Self-sustained magnetoelectric oscillations in magnetic resonant tunneling structures

The dynamic interplay of transport, electrostatic, and magnetic effects in the resonant tunneling through ferromagnetic quantum wells is theoretically investigated. It is shown that the carrier-mediated magnetic order in the ferromagnetic region not only induces, but also takes part in intrinsic, robust, and sustainable high-frequency current oscillations over a large window of nominally steady bias voltages. This phenomenon could spawn a new class of quantum electronic devices based on ferromagnetic semiconductors.Comment: 5 pages, 4 figure

Ion-supported tori: a thermal bremsstrahlung model for the X-ray Background

We discuss the possibility that a significant contribution of the hard X-ray Background is the integrated emission from a population of galaxies undergoing advection-dominated accretion in their nuclei. Owing to poor coupling between ions and electrons and to efficient radiative cooling of the electrons, the accreting plasma is two-temperature, with the ions being generally much hotter than the electrons and forming an ion-supported torus. We show that the electron te mperature then saturates at approximately 100keV independent of model parameters. At this temperature the hard X-ray emission is dominated by bremsstrahlung radiation. We find that this physical model gives an excellent fit to the spectrum of the XRB in the 3-60 keV range, provided that there is some evolution associated with the spectral emissivity which must peak at a redshift of about 2. We estimate that such galaxies contribute only to a small fraction of the local X-ray volume emissivity. The model implies a higher mean black hole mass than is obtained from the evolution of quasars alone.Comment: 7 pages, 7 ps figures, uses mn.sty (included). Submitted for publication to MNRA

Magnetic flares in accretion disc coronae and the Spectral States of black hole candidates: the case of GX 339-4

We present a model for the different X-ray spectral states displayed by Galactic Black Hole Candidates (GBHC). We discuss the physical and spectral implications for a magnetically structured corona in which magnetic flares result from reconnection of flux tubes rising from the accretion disk by the magnetic buoyancy instability. Using observations of one of the best studied examples, GX339-4, we identify the geometry and the physical conditions characterizing each of these states. We find that, in the Soft state, flaring occurs at small scale heights above the accretion disk. The soft thermal-like spectrum is the result of heating and consequent re-radiation of the hard X-rays produced by such flares. The hard tail is produced by Comptonization of the soft field radiation. Conversely, the hard state is the result of flares triggered high above the underlying accretion disk which produce X-rays via Comptonization of either internal synchrotron radiation or soft disk photons. The spectral characteristics of the different states are naturally accounted for by the choice of geometry: when flares are triggered high above the disk the system is photon-starved, hence the hard Comptonized spectrum of the hard state. Intense flaring close to the disk greatly enhances the soft-photon field with the result that the spectrum softens. We interpret the two states as being related to two different phases of magnetic energy dissipation. In the Soft state, Parker instability in the disk favours the emergence of large numbers of relatively low magnetic field flux tubes. In the hard state, only intense magnetic fields become buoyant. The model can also qualitatively account for the observed short timescale variability and the characteristics of the X-ray reflected component of the hard state.Comment: submitted to MNRAS, Feb. 1998, 10 pages, 3 figures in MNRAS LaTex styl

Electric field control of spin lifetimes in Nb-SrTiO3_3 by spin-orbit fields

We show electric field control of the spin accumulation at the interface of the oxide semiconductor Nb-SrTiO$_{3}$ with Co/AlO$_{x}$ spin injection contacts at room temperature. The in-plane spin lifetime $\tau_\parallel$ as well as the ratio of the out-of-plane to in-plane spin lifetime $\tau_\perp/\tau_\parallel$ is manipulated by the built-in electric field at the semiconductor surface, without any additional gate contact. The origin of this manipulation is attributed to Rashba Spin-Orbit Fields (SOFs) at the Nb-SrTiO$_3$ surface and shown to be consistent with theoretical model calculations based on SOF spin flip scattering. Additionally, the junction can be set in a high or low resistance state, leading to a non-volatile control of $\tau_\perp/\tau_\parallel$, consistent with the manipulation of the Rashba SOF strength. Such room temperature electric field control over the spin state is essential for developing energy-efficient spintronic devices and shows promise for complex oxide based (spin)electronicsComment: 5 pages, 4 figure