Current Projects

Structural, Morphological, and Magnetic Characterization of the Exchange Coupled Structures

Project Summary A fundamental problem for layered magnetic materials is the influence of the interfaces on the magnetic properties of the composite structure. This is a complex problem since there are many different types of interfaces and the structural interface properties may be quite different from the magnetic interface properties. To determine the correlation between the structural and magnetic interface properties we are investigating the magnetic and structural properties of Fe/Pd/Fe and Fe/Ag/Fe structures. These structures can be grown with a variety of well-characterized interfaces and, depending on growth conditions, can show terrace widths varying from a few nanometers to a micron. A key measurement tool is the use of ferromagnetic resonance to determine magnetic interface quality through measurements of the linewidth. An important application of our results is to correlate magnetic and structural interface quality with the size of the giant magnetoresistance in these layered structures.

High Frequency Devices

Project Summary We are interested in both the fundamental and applied aspects of the physics of high frequency signal processing of electromagnetic waves in the 10-100 GHz range. This frequency range is particularly interesting and important as there are a number of possible military and civilian applications.

In terms of applied physics, we are working on the development of three different signal processing devices: a tunable notch filter, a tunable band pass filter and a tunable phase shifter. These devices use a magnetic multilayer system as their active element. This allows significant flexibility in achieving the required device parameters. The multilayers are produced by a variety of different methods including Molecular Beam Epitaxy, sputtering and e-beam evaporation. They are characterized by a series of ferromagnetic resonance measurements and by a microwave network analyzer. Our initial work on each of these devices already indicates substantial promise for practical devices in the 10-80 GHz range, since these magnetic multilayers are fundamentally new materials with properties which can be tailored by altering the components or by altering the layering pattern. We hope that this flexibility can be used in the future to explore the creation of fundamentally new devices. In order to make good devices, however, it is necessary to understand the fundamental physics that governs the properties of high frequency waves in magnetic materials. In particular, we are studying the mechanisms that govern the damping of the spin waves at high frequencies. In addition, we are carrying out theoretical calculations of the efficiency of the launching of electromagnetic waves in these devices.

Exchange Biasing in Fe/KCoF3 and Fe/KMnF3 Structures

Project Summary Despite the enormous efforts in the field of the ultrathin, artificially structured magnetic layers, there are still outstanding issues of paramount importance. One of the most significant is the fundamental problem of the interaction between a ferromagnet and an antiferromagnet. The aim of our experimental and theoretical project is to understand the nature of the interaction at the interface between ferromagnets and antiferromagnets.

To do this we have chosen to investigate the Fe/KMn1-xNixF3 and Fe/KCoF3 systems due to their unique properties. For example, in the Fe/KMn1-xNixF3 system we measure the magnetic excitations in both materials and study how the different magnetic states of the antiferromagnet (paramagnetic, antiferromagnetic, and spin canted) modify the coupling and magnetic configuration of the entire structure. We expect to correlate the strength of the observed exchange coupling with the magnetic structure of the antiferromagnet and the morphology of the ferromagnet/antiferromagnet interface. The structures are prepared using Molecular Beam Epitaxy and magnetically characterized by both dynamic and static techniques: Brillouin Light Scattering (BLS), Ferromagnetic Resonance (FMR), Magneto-Optical Kerr Effect, and SQUID.

Magnetic Multilayers and Thin Films

Project Summary We are investigating theoretically the properties of magnetic multilayers and thin films. These research projects are closely tied to experimental work performed in our CMMNS laboratory. The projects fall into three broad areas:

1) Static and Dynamic Magnetic Response of Multilayers and other Microstructured Materials

Magnetic multilayers have a diverse variety of interesting and complex magnetic phases. The determination of the static (phase diagram, hysteresis curves) and dynamic (resonance frequencies) behaviors in these different phases has only been explored in a limited manner so far. Partly this is because of the complexity of the necessary calculations. We use one calculation to determine both the equilibrium spin structure and its dynamic response. This allows us to study linear and nonlinear behavior as well. This method is used in studying surface phase transitions in Fe/Cr(211) structures where we calculate the dynamic susceptibility, hysteresis curves, and the possibility of obtaining chaotic behavior in the surface spin flop state. We also use this behavior to calculate the structure and response characteristics of arrays of magnetic dots where control of the lattice structure should allow the construction of materials with very different properties.

2) Magnetism in New Multilayer Structures and Ultrathin Films

Recently, ultrathin films and multilayers of antiferromagnetic insulators have been grown. The quality of these structures is now so high that one can discriminate between films which have an odd or even number of magnetic layers. There are now only a few studies of the static properties of these structures and no experimental results on the dynamic properties. This is, in part, due to the difficulty of such measurements, even though dynamic measurements will provide key material parameters such as exchange and anisotropy fields. We study the dynamic properties of ultrathin antiferromagnetic insulators in a waveguide structure. Such a structure should enhance the coupling between electromagnetic waves and the antiferromagnet by concentrating the wave in the antiferromagnet. These studies should help to understand the exchange biasing effect between ferromagnets and antiferromagnets. In addition such structures can be used for signal processing in the millimeter range.

3) Attenuation Lengths, Linewidths, and Pulse Distortion in High Frequency Microwave Structures

There is current interest in the use of very thin films of ferromagnetic materials such as Fe as the basis for high frequency microwave devices. Many of the key properties in such devices depend on the strength of the damping in these systems. We study damping due to eddy current losses and due to the roughness of the surfaces of the thin films.