T. J. Fal

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.

Millimeter wave phase shifter based on ferromagnetic resonance in a hexagonal barium ferrite thin film

A hexagonal ferrite thin film-based planar millimeter-wave phase shifter was demonstrated. The device made use of an M-type barium ferrite (BaM) thin film prepared by pulsed laser deposition and a coplanar waveguide geometry. The phase tuning relied on ferromagnetic resonance in the BaM film. The device showed a phase tuning rate of 43°/(mm kOe) and an insertion loss of 3.1 dB/mm in the on-resonance regime. In off-resonance regimes, the device showed smaller loss and smaller tuning rates. The experimental results were confirmed by theoretical calculations.

Hexagonal ferrites for use in microwave notch filters and phase shifters

Recently there has been interest in small, planar, high frequency (10–50 GHz) signal processing devices which are based on metallic ferromagnets. This paper, in contrast, presents theoretical results for devices utilizing a hexagonal ferrite. The notch filter results show attenuation of frequencies in the 40–60 GHz range with applied fields in the 1–5 kOe range. For geometries similar to the ultrasmall metallic devices, the transmission loss at the notch can be from 20 to 140 dB/cm depending on the thickness of the hexagonal ferrite film and the inclusion of dielectric spacers. The phase shifter results show phase shifts of up to 360°, with losses below 2 dB/cm. These results are obtained for devices using thin films around 1 μm in thickness for the hexagonal ferrite, and for reasonable linewidths below 200 Oe.

Effects of adjusting the position of the magnetic layer in magnetic notch filters

We study small thin-layer magnetic notch filters which operate in the 5–40 GHz range. Past theoretical work has concentrated on a structure where the magnetic film was right next to one of the conductive films in a waveguide. Here we present a theoretical model, which investigates the properties of a waveguide with two dielectric films and one magnetic film placed between two outer conductive layers. The results show this more general structure produces a deeper attenuation and a narrower peak compared to the earlier structure. The additional attenuation varies from 0 to 30 dB/cm, depending on the thickness and position of the magnetic film. This article also examines the reflection of the guided waves as they enter the notch filter. The results from an effective medium calculation show that a signal experiences the largest return losses near the ferromagnetic resonance frequency of the magnetic film, with typical losses below −4 dB. The return loss can be reduced significantly if the linewidth in the fer...

Domain wall and microwave assisted switching in an exchange spring bilayer

We explore the response of a magnetic bilayer to a driving microwave field using micromagnetic simulations. The bilayer consists of 8 nm of a material with a high uniaxial anisotropy and 56 nm of a material with a lower uniaxial anisotropy. The width and length of the structure is 100 × 100 square microns. A small applied field, opposite to the magnetization, switches most of the lower anisotropy material but not the higher anisotropy material, forming a domain wall between the two materials. We evaluate the frequencies of the magnetic eigenmodes for the entire system using Fourier analysis and then drive the structure with an oscillating magnetic field at each of the eigenfrequencies. When the oscillating microwave field is added, the static switching field required to align both layers is decreased compared to the undriven case. With a driving field strength of 120 Oe the switching field is reduced by about 40%, from 1.12 kOe for the undriven case to 0.55 Oe for the driven case.

Non-reciprocal devices using attenuated total reflection and thin film magnetic layered structures

There is need for non-reciprocal devices such as circulators and isolators. Although such devices are common at frequencies below 10 GHz, there is a lack of compact, low-weight, devices at higher microwave frequencies. This paper examines the non-reciprocal behavior associated with attenuated total reflection (ATR) for multi-layered dielectric and magnetic structures. Non-reciprocal behaviors produced by ATR have been explored for semi-infinite magnetic materials. This paper focuses on ATR behavior with magnetic films of finite thickness, from thick layers of around 3 cm to thin layers of about 1 µm. The results show significant non-reciprocity even for magnetic layers less than 0.1 cm thick, with reflection loss differences of more than 30 dB between positive and negative signal propagation. Results are presented for yttrium iron garnet and M type barium hexagonal ferrites. The two materials allow nonreciprocal behavior at different frequencies, 5–20 GHz for the garnet and 45–80 GHz for the hexagonal fer...

Iron based microstrip phase shifter; optimization of phase shift

A microwave phase shifter is a device used to introduce phase change in a propagating electromagnetic wave in a waveguide. A series of microstrip transmission lines, with an iron film of 300 nm thickness placed at various places inside a SiO2 dielectric layer, was fabricated and tested as phase shifters based on ferromagnetic resonance principle. It is observed that the differential phase shift obeys a Sin2 – law (derived from perturbation theory) given by; Δβ ∝ Sin2(πx/h). Here, x is the distance of Fe film inside SiO2 dielectric from the Cu conductors, h is the height of SiO2 dielectric. This give Δβ as minimum when Fe film is at the two edges of the dielectric and maximum when Fe is at the center of the dielectric. The differential phase shift varies as high as 350° at resonance (20 GHz), when Fe is at the middle of the dielectric. For high (at 25 GHz) and low (at 8 GHz) frequency operation, far above and far below resonance, the differential phase shift is ∼125° and 250° when Fe at middle of the diele...

Microwave assisted switching In bit patterned media: Accessing multiple states

Using a micromagnetics calculation, we explore the properties of a submicron magnetic square with microwave assisted switching. For a 10×160×160 nm3 structure of Fe–Ti–N, there are three particular stable magnetic states for reversal fields up to −320 Oe. One can switch between these different states by adding a microwave field. The strength and the frequency of the microwave field determine the final state. A microwave field of up to 30 Oe does not change the magnetization. Fields of 50 to 75 Oe result in an intermediate state, while larger microwave fields produce a reversed ground state.

Ultrathin magnetic multilayer films for low-field microwave notch filters

Microwave filters that use thin films of ferromagnetic metals are now being established as a valuable option compared to yttrium iron garnet based filters due to their higher frequency response. In these filters the signal propagation is inhibited over a wide frequency band, depending on the applied dc magnetic field. However, the continuous application of an applied field to achieve an operating frequency in the higher gigahertz range increases the power consumption of the device. The main contribution of this article is to provide techniques which significantly boost the operating frequency of notch filters in zero or very low applied magnetic fields. To do this, the authors fabricated high quality epitaxial Fe films which are interlayer exchange coupled through nonmagnetic Si layer of different thicknesses. The films were used in flip-chip geometry on top of a Cu-coplanar waveguide to create band-stop filters. In contrast to filters based on Fe alone, the multilayer filters can operate above 25GHz with...