons. As-cast and previously nanocrystallized samples were submitted to different annealing conditions (T=500, 530, and 560 °C and tensile stress σ=150 MPa) in order to induce different magnetic anisotropy. The magnetoimpedance ratio [ΔZ/Z] was measured at 3 MHz using a driving current Irms=10 mA. An hysteretic behavior of the magnetoimpedance ratio is observed. This hysteretic behavior is interpreted considering the magnetization process for samples exhibiting different magnetic anisotropies.

Influence of magnetization processes and device geometry on the GMI effect

Summary form only given. The use of Giant Magneto-Impedance as a mechanism for sensor design has been stressed in the past few years. Nowadays, some devices are already in the market and many different materials with various shapes have been tested for these sensors. In this paper we review the different geometries (wires, ribbons, films) and structures (either homostructures or sandwiches and plated wires) as well as the different materials (either amorphous or crystalline) and magnetization processes occurring in the GMI elements. The different character of the magnetic anisotropy in crystalline and amorphous alloys determines the type of magnetization process dominant in the material (either wall movement or magnetization rotation) as well as the single peak or double peak behaviour of the GMI. The wall movement results typically in larger permeability values and then in improved GMI ratios. However the damping of the walls rapidly reduces the effective permeability that becomes the one corresponding to that of rotation processes.

Frequency dependence of giant magnetoimpedance effect in CuBe/CoFeNi plated wire with different types of magnetic anisotropy

A giant magnetoimpedance effect (GMI) has been measured in a CuBe 0.1 mm diameter wire, electroplated with a Co6Fe20Ni74 layer 1 μm thick. Heat treatments after the electrodeposition were performed under the dc or ac field. The GMI has been measured in the frequency range 0.25 to 9 MHz, using ac driving current from 5 to 35 mA. The maximum GMI ratio depends strongly on the heat treatments, the intensity and the frequency of the driving current, reaching the value of 165% for optimal conditions. Results are discussed, taking into account the anisotropy distribution in the magnetic layer induced by the heat treatments under the field and a simple rotation model.

Spherical magnetic nanoparticles fabricated by laser target evaporation

Magnetic nanoparticles of iron oxide (MNPs) were prepared by the laser target evaporation technique (LTE). The main focus was on the fabrication of de-aggregated spherical maghemite MNPs with a narrow size distribution and enhanced effective magnetization. X-ray diffraction, transmission electron microscopy, magnetization and microwave absorption measurements were comparatively analyzed. The shape of the MNPs (mean diameter of 9 nm) was very close to being spherical. The lattice constant of the crystalline phase was substantially smaller than that of stoichiometric magnetite but larger than the lattice constant of maghemite. High value of M s up to 300 K was established. The 300 K ferromagnetic resonance signal is a single line located at a field expected from spherical magnetic particles with negligible magnetic anisotropy. The maximum obtained concentration of water based ferrofluid was as high as 10g/l of magnetic material. In order to understand the temperature and field dependence of MNPs magnetization, we invoke the core-shell model. The nanoparticles is said to have a ferrimagnetic core (roughly 70 percent of the caliper size) while the shell consists of surface layers in which the spins are frozen having no long range magnetic order. The core-shell interactions were estimated in frame of random anisotropy model. The obtained assembly of de-aggregated nanoparticles is an example of magnetic nanofluid stable under ambient conditions even without an electrostatic stabilizer.

FeNi-based magnetoimpedance multilayers: Tailoring of the softness by magnetic spacers

The microstructure and magnetic properties of sputtered permalloy films and FeNi(170 nm)/X/FeNi(170 nm) (X = Co, Fe, Gd, Gd-Co) sandwiches were studied. Laminating of the thick FeNi film with various spacers was done in order to control the magnetic softness of FeNi-based multilayers. In contrast to the Co and Fe spacers, Gd and Gd-Co magnetic spacers improved the softness of the FeNi/X/FeNi sandwiches. The magnetoimpedance responses were measured for [FeNi/Ti(6 nm)]2/FeNi and [FeNi/Gd(2 nm)]2/FeNi multilayers in a frequency range of 1–500 MHz: for all frequencies under consideration the highest magnetoimpedance variation was observed for [FeNi/Gd(2 nm)]2/FeNi multilayers.

Giant magnetoimpedance: A label-free option for surface effect monitoring

The change in magnetoimpedance (MI) during surface modification of the magnetic sensitive element caused by highly corrosive fluid was studied with the aim of creating a robust method to monitor the surface effects. A MI-based sensor prototype with an as-quenched FeCoSiB or FeCoCrSiB amorphous ribbon sensitive element was designed and calibrated for a frequency range of 0.5–10MHz at an intensity of the current of 10–60mA without bath for fluids. Measurements as a function of the exposure time were also made in a regime where chemical surface modification and sensing were not separated (in a bath for fluids). The MI variation was explained by the change of the surface magnetic anisotropy and the geometry of the sensitive element. A simple model was developed to describe MI change. It was shown that the magnetoimpedance effect can be employed as useful method to probe the electric features of surface-modified magnetic electrodes when the corrosive fluid, the material of the sensitive element, and the detect...