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In viscous dynamics, velocity is proportional to the force. An ideal memristor is a device whose resistance changes at a rate proportional to the driving input. We present a proof-of-principle demonstration of the connection between viscous dynamics and memristive functionality by utilizing a thin-film ferromagnet/antiferromagnet bilayer, where viscous magnetization dynamics results from the frustration at the magnetic interface, and driving is provided by an external magnetic field. Thanks to the atomic scale of frustration effects, the presented approach is amenable to downscaling. It can also be adapted for electronic driving by spin torque, making it attractive for applications in neuromorphic circuits. [ABSTRACT FROM AUTHOR]

Highlights • Variable-temperature, variable-frequency susceptibility of a thin-film F/AF bilayer. • Unidirectional anisotropy is significantly smaller than the exchange bias. • Exchange bias caused by the different reversal mechanisms between two states. • Magnetic viscosity of AF increases by 4 orders of magnitude over 20 K. • Results indicate that the Heisenberg domain state of AF is a magnetic glass. Abstract We utilize variable-temperature, variable-frequency magneto-optical transverse magnetic susceptibility technique to study the static and dynamical magnetic properties of a thin-film CoO/Permalloy bilayer. Our measurements demonstrate that in the studied system, the directional asymmetry of the hysteresis loop is associated mainly with the difference in the reversal mechanisms between the two reversed states of magnetization stabilized by the exchange-induced uniaxial anisotropy. The latter is found to be much larger than the exchange-induced unidirectional anisotropy of the ferromagnet. We also observe an abrupt variation of the frequency-dependent imaginary part of ac susceptibility near the exchange bias blocking temperature, consistent with the magnetic freezing transition inferred from the previous time-domain studies of magnetic aging in similar systems. The developed measurement approach enables precise characterization of the dynamical and static characteristics of thin-film magnetic heterostructures that can find applications in reconfigurable magnonic and neuromorphic circuits. [ABSTRACT FROM AUTHOR]

We demonstrate that magnetic multilayer nanopillars can be efficiently protected from oxidation by coating with silicon. Both the protected and the oxidized nanopillars exhibit an increase of reversal current at cryogenic temperatures. However, the magnetic excitation onset current increases only in the oxidized samples. We show that oxidized nanopillars exhibit anomalous switching statistics at low temperature, providing a simple test for the quality of magnetic nanodevices. [ABSTRACT FROM AUTHOR]

With the advent of pure-spin-current sources, spin-based electronic (spintronic) devices no longer require electrical charge transfer, opening new possibilities for both conducting and insulating spintronic systems. Pure spin currents have been used to suppress noise caused by thermal fluctuations in magnetic nanodevices, amplify propagating magnetization waves, and to reduce the dynamic damping in magnetic films. However, generation of coherent auto-oscillations by pure spin currents has not been achieved so far. Here we demonstrate the generation of single-mode coherent auto-oscillations in a device that combines local injection of a pure spin current with enhanced spin-wave radiation losses. Counterintuitively, radiation losses enable excitation of auto-oscillation, suppressing the nonlinear processes that prevent auto-oscillation by redistributing the energy between different modes. Our devices exhibit auto-oscillations at moderate current densities, at a microwave frequency tunable over a wide range. These findings suggest a new route for the implementation of nanoscale microwave sources for next-generation integrated electronics. [ABSTRACT FROM AUTHOR]

We demonstrate that microwave pumping of a spin-torque nano-oscillator can lead to transfer of the spin-wave energy generated by the oscillator into a mode with frequency given by the difference between the pumping frequency and that of the auto-oscillation. The decay length of spin waves at the combination frequency is significantly increased, while the directionality of emission is preserved. The observed phenomenon provides a route for the implementation of nanosized spin-wave emitters with controllable emission characteristics for applications in next-generation microwave electronic devices. [ABSTRACT FROM AUTHOR]

Dynamics induced by spin-transfer torque is a quickly developing topic in modern magnetism, which has initiated several new approaches to magnetic nanodevices. It is now well established that a spin-polarized electric current injected into a ferromagnetic layer through a nanocontact exerts a torque on the magnetization, leading to microwave-frequency precession detectable through the magnetoresistance effect. This phenomenon provides a way for the realization of tunable nanometre-size microwave oscillators, the so-called spin-torque nano-oscillators (STNOs). Present theories of STNOs are mainly based on pioneering works predicting emission of spin waves due to the spin torque. Despite intense experimental studies, until now this spin-wave emission has not been observed. Here, we report the first experimental observation and two-dimensional mapping of spin waves emitted by STNOs. We demonstrate that the emission is strongly directional, and the direction of the spin-wave propagation is steerable by the magnetic field. The information about the emitted spin waves obtained in our measurements is of key importance for the understanding of the physics of STNOs, and for the implementation of coupling between individual oscillators mediated by spin waves. Analysis shows that the observed directional emission is a general property inherent to any dynamical system with strongly anisotropic dispersion. [ABSTRACT FROM AUTHOR]

The advent of free electron lasers has opened the opportunity to explore interactions between extreme ultraviolet (EUV) photons and collective excitations in solids. While EUV transient grating spectroscopy, a noncollinear four-wave mixing technique, has already been applied to probe coherent phonons, the potential of EUV radiation for studying nanoscale spin waves has not been harnessed. Here we report EUV transient grating experiments with coherent magnons in Fe/Gd ferrimagnetic multilayers. Magnons with tens of nanometers wavelengths are excited by a pair of femtosecond EUV pulses and detected via diffraction of a probe pulse tuned to an absorption edge of Gd. The results unlock the potential of nonlinear EUV spectroscopy for studying magnons and provide a tool for exploring spin waves in a wave vector range not accessible by established inelastic scattering techniques. [ABSTRACT FROM AUTHOR]

We simulate scattering of electrons by a chain of antiferromagnetically coupled quantum Heisenberg spins, to analyze spin-transfer effects not described by the classical models of magnetism. Our simulations demonstrate efficient excitation of dynamical states that would be forbidden by the semiclassical symmetries, such as generation of multiple magnetic excitation quanta by a single electron. Furthermore, quantum interference of spin wave functions enables generation of magnetization dynamics with amplitudes exceeding the transferred magnetic moment. The efficiency of excitation is almost independent of the electron spin polarization, and is governed mainly by the transfer of energy. Nonclassical spin transfer may thus enable efficient electronic control of antiferromagnets not limited by the classical constraints. [ABSTRACT FROM AUTHOR]

We demonstrate experimentally the ability to control by electric current the phase and the wavelength of spin waves propagating in submicrometer magnetic waveguides. The dependence of the spin-wave characteristics on the current and on the excitation frequency was determined by phase-resolved microfocus Brillouin light scattering spectroscopy. We show that moderately small currents are sufficient to induce nearly twofold changes of the wavelength and spin-wave phase shift of more than ±π radians over a propagation distance of several micrometers. [ABSTRACT FROM AUTHOR]

We show that ideal memristors—devices whose resistance is proportional to the charge that flows through them—can be realized using spin torque-driven viscous magnetization dynamics. The latter can be accomplished in the spin liquid state of thin-film heterostructures with frustrated exchange, where the memristive response is tunable by proximity to the glass transition, while current-induced Joule heating facilitates non-volatile operation and second-order memristive functionality beneficial for neuromorphic applications. Ideal memristive behaviors can be achieved in other systems characterized by viscous dynamics of physical, electronic, or magnetic degrees of freedom. [ABSTRACT FROM AUTHOR]

The article comments on the report "Molecular Beam Epitaxy and Characterization of Thin Bi2Se3 Films on Al2O3 (110)" published in the previous issue.

We utilized Brillouin light-scattering spectroscopy to study the magnetization dynamics in nanoscale permalloy elements driven by a large microwave magnetic field. We demonstrate that at large precession angles, the magnons excited by the microwaves scatter into the lowest-frequency state by a process similar to the scattering that leads to the Bose-Einstein condensation of magnons in the low-loss garnet films. The discovered effect is important for understanding the nonlinear processes in strongly driven nanomagnetic devices such as spin-torque nano-oscillators and magnetic memory. [ABSTRACT FROM AUTHOR]

We study the effects of spin transfer on thermally activated dynamics of magnetic nanopillars with identical thicknesses of the magnetic layers. The symmetric nanopillars exhibit anomalous dependencies of switching statistics on magnetic field and current. We interpret our data in terms of simultaneous current-induced excitation of both layers. We also find evidence for coupling between the fluctuations of the layers due to the spin transfer. [ABSTRACT FROM AUTHOR]

We utilize nanoscale spin valves with Pt spacer layers to characterize spin relaxation in Pt. Analysis of the spin lifetime indicates that Elliott-Yafet spin scattering is dominant at room temperature, but an unexpected intrinsic Dyakonov-Perel-like spin relaxation becomes dominant at cryogenic temperatures. We also observe suppression of spin relaxation in a Pt layer interfaced with a ferromagnet, likely caused by the competition between the effective exchange and spin-orbit fields. [ABSTRACT FROM AUTHOR]

We utilize a nanoscale magnetic spin-valve structure to demonstrate that current-induced magnetization fluctuations at cryogenic temperatures result predominantly from the quantum fluctuations enhanced by the spin transfer effect. The demonstrated spin transfer due to quantum magnetization fluctuations is distinguished from the previously established current-induced effects by a nonsmooth piecewise-linear dependence of the fluctuation intensity on current. It can be driven not only by the directional flows of spin-polarized electrons, but also by their thermal motion and by scattering of unpolarized electrons. This effect is expected to remain non-negligible even at room temperature, and entails a ubiquitous inelastic contribution to spin-polarizing properties of magnetic interfaces. [ABSTRACT FROM AUTHOR]

We demonstrate that thin polycrystalline films of antiferromagnet CoO, in bilayers with ferromagnetic Permalloy, exhibit slow power-law aging of their magnetization state. The aging characteristics are remarkably similar to those previously observed in thin epitaxial Fe50Mn50 films, indicating that these behaviors are likely generic to ferromagnet/antiferromagnet bilayers. In very thin films, aging is observed over a wide temperature range. In thicker CoO, aging effects become reduced at low temperatures. Aging entirely disappears for large CoO thicknesses. We also investigate the dependence of aging characteristics on temperature and magnetic history. Analysis shows that the observed behaviors are inconsistent with the Neel-Arrhenius model of thermal activation, and are instead indicative of cooperative aging of the antiferromagnet. Our results provide new insights into the mechanisms controlling the stationary states and dynamics of ferromagnet/antiferromagnet bilayers, and potentially other frustrated magnetic systems. [ABSTRACT FROM AUTHOR]