A.V. Svalov

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.

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.

Sensor applications of soft magnetic materials based on magneto-impedance, magneto-elastic resonance and magneto-electricity.

The outstanding properties of selected soft magnetic materials make them successful candidates for building high performance sensors. In this paper we present our recent work regarding different sensing technologies based on the coupling of the magnetic properties of soft magnetic materials with their electric or elastic properties. In first place we report the influence on the magneto-impedance response of the thickness of Permalloy films in multilayer-sandwiched structures. An impedance change of 270% was found in the best conditions upon the application of magnetic field, with a low field sensitivity of 140%/Oe. Second, the magneto-elastic resonance of amorphous ribbons is used to demonstrate the possibility of sensitively measuring the viscosity of fluids, aimed to develop an on-line and real-time sensor capable of assessing the state of degradation of lubricant oils in machinery. A novel analysis method is shown to sensitively reveal the changes of the damping parameter of the magnetoelastic oscillations at the resonance as a function of the oil viscosity. Finally, the properties and performance of magneto-electric laminated composites of amorphous magnetic ribbons and piezoelectric polymer films are investigated, demonstrating magnetic field detection capabilities below 2.7 nT.

Nanostructured giant magneto-impedance multilayers deposited onto flexible substrates for low pressure sensing.

Nanostructured FeNi-based multilayers are very suitable for use as magnetic sensors using the giant magneto-impedance effect. New fields of application can be opened with these materials deposited onto flexible substrates. In this work, we compare the performance of samples prepared onto a rigid glass substrate and onto a cyclo olefin copolymer flexible one. Although a significant reduction of the field sensitivity is found due to the increased effect of the stresses generated during preparation, the results are still satisfactory for use as magnetic field sensors in special applications. Moreover, we take advantage of the flexible nature of the substrate to evaluate the pressure dependence of the giant magneto-impedance effect. Sensitivities up to 1 Ω/Pa are found for pressures in the range of 0 to 1 Pa, demostrating the suitability of these nanostructured materials deposited onto flexible substrates to build sensitive pressure sensors.

FeNi-based magnetic layered nanostructures: Magnetic properties and giant magnetoimpedance

Magnetic properties and the magnetoimpedance (MI) effect were studied for a series of [Fe20Ni80/Ti]n/Fe20Ni80 (n=0 to 5) nanostructures, prepared by dc magnetron sputtering. The thickness of the FeNi layers was selected as 170 nm in order to avoid the appearance of the “transcritical” state that takes place for thicker layers. First, the influence of the Ti layer thickness was determined for n=1 trilayers, with Ti layers ranging from 2 to 20 nm. The minimum coercivity corresponded to a Ti layer of about 6 nm. Second, the magnetic properties and MI responses were studied for different [FeNi/Ti]n/FeNi structures at a fixed Ti layer thickness of 6 nm. The complex impedance was measured for a frequency range of 1–300 MHz. The highest value of the MI was obtained in the sample [FeNi/Ti]5/FeNi with the largest total thickness.

GMI in Nanostructured FeNi/Ti Multilayers With Different Thicknesses of the Magnetic Layers

Nanostructured multilayers are proposed as a competitive alternative to magnetoimpedance (MI) thick films. However, the effect of the thickness of the magnetic layers on the properties of MI multilayers is still unclear. To perform a systematic study, three multilayered structures consisting of magnetic Fe20Ni80 layers and nonmagnetic Ti layers were prepared by sputtering. In each structure, FeNi layers have different thickness (25, 50, and 100 nm) keeping the same total thickness. Magnetic measurements and magnetic domain structure observations were used for the characterization of the samples. The MI of FeNi/Ti- based multilayers was evaluated in terms of the magnitude of the impedance variations and its sensitivity with respect to the applied magnetic field. The results show that the multilayer with 50 nm FeNi layers has a slightly better properties than the sample with a 100 nm FeNi layers, while the sample with a 25 nm thick FeNi layers performs considerably worse. The sample [FeNi(50 nm)/Ti(6 nm)]7FeNi(50 nm) displays a MI ratio of 24% at 200 MHz and a sensitivity of 135% kAm-1 at 140 MHz.

High-yield fabrication of 60 nm Permalloy nanodiscs in well-defined magnetic vortex state for biomedical applications

Permalloy disc structures in magnetic vortex state constitute a promising new type of magnetic nanoparticles for biomedical applications. They present high saturation magnetisation and lack of remanence, which ease the remote manipulation of the particles by magnetic fields and avoid the problem of agglomeration, respectively. Importantly, they are also endowed with the capability of low-frequency magneto-mechanical actuation. This effect has already been shown to produce cancer cell destruction using functionalized discs, about 1 μm in diameter, attached to the cell membrane. Here, Permalloy nanodiscs down to 60 nm in diameter are obtained by hole-mask colloidal lithography, which is proved to be a cost-effective method for the uniform patterning of large substrate areas, with a high production yield of nanostructures. The characterisation of the magnetic behaviour of the nanodiscs, complemented with micromagnetic simulations, confirms that they present a very well defined magnetic vortex configuration, unprecedented, to our knowledge, for nanostructures of this size prepared by a high-yield method. The successful detachment of the gold-covered nanodiscs from the substrate is also demonstrated by the use of sacrificial layers.

Equivalent Magnetic Noise of Micro-Patterned Multilayer Thin Films Based GMI Microsensor

The detection limit of thin film giant magnetoimpedance (GMI) magnetic sensors has been investigated by measuring their magnetic noise. The GMI sensing elements are multilayers based on Fe20Ni80 Permalloy (Py), deposited by sputtering and patterned by photolithography in the form of 2-mm long and 50- to 130-μm wide stripes. The multilayered samples had the following structure: [Py(170 nm)/Ti(6 nm)]3/Cu(250 nm)/[Ti(6 nm)/Py(170 nm)]3. As electronic conditioning circuits, different configurations of oscillators and detectors were tested in order to determine their influence on the total output voltage sensor noise. The latter was measured using a dynamic signal analyzer and the equivalent magnetic noise of the sensor was determined through the measured Fourier transfer function of the device at the operating point. Filtering and impedance matching strategies were implemented to minimize the equivalent magnetic sensor noise. Presently, a white noise level of 120 pT/√Hz at 2 kHz is obtained.

High-Frequency Magnetoimpedance Response of Thin-Film Microstructures Using Coplanar Waveguides

Development of accurate measuring techniques is an important task for the high-frequency materials characterization. The magnetoimpedance (MI) of [Py(170 nm)/Ti(6 nm)]3/Cu(250 nm)/[Ti(6 nm)/Py(170 nm)]3 multilayer sandwiched structures is measured using two different high-frequency test fixtures and the results are compared for both cases. Sets of rectangular samples with different lengths and widths are fabricated by photolithography and inserted in test fixtures based either on microstrip or coplanar waveguides (CPWs). Measurements with CPWs ensure higher MI values, since their contribution to the total impedance is lower. Besides, we describe the de-embedding procedure that allows the subtraction of the external impedance brought about by the CPW from the total impedance that is measured using the test fixture. The intrinsic MI ratio of the thin-film structures, obtained by this de-embedding procedure, reaches 550%.

Transformation volume strain in Ni-Mn-Ga thin films

The temperature dependences of the lattice parameters and residual stress have been measured for a fine-grained Ni52.2Mn26.8Ga21.0 (at. %) thin film fabricated by sputter deposition onto a heated silicon wafer with SiNx buffer layer. The transformation volume strain in the film was found to be a lattice expansion during the forward martensitic transformation which is opposite to a volume contraction exhibited by bulk Ni-Mn-Ga alloys. This unusual effect can be explained by the substrate-induced residual stresses in the film and the difference in the elastic modulus of austenite and martensite.