R. E. Camley

Characterization of a novel metabolic strategy used by drug-resistant tumor cells

Acquired or inherent drug resistance is the major problem in achieving successful cancer treatment. However, the mechanism(s) of pleiotropic drug resistance remains obscure. We have identified and characterized a cellular metabolic strategy that differentiates drug‐resistant cells from drug‐sensitive cells. This strategy may serve to protect drug‐resistant cells from damage caused by chemotherapeutic agents and radiation. We show that drug‐resistant cells have low mitochondrial membrane potential, use nonglucose carbon sources (fatty acids) for mitochondrial oxygen consumption when glucose becomes limited, and are protected from exogenous stress such as radiation. In addition, drug‐resistant cells express high levels of mitochondrial uncoupling protein 2 (UCP2). The discovery of this metabolic strategy potentially facilitates the design of novel therapeutic approaches to drug resistance.—Harper, M.‐E., Antoniou, A., Villalobos‐Menuey, E., Russo, A., Trauger, R., Vendemelio, George, A. M., Bartholomew, R., Carlo, D., Shaikh, A., Kupperman, J., Newell, E. W., Bespalov, I. A., Wallace, S. S., Liu, Y., Rogers, J. R., Gibbs, G. L., Leahy, J. L., Camley, R. E., Melamede, R., Newell, M. K. Characterization of a novel metabolic strategy used by drug‐resistant tumor cells. FASEB J. 16, 1550–1557 (2002)

Magnetic multilayers: spin configurations, excitations and giant magnetoresistance

The authors discuss some of the fundamental properties unique to magnetic multilayers. Complex spin configurations are examined for many different systems and are shown to arise from a simple competition between exchange and Zeeman energies. The spin configurations found in multilayer systems determine macroscopic properties such as the static susceptibility and magnetization, and can lead to anomalous field and temperature behaviour. The authors also discuss the dynamic behavior of magnetic multilayers. Emphasis is placed on spin waves in magnetic multilayers with canted spin configurations and the softening of modes at magnetic phase transitions. Furthermore they show that spin wave excitations provide a powerful method for studying exchange interactions and spin configurations. Finally, the phenomenon of giant magnetoresistance in magnetic multilayers where the resistivity of the metallic structure can be changed by over 60% at room temperature, is discussed. Simple theoretical approaches are used to understand and predict the properties of the multilayer systems and comparisons between theory and experiment are stressed.

Nonreciprocal surface waves

Some surface waves have the property that a simple reversal of the direction of propagation leads to a surface wave with a different frequency. In this paper we explore the properties of such nonreciprocal waves for a variety of excitations and structures. For a semi-infinite geometry the nonreciprocity can be quite dramatic. For example in some magnetic systems, a surface wave exists for propagation in one direction, but no surface wave exists for propagation in the reverse direction. In thin films, it is generally the localization of the surface wave which is nonreciprocal. Thus a surface wave travelling along the + x axis may be localized at the upper surface of the film, while a surface wave travelling along the - x axis is localized at the lower surface of the film. We will discuss nonreciprocal surface excitations which occur in a variety of systems. The elementary properties of nonreciprocal surface plasmons, polaritons, phonons and magnons are derived. We briefly review some of the applications of nonreciprocal surface waves to practical signal processing devices. Finally we discuss the microscopic origin of nonreciprocity.

Tunable high-frequency band-stop magnetic filters

We present results on Fe- and Permalloy™-based microstrip microwave band-stop filters. These structures, prepared on GaAs substrates, are compatible in size and growth process with on-chip high-frequency electronics. We observed power attenuation of 100 dB/cm for Permalloy and 180 dB/cm for Fe. The insertion loss is low: 2–3 dB for Permalloy and 3–5 dB for the Fe-based structures. Our geometry includes a significant boost to the zero-field operational frequency due to the shape anisotropy of the magnetic element in the microstrip. Using the shape anisotropy, we create a Fe-based filter that operates at 11 GHz with zero applied field.

Nonreciprocal microwave devices based on magnetic nanowires

We use magnetic nanowires in an alumina matrix as the active element in microwave nonreciprocal resonance isolators. The design is related to waveguide E-plane isolators but is planar and much smaller than typical waveguide isolators. There is a nonreciprocal attenuation of the wave in forward and reverse directions. The isolation is about 6 dB/cm at 23 GHz. The bandwidth of the device is relatively large (5–7 GHz) in comparison to ferrite-based devices. The central frequency of the device can be tuned with the application of magnetic field.

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.

Theory of microwave propagation in dielectric/magnetic film multilayer structures

We explore the theory of microwave propagation in dielectric films, on which thin metallic ferromagnetic films have been deposited. Our aim is to study coupling between the microwave electromagnetic fields, and spin excitations in the ferromagnetic films. We present quantitative studies of attenuation provided by coupling to spin excitations, for various model structures including superlattices. We find strong attenuation of the microwaves, for frequencies near the ferromagnetic resonance frequency of Fe. Modest magnetic fields place this resonance above 20 GHz, and allow its frequency to be tuned. We note a transmission minimum occurs near the frequency γ(H0+4πMs), which is in the 70 GHz range for external magnetic fields H0 of a few kilograms. We explore the dependence of these phenomena on film thicknesses, and argue that such structures will move suitably for high frequency microwave devices.

Exchange bias of NiO/NiFe: Linewidth broadening and anomalous spin-wave damping

We studied sputtered NiO(150 nm)/NiFe exchange-biased films using Network Analyzer ferromagnetic resonance spectroscopy (NA-FMR) and Brilliouin light scattering (BLS) techniques. The complex permeability spectra were obtained for NiO/NiFe films and were fitted to the Landau–Lifshitz–Gilbert equation to determine intrinsic and extrinsic contribution to Gilbert damping, in addition to other magnetic parameters. The exchange anisotropy (HEX) was determined from the field variation data of NA-FMR resonance frequency (fres) and BLS mode frequency (fm). HEX was observed to decreases as 1/thickness, from where we derive macroscopic interfacial exchange energy JE=0.021 erg/cm2. Second, we investigated the relaxation mechanism in NiO/NiFe films from NA-FMR linewidth (Δfres) for wave vector k=0 mode and from BLS mode linewidth (Δfm) for k≠0 modes. Interestingly, we observed Δfres to increase with increasing magnetic field but Δfm was observed to decrease with increasing magnetic field. Therefore, it is confirmed th...

Liquid crystal phase shifters at millimeter wave frequencies

We demonstrate an on-wafer liquid crystal phase shifter which has a tunable 0–300°/cm phase shift at 110 GHz. The results show no dispersion over the entire frequency range indicating a tunable “true time delay” of up to 2.5 ps/cm at all frequencies. The inherent losses in the liquid crystal are small, less than 1 dB/cm over the range of 1–110 GHz. The full tunability is achieved using small voltages, close to 10 V. We anticipate that one could achieve a phase shift of 600°/cm at 220 GHz.

EXCHANGE-SPRING SYSTEMS : COUPLING OF HARD AND SOFT FERROMAGNETS AS MEASURED BY MAGNETIZATION AND BRILLOUIN LIGHT SCATTERING (INVITED)

An experimental and theoretical study is presented of the normal magnetic modes in spiral ferromagnetic structures. The bilayer system studied consists of Fe layers (25, 50, 100, and 200 A thick) that are exchange coupled to 200 A thick SmCo films that have ≈200 kOe anisotropies. The Fe spiral—induced by an external magnetic field that is applied opposite to the direction of the magnetized film—results in a structure similar to that encountered in a Bloch domain wall. The magnetization and the field dependence of the magnons in various Fe films are explained by the theoretical model.