Michael Frommberger

Nanostructured magnetic Fe–Ni–Co/Teflon multilayers for high-frequency applications in the gigahertz range

Thin multilayer films of sputtered polytetrafluoroethylene (Teflon) and Fe54Ni27Co19 with different layer thicknesses were prepared by vapor-phase tandem deposition. The films show ferromagnetic resonance frequencies from 3.0to4.7GHz and a high-frequency permeability in the range from 100 to 175, while having negligible losses up to 700MHz and a quality factor Q up to 12 at 1GHz. Thus these films could be promising candidates as high-frequency components used, for example, in mobile communication electronics.

Magnetic anisotropy and domain patterning of amorphous films by He-ion irradiation

The magnetic anisotropy in amorphous soft magnetic FeCoSiB films was modified by He-ion irradiation. A rotation of uniaxial anisotropy depending on the applied field direction in the irradiated areas is observed by magnetometry and complementary domain observation by Kerr microscopy. No significant degradation in magnetic properties relative to the as-deposited state is found from the magnetization loops on nonpatterned films. Using irradiation together with photolithography, the films were treated locally, resulting in “anisotropy patterned” structures. Complicated periodic domain patterns form due to the locally varying anisotropy distribution. Overall magnetic properties and domain patterns are adjusted.

Integration of crossed anisotropy magnetic core into toroidal thin-film inductors

A new approach to RF thin-film inductors with an integrated magnetic core has been investigated. A toroidal inductor design was realized in thin-film technology aiming at small-signal applications in the frequency range from 10 MHz to 1 GHz. The magnetic core consists of a multilayer of sputter deposited soft magnetic FeCoBSi. The individual magnetic films were deposited in a way to realize a crossed magnetic anisotropy in the core. High-frequency measurements of the multilayers already illustrated the advantages of the unique magnetic geometry. The influence and benefit of such a magnetic core on the toroid microinductor is discussed. The results show that such crossed anisotropy microinductors are a very promising alternative to common planar spiral inductors in the RF range.

High-frequency magnetoelastic multilayer thin films and applications

For the realization of new applications in the field of high-frequency devices with incorporated magnetic layers, like for example coils, transformers, transmission-line structures and especially non-contacting stress sensors, the necessary thin film materials have to be tailored in order to satisfy simultaneously a broad set of materials requirements. These are: a high ferromagnetic resonance frequency, a high magnetic permeability in combination with low losses at the working frequency (0.1 up to 2.45 GHz), a controllable magnetic anisotropy and domain structure, in case of films for mechanical sensors a high change of permeability when stress is applied. Furthermore these materials have to be compatible to the surrounding materials and process steps, which are needed to fabricate the devices.

Stress-induced remagnetization in magnetostrictive films

Strictly stress-induced magnetization reversal in magnetostrictive Co50Fe50/Co80B20 multilayers was studied by magneto-optical Kerr microscopy. The remagnetization takes place by rotational processes in externally applied stress in the absence of magnetic fields. Multiple small-angle domains develop at low magnetic anisotropy. Cross-tie and “fir-tree”-like domain structures appear. Magnetoelastic energy contributions in the substrate–film structure are responsible for the low-angle domain wall patterns. Additional effects from the crystalline nature of the films are discussed.

High-frequency properties of FeCoSiB thin films with crossed anisotropy

Investigations on amorphous FeCoSiB/SiO/sub 2//FeCoSiB thin-film samples with crossed anisotropies are presented in this paper. The samples consist of two magnetic layers, 40 nm thick each, where the second magnetic layers uniaxial anisotropy is rotated by an angle of 90/spl deg/ with respect to the anisotropy of the first layer. The individual layers were separated by a 20 nm SiO/sub 2/ thin film. The angular dependent magnetic properties of the samples have been characterized by vibrating sample and inductive magnetometry. The high-frequency (HF) response was determined by HF-permeameter measurements and pulsed inductive microwave magnetometry (PIMM). Good HF-response independent of the magnetic field excitation angle is achieved. The results are compared to an 80 nm FeCoSiB reference film with uniaxial anisotropy. Our results provide new insights important for the design of HF magnetic devices.

Processing and application of magnetoelastic thin films in high-frequency devices

Magnetoelastic thin films and multilayers were optimized to exhibit a high inverse magnetostrictive effect and a high ferromagnetic resonance frequency. FeCo/CoB and FeCoBSi films were used in two promising applications, a resonant LC circuit and a transmission line. In both cases the basic structure is a strip line with magnetic sandwich layers, which increase the inductance. In the resonant circuit design, a magnetic field or stress applied to the device changes the permeability of the magnetic layers and therefore the inductance and the resonance frequency of the device. Resonance frequencies of about 500 MHz with quality factors of up to 8 could be achieved. Such LC circuits can be used as remote interrogated stress or torque sensors. The second design is a transmission-line device, which shows a large wavelength shortening effect and can be used as a band filter in GHz applications.

Magnetoelastic thin films and multilayers for high-frequency applications

Magnetoelastic and magnetic high-frequency properties of single and multilayer thin films are investigated with respect to applications in micro-inductors and as remote-interrogated sensors for mechanical quantities. The basic structure is a thin film inductor incorporating a magnetic material in order to enhance the inductance. In case of a sensor the inverse magnetostriction is used to change the permeability of the magnetic material and thus the inductance of the device. Materials properties like saturation magnetization, anisotropy field, resulting domain structures, electrical resistivity as well as the stress state of the films have to be adjusted carefully in order to meet certain requirements like high cut-off frequencies combined with low losses, and a high and controllable inverse magnetostrictive effect. In this paper (Fe-Co/Fe-Co- B-Si) multilayers are investigated in terms of their magnetic, high-frequency, and structural properties. (Fe50Co50Cp80B20) multilayers, e.g., have been fabricated which show ferromagnetic resonance frequencies up to 5 GHz while exhibiting a high inverse magnetostrictive effect up to the GHz frequency range. The obtained results for first LC circuits realized using (Fe50Co50/Co80B20) multilayers are discussed.

Magnetic properties and domain formation in amorphous films anisotropy patterned by ion irradiation

The amorphous soft magnetic films of Fe/sub 70/Co/sub 8/B/sub 10/Si/sub 12/ were prepared on silicon substrate by rf magnetron sputtering. The magnetic properties and domain formation in the films patterned by cobalt and helium ion irradiation were investigated by magneto-optical Kerr effect magnetometry.

Magnetostrictive LC-circuits as mechanical sensors

A remote interrogatable highly sensitive mechanical sensor based on softmagnetic magnetostrictive thin films has been developed. This sensor is very interesting for stress and torque measurements on rotating or hidden objects. It consists of a resonating circuit (LC-tag) with a magnetically sandwiched strip inductor and a thin film capacitor. The resonance frequency of this device is about 500 MHz with quality factors about 8. First measurements with the sensor in a bending test jig have shown high sensitivity to applied strain. A figure of merit (defined as ∆f/f 0/∆e on the order of 1000 could be obtained.