arXiv subjects

Andrew D. Kent

Publications and source records attributed to Andrew D. Kent.

At least 55 records · Page 3Linked to original sources

arXiv2019

Planar Hall driven torque in a FM/NM/FM system

An important goal of spintronics is to covert a charge current into a spin current with a controlled spin polarization that can exert torques on an adjacent magnetic layer. Here we demonstrate such torques in a two ferromagnet system. A CoNi multilayer is used as a spin current source in a sample with structure CoNi/Au/CoFeB. Spin torque ferromagnetic resonance is used to measure the torque on the CoFeB layer. The response as a function of the applied field angle and current is consistent with the symmetry expected for a torques produced by the planar Hall effect originating in CoNi. We find the strength of this effect to be comparable to that of the spin Hall effect in platinum, indicating that the planar Hall effect holds potential as a spin current source with a controllable polarization direction.

cond-mat.mes-hall↗

arXiv2019

A Low Temperature Functioning CoFeB/MgO Based Perpendicular Magnetic Tunnel Junction for Cryogenic Nonvolatile Random Access Memory

We investigated the low temperature performance of CoFeB/MgO based perpendicular magnetic tunnel junctions (pMTJs) by characterizing their quasi-static switching voltage, high speed pulse write error rate and endurance down to 9 K. pMTJ devices exhibited high magnetoresistance (>120%) and reliable (error rate<10-4) bi-directional switching with 2 to 200 ns voltage pulses. The endurance of the devices at 9 K surpassed that at 300 K by three orders of magnitude under the same write conditions, functioning for more than 10^12 cycles with 10 ns write pulses. The critical switching voltage at 9 K was observed to increase by 33% to 93%, depending on pulse duration, compared to that at 350 K. Ferromagnetic resonance and magnetization measurements on blanket pMTJ film stacks suggest that the increased switching voltage is associated with an increase in effective magnetic anisotropy and magnetization of free layer with decreasing temperature. Our work demonstrates that CoFeB/MgO based pMTJs have great potential to enable cryogenic MRAM and that their low temperature magnetization and effective magnetic anisotropy can be further optimized to lower operating power and improve endurance.

cond-mat.mes-hall↗

arXiv2019

Sub-nanosecond spin-torque switching of perpendicular magnetic tunnel junction nanopillars at cryogenic temperatures

Spin-transfer magnetic random access memory is of significant interest for cryogenic applications where a persistent, fast, low-energy consumption and high device density is needed. Here we report the low-temperature nanosecond duration spin-transfer switching characteristics of perpendicular magnetic tunnel junction (pMTJ) nanopillar devices (40 to 60 nm in diameter) and contrast them to their room temperature properties. Interestingly, at fixed pulse voltage overdrive the characteristic switching time decreases with temperature, in contrast to macrospin model predictions, with the largest reduction in switching time occurring between room temperature and 150 K. The switching energy increases with decreasing temperature, but still compares very favorably to other types of spin-transfer devices at 4 K, with < 300 fJ required per switch. Write error rate (WER) measurements show highly reliable (WER <= 5x10^-5 with 4 ns pulses at 4 K) demonstrating the promise of pMTJ devices for cryogenic applications and routes to further device optimization.

physics.app-ph↗

arXiv2019

Electrical control of Majorana Bound States Using Magnetic Stripes

A hybrid semiconductor-superconductor nanowire on the top of a magnetic film in the stripe phase experiences a magnetic texture from the underlying fringing fields. The Zeeman interaction with the highly inhomogeneous magnetic textures generates a large synthetic spin-orbit coupling. We show that this platform can support the formation of Majorana bound states (MBS) localized at the ends of the nanowire. The transition to the topological superconducting phase not only depends on the nanowire parameters and stripe size but also on the relative orientation of the stripes with respect to the nanowire axis. Topological phase transitions with the corresponding emergence or destruction of MBS can be induced by reorienting the stripes or shifting their position, which can be achieved by passing a charge current through the magnetic film or by applying electrically-controlled strain to it. The proposed platform removes the need for external magnetic fields and offers a non-invasive electrical tuning of MBS with the perturbation (current or strain) acting only on thee magnetic film.

cond-mat.supr-con↗

arXiv2019

Spin transport in an insulating ferrimagnetic-antiferromagnetic-ferrimagnetic trilayer as a function of temperature

We present a study of the transport properties of thermally generated spin currents in an insulating ferrimagnetic-antiferromagnetic-ferrimagnetic trilayer over a wide range of temperature. Spin currents generated by the spin Seebeck effect (SSE) in a yttrium iron garnet (YIG) YIG/NiO/YIG trilayer on a gadolinium gallium garnet (GGG) substrate were detected using the inverse spin Hall effect in Pt. By studying samples with different NiO thicknesses, the NiO spin diffusion length was determined to be 4.2 nm at room temperature. Interestingly, below 30 K, the inverse spin Hall signals are associated with the GGG substrate. The field dependence of the signal follows a Brillouin function for a S=7/2 spin ($\mathrm{Gd^{3+}}$) at low temperature. Sharp changes in the SSE signal at low fields are due to switching of the YIG magnetization. A broad peak in the SSE response was observed around 100 K, which we associate with an increase in the spin-diffusion length in YIG. These observations are important in understanding the generation and transport properties of spin currents through magnetic insulators and the role of a paramagnetic substrate in spin current generation.

cond-mat.mes-hall↗

arXiv2019

Sub-nanosecond switching in a cryogenic spin-torque spin-valve memory element with a dilute permalloy free layer

We present a study of the pulsed current switching characteristics of spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted permalloy free layers in the ballistic regime at low temperature. The dilute permalloy free layer device switches much faster: the characteristic switching time for a permalloy free (Ni0.83Fe0.17) layer device is 1.18 ns, while that for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns. A ballistic macrospin model can capture the data trends with a reduced spin torque asymmetry parameter, reduced spin polarization and increased Gilbert damping for the dilute permalloy free layer relative to the permalloy devices. Our study demonstrates that reducing the magnetization of the free layer increases the switching speed while greatly reducing the switching energy and shows a promising route toward even lower power magnetic memory devices compatible with superconducting electronics.

physics.app-ph↗

arXiv2019

Reduced Exchange Interactions in Magnetic Tunnel Junction Free Layers with Insertion Layers

Perpendicularly magnetized CoFeB layers with ultra-thin non-magnetic insertion layers are very widely used as the electrodes in magnetic tunnel junctions for spin transfer magnetic random access memory devices. Exchange interactions play a critical role in determining the thermal stability of magnetic states in such devices and their spin torque switching efficiency. Here the exchange constant of free layers incorporated in full magnetic tunnel junction layer stacks, specifically CoFeB free layers with W insertion layers is determined by magnetization measurements in a broad temperature range. A significant finding is that the exchange constant decreases significantly and abruptly with W insertion layer thickness. The perpendicular magnetic anisotropy shows the opposite trend; it initially increases with W insertion layer thickness and shows a broad maximum for approximately one monolayer (0.3 nm) of W. These results highlight the interdependencies of magnetic characteristics required to optimize the performance of magnetic tunnel junction devices.

cond-mat.mes-hall↗

arXiv2018

Magnetization Dynamics

Magnetism primarily describes the physics and materials science of systems presenting a magnetization -- a macroscopic order parameter characterizing electron angular momentum. The order parameter is associated with the electronic exchange interactions, which is fundamentally quantum mechanical. Its dynamic behavior bridges the macroscopic and the microscopic worlds. On macroscopic length and time-scales, it interacts with electromagnetic fields dictated by the Maxwells equations. On a microscopic scale, it involves the quantum-mechanical electronic states both in spin-space and momentum space, thus giving rise to a wide range of behavior that extend down to femtoseconds. Thanks to the development of modern metrology, there have been many new and noteworthy observations of magnetism-related phenomena across the entire range -- from spin-torque induced antidamping dynamics, to ultrafast laser induced femtosecond electron dynamics that involve spin current and angular momentum conservation. In this review we introduce some observations on magnetodynamics, and the scientific subjects these new results give rise to.

cond-mat.mes-hall↗

arXiv2018

First harmonic measurements of the spin Seebeck effect

We present measurements of the spin Seebeck effect (SSE) by a technique that combines alternating currents (AC) and direct currents (DC). The method is applied to a ferrimagnetic insulator/heavy metal bilayer, Y$_3$Fe$_5$O$_{12}$(YIG)/Pt. Typically, SSE measurements use an AC current to produce an alternating temperature gradient and measure the voltage generated by the inverse spin-Hall effect in the heavy metal at twice the AC frequency. Here we show that when Joule heating is associated with AC and DC bias currents, the SSE response occurs at the frequency of the AC current drive and can be larger than the second harmonic SSE response. We compare the first and second harmonic responses and show that they are consistent with the SSE. The field dependence of the voltage response is used to characterize the damping-like and field-like torques. This method can be used to explore nonlinear thermoelectric effects and spin dynamics induced by temperature gradients.

cond-mat.mes-hall↗

arXiv2018

Ferromagnetic resonance linewidth in coupled layers with easy-plane and perpendicular magnetic anisotropies

Magnetic bilayers with different magnetic anisotropy directions are interesting for spintronic appli- cations as they offer the possibility to engineer tilted remnant magnetization states. We investigate the ferromagnetic resonance (FMR) linewidth of modes associated with two interlayer exchange- coupled ferromagnetic layers, the first a CoNi multilayer with a perpendicular magnetic anisotropy, and the second a CoFeB layer with an easy-plane anisotropy. For antiferromagnetic interlayer ex- change coupling, elevated FMR linewidths are observed below a characteristic field. This is in contrast to what is found in uncoupled, ferromagnetically coupled and single ferromagnetic layers in which the FMR linewidth increases monotonically with field. We show that the characteristic field at which there is a dramatic increase in FMR linewidth can be understood using a macrospin model with Heisenberg-type exchange coupling between the layers.

cond-mat.mes-hall↗

arXiv2018

Generation and annihilation time of magnetic droplet solitons

Magnetic droplet solitons were first predicted to occur in materials with uniaxial magnetic anisotropy due to a long-range attractive interaction between elementary magnetic excitations, magnons. A non-equilibrium magnon population provided by a spin-polarized current in nanocontacts enables their creation and there is now clear experimental evidence for their formation, including direct images obtained with scanning x-ray transmission microscopy. Interest in magnetic droplets is associated with their unique magnetic dynamics that can lead to new types of high frequency nanometer scale oscillators of interest for information processing, including in neuromorphic computing. However, there are no direct measurements of the time required to nucleate droplet solitons or their lifetime---experiments to date only probe their steady-state characteristics, their response to dc spin currents. Here we determine the timescales for droplet annihilation and generation using current pulses. Annihilation occurs in a few nanoseconds while generation can take several nanoseconds to a microsecond depending on the pulse amplitude. Micromagnetic simulations show that there is an incubation time for droplet generation that depends sensitively on the initial magnetic state of the nanocontact. An understanding of these processes is essential to utilizing the unique characteristics of magnetic droplet solitons oscillators, including their high frequency, tunable and hysteretic response.

cond-mat.mes-hall↗

arXiv2018

Generation and stability of dynamical skyrmions and droplet solitons

A spin-polarized current in a nanocontact to a magnetic film can create collective magnetic oscillations by compensating the magnetic damping. In particular, in materials with uniaxial magnetic anisotropy, droplet solitons have been observed a self-localized excitation consisting of partially reversed magnetization that precesses coherently in the nanocontact region. It is also possible to generate topological droplet solitons, known as \emph{dynamical skyrmions}. Here we study the conditions that promote either droplet or dynamical skyrmion formation and describe their stability in magnetic films without Dzyaloshinskii-Moriya interactions. We show that Oersted fields from the applied current as well as the initial magnetization state can determine whether a droplet or dynamical skyrmion forms. Dynamical skyrmions are found to be more stable than droplets. We also discuss electrical characteristics that can be used distinguish these magnetic objects.

cond-mat.mes-hall↗

arXiv2018

Voltage-Controlled Topological-Spin Switch for Ultra-Low-Energy Computing--Performance Modeling and Benchmarking

A voltage-controlled topological-spin switch (vTOPSS) that uses a hybrid topological insulator-magnetic insulator multiferroic is presented that can implement Boolean logic operations with sub-10 aJ energy-per-bit and energy-delay product on the order of $10^{-27}$ Js. The device uses a topological insulator (TI), which has the highest efficiency of conversion of electric field to spin torque yet observed at room temperature, and a low-moment magnetic insulator (MI) that can respond rapidly to a given spin torque. We present the theory of operation of vTOPSS, develop analytic models of its performance metrics, elucidate performance scaling with dimensions and voltage, and benchmark vTOPSS against existing spin-based and CMOS devices. Compared to existing spin-based devices, such as all-spin logic and charge-spin logic, vTOPSS offers 100$\times$ lower energy dissipation and (40-100)$\times$ lower energy-delay product. With experimental advances and improved material properties, we show that the energy-delay product of vTOPSS can be lowered to $10^{-29}$ Js, competitive against existing CMOS technology. Finally, we establish that interconnect issues that dominate the performance in CMOS logic are relatively less significant for vTOPSS, implying that highly resistive materials can indeed be used to interconnect vTOPSS devices.

physics.app-ph↗

arXiv2017

Coherent spin-transfer precession switching in orthogonal spin-torque devices

We present experimental results and macrospin simulations of the switching characteristics of orthogonal spin-transfer devices incorporating an out-of-plane magnetized polarizing layer and an in-plane magnetized spin valve device at cryogenic temperatures. At $T=4$ K we demonstrate: high speed deterministic switching at short pulse lengths - down to 100 ps - with sufficient measurement statistics to establish a switching error rate of $10^{-5}$; coherent precessional switching at longer times; and ensemble decoherence effects at even longer times. Finite temperature macrospin models capture the precessional switching well but fail to fully reproduce all the decoherence and switching error behavior.

cond-mat.mes-hall↗

arXiv2017

Emerging Magnetic Order In Copper Induced By Proximity To Cobalt: A Detailed Soft X-Ray Spectroscopy Study

We present an x-ray magnetic dichroism (XMCD) and soft x-ray absorption spectroscopy (XAS) study to address the nature of emerging magnetic order in metallic Copper as Cobalt is added to the matrix. For this purpose line shape and energy position of XAS and XMCD spectra will be analyzed for a series of Co/Cu alloys as well as a multilayer reference. We observe an increased hybridization between Cu and Co sites as well as increased localization of the Cu d-electrons and an induced magnetic moment in Cu. The emergence of long range magnetic order in non-magnetic materials that are in proximity to a ferromagnet is significant for a comprehensive interpretation of transport phenomena at ferromagnetic/non-magnetic interfaces, like e.g. the giant magnetoresistance effect. The presented results will further enable us to interpret Cu XMCD and XAS spectra acquired from unknown Co/Cu samples to identify the environment of Cu atoms exhibiting proximity induced magnetism.

cond-mat.mtrl-sci↗

arXiv2017

Tunable Magnetic Textures: From Majorana Bound States to Braiding

A versatile control of magnetic systems, widely used to store information, can also enable manipulating Majorana bounds states (MBS) and implementing fault-tolerant quantum information processing. The proposed platform relies on the proximity-induced superconductivity in a two-dimensional electron gas placed next to an array of magnetic tunnel junctions (MTJs). A change in the magnetization configuration in the MTJ array creates tunable magnetic textures thereby removing several typical requirements for MBS: strong spin-orbit coupling, applied magnetic field, and confinement by one-dimensional structures which complicates demonstrating non-Abelian statistics through braiding. Recent advances in fabricating two-dimensional epitaxial superconductor/semiconductor heterostructures and designing tunable magnetic textures support the feasibility of this novel platform for MBS.

cond-mat.mes-hall↗

arXiv2016

Time-Resolved Studies of the Spin-Transfer Reversal Mechanism in Perpendicularly Magnetized Magnetic Tunnel Junctions

Pulsed spin-torque switching has been studied using single-shot time-resolved electrical measurements in perpendicularly magnetized magnetic tunnel junctions as a function of pulse amplitude and junction size in 50 to 100 nm diameter circular junctions. The mean switching time depends inversely on pulse amplitude for all junctions studied. However, the switching dynamics is found to be strongly dependent on junction size and pulse amplitude. In 50 nm diameter junctions the switching onset is stochastic but the switching once started, is fast; after being initiated it takes less than 2 ns to switch. In larger diameter junctions the time needed for complete switching is strongly dependent on the pulse amplitude, reaching times less than 2 ns at large pulse amplitudes. Anomalies in the switching rate versus pulse amplitude are shown to be associated with the long lived (> 2 ns) intermediate junction resistance states.

cond-mat.mes-hall↗

arXiv2016

Spin transport and dynamics in all-oxide perovskite La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$ bilayers probed by ferromagnetic resonance

Thin films of perovskite oxides offer the possibility of combining emerging concepts of strongly correlated electron phenomena and spin current in magnetic devices. However, spin transport and magnetization dynamics in these complex oxide materials are not well understood. Here, we experimentally quantify spin transport parameters and magnetization damping in epitaxial perovskite ferromagnet/paramagnet bilayers of La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$ (LSMO/SRO) by broadband ferromagnetic resonance spectroscopy. From the SRO thickness dependence of Gilbert damping, we estimate a short spin diffusion length of $\lesssim$1 nm in SRO and an interfacial spin-mixing conductance comparable to other ferromagnet/paramagnetic-metal bilayers. Moreover, we find that anisotropic non-Gilbert damping due to two-magnon scattering also increases with the addition of SRO. Our results demonstrate LSMO/SRO as a spin-source/spin-sink system that may be a foundation for examining spin-current transport in various perovskite heterostructures.

cond-mat.mtrl-sci↗