arXiv subjects

Mingzhong Wu

Publications and source records attributed to Mingzhong Wu.

33 records · Page 2Linked to original sources

arXiv2017

Spin caloritronic nano-oscillator

Energy loss due to ohmic heating is a major bottleneck limiting down-scaling and speed of nano-electronic devices, and harvesting ohmic heat for signal processing is a major challenge in modern electronics. Here we demonstrate that thermal gradients arising from ohmic heating can be utilized for excitation of coherent auto-oscillations of magnetization and for generation of tunable microwave signals. The heat-driven dynamics is observed in $\mathrm{Y_{3}Fe_{5}O_{12}/Pt}$ bilayer nanowires where ohmic heating of the Pt layer results in injection of pure spin current into the $\mathrm{Y_{3}Fe_{5}O_{12}}$ layer. This leads to excitation of auto-oscillations of the $\mathrm{Y_{3}Fe_{5}O_{12}}$ magnetization and generation of coherent microwave radiation. Our work paves the way towards spin caloritronic devices for microwave and magnonic applications.

cond-mat.mes-hall↗

arXiv2017

Long-distance spin transport in a disordered magnetic insulator

Spin transport through magnetic insulators via magnons has recently been explored for a growing variety of magnetic systems with long-range order and well-understood spin excitation spectra. Here we show dramatic effects of spin transport through an amorphous magnetic insulator, which is both magnetically and structurally disordered. We generate and detect spin flow though amorphous yttrium-iron-garnet ($a$-YIG) thin films in a non-local geometry by use of the spin Hall and inverse spin Hall effects in platinum strips separated by 10 or more microns. By comparing non-local spin transport in $a$-YIG on suspended micromachined thermal isolation platforms to the same experiment performed on a bulk substrate, we show strong effects of in-plane thermal gradients on spin transport in the disordered magnetic insulator. The resulting non-local voltage signals are orders of magnitude larger than those seen in crystalline magnetic insulators, with easily measurable spin signals seen even at distances in excess of $100$ microns. In analogy to heat transport, where disordered materials support a range of vibrational excitations that can allow large thermal conductivities, we suggest that efficient spin transport in disordered magnetic systems can occur via a similar spectrum of excitations that relies on strong local exchange interactions and does not require long-range order. This work not only opens a new area for fundamental experimental and theoretical studies of spin transport, but also sets a new direction in materials science for magnonic and spintronic devices.

cond-mat.mes-hall↗

arXiv2017

Patterned growth of crystalline Y$_{3}$Fe$_{5}$O$_{12}$ nanostructures with engineered magnetic shape anisotropy

We demonstrate patterned growth of epitaxial yttrium iron garnet (YIG) thin films using lithographically defined templates on gadolinium gallium garnet (GGG) substrates. The fabricated YIG nanostructures yield the desired crystallographic orientation, excellent surface morphology, and narrow ferromagnetic resonance (FMR) linewidth (~ 4 Oe). Shape-induced magnetic anisotropy is clearly observed in a patterned array of nanobars engineered to exhibit the larger coercivity (40 Oe) compared with that of continuous films. Both hysteresis loop and angle-dependent FMR spectra measurements indicate that the easy axis aligns along the longitudinal direction of the nanobars, with an effective anisotropy field of 195 Oe. Our work overcomes difficulties in patterning YIG thin films and provides an effective means to control their magnetic properties and magnetic bias conditions.

cond-mat.mes-hall↗

arXiv2017

Observation of Self-Cavitating Envelope Dispersive Shock Waves in Yttrium Iron Garnet Thin Films

The formation and properties of envelope dispersive shock wave (DSW) excitations from repulsive nonlinear waves in a magnetic film are studied. Experiments involve the excitation of a spin-wave step pulse in a low-loss magnetic Y$_3$Fe$_5$O$_{12}$ thin film strip, in which the spin-wave amplitude increases rapidly, realizing the canonical Riemann problem of shock theory. Under certain conditions, the envelope of the spin-wave pulse evolves into a DSW that consists of an expanding train of nonlinear oscillations with amplitudes increasing from front to back, terminated by a black soliton. The onset of DSW self-cavitation, indicated by a point of zero power and a concomitant 180 phase jump, is observed for sufficiently large steps, indicative of the bidirectional dispersive hydrodynamic nature of the DSW. The experimental observations are interpreted with theory and simulations of the nonlinear Schrödinger equation.

cond-mat.mes-hall↗

arXiv2017

Influence of heavy metal materials on magnetic properties of Pt/Co/heavy metal tri-layered structures

Pt/Co/heavy metal (HM) tri-layered structures with interfacial perpendicular magnetic anisotropy (PMA) are currently under intensive research for several emerging spintronic effects, such as spinorbit torque, domain wall motion, and room temperature skyrmions. HM materials are used as capping layers to generate the structural asymmetry and enhance the interfacial effects. For instance, the Pt/Co/Ta structure attracts a lot of attention as it may exhibit large Dzyaloshinskii-Moriya interaction. However, the dependence of magnetic properties on different capping materials has not been systematically investigated. In this paper, we experimentally show the interfacial PMA and damping constant for Pt/Co/HM tri-layered structures through time-resolved magneto-optical Kerr effect measurements as well as magnetometry measurements, where the capping HM materials are W, Ta, and Pd. We found that the Co/HM interface plays an important role on the magnetic properties. In particular, the magnetic multilayers with a W capping layer features the lowest effective damping value, which may be attributed to the different spin-orbit coupling and interfacial hybridization between Co and HM materials. Our findings allow a deep understanding of the Pt/Co/HM tri-layered structures. Such structures could lead to a better era of data storage and processing devices.

cond-mat.mes-hall↗

arXiv2016

Spin transport in antiferromagnetic NiO and magnetoresistance in Y$_3$Fe$_5$O$_{12}$/NiO/Pt structures

We have studied spin transport and magnetoresistance in yttrium iron garnet (YIG)/NiO/Pt trilayers with varied NiO thickness. To characterize the spin transport through NiO we excite ferromagnetic resonance in YIG with a microwave frequency magnetic field and detect the voltage associated with the inverse spin-Hall effect (ISHE) in the Pt layer. The ISHE signal is found to decay exponentially with the NiO thickness with a characteristic decay length of 3.9 nm. This is contrasted with the magnetoresistance in these same structures. The symmetry of the magnetoresistive response is consistent with spin-Hall magnetoresistance (SMR). However, in contrast to the ISHE response, as the NiO thickness increases the SMR signal goes towards zero abruptly at a NiO thickness of $\simeq$ 4 nm, highlighting the different length scales associated with the spin-transport in NiO and SMR in such trilayers.

cond-mat.mtrl-sci↗

arXiv2016

Surface state dominated spin-charge current conversion in topological insulator/ferromagnetic insulator heterostructures

We report the observation of ferromagnetic resonance-driven spin pumping signals at room temperature in three-dimensional topological insulator thin films -- Bi2Se3 and (Bi,Sb)2Te3 -- deposited by molecular beam epitaxy on yttrium iron garnet thin films. By systematically varying the Bi2Se3 film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse Rashba-Edelstein effect length (lambda_IREE), increases dramatically as the film thickness is increased from 2 quintuple layers, saturating above 6 quintuple layers. This suggests a dominant role of surface states in spin and charge interconversion in topological insulator/ferromagnet heterostructures. Our conclusion is further corroborated by studying a series of YIG/(BiSb)2Te3 heterostructures. Finally, we use the ferromagnetic resonance linewidth broadening and the inverse Rashba-Edelstein signals to determine the effective interfacial spin mixing conductance and lambda_IREE.

cond-mat.mes-hall↗

arXiv2015

Driving and detecting ferromagnetic resonance in insulators with the spin Hall effect

We demonstrate the generation and detection of spin-torque ferromagnetic resonance in Pt/YIG bilayers. A unique attribute of this system is that the spin Hall effect lies at the heart of both the generation and detection processes and no charge current is passing through the insulating magnetic layer. When the YIG undergoes resonance, a dc voltage is detected longitudinally along the Pt that can be described by two components. One is the mixing of the spin Hall magnetoresistance with the microwave current. The other results from spin pumping into the Pt being converted to a dc current through the inverse spin Hall effect. The voltage is measured with applied magnetic field directions that range in-plane to nearly perpendicular. We find that for magnetic fields that are mostly out-of-plane, an imaginary component of the spin mixing conductance is required to model our data.

cond-mat.mes-hall↗

arXiv2015

Formation of Bright Solitons from Wave Packets with Repulsive Nonlinearity

Formation of bright envelope solitons from wave packets with a repulsive nonlinearity was observed for the first time. The experiments used surface spin-wave packets in magnetic yttrium iron garnet (YIG) thin film strips. When the wave packets are narrow and have low power, they undergo self-broadening during the propagation. When the wave packets are relatively wide or their power is relatively high, they can experience self-narrowing or even evolve into bright solitons. The experimental results were reproduced by numerical simulations based on a modified nonlinear Schrödinger equation model.

cond-mat.mes-hall↗

arXiv2014

Spin waves in micro-structured yttrium iron garnet nanometer-thick films

We investigated the spin-wave propagation in a micro-structured yttrium iron garnet waveguide of $40$ nm thickness. Utilizing spatially-resolved Brillouin light scattering microscopy, an exponential decay of the spin-wave amplitude of $(10.06 \pm 0.83)$ $μ$m was observed. This leads to an estimated Gilbert damping constant of $α=(8.79\pm 0.73)\times 10^{-4}$, which is larger than damping values obtained through ferromagnetic resonance measurements in unstructured films. The theoretically calculated spatial interference of waveguide modes was compared to the spin-wave pattern observed experimentally by means of Brillouin light scattering spectroscopy.

cond-mat.mes-hall↗

arXiv2013

Complex Solitary Wave Dynamics, Pattern Formation, and Chaos in the Gain-Loss Nonlinear Schrödinger Equation

A numerical exploration of a gain-loss nonlinear Schrödinger equation was carried out utilizing over 180000 core hours to conduct more than 10000 unique simulations in an effort to characterize the model's six dimensional parameter space. The study treated the problem in full generality, spanning a minimum of eight orders of magnitude for each of three linear and nonlinear gain terms and five orders of magnitude for higher order nonlinearities. The gain-loss nonlinear Schrödinger equation is of interest as a model for spin wave envelopes in magnetic thin film active feedback rings and analogous driven damped nonlinear physical systems. Bright soliton trains were spontaneously driven out of equilibrium and behaviors stable for tens of thousands of round trip times were numerically identified. Nine distinct complex dynamical behaviors with lifetimes on the order of ms were isolated as part of six identified solution classes. Numerically located dynamical behaviors include: (1) Low dimensional chaotic modulations of bright soliton trains; (2) spatially symmetric/asymmetric interactions of solitary wave peaks; (3) dynamical pattern formation and recurrence; (4) steady state solutions and (5) intermittency. Simulations exhibiting chaotically modulating bright soliton trains were found to qualitatively match previous experimental observations. Ten new dynamical behaviors, eight demonstrating long lifetimes, are predicted to be observable in future experiments.

nlin.PS↗

arXiv2011

Chaotic Spin-Wave Solitons in Magnetic Film Feedback Rings

Chaotic spin-wave solitons in magnetic film active feedback rings were observed for the first time. At some ring gain level, one observes the self-generation of a single spin-wave soliton pulse in the ring. When the pulse circulates in the ring, its amplitude varies chaotically with time. Numerical simulations based on a gain-loss nonlinear Schrödinger equation reproduce the observed responses.

cond-mat.mes-hall↗

arXiv2009

Formation of Random Dark Envelope Solitons from Incoherent Waves

This letter reports experimental results on a new type of soliton: the random temporal dark soliton. One excites an incoherent large-amplitude propagating spin-wave packet in a ferromagnetic film strip with a repulsive, instantaneous nonlinearity. One then observes the random formation of dark solitons from this wave packet. The solitons appear randomly in time and in position relative to the entire wave packet. They can be gray or black. For wide and/or very strong spin-wave packets, one also observes multiple dark solitons. In spite of the randomness of the initial wave packets and the random formation processes, the solitons show signatures that are found for conventional coherent dark solitons.

cond-mat.mes-hall↗

arXiv2009

Giant adiabatic spin torque in magnetic tunnel junctions with a hybrid free layer structure

The discovery of spin torque transfer (STT) has lead to a significant advance in the development of spintronic devices. Novel structures and materials have been studied in order to improve the performance of the magnetic tunnel junctions (MTJs) performances and understand the fundamental physics in spin torque transfer. The adiabatic spin torque effect, which is due to the spatial non-uniformity of magnetic properties, has been predicted in theory and demonstrated experimentally in magnetic nanowires. However, this important spin torque has been rarely concerned in the magnetic tunnel junctions (MTJ) because of its extremely weak effect in conventional MTJs. This paper reports for the first time a giant adiabatic spin torque in MTJ devices with a hybrid free layer structure. The generation of the giant adiabatic spin torque was realized through the introduction of a spatial magnetic non-uniformity in a hybrid free layer along the current direction. It is observed that the giant adiabatic spin torque can substantially promote the current-induced switching process in the MTJ devices: the adiabatic spin torque can be larger than the in-plane spin torque, which allows for the switching with a single-polar current under different bias fields. Moreover, the adiabatic spin torque, which is proportional to the level of spatial non-uniformity, increases during the switching process. The observed effects are confirmed by numerical simulations. These results have far-reaching implications for the future of high-density STT-MRAM devices.

cond-mat.mes-hall↗

arXiv2007

Observation of Spin Wave Soliton Fractals in Magnetic Film Active Feedback Rings

The manifestation of fractals in soliton dynamics has been observed for the first time. The experiment utilized self-generated spin wave envelope solitons in a magnetic film based active feedback ring. At high ring gain, the soliton that circulates in the ring breathes in a fractal pattern. The corresponding power frequency spectrum shows a comb structure, with each peak in the comb having its own comb, and so on, to finer and finer scales.

cond-mat.other↗