Eduardo Herrera Fernández

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.

Magnetic Properties and Giant Magnetoimpedance of FeNi-Based Nanostructured Multilayers With Variable Thickness of the Central Cu Lead

The magnetoimpedance effect is attractive for thin film-based magnetic sensor applications. Recently a significant progress has been made in the development of appropriate theories, preparation and characterization of MI thin film-based structures. In the present work FeNi(100 nm)/Cu(3.2 nm)]4/FeNi(100 nm)/Cu(LCu)/[FeNi(100 nm)/Cu(3.2 nm)]4/FeNi(100 nm) multilayered structures with open magnetic flux have been prepared by RF-sputtering. Their magnetic properties and MI were studied as a function of the thickness of the central Cu lead. It was shown that the thickness of the Cu lead is an important parameter. The highest sensitivity (≈ 50%/Oe, f=160 MHz) was observed for the sample with a central Cu layer thickness of about a half-thickness of a magnetic layer (LCu ≈ 250 nm). The maximum sensitivity of the real part of the impedance was also obtained for this thickness (≈ 75%/Oe).

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.

Differences in the Magneto-Impedance of FeNi/Cu/FeNi Multilayers With Open and Closed Magnetic Path

The performance of the magnetoimpedance effect in terms of magnitude and sensibility is evaluated for two FeNi/Cu/FeNi multilayer structures with different geometries, such that in one of them the magnetic layers are in contact at the edges, presenting a closed magnetic path that is absent in the other. Magnetization measurements by Kerr effect and magnetic domain observations are used to characterize the samples. The results show that the sample with the closed magnetic path performs better, reaching a value of 38 % of variation of the impedance and a sensibility of about 5%/Oe at 325 MHz. However, at lower frequencies the sample with open magnetic path presents higher values of the magnetoimpedance effect and its sensitivity than the one with the closed magnetic path. Finite element simulations confirm that this behavior is a direct consequence of the geometry of the samples.

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.

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%.

Comparison of Micro-Fabrication Routes for Magneto-Impedance Elements: Lift-Off and Wet-Etching

We compare two photolithographic methods to pattern magneto-impedance elements with micrometric dimensions, suitable to build magnetic microsensors. The magnetic material to be patterned is deposited by sputtering onto silicon wafers. It has a sandwiched structure where the outer magnetic layers are composed by a stack of permalloy and thin titanium layers and the central non-magnetic layer is made of copper. The total thickness is 1.31 μm. After preparation it displays an excellent magneto-impedance performance that is intended to be retained after the patterning procedure. For the lift-off method, a negative-type resist is deposited and patterned onto the silicon wafer prior to the material deposition. The undesired parts of the material are peeled off when removing the resist. Even for such a thick film, the quality of the obtained samples is quite good and the magneto-impedance response of the micro-shaped samples is great. In the wet-etching method, a positive-type resist is deposited and patterned onto the sputtered material. The parts that are not protected by the resist are etched using an acid solution. The different chemical behavior of the metals that constitute the sample severely challenges the wet-etching process. The results are poor, and the results compare unfavorably to those obtained by the lift-off process.

FeNi-Based Film Nanostructures for High Frequency Applications: Design and Characterization

FeNi films were deposited by DC magnetron sputtering at different Ar pressures. The structure and magnetic properties of the FeNi films are affected by the Ar pressure. Ferromagnetic resonance (FMR) measurements were done at a frequency of about 8.85 GHz. Both the value of resonance field and resonance line width show strong dependence on the Ar pressure: the lowest value of the resonance field and the narrowest resonance width correspond to the smallest argon pressure. Increase of the Ar pressure causes the films to have a significant perpendicular anisotropy with the easy axis pointing out of the plane. The magnetic properties and FMR were also studied for the [FeNi(170 nm)/Ti]n/FeNi(170 nm) (n = 1, 2, 5) structures prepared at the smallest Ar pressure. The FMR studies showed that the obtained multilayers are very robust: the value of the resonance field and resonance line width of the [FeNi/Ti]n/FeNi multilayers are very close to the corresponding values for the FeNi films.