Ian Harward

High-frequency signal processing using ferromagnetic metals

We present results for tunable microwave band-stop and bandpass filters on a microstrip geometry. These structures, prepared by sputtering on GaAs substrates, are compatible in size and growth process with on-chip high-frequency electronics. For the notch filters, we observed power attenuation up to /spl sim/100 dB/cm and an insertion loss on the order of /spl sim/2-3 dB for both Permalloy- and Fe-based structures. The operational frequency ranges from 5 to 35 GHz for external fields below 5 kOe. We discuss methods to increase operational frequency and reduce device linewidth. Using these techniques we are able, for example, to obtain an operational frequency of 11GHz at zero applied field and to narrow the device linewidth from 3 GHz to 330 MHz. The operational frequency, which can be obtained from the ferromagnetic resonance condition, is set by material properties such as saturation magnetization M/sub s/, anisotropy fields, the gyromagnetic ratio, and the magnitude of an applied field H. Thus, by using different materials and external fields one can create devices which function over a wide range of frequencies.

A broadband ferromagnetic resonance spectrometer to measure thin films up to 70 GHz.

We report the development of a broadband ferromagnetic resonance (FMR) system operating in the frequency range from 10 MHz to 70 GHz using a closed-cycle He refrigeration system for measurements of thin films and micron/nano structures. The system is capable of carrying out measurements in frequency and field domain. Using two coplanar waveguides, it is capable of simultaneously measuring two samples in the out of plane and in plane FMR geometries. The system operates in the temperature range of 27-350 K and is sensitive to less than one atomic monolayer of a single crystal Fe film.

Physical properties of Al doped Ba hexagonal ferrite thin films

We developed the thin film microwave magnetic material, M-type barium hexagonal ferrite (BaM) doped with Al, for signal processing devices operating above 40 GHz with little to no applied magnetic field. Al was chosen as the dopant material because it significantly increases the already strong anisotropy field of BaM. A series of thin film BaAlxFe12-xO19 samples, x ranging from 0 to 2 in 0.25 steps, were deposited on Pt templates using a metal-organic decomposition growth technique. The resulting films are polycrystalline and highly textured, with the hexagonal c-axis directed out of plane. These films are also self-biasing; easy axis hysteresis loops have a high squareness ratio, s, in the 0.83-0.92 range. As expected, the anisotropy field increases with x, ranging from 1.34 to 2.19 × 106 A/m (16.9-27.5 kOe) for x = 0-2, while the saturation magnetization Ms decreases with x, ranging from 0.334 to 0.175 × 106 A/m (4πMs = 4.2-2.2 kG) for x = 0-2. These values were measured at room temperature, but the tem...

Iron-based microstrip band-stop filters at higher microwave frequency range: Design optimization using shape anisotropy

Microwave filters that use thin films of ferromagnetic metals are now being established as an option compared to YIG based filters due to their higher frequency response at very small magnetic fields. The aim of the present investigation is to significantly boost the operating frequency of notch filters in very low applied magnetic fields. To do this, we fabricated a series of notch filters with Fe films of different thicknesses and shapes. The stop-band response of each of these filters cover four waveguide bands (X, KU, K and Ka) ranging from 8to40GHz with an applied magnetic field up to 4.5kOe. The frequency characteristics of these filters at zero field can be significantly changed by changing the geometry of the magnetic element. For example, an Fe film which is 7.5μm wide, 100nm thick, and 3mm long has a stop band centered at 9.5GHz, but this is increased to 22GHz for an 800nm thick Fe film. For narrower signal lines (4.5μm wide), we observed further increases in the frequencies: 11GHz for a 100nm F...

Magnetically tunable micro-strip band-stop filter : Design optimization and characterization

We investigate ultra-small band-stop filters made with continuous Fe films and with multilayered Fe∕Cu films. In ferromagnetic resonance measurements, the continuous Fe(100nm) film had a larger linewidth, on the order of 200Oe at 10GHz, resulting in a device linewidth, which typically was close to 2GHz in the operational device. In contrast a [Fe(5nm)∕Cu(0.8nm)]20 multilayer structure showed a ferromagnetic resonance linewidth of 50Oe at 10GHz and had a device linewdith of about 0.5GHz. We expect that the breaking of the Fe films by Cu reduces the typical size of crystallites in Fe, thus reducing the linewidth. The filter demonstrates wide tuning range of 10–30GHz with bias fields up to 4kOe. This work demonstrates a single notch-filter, which can operate from X- to K-band with a linewidth of 1.8GHz at 0.07kOe and 0.33GHz at 4.0kOe. The measured minimum insertion loss in each case is ∼2.5dB, with greater than 15dB return loss in the entire pass-band frequency.

Nonlinear amplification and mixing of spin waves in a microstrip geometry with metallic ferromagnets

We explore the nonlinear mixing and amplification of magnetic polariton modes in ultrasmall waveguides. Ultrasmall waveguide geometries can produce large oscillating microwave fields—up to about 500 Oe. Using these large fields, we examine nonlinear ferromagnetic dynamics in ribbons of Permalloy and Fe. In particular if two microwave signals at different frequencies are sent into the waveguide, we can increase the transmission of one wave by adding energy to the other wave. We also demonstrate the creation of new frequencies and the development of a comb of equally spaced frequencies. These experimental results are explained with perturbation theory and micromagnetics calculations.

On-wafer magnetically tunable millimeter wave notch filter using M-phase Ba hexagonal ferrite/Pt thin films on Si

A prototype of a fully integrated on-wafer, magnetically tunable band-stop filter operating at millimeter wave frequencies is demonstrated on a Si substrate. In contrast to earlier studies, the filter uses a very thin barium hexagonal ferrite film incorporated into the dielectric layer of a microstrip transmission line to filter the signal. The zero-field operational frequency is about 34 GHz, increasing linearly with the strength of a static, perpendicularly applied magnetic field at a rate of about 2.7 GHz/kOe. Experimentally, high signal attenuation (33–67 dB/cm) at the resonance frequency and insertion losses as low as 4.5 dB were simultaneously observed, while the 3 dB device bandwidths were generally below 1 GHz. Our calculations are in quantitative agreement with the experimental results. We also find an important result that the thickness and conductivity of the Pt ground plane plays a key role in insertion losses, indicating directions for further improvements.

Exponentially decaying magnetic coupling in sputtered thin film FeNi/Cu/FeCo trilayers

Magnetic coupling in trilayer films of FeNi/Cu/FeCo deposited on Si/SiO2 substrates have been studied. While the thicknesses of the FeNi and FeCo layers were kept constant at 100 angstrom, the thic ...

Increasing Operational Frequency in Microwave Devices by Using [SmCo/NiFe] Multilayered Structures

We present here the application of an exchange spring multilayer system in an on-chip microwave device. The microwave devices were made in a coplanar geometry using a [SmCo/NiFe]10 sputtered multilayer structure as the active material. At low fields we find an up shift of the operational frequency by more than 15 GHz for the multilayer system compared to the NiFe alone. For higher fields (above 2 kOe) the increase in operational frequency is about 8-10 GHz. In contrast to previous results using an oriented SmCo film, we find in our polycrystalline film that there is not a large difference between frequencies measured with positive magnetic field compared to those measured with negative magnetic field. We studied multilayer systems with different thicknesses of NiFe. Magneto-optical Kerr effect measurements show a distinct uniaxial anisotropy for structures with 30-nm NiFe. Thinner NiFe films did not result in a clear anisotropy. Nonetheless, a substantial frequency shift was measured for all the samples. These measurements indicate that exchange coupled structures can substantially increase the frequency of signal processing devices

Analytical analysis of a multilayer structure with ultrathin Fe film for magneto-optical sensing.

Magneto-optic (MO) response in nanostructures with ultrathin Fe considered for the MO mapping of current pulses with a two-dimensional diffraction limited resolution is investigated in detail. The structures consist of an ultrathin Fe layer sandwiched with dielectric layers, deposited on a reflector and covered by a noble metal protecting layer. The structures are modeled as five-layer systems with abrupt interfaces. Analytical expressions are provided that are useful in the search for the maximum of MO reflected wave amplitude polarized perpendicular to the incident linearly polarized wave, |ryx((05))|. The procedure of finding the maximal |ryx((05))| is illustrated on the structures with ultrathin Fe at the laser wavelength of 632.8 nm. The maximal |ryx((05))| of 0.018347 was achieved in the structure AlN(52 nm)/Fe(15 nm)/AlN(26 nm)/Au. The deposition of a 5 nm protecting Au layer reduced |ryx((05))| by 6 per cent.