Sergiy Cherepov

Electric-field-induced spin wave generation using multiferroic magnetoelectric cells

In this work, we report on the demonstration of voltage-driven spin wave excitation, where spin waves are generated by multiferroic magnetoelectric (ME) cell transducers driven by an alternating voltage, rather than an electric current. A multiferroic element consisting of a magnetostrictive Ni film and a piezoelectric [Pb(Mg1/3Nb2/3)O3](1−x)–[PbTiO3]x substrate was used for this purpose. By applying an AC voltage to the piezoelectric, an oscillating electric field is created within the piezoelectric material, which results in an alternating strain-induced magnetic anisotropy in the magnetostrictive Ni layer. The resulting anisotropy-driven magnetization oscillations propagate in the form of spin waves along a 5 μm wide Ni/NiFe waveguide. Control experiments confirm the strain-mediated origin of the spin wave excitation. The voltage-driven spin wave excitation, demonstrated in this work, can potentially be used for low-dissipation spin wave-based logic and memory elements.

Voltage-induced switching of nanoscale magnetic tunnel junctions

We demonstrate voltage-induced (non-STT) switching of nanoscale, high resistance voltage-controlled magnetic tunnel junctions (VMTJs) with pulses down to 10 ns. We show ~10x reduction in switching energies (compared to STT) with leakage currents <; 105 A/cm2. Switching dynamics, from quasi-static to the nanosecond regime, are studied in detail. Finally, a strategy for eliminating the need for external magnetic-fields, where switching is performed by set/reset voltages of different amplitudes but same polarity, is proposed and verified experimentally.

Core-Core Dynamics in Spin Vortex Pairs

We investigate nanopillars in which two thin ferromagnetic particles are separated by a nanometer thin nonmagnetic spacer and can be set into stable spin vortex-pair configurations. We find that the previously unexplored limit of strong vortex core-core coupling can dominate the spin dynamics in the system. We observe experimentally and explain analytically and numerically how the 0.2 GHz gyrational resonance modes of the individual vortices are transformed into a 2 GHz collective rotational resonance mode in the configurations where the two cores form a bound pair.

Diode-MTJ Crossbar Memory Cell Using Voltage-Induced Unipolar Switching for High-Density MRAM

This letter presents a diode-magnetic tunnel junction (MTJ) magnetic random access memory cell in a 65-nm complimentary metal-oxide-semiconductor compatible process. A voltage-controlled magnetic anisotropy switching mechanism, in addition to STT, allows for a unipolar set/reset write scheme, where voltage pulses of the same polarity, but different amplitudes, are used to switch the MTJs. A small crossbar array is constructed from 65-nm MTJs fabricated on a silicon wafer, with switching voltages ~ 1 V and thermal stability greater than 10 years, with discrete germanium diodes as access devices to allow for read/write operations. The crossbar architecture can be extended to multiple layers to create a 3-D stackable, nonvolatile memory with a sub-1F2 effective cell size.

Spin wave nanofabric update

We provide a progress update on the spin wave nanofabric. The nanofabric comprises magneto-electric cells and spin wave buses serving for spin wave propagation. The magneto-electric cells are used as the input/output ports for information transfer between the charge and the spin domains, while information processing inside the nanofabric is via spin waves only. Information is encoded into the phase of the propagating spin wave, which makes it possible to utilize waveguides as passive logic elements and take the advantage of using wave superposition for data processing. This provides a fundamental advantage over the conventional transistor-based logic circuitry allowing for functional throughput enhancement and power consumption minimization at the same time. We present recent accomplishments in the magneto-electric element development and integration with spin wave buses. In particular, we show the excitation and detection of the spin waves via multiferroic elements. In addition, we present different approaches to magnonic logic circuit engineering and provide the comparison with CMOS by mapping the designs to 45nm NANGATE standard cell libraries. The estimates show more than 40X power reduction and 53X area reduction for magnonic circuits. These results illustrate the potential advantages over conventional charge based electronics that could be a route to beyond CMOS logic circuitry.

Micromagnetics of Spin-Flop Bilayers: S, C, and Vortex Spin States

Spin-flop tunnel junctions subjected to strong gigahertz excitations are found to exhibit highly stable resistance states, intermediate between the two spin-uniform ground states of high and low resistance. The associated spin distributions are necessarily nonuniform and differ significantly from the ground-state anti-parallel spin configuration in their static and dynamic properties. Detailed micromagnetic modeling reveals that inplane spin vortices in dipole-coupled thin-film bilayers are stable spin configurations, where the orientation of the vortex cores and the vortex chirality play an important role in the response of the system to external magnetic fields.

Resonant Switching of Two Dipole-Coupled Nanomagnets

The storage layer of recently developed spin-flop magnetic random-access memory consists of two closely spaced dipole-coupled nanomagnets and is highly stable in the ground state as well as in quasistatic fields applied off the easy axis. We show experimentally and confirm by using micromagnetic simulations that these spin-flop bilayers can be switched relatively easily by dynamic fields, applied at the frequency of the optical spin resonance of the bilayer. The field amplitude sufficient for this resonant switching can be an order of magnitude lower than the fields necessary for quasistatic reversal. Our data and micromagnetic analysis suggest that thermal agitation can play a role in the observed resonant switching behavior.

Tunneling Spectroscopy of Magnetic Double Barrier Junctions

Scanning tunneling microscopy (STM) is used to study transport in magnetic double tunnel junctions (DTJs) formed using a fixed transparency barrier of a patterned tunnel junction (TJ), and a variable tunnel barrier between the top electrode of the patterned junction and the STM tip. A sufficiently thin top electrode has been predicted to result in a rectification of charge current through a DTJ when the two barriers have different transparency. Our measurements indeed show a high current rectification ratio for 3-nm-thick, continuous film top electrodes, which is observed for junctions with asymmetric tunnel barriers

Ultrafast all-optical magnetization reversal in GdFeCo films aroundplasmonic nanostructures

It has recently been experimentally demonstrated that reproducible and controllable all-optical magnetization reversal in GdFeCo films can be achieved with a single ultrafast (from 40fs to 3ps) femtosecond laser pulse. While the microscopic origin of the effect is still unclear, we suggest that the effect is caused by a combination of light-induced quasi-static magnetic field, with dynamic thermal effects due to laser heating, as well as magnetic fields generated by thermoelectric effect-caused electrical currents. This finding reveals great potential for ultrafast data storage through magnetic switching without the aid of an external magnetic field. It was further recently predicted that utilization of plasmonic nanostructures may provide the way to achieve fast all-optical magnetization switching with smaller/cheaper laser sources with longer pulse durations. We will present the simulations of temporal dynamics of magnetization reversal around plasmonic nanostructures with the combination of Landau Lifshitz Bloch and finite element modeling. Our modeling results predict that plasmonic nanostructures can significantly alter all-optical magnetization switching process and may help achieve a number of technologically important effects that cannot be achieved otherwise. Results of experimental studies of optical magnetization reversal in GdFeCo films around plasmonic nanostructures are also provided.