Vladislav Korenivski

Design of high frequency inductors based on magnetic films

Magnetically coated stripe inductors with conductor insulated from the magnetic film and with conductor in direct electrical contact with the magnetic film are analyzed. A simple equivalent circuit model shows that for the structure without insulation, regardless of the fact that the dc resistivity of the magnetic film is much higher than that of the conductor, most of the driving current flows in the magnetic film at frequencies where the impedance of the device is dominated by inductance, i.e., where the quality factor Q>1. This effect of current re-distribution significantly lowers the performance of the device. A design is proposed having insulation layers to avoid current flow in the magnetic film, which is broken up in sections along the conductor stripe to avoid displacement currents in the system.

Electromagnetic analysis of layered magnetic/conductor structures

A method is described for calculating the impedance of conductor structures comprised of ferromagnetic and non-magnetic layers of arbitrary cross sectional shape with transverse dimensions which are small compared to the conductors length. The method is exemplified on a conductor of axial symmetry enclosed in a high permeability film of uniaxial anisotropy along the conductor axis. To find the impedance the full boundary value problem for the vector potential is solved, allowing driving and eddy currents in both the conductor and the magnetic film, and taking into account the dynamics of the magnetization governed by the Landau-Lifshitz equations. The results obtained are used to model the giant magneto impedance effect in ferromagnetically coated wires.

Thermally activated switching in spin-flop tunnel junctions

Magnetic tunnel junctions with a flux-closed sandwich replacing the single-soft layer have been proposed for increasing the magnetic stability of the junctions. Such flux-closed soft layers have two antiferromagnetic ground states and reverse by spin-flop switching. They are expected to essentially eliminate the critical issue of thermally activated reversal under half-select fields and ease the scaling of the emerging technology of magnetic random access memory. High-speed pulse measurements of such submicron spin-flop junctions performed over nine decades in time are reported here. The switching probability as a function of the pulse field amplitude and duration is analyzed to estimate the energy barrier to thermal activation. This activation energy is found to be substantially enhanced compared to the single-free layer case, and agrees well with the single-domain model prediction.

Impedance of a ferromagnetic sandwich strip

A theoretical approach for calculating the impedance of a three-layer sandwich, consisting of two metallic ferromagnetic layers separated by a non-magnetic conductive layer, is presented. The Maxwell equations for the electromagnetic field coupled with the Landau–Lifshitz equations for the magnetization dynamics are solved, which enables one to describe the system over a wide frequency range, including the ferromagnetic resonance. Two sandwich strip structures are analyzed, both having thickness much less than the width, and the width much less than the length: a “closed” structure with the magnetic film closing at the edges along the width, and an “open” structure without flux closures where all the layers have the same width. The impedance for the two structures is calculated and analyzed as a function of the physical parameters of the device and frequency. The “closed” structure is more efficient magnetically and exhibits a highly inductive response to much higher frequencies than the “open” structure....

Impedance and surface impedance of ferromagnetic multilayers: the role of exchange interaction

A theory of impedance and surface impedance in conductive ferromagnetic films and layered magnetic/conductor structures is presented. The theory is based on solving Maxwells equations for the magnetic and electric fields and the Landau-Lifshitz equations for the magnetization. The exchange interaction in the magnetic films as well as pinning at surfaces are taken into account. The analytical results obtained demonstrate the importance of the exchange, pinning and also the type of excitation the magnetic systems are subject to in describing their properties at ultrahigh frequencies.

Analysis of current distribution in magnetic film inductors

We have extended our electromagnetic model for high-frequency magnetic film inductors [A. Gromov, V. Korenivski, K. V. Rao, R. B. van Dover, and P. M. Mankiewich, IEEE Trans. Magn. 34, 1246 (1998)] to account for driving current redistribution in structures where the conductor and the magnetic film are in direct electrical contact. We consider a stripe conductor of a rectangular cross section enclosed in a magnetic film. A potential difference of fixed amplitude is applied to both the conductor and the magnetic film. Maxwell’s equations for this geometry are solved analytically, and a simple expression for the impedance is obtained. The inductance at low frequencies is practically unchanged compared to the case where the magnetic film is insulated from the conductor. However, the maximum achievable quality factor is found to be higher than that for the insulated case, even when a significant portion of the driving current flows through the magnetic film. Magnetic film inductors without insulation layers a...

Suppression of superconductivity due to spin imbalance in Co/Al/Co single electron transistor

Transport properties of ferromagnetic/nonmagnetic/ferromagnetic single electron transistors are investigated as a function of external magnetic-field, temperature, bias, and gate voltage. By designing the magnetic electrodes to have different switching fields, a two-mode device is realized having two stable magnetization states, with the electrodes aligned in parallel and antiparallel. Magnetoresistance of approximately 100% is measured in Co/AlOX/Al/AlOX/Co double tunnel junction spin valves at low bias, with the Al spacer in the superconducting state. The effect is substantially reduced at high bias and temperatures above the TC of the Al. The experimental results are interpreted as due to spin imbalance of charge carriers resulting in suppression of the superconducting gap of the Al island.

Thermoelectrical manipulation of nanomagnets

We investigate the interplay between the thermodynamic properties and spin-dependent transport in a mesoscopic device based on a magnetic multilayer (F/f/F), in which two strongly ferromagnetic layers (F) are exchange-coupled through a weakly ferromagnetic spacer (f) with the Curie temperature in the vicinity of room temperature. We show theoretically that the Joule heating produced by the spin-dependent current allows a spin-thermoelectronic control of the ferromagnetic-to-paramagnetic (f/N) transition in the spacer and, thereby, of the relative orientation of the outer F-layers in the device (spin-thermoelectric manipulation of nanomagnets). Supporting experimental evidence of such thermally-controlled switching from parallel to antiparallel magnetization orientations in F/f(N)/F sandwiches is presented. Furthermore, we show theoretically that local Joule heating due to a high concentration of current in a magnetic point contact or a nanopillar can be used to reversibly drive the weakly ferromagnetic spacer through its Curie point and thereby exchange couple and decouple the two strongly ferromagnetic F-layers. For the devices designed to have an antiparallel ground state above the Curie point of the spacer, the associated spin-thermionic parallel to antiparallel switching causes magnetoresistance oscillations whose frequency can be controlled by proper biasing from essentially dc to GHz. We discuss in detail an experimental realization of a device that can operate as a thermomagnetoresistive switch or oscillator.

Spin-flip scattering at Al surfaces

Nonlocal measurements are performed on a multiterminal device to in situ determine the spin-diffusion length and in combination with resistivity measurements also the spin-relaxation time in Al films. By varying the thickness of Al we determine the contribution to spin relaxation from surface scattering. From the temperature dependence of the spin-diffusion length it is established that the spin relaxation is impurity dominated at low temperature. A comparison of the spin- and momentum-relaxation lengths for different thicknesses reveals that the spin-flip scattering at the surfaces is weak compared to that within the bulk of the Al films.

Electronic structure, optical and magnetic properties of Co2FeGe Heusler alloy films

Optical properties of ferromagnetic half-metallic full-Heusler Co