Anna A. Chlenova

The structure, magnetic properties, and magnetic domains in rapidly quenched amorphous Co_78.5Ni_12.4Fe_5.5Cr_0.8Mn_1.4Si_1.1B₃ ribbons were experimentally studied in as-quenched state and after treatment with argon ion flows generated in the vacuum chamber of an ion-plasma sputtering installation under specially selected conditions near the threshold for Ar ion energy at which sputtering of the surface of the ribbon samples occurs (approximately 1.3 keV). An explanation is proposed for the effectiveness of Ar ion treatment of the surface of rapidly quenched ribbon under the considered conditions. Obtained results can be useful for development magnetically soft layered composites for magnetic field sensors using magnetoimpedance and thermomagnetic effects.

FeNi films were prepared using the DC magnetron sputtering technique with an oblique deposition arrangement. Multilayers with different orientations of the magnetic anisotropy axes were obtained thanks to a rotary sample holder inside the vacuum chamber. Magnetic properties were studied using magneto–optical Kerr microscopy and a vibrating sample magnetometer. Single-layered FeNi films having thicknesses as high as 10 nm and 40 nm show in-plane uniaxial easy magnetization axes produced by the oblique incidence of incoming components of the beams. Magnetic anisotropy field for four-layered samples with orthogonal uniaxial magnetic anisotropy axes in the adjacent layers and the thickness of individual layers of 10 nm and 40 nm turned out to be less than in single-layered films. The magnetic properties peculiarities of the eight-layered sample FeNi (10 nm) × 8 obtained by rotation of the sample holder by 45° before deposition of each subsequent layer suggest the formation of a helix-like magnetic structure through the thickness of the multilayered sample similar to the magnetization arrangement in the Bloch-type magnetic domain wall.

The structure, magnetic, and magnetoimpedance properties of Fe₃Co₆₇Cr₃Si₁₅B₁₂ amorphous ribbons are studied using samples differing in the geometry, which are prepared by rapid quenching of the melt on a rotating wheel. The comparative analysis is performed for the as-quenched ribbons without additional treatments (A) and after relaxation annealing at 350°C (B) or thermomechanical treatment at 350°C under a specific load of 230 MPa (C). The selected treatments A and B correspond to the longitudinal magnetic anisotropy; in the case of annealed samples, the level of residual stresses is lower. Treatment C leads to the formation of transverse uniaxial magnetic anisotropy and record high sensitivity of giant magnetoimpedance effect (~200%/Oe) to the external magnetic field in the range of low fields ~2–3 Oe.

Magnetic properties and peculiarities of the domain structure of multilayer [Ti/FeNi]_х/Cu/[FeNi/Ti]_x elements, the geometry of which ensures the high magnetoimpedance effect, are studied by varying the number of layers in the range x = 0–5. Peculiarities of the magnetization processes of the multilayer structures, which are magnetized at both substrate and free surfaces, are analyzed. It has been found that elements in which the magnetization reversal from the free side occurs by rotation of the magnetization vector have a higher magnetoimpedance effect compared to elements in which the magnetization reversal occurs with the formation of 180° domain walls both in the layer from the side of the substrate and the free layer as well.

The ferromagnetic resonance (FMR) in the frequency range of 0.5 to 12.5 GHz has been investigated as a function of external magnetic field for rapidly quenched Fe₃Co₆₇Cr₃Si₁₅B₁₂ amorphous ribbons with different features of the effective magnetic anisotropy. Three states of the ribbons were considered: as-quenched without any treatment; after relaxation annealing without stress at the temperature of 350 °C during 1 h; and after annealing under specific stress of 230 MPa at the temperature of 350 °C during 1 h. For FMR measurements, we adapted a technique previously proposed and tested for the case of microwires. Here, amorphous ribbons were studied using the sample holder based on a commercial SMA connector. On the basis of the measurements of the reflection coefficient S_11, the total impedance including its real and imaginary components was determined to be in the frequency range of 0.5 to 12.5 GHz. In order to confirm the validity of the proposed technique, FMR was also measured by the certified cavity perturbation technique using a commercial Bruker spectrometer operating at X-band frequency of 9.39 GHz. As part of the characterization of the ribbons used for microwave measurements, comparative analysis was performed of X-ray diffraction, optical microscopy, transmission electron microscopy, inductive magnetic hysteresis loops, vibrating sample magnetometry, magneto-optical Kerr effect (including magnetic domains) and magnetoimpedance data for of all samples.

Magnetometry and ferromagnetic resonance are used to quantitatively study magnetic anisotropy with an easy axis both in the film plane and perpendicular to it. In the study of single-layer and multilayer permalloy films, it is demonstrated that these methods make it possible not only to investigate the average field of perpendicular and in-plane anisotropy, but also to characterize their inhomogeneity. It is shown that the quantitative data from direct integral and local measurements of magnetic anisotropy are consistent with the direct and indirect estimates based on processing of the magnetization curves. The possibility of estimating the perpendicular magnetic anisotropy constant from the width of stripe domains in a film in the transcritical state is demonstrated. The average in-plane magnetic anisotropy field of permalloy films prepared by magnetron sputtering onto a Corning glass is almost unchanged with the thickness of a single-layer film. The inhomogeneity of the perpendicular anisotropy field for a 500 nm film is greater than that for a 100 nm film, and for a multilayer film with a total permalloy thickness of 500 nm, it is greater than that for a homogeneous film of the same thickness.

Soft magnetic materials are widely requested in electronic and biomedical applications. Co-based amorphous ribbons are materials which combine high value of the magnetoimpedance effect (MI), high sensitivity with respect to the applied magnetic field, good corrosion stability in aggressive environments, and reasonably low price. Functional properties of ribbon-based sensitive elements can be modified by deposition of additional magnetic and non-ferromagnetic layers with required conductivity. Such layers can play different roles. In the case of magnetic biosensors for magnetic label detection, they can provide the best conditions for self-assembling processes in biological experiments. In this work, magnetic properties and MI effect were studied for the cases of rapidly quenched Co₆₇Fe₃Cr₃Si₁₅B₁₂ amorphous ribbons and magnetic Fe₂₀Ni₈₀/Co₆₇Fe₃Cr₃Si₁₅B₁₂/Fe₂₀Ni₈₀ composites obtained by deposition of Fe₂₀Ni₈₀ 1 μm thick films onto both sides of the ribbons by magnetron sputtering technique. Their comparative analysis was used for finite element computer simulations of MI responses with different types of magnetic and conductive coatings. The obtained results can be useful for the design of MI sensor development, including MI biosensors for magnetic label detection.

By examples of multilayered film structure Cu/[FeNi/Cu]₁₀ deposited by the method of ion-plasma sputtering, a comparative analysis of the magnetic hysteresis loops measured on a vibrating magnetometer, a magnetic measuring complex with primary converter on the basis of SQUID (SQUID magnetometer), and a magneto-optical Kerr microscope is carried out. The use of a SQUID magnetometer made it possible to detect the step-by-step nature of remagnetization of the film structure, the presence of which was confirmed by observations on the Kerr microscope in the quasi-static remagnetization mode.

Thin films of Fe_xNi_{100 – x} (x = 20, 17, 15%) deposited by magnetron sputtering onto glass and polymeric substrates are investigated for thicknesses ranging from 50 to 500 nm. On the basis of analysis of the angular dependences of magnetic resonance spectra, the multilayer structure [Cu/Fe₂₀Ni₈₀]₅/Cu is selected as the most promising for sensor applications.

Amorphous and nanocrystalline soft magnetic materials have attracted much attention in the area of sensor applications. In this work, the magnetoimpedance (MI) effect of patterned soft ferromagnetic meander-shaped sensor elements has been investigated. They were fabricated starting from the cobalt-based amorphous ribbon using the lithography technique and chemical etching. Three-turn (S1: spacing s = 50 μm, width w = 300 μm, length l = 5 mm; S2: spacing s = 50 μm, width w = 400 μm, length l = 5 mm) and six-turn (S3: s = 40 μm, w = 250 μm, length l = 5 mm; S4: s = 40 μm, w = 250 μm and l = 8 mm) meanders were designed. The ‘n’ shaped meander part was denominated as “one turn”. The S4 meander possesses a maximum MI ratio calculated for the total impedance ΔZ/Z ≈ 250% with a sensitivity of about 36%/Oe (for the frequency of about 45 MHz), and an MI ratio calculated for the real part of the total impedance ΔR/R ≈ 250% with the sensitivity of about 32%/Oe (for the frequency of 50 MHz). Chemical etching and the length of the samples had a strong impact on the surface magnetic properties and the magnetoimpedance. A comparative analysis of the surface magnetic properties obtained by the magneto-optical Kerr technique and MI data shows that the designed ferromagnetic meander-shaped sensor elements can be recommended for high frequency sensor applications focused on the large drop analysis. Here we understand a single large drop as the water-based sample to analyze, placed onto the surface of the MI sensor element either by microsyringe (volue range 0.5–500 μL) or automatic dispenser (volume range 0.1–50 mL).