arXiv subjects

Lijun Zhu

Publications and source records attributed to Lijun Zhu.

At least 73 records · Page 4Linked to original sources

arXiv2021

Maximizing Spin-Orbit Torque Generated by the Spin Hall Effect of Pt

Efficient generation of spin-orbit torques (SOTs) is central for the exciting field of spin-orbitronics. Platinum, the archetypal spin Hall material, has the potential to be an outstanding provider for spin-orbit torques due to its giant spin Hall conductivity, low resistivity, high stabilities, and the ability to be compatible with CMOS circuits. However, pure clean-limit Pt with low resistivity still provides a low damping-like spin-orbit torque efficiency, which limits its practical applications. The efficiency of spin-orbit torque in Pt-based magnetic heterostructures can be improved considerably by increasing the spin Hall ratio of Pt and spin transmissivity of the interfaces. Here we reviews recent advances in understanding the physics of spin current generation, interfacial spin transport, and the metrology of spin-orbit torques, and summarize progress towards the goal of Pt-based spin-orbit torque memories and logic that are fast, efficient, reliable, scalable, and non-volatile.

cond-mat.mes-hall↗

arXiv2021

Unveiling the mechanism of bulk spin-orbit torques within chemically disordered Fe$_x$Pt$_{1-x}$ single layers

Recent discovery of spin-orbit torques (SOTs) within magnetic single-layers has attracted attention in the field of spintronics. However, it has remained elusive as to how to understand and how to tune the SOTs. Here, utilizing the single layers of chemically disordered Fe$_x$Pt$_{1-x}$, we unveil the mechanism of the "unexpected" bulk SOTs by studying their dependence on the introduction of a controlled vertical composition gradient and on temperature. We find that the bulk damping like SOT arises from an imbalanced internal spin current that is transversely polarized and independent of the magnetization orientation. The torque can be strong only in the presence of a vertical composition gradient and the SOT efficiency per electric field is insensitive to temperature but changes sign upon reversal of the orientation of the composition gradient, which are in analogue to behaviors of the strain. From these characteristics we conclude that the imbalanced internal spin current originates from a bulk spin Hall effect and that the associated inversion asymmetry that allows for a non-zero net torque is most likely a strain non-uniformity induced by the composition gradient. The fieldlike SOT is a relatively small bulk effect compared to the dampinglike SOT. This work points to the possibility of developing low-power single-layer SOT devices by strain engineering.

cond-mat.mes-hall↗

arXiv2021

A multi-channel approach for automatic microseismic event association using RANSAC-based arrival time event clustering(RATEC)

In the presence of background noise, arrival times picked from a surface microseismic data set usually include a number of false picks that can lead to uncertainty in location estimation. To eliminate false picks and improve the accuracy of location estimates, we develop an association algorithm termed RANSAC-based Arrival Time Event Clustering (RATEC) that clusters picked arrival times into event groups based on random sampling and fitting moveout curves that approximate hyperbolas. Arrival times far from the fitted hyperbolas are classified as false picks and removed from the data set prior to location estimation. Simulations of synthetic data for a 1-D linear array show that RATEC is robust under different noise conditions and generally applicable to various types of subsurface structures. By generalizing the underlying moveout model, RATEC is extended to the case of a 2-D surface monitoring array. The effectiveness of event location for the 2-D case is demonstrated using a data set collected by the 5200-element dense Long Beach array. The obtained results suggest that RATEC is effective in removing false picks and hence can be used for phase association before location estimates.

physics.geo-ph↗

arXiv2021

BCNN: A Binary CNN with All Matrix Ops Quantized to 1 Bit Precision

This paper describes a CNN where all CNN style 2D convolution operations that lower to matrix matrix multiplication are fully binary. The network is derived from a common building block structure that is consistent with a constructive proof outline showing that binary neural networks are universal function approximators. 71.24% top 1 accuracy on the 2012 ImageNet validation set was achieved with a 2 step training procedure and implementation strategies optimized for binary operands are provided.

cs.LG↗

arXiv2021

Fully Spin-transparent magnetic interfaces enabled by insertion of a paramagnetic NiO layer

Spin backflow and spin-memory loss have been well established to considerably lower the interfacial spin transmissivity of metallic magnetic interfaces and thus the energy efficiency of spin-orbit torque technologies. Here we report that spin backflow and spin-memory loss at Pt-based heavy metal/ferromagnet interfaces can be effectively eliminated by inserting an insulating paramagnetic NiO layer of optimum thickness. The latter enables the thermal magnon-mediated essentially unity spin-current transmission at room temperature due to considerably enhanced effective spin-mixing conductance of the interface. As a result, we obtain dampinglike spin-orbit torque efficiency per unit current density of up to 0.8 as detected by the standard technology ferromagnet FeCoB and others, which reaches the expected upper-limit spin Hall ratio of Pt. We establish that Pt/NiO and Pt-Hf/NiO are two energy-efficient, integration-friendly, and high-endurance spin-current generators that provide >100 times greater energy efficiency than sputter-deposited topological insulators BiSb and BiSe. Our finding will benefit spin-orbitronic research and advance spin-torque technologies.

physics.app-ph↗

arXiv2021

Absence of significant spin current generation in Ti/FeCoB bilayers with strong interfacial spin-orbit coupling

After one decade of the intensive theoretical and experimental explorations, whether interfacial spin-orbit coupling (ISOC) at metallic magnetic interfaces can effectively generate a spin current has remained in dispute. Here, utilizing the Ti/FeCoB bilayers that are unique for the negligible bulk spin Hall effect and the strong tunable ISOC, we establish that there is no significant charge-to-spin conversion at magnetic interfaces via spin-orbit filtering effect or Rashba-Edelstein(-like) effect even when the ISOC is stronger than that of a typical Pt/ferromagnet interface and can promote strong perpendicular magnetic anisotropy. We also find a minimal orbital Hall effect in 3d Ti.

cond-mat.mes-hall↗

arXiv2021

Lack of simple correlation between switching current density and spin-orbit torque efficiency of perpendicularly magnetized spin-current generator/ferromagnet heterostructures

Spin-orbit torque can drive electrical switching of magnetic layers. Here, we report that at least for micrometer-sized samples there is no simple correlation between the efficiency of dampinglike spin-orbit torque (ξ_DL^j) and the critical switching current density of perpendicularly magnetized spin-current generator/ferromagnet heterostructures. We find that the values of ξ_DL^j based on switching current densities can either under- or over-estimated ξ_DL^j by up to tens of times in a domain-wall depinning analysis, while in the macrospin analysis based on the switching current density ξ_DL^j can be overestimated by up to thousands of times. When comparing the relative strengths of ξ_DL^j of spin-current generators, the critical switching current densities by themselves are a poor predictor.

cond-mat.mtrl-sci↗

arXiv2020

Event-triggered controllers based on the supremum norm of sampling-induced error

The paper proposes a novel event-triggered control scheme for nonlinear systems based on the input-delay method. Specifically, the closed-loop system is associated with a pair of auxiliary input and output. The auxiliary output is defined as the derivative of the continuous-time input function, while the auxiliary input is defined as the input disturbance caused by the sampling or equivalently the integral of the auxiliary output over the sampling period. As a result, a cyclic mapping forms from the input to the output via the system dynamics and back from the output to the input via the integral. The event-triggering law is constructed to make the mapping contractive such that the stabilization is achieved and an easy-to-check Zeno-free condition is provided. With this idea, we develop a theorem for the event-triggered control of interconnected nonlinear systems which is employed to solve the event-triggered control for lower-triangular systems with dynamic uncertainties.

eess.SY↗

arXiv2020

Observation of strong bulk damping-like spin-orbit torque in chemically disordered ferromagnetic single layers

Strong damping-like spin-orbit torque (τDL) has great potential for enabling ultrafast energy-efficient magnetic memories, oscillators, and logic. So far, the reported τDL exerted on a thin-film magnet must result from an externally generated spin current or from an internal non-equilibrium spin polarization in noncentrosymmetric GaMnAs single crystals. Here, we for the first time demonstrate a very strong, unexpected τDL from current flow within ferromagnetic single layers of chemically disordered, face-centered-cubic CoPt. We establish that the novel τDL is a bulk effect, with the strength per unit current density increasing monotonically with the CoPt thickness, and is insensitive to the presence or absence of spin sinks at the CoPt surfaces. This τDL most likely arises from a net transverse spin polarization associated with a strong spin Hall effect (SHE), while there is no detectable long-range asymmetry in the material. These results broaden the scope of spin-orbitronics and provide a novel avenue for developing single-layer-based spin-torque memory, oscillator, and logic technologies.

cond-mat.mtrl-sci↗

arXiv2020

Operando Control of Skyrmion Density in a Lorentz Transmission Electron Microscope with Current Pulses

Magnetic skyrmions hold promise for spintronic devices. To explore the dynamical properties of skyrmions in devices, a nanoscale method to image spin textures in response to a stimulus is essential. Here, we apply a technique for operando electrical current pulsing of chiral magnetic devices in a Lorentz transmission electron microscope. In ferromagnetic multilayers with interfacial Dzyaloshinskii-Moriya interaction (DMI), we study the creation and annihilation of skyrmions localized by point-like pinning sites due to defects. Using a combination of experimental and micromagnetic techniques, we establish a thermal contribution for the creation and annihilation of skyrmions in our study. Our work reveals a mechanism for controlling skyrmion density, which enables an examination of skyrmion magnetic field stability as a function of density. We find that high-density skyrmion states are more stable than low-density states or isolated skyrmions resisting annihilation over a magnetic field range that increases monotonically with density.

cond-mat.mes-hall↗

arXiv2020

Origin of Strong Two-Magnon Scattering in Heavy Metal/Ferromagnet/Oxide Heterostructures

We experimentally investigate the origin of the two-magnon scattering (TMS) in heavy-metal (HM)/ferromagnet (FM)/oxide heterostructures (FM = Co, Ni81Fe19, or Fe60Co20B20) by varying the materials located above and below the FM layers. We show that strong TMS in HM/FM/oxide systems arises primarily at the HM/FM interface and increases with the strength of interfacial spin-orbit coupling and magnetic roughness at this interface. TMS at the FM/oxide interface is relatively weak, even in systems where spin-orbit coupling at this interface generates strong interfacial magnetic anisotropy. We also suggest that the spin-current-induced excitation of non-uniform short-wavelength magnon at the HM/FM interface may function as a mechanism of spin memory loss for the spin-orbit torque exerted on the uniform mode.

cond-mat.mtrl-sci↗

arXiv2019

Energy-efficient ultrafast SOT-MRAMs based on low-resistivity spin Hall metal Au0.25Pt0.75

Many key electronic technologies (e.g., large-scale computing, machine learning, and superconducting electronics) require new memories that are fast, reliable, energy-efficient, and of low-impedance at the same time, which has remained a challenge. Non-volatile magnetoresistive random access memories (MRAMs) driven by spin-orbit torques (SOTs) have promise to be faster and more energy-efficient than conventional semiconductor and spin-transfer-torque magnetic memories. This work reports that the spin Hall effect of low-resistivity Au0.25Pt0.75 thin films enables ultrafast antidamping-torque switching of SOT-MRAM devices for current pulse widths as short as 200 ps. If combined with industrial-quality lithography and already-demonstrated interfacial engineering, our results show that an optimized MRAM cell based on Au0.25Pt0.75 can have energy-efficient, ultrafast, and reliable switching, e.g. a write energy of < 1 fJ (< 50 fJ) for write error rate of 50% (<1e-5) for 1 ns pulses. The antidamping torque switching of the Au0.25Pt0.75 devices is 10 times faster than expected from a rigid macrospin model, most likely because of the fast micromagnetics due to the enhanced non-uniformity within the free layer. These results demonstrate the feasibility of Au0.25Pt0.75-based SOT-MRAMs as a candidate for ultrafast, reliable, energy-efficient, low-impedance, and unlimited-endurance memory.

physics.app-ph↗

arXiv2019

Maximizing the spin-orbit torque efficiency of Pt/Ti multilayers by optimization of the tradeoff between the intrinsic spin Hall conductivity and carrier lifetime

We report a comprehensive study of the maximization of the spin Hall ratio (θSH) in Pt thin films by the insertion of sub-monolayer layers of Ti to decrease carrier lifetime while minimizing the concurrent reduction in the spin Hall conductivity. We establish that the intrinsic spin Hall conductivity of Pt, while robust against the strain and the moderate interruption of crystal order caused by these insertions, begins to decrease rapidly at high resistivity level because of the shortening carrier lifetime. The unavoidable trade-off between the intrinsic spin Hall conductivity and carrier lifetime sets a practical upper bound of θSH >=0.8 for heterogeneous materials where the crystalline Pt component is the source of the spin Hall effect and the resistivity is increased by shortening carrier lifetime. This work also establishes a very promising spin-Hall metal of [Pt 0.75 nm/Ti 0.2 nm]7/Pt 0.75 nm for energy-efficient, high-endurance spin-orbit torque technologies (e.g., memories, oscillators, and logic) due to its combination of a giant θSH of 0.8, or equivalently a dampinglike spin torque efficiency per unit current density of 0.35, with a relatively low resistivity (90 uOhm cm) and high suitability for practical technology integration.

cond-mat.mtrl-sci↗

arXiv2019

Influence of chemical stability on the fabrication of MnGa-based devices

Ferromagnetic films of L10-ordered MnGa have shown promise not only in the applications in ultrahigh-density magnetic recording and spintronic memories, oscillators, and sensors, but also in controllable studies of novel electrical transport phenomena. However, the stability of MnGa in chemicals and oxygen plasma that are commonly used in the standard micro-/nano-fabrication process has unsettled. In this work, we report a systematic study on the chemical stability of the MnGa films in acids, acetone, ethanol, deionized water, tetramethylammonium hydroxide (TMAOH) and oxygen plasma. We find that MnGa is very stable in acetone and ethanol, while can be attacked substantially if soaked in TMAOH solution for sufficiently long time. Deionized water and acids (e.g., HCl, H3PO4 and H2SO4 solutions) attack MnGa violently and should be avoided whenever possible. In addition, oxygen plasma can passivate the MnGa surface by oxidizing the surface. These results provide important information for the fabrication and the integration of MnGa based spintronic devices.

physics.app-ph↗

arXiv2019

Effective spin-mixing conductance of heavy-metal-ferromagnet interfaces

The effective spin-mixing conductance (G_eff) of a heavy metal/ferromagnet (HM/FM) interface characterizes the efficiency of the interfacial spin transport.Accurately determining G_eff is critical to the quantitative understanding of measurements of direct and inverse spin Hall effects. G_eff is typically ascertained from the inverse dependence of magnetic damping on the FM thickness under the assumption that spin pumping is the dominant mechanism affecting this dependence.Here we report that, this assumption fails badly in many in-plane magnetized prototypical HM/FM systems in the nm-scale thickness regime. Instead, the majority of the damping is from two-magnon scattering at the FM interface, while spin-memory-loss scattering at the interface can also be significant.If these two effects are neglected, the results will be an unphysical "giant" apparent G_eff and hence considerable underestimation of both the spin Hall ratio and the spin Hall conductivity in inverse/direct spin Hall experiments.

cond-mat.mtrl-sci↗

arXiv2019

Interface-driven unusual anomalous Hall effect in MnxGa/Pt bilayers: No correlation with chiral spin structures

The effects of spin-orbit coupling and symmetry breaking at the interface between a ferromagnet and heavy metal are particularly important for spin-based information storage and computation. Recent discoveries suggest they can create chiral spin structures (e.g. skyrmions), which have often been identified through the appearance of the bump/dip features of Hall signals, the so-called topological Hall effect (THE). In this work, however, we have present an unusual anomalous Hall effect (UAHE) in MnxGa/Pt bilayers and demonstrated that the features extremely similar to THE can be generated without involving any chiral spin structures. The low temperature magnetic force microscopy has been used to explore the magnetic field-dependent behavior of spin structures, and the UAHE as a function of magnetic field does not peak near the maximal density of magnetic bubbles. The results unambiguously evidence that the UAHE in MnxGa/Pt bilayers shows no correlation with chiral spin structures but is driven by the modified interfacial properties. The bump/dip features of Hall signals cannot be taken as an unambiguous signature for the emergence of chiral spin structures, and a wealth of underlying and interesting physics need explored.

cond-mat.mtrl-sci↗

arXiv2019

Cryogenic Memory Architecture Integrating Spin Hall Effect based Magnetic Memory and Superconductive Cryotron Devices

One of the most challenging obstacles to realizing exascale computing is minimizing the energy consumption of L2 cache, main memory, and interconnects to that memory. For promising cryogenic computing schemes utilizing Josephson junction superconducting logic, this obstacle is exacerbated by the cryogenic system requirements that expose the technology's lack of high-density, high-speed and power-efficient memory. Here we demonstrate an array of cryogenic memory cells consisting of a non-volatile three-terminal magnetic tunnel junction element driven by the spin Hall effect, combined with a superconducting heater-cryotron bit-select element. The write energy of these memory elements is roughly 8 pJ with a bit-select element, designed to achieve a minimum overhead power consumption of about 30%. Individual magnetic memory cells measured at 4 K show reliable switching with write error rates below $10^{-6}$, and a 4x4 array can be fully addressed with bit select error rates of $10^{-6}$. This demonstration is a first step towards a full cryogenic memory architecture targeting energy and performance specifications appropriate for applications in superconducting high performance and quantum computing control systems, which require significant memory resources operating at 4 K.

physics.app-ph↗

arXiv2019

Intrinsic nature of the giant spin Hall conductivity of Pt

More than a decade after the first theoretical and experimental studies of the spin Hall conductivity (SHC) of Pt, both its dominant origin and amplitude remain in dispute. Resolving these questions is of fundamental importance for advancing understanding of very strong spin-orbit effects in conducting systems and for maximizing the spin Hall effect for energy-efficient spintronics applications. Here, we report the experimental determination of the rapid variation of the intrinsic SHC of Pt with the carrier lifetime (τ) in the dirty-metal regime by incorporating finely dispersed MgO inter-site impurities into the Pt while maintaining the essential elements of its band structure (face-centered-cubic order). This findings conclusively validate the theoretical prediction that the SHC in Pt in the dirty-metal regime should be dominated by the intrinsic Berry curvature contribution and should decrease rapidly with shortening τ. This also establishes the limit to which the spin Hall ratio θSH of pure Pt can be increased by shortening τ. When the spin backflow at the Pt/ferromagnet interface due to the finite interfacial spin-mixing conductance is taken into account, the amplitude of the intrinsic SHC of Pt in the clean limit is found to be at least 1.6x10^6 (h_bar/2e) Ω-1 m-1, more than 3.5 times greater than the available theoretical predictions. Our work also establishes a compelling spin Hall metal Pt0.6(MgO)0.4 that combines a giant θSH (0.73) with a moderate resistivity (74 μΩ cm), a strong Dzyaloshinskii-Moriya interaction, easy growth, and good integration compatibility for spintronics technology.

cond-mat.mtrl-sci↗