N. Cramer

High attenuation tunable microwave notch filters utilizing ferromagnetic resonance

We have constructed a series of microstrips for transmission of microwaves. These microstrips incorporate ferromagnetic and dielectric layers and therefore absorb microwave energy at the ferromagnetic resonance (FMR) frequency. The absorption notch in transmission can be tuned to various frequencies by varying an external applied magnetic field. For our devices, which incorporate Fe as the ferromagnetic material, the resultant FMR frequencies range from 10–20 GHz for applied fields up to only 1000 Oe. This frequency range is substantially higher than those found in devices utilizing a dielectric ferrimagnet such as YIG. We constructed devices using monocrystalline Fe films grown in a molecular beam epitaxy system. Our devices are of different construction than other Fe dielectric microstrips and show much improvement in terms of notch width and depth. We observed maximum attenuation on the order of 100 dB/cm, much larger than previously reported values of 4 dB/cm.

Tunable Ferroelectric Capacitor-Based Voltage-Controlled Oscillator

Ferroelectric capacitors made from Ba10.5Sr0.5 TiO3 (BST) are applied as varactors in tunable, high-frequency circuit applications. In this context, a voltage-controlled oscillator (VCO) has been designed and implemented using discrete RF bipolar junction transistor (BJTs) and tunable ferroelectric capacitor. The designed VCO has a tuning range from 205 MHz to 216.3 MHz with a power dissipation of 5.1 mW. The measured phase noise is -90 dBc/Hz at 100 kHz and -140 dBc/Hz at 1 MHz offset

Thermally induced modification of GMR in Co/Cu multilayers: correlation among structural, transport, and magnetic properties

Co/Cu multilayers with individual layer thickness corresponding to the second maximum of the antiferromagnetic coupling were investigated as an attractive system for applications utilizing the giant magnetoresistance (GMR). Annealing causes distinct changes in the transport and magnetic properties of the multilayers. To obtain a deeper understanding of the triggering mechanisms, the variations in the layer and lattice structure occurring during heat treatment were studied by x-ray diffraction and electron microscopy. Structural and corresponding magnetic changes were observed according to characteristic temperature regions. The initial GMR of about 25% increased up to an annealing temperature of 400°C in correlation with the sharpening of the interfaces. Above 300°C grain growth sets in, and high coherency strains give rise to a transition from a 111 to a 100 texture. Above an annealing temperature of 400°C increasing layer defects cause a reduction of the GMR, and exceeding 600°C results in a complete transition from the multilayer configuration to a granular-like structure with large (~100 nm) domains of Co and Cu.

Incorporation of ferromagnetic metallic films in planar transmission lines for microwave device applications

We constructed a series of microstrip and coplanar microwave waveguides. These structures use metallic ferromagnets and therefore exhibit strongly frequency-dependent attenuation and phase-shift effects. The lines have maximum attenuation peaks occurring at the ferromagnetic resonance frequency, which increases with applied magnetic field. Such properties are used in band-stop filters. The devices used monocrystalline Fe films grown by molecular beam epitaxy and polycrystalline sputtered permalloy films. For our devices that incorporated Fe the band-stop frequencies ranged from 10-20 GHz for applied fields up to only 80 kA/m (1000 Oersted). For devices using permalloy, the band-stop frequency was in the 5-10 GHz range for applied fields less than 80 kA/m. The maximum power attenuation was about 100 dB/cm, much larger than the previously reported values of 4 dB/cm. The resonance condition also affects the phase of the transmitted wave, strongly changing phase above and below the resonance frequency. The result is a phase-shifter that is tunable with applied magnetic field. We observed phase changes of over 360/spl deg//cm with an applied field of less than 40 kA/m.

A 1-GHz active phase shifter with a ferroelectric varactor

An active phase shifter circuit implemented with discrete components is reported. The tuning element, a ferroelectric varactor, is a parallel plate capacitor with Ba/sub 0.5/Sr/sub 0.5/TiO/sub 3/ (BST) as the dielectric. The circuit consists of two bipolar junction transistors coupled with a feedback network, which contains the varactor and thus produces a transfer function that can be varied with a control voltage. The active nature of the circuit allows for signal gain, while the BST varactor provides a high-Q tuning element. This represents an improvement over strictly passive phase shifters with ferroelectric elements. Circuit simulation results are presented and compared with measured data from the implemented system. The network, even with markedly nonideal transistors, can provide a true all-pass response over the frequency band of interest (200-1100 MHz). The measurement results demonstrate an analog tunability of about 100/spl deg/ with a gain variation of about 0.6 dB at I GHz when using a BST capacitor with a tunability of 2.75:1.

Verilog-A Modeling of Ferroelectric High-K Capacitors for Tunable Circuit Applications

Ferroelectric capacitors have shown very good prospect for use in circuits where voltage dependent tuning is required. We are studying high-K capacitors made from Ba1 - xCaxTi1 - yZryO3 (BCTZ) to fully characterize their tunable and high frequency behavior. A model for capacitance-voltage, current-voltage, and impedance-frequency relations is presented. The model can be used for simulation of high frequency tuning circuits.

Effect of annealing on leakage current in Ba0.5Sr0.5TiO3 and Ba0.96Ca0.04Ti0.84Zr0.16O3 thin films with Pt electrodes

Ba0.96Ca0.04Ti0.84Zr0.16O3 (BCTZ) and Ba0.5Sr0.5TiO3 (BST) thin films were deposited at a substrate temperature of 450°C via rf magnetron sputtering to form Pt∕BCTZ∕Pt and Pt∕BST∕Pt capacitors. BCTZ thin films hold promise as an alternative to BST in capacitor applications due to the resistance of BCTZ to reducing atmospheres. In order to produce BST films with low dc leakage current, oxygen is routinely used during film growth and often afterwards during anneals. The effect of postannealing the capacitors in either forming gas or oxygen at temperatures up to 700°C (BST) and 800°C (BCTZ) were studied. The leakage mechanism is shown to be dominated by Schottky emission and the Schottky barrier height is reported as a function of anneal gas composition and anneal temperature.

Thermally activated deterioration processes in Co/Cu GMR multilayers

The magnitude of the giant magnetoresistance (GMR) observed in multilayers is known to change irreversibly at elevated temperatures. To improve the thermal stability of devices, a fundamental understanding of the GMR and its correlation to the temperature-induced structural and morphological changes in a given system is mandatory. We therefore investigated the structural and magnetic properties of sputtered Co/Cu multilayers in the pre- and postannealed states (temperature regime for annealing up to 750 °C) by in situ x-ray diffraction, transport measurements, ferromagnetic resonance (FMR), and magneto-optical Kerr effect (MOKE). We were able to identify a sequence of distinct structural changes each of which sets in above a characteristic critical temperature. These critical temperatures depend strongly on the thickness of the individual layers. The structural alterations observed range from interfacial sharpening through texture reorientations up to the formation of a granular state, and are associated ...

Size dependence of magnetic domain patterns in exchange-biased Permalloy/NiO microstructures

The magnetic domain structure in Permalloy (Ni81Fe19) micropatterns (10?100??m) on NiO has been investigated by means of soft x-ray photoemission electron microscopy. The exchange anisotropy between the Ni81Fe19 patterns and the NiO layer results in the formation of complex domain structures which markedly differ from the simple Landau?Lifshitz configurations. The domain structures reflect the competition between the exchange anisotropy and the dipole?dipole interaction in a weakly coupled system. The observed domain structures change with the feature size, as the domain patterns lose complexity in the smaller structures.

Temperature dependence of anisotropy in exchange biased Fe/KCoF3 system

We used molecular beam epitaxy to deposit a novel ferro-/antiferromagnet (Fe/KCoF3) system on gallium terminated GaAs (100) substrates. We varied the thicknesses of single crystal Fe (001) layers from 1.05 to 3 nm. The antiferromagnetic fluoride, with a thickness of 30 nm, was deposited either in a single-crystal or a polycrystalline form, depending on the deposition conditions. KCoF3 is an antiferromagnet with a Neel temperature of 114 K. Its cubic structure almost perfectly matches the Fe film structure. The growth was monitored by reflection high energy electro diffraction. The magnetic properties of the system were studied using ferromagnetic resonance. Uniaxial and unidirectional anisotropy fields, due to exchange bias, were measured at low temperatures in the field-cooled samples and were smaller than 45 and 72 Oe, respectively. Both anisotropy fields, unidirectional and uniaxial, decreased with increasing thickness of the Fe film or with increases in temperature. The temperature dependence of the f...