arXiv subjects

Zhiyong Qiu

Publications and source records attributed to Zhiyong Qiu.

At least 55 records · Page 3Linked to original sources

arXiv2019

Nonlinear excitation of geodesic acoustic mode by toroidal Alfvén eigenmodes and impact on plasma performance

Spontaneous nonlinear excitation of geodesic acoustic mode (GAM) by toroidal Alfvén eigenmode (TAE) is investigated using nonlinear gyrokinetic theory. It is found that, the nonlinear decay process depends on thermal ion βvalue. Here, βis the plasma thermal to magnetic pressure ratio. In the low-βlimit, TAE decays into a GAM and a lower TAE sideband in the toroidicity induced shear Alfvén wave continuous spectrum gap; while in the high-βlimit, TAE decays into a GAM and a propagating kinetic TAE in the continuum. Both cases are investigated for the spontaneous decay conditions. The nonlinear saturation levels of both GAM and daughter wave are derived. The corresponding power balance and wave particle power transfer to thermal plasma are computed. Implications on thermal plasma heating are also discussed.

physics.plasm-ph↗

arXiv2018

Nonlinear saturation of toroidal Alfven eigenmodes via nonlinear mode couplings

Gyrokinetic theory of nonlinear mode coupling as a mechanism for toroidal Alfven eigenmode (TAE) saturation in the fusion plasma related parameter regime is presented, including 1) para- metric decay of TAE into lower kinetic TAE (LKTAE) and geodesic acoustic mode (GAM), and 2) enhanced TAE coupling to shear Alfven wave (SAW) continuum via ion induced scattering. Our theory shows that, for TAE saturation in the parameter range of practical interest, several processes with comparable scattering cross sections can be equally important.

physics.plasm-ph↗

arXiv2018

Nonlinear dynamics of Shear Alfvén fluctuations in Divertor Tokamak Test facility plasmas

Following the analysis on linear spectra of shear Alfvén fluctuations excited by energetic particles (EPs) in the Divertor Tokamak Test (DTT) facility plasmas [T. Wang et al., Phys. Plasmas 25, 062509 (2018)], in this work, nonlinear dynamics of the corresponding mode saturation and the fluctuation induced EP transport is studied by hybrid magnetohydrodynamic-gyrokinetic simulations. For the reversed shear Alfvén eigenmode driven by magnetically trapped EP precession resonance in the central core region of DTT plasmas, the saturation is mainly due to radial decoupling of resonant trapped EPs. Consistent with the wave-EP resonance structure, EP transport occurs in a similar scale to the mode width. On the other hand, passing EP transport is analyzed in detail for toroidal Alfvén eigenmode in the outer core region, with mode drive from both passing and trapped EPs. It is shown that passing EPs experience only weak redistributions in the weakly unstable case; and the transport extends to meso-scale diffusion in the strongly unstable case, due to orbit stochasticity induced by resonance overlap. Here, weakly/strongly unstable regime is determined by Chirikov condition for resonance overlap. This work then further illuminates rich and diverse nonlinear EP dynamics related to burning plasma studies, and the capability of DTT to address these key physics.

physics.plasm-ph↗

arXiv2018

Gyrokinetic theory of the nonlinear saturation of toroidal Alfven eigenmode

Nonlinear saturation of toroidal Alfven eigenmode (TAE) via ion induced scatterings is investigated in the short-wavelength gyrokinetic regime. It is found that the nonlinear evolution depends on the thermal ion \b{eta} value. Here, \b{eta} is the plasma thermal to magnetic pressure ratio. Both the saturation levels and associated energetic-particle transport coefficients are derived and estimated correspondingly.

physics.plasm-ph↗

arXiv2018

Spin colossal magnetoresistance in an antiferromagnetic insulator

Colossal magnetoresistance (CMR) refers to a large change in electrical conductivity induced by a magnetic field in the vicinity of a metal-insulator transition and has inspired extensive studies for decades\cite{Ramirez1997, Tokura2006}. Here we demonstrate an analogous spin effect near the Néel temperature $T_{\rm{N}}$=296 K of the antiferromagnetic insulator \CrO. Using a yttrium iron garnet \YIG/\CrO/Pt trilayer, we injected a spin current from the YIG into the \CrO layer, and collected via the inverse spin Hall effect the signal transmitted in the heavy metal Pt. We observed a change by two orders of magnitude in the transmitted spin current within 14 K of the Néel temperature. This transition between spin conducting and nonconducting states could be also modulated by a magnetic field in isothermal conditions. This effect, that we term spin colossal magnetoresistance (SCMR), has the potential to simplify the design of fundamental spintronics components, for instance enabling the realization of spin current switches or spin-current based memories.

cond-mat.mtrl-sci↗

arXiv2018

Spin transport across antiferromagnets induced by the spin Seebeck effect

For prospective spintronics devices based on the propagation of pure spin currents, antiferromagnets are an interesting class of materials that potentially entail a number of advantages as compared to ferromagnets. Here, we present a detailed theoretical study of magnonic spin current transport in ferromagnetic-antiferromagnetic multilayers by using atomistic spin dynamics simulations. The relevant length scales of magnonic spin transport in antiferromagnets are determined. We demonstrate the transfer of angular momentum from a ferromagnet into an antiferromagnet due to the excitation of only one magnon branch in the antiferromagnet. As an experimental system, we ascertain the transport across an antiferromagnet in YIG$|$Ir$_{20}$Mn$_{80}|$Pt heterostructures. We determine the spin transport signals for spin currents generated in the YIG by the spin Seebeck effect and compare to measurements of the spin Hall magnetoresistance in the heterostructure stack. By means of temperature-dependent and thickness-dependent measurements, we deduce conclusions on the spin transport mechanism across IrMn and furthermore correlate it to its paramagnetic-antiferromagnetic phase transition.

cond-mat.mtrl-sci↗

arXiv2018

Kinetic theory of geodesic acoustic modes in toroidal plasmas: a brief review

Geodesic acoustic modes (GAM) are oscillating zonal structures unique to toroidal plasmas, and have been extensively studied in the past decades due to their potential capabilities of regulating microscopic turbulences and associated anomalous transport. This article reviews linear and nonlinear theories of GAM; with emphases on kinetic treatment, system nonuniformity and realistic magnetic geometry, in order to reflect the realistic experimental conditions. Specifically, in the linear physics, the resonant wave-particle interactions are discussed, with the application to resonant excitation by energetic particles (EPs). The theory of EP-induced GAM (EGAM) is applied to realistic devices for the interpretation of experimental observations, and global effects due to coupling to GAM continuum are also discussed. Meanwhile, in the nonlinear physics, the spontaneous GAM excitation by microscale turbulences is reviewed, including the effects of various system nonuniformities. A unified theoretical framework of GAM/EGAM is then constructed based on our present understandings. The first-principle-based GAM/EGAM theories reviewed here, thus, provide the tools needed for the understanding and interpretation of experimental/numerical results.

physics.plasm-ph↗

arXiv2017

Short Wavelength Geodesic Acoustic Mode Excitation by Energetic Particles

Taking the collisionless damping of geodesic acoustic mode (GAM) as an example, the physics processes underlying wave particle resonances in the short wavelength limit are clarified. As illus- trative application, GAM excitation by energetic particles in short wavelength limit is investigated assuming a single pitch angle slowing-down fast ion equilibrium distribution function. Conditions for this energetic particle-induced GAM (EGAM) to be unstable are discussed.

physics.plasm-ph↗

arXiv2017

Antiferromagnetic anisotropy determination by spin Hall magnetoresistance

An electric method for measuring magnetic anisotropy in antiferromagnetic insulators (AFIs) is proposed. When a metallic film with strong spin-orbit interactions, e.g., platinum (Pt), is deposited on an AFI, its resistance should be affected by the direction of the AFI N eel vector due to the spin Hall magnetoresistance (SMR). Accordingly, the direction of the AFI N eel vector, which is affected by both the external magnetic field and the magnetic anisotropy, is reflected in resistance of Pt. The magnetic field angle dependence of the resistance of Pt on AFI is calculated by consider- ing the SMR, which indicates that the antiferromagnetic anisotropy can be obtained experimentally by monitoring the Pt resistance in strong magnetic fields. Calculations are performed for realistic systems such as Pt/Cr2O3, Pt/NiO, and Pt/CoO.

cond-mat.mtrl-sci↗

arXiv2017

Ferroic collinear multilayer magnon spin valve

Information transport and processing by pure magnonic spin currents in insulators is a promising alternative to conventional charge-current driven spintronic devices. The absence of Joule heating as well as the reduced spin wave damping in insulating ferromagnets has been suggested to enable the implementation of efficient logic devices. After the proof of concept for a logic majority gate based on the superposition of spin waves has been successfully demonstrated, further components are required to perform complex logic operations. A key component is a switch that corresponds to a conventional magnetoresistive spin valve. Here, we report on magnetization orientation dependent spin signal detection in collinear magnetic multilayers with spin transport by magnonic spin currents. We find in Y3Fe5O12|CoO|Co tri-layers that the detected spin signal depends on the relative alignment of Y3Fe5O12 and Co. This demonstrates a spin valve behavior with an effect amplitude of 120% in our systems. We demonstrate the reliability of the effect and investigate the origin by both temperature and power dependent measurements, showing that spin rectification effects and a magnetic layer alignment dependent spin transport effect result in the measured signal.

cond-mat.mtrl-sci↗

arXiv2017

Detection of induced paramagnetic moments in Pt on Y$_3$Fe$_5$O$_{12}$ via x-ray magnetic circular dichroism

Magnetic moments in an ultra-thin Pt film on a ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ (YIG) have been investigated at high magnetic fields and low temperatures by means of X-ray magnetic circular dichroism (XMCD). We observed an XMCD signal due to the magnetic moments in a Pt film at the Pt $L_{3}$- and $L_{2}$-edges. By means of the element-specific magnetometry, we found that the XMCD signal at the Pt $L_{3}$-edge gradually increases with increasing the magnetic field even when the field is much greater than the saturation field of YIG. Importantly, the observed XMCD intensity was found to be much greater than the intensity expected from the Pauli paramagnetism of Pt when the Pt film is attached to YIG. These results imply the emergence of induced paramagnetic moments in Pt on YIG and explain the characteristics of the unconventional Hall effect in Pt/YIG systems.

cond-mat.mtrl-sci↗

arXiv2017

Non-local magnon transport in the compensated ferrimagnet GdIG

We study the diffusive transport of magnons through the compensated ferrimagnetic insulator Gd3Fe5O12 (GdIG). The magnons are injected and detected electrically in a non-local measurement configuration via two parallel Pt strips deposited on top of the ferrimagnet. GdIG exhibits a rich magnon spectrum, with several thermally populated magnon bands at room temperature. We observe a strong temperature and field dependence of the non-local voltage in the detector strip. Just below the magnetization compensation temperature we find that the increasing magnetic field causes an unexpected enhancement of the non-local signal. A comparison with GdIG spin wave spectra obtained from atomistic modeling indicates that the thermal magnon population is important for understanding the non-local voltage signal.

cond-mat.mes-hall↗

arXiv2017

Fabrication and magnetic control of Y3Fe5O12 cantilevers

We have fabricated ferrite cantilevers in which their vibrational properties can be controlled by external magnetic fields. Submicron-scale cantilever structures were made from Y3Fe5O12 (YIG) films by physical etching combined with use of a focused ion beam milling technique. We found that the cantilevers exhibit two resonance modes which correspond to horizontal and vertical vibrations. Under external magnetic fields, the resonance frequency of the horizontal mode increases, while that of the vertical mode decreases, quantitatively consistent with our numerical simulation for magnetic forces. The changes in resonance frequencies with magnetic fields reach a few percent, showing that efficient magnetic control of resonance frequencies was achieved.

cond-mat.mes-hall↗

arXiv2017

Tunable sign change of spin Hall magnetoresistance in Pt/NiO/YIG structures

Spin Hall magnetoresistance (SMR) has been investigated in Pt/NiO/YIG structures in a wide range of temperature and NiO thickness. The SMR shows a negative sign below a temperature which increases with the NiO thickness. This is contrary to a conventional SMR theory picture applied to Pt/YIG bilayer which always predicts a positive SMR. The negative SMR is found to persist even when NiO blocks the spin transmission between Pt and YIG, indicating it is governed by the spin current response of NiO layer. We explain the negative SMR by the NiO 'spin-flop' coupled with YIG, which can be overridden at higher temperatures by positive SMR contribution from YIG. This highlights the role of magnetic structure in antiferromagnets for transport of pure spin current in multilayers.

cond-mat.mes-hall↗

arXiv2017

On Zero Frequency Zonal Flow and Second Harmonic Generation by Finite Amplitude Energetic Particle Induced Geodesic Acoustic Mode

Nonlinear self-interaction of finite amplitude energetic particle induced geodesic acoustic mode (EGAM) is investigated using nonlinear gyrokinetic theory. It is found that both zero frequency zonal flow(ZFZF) and second harmonic can be driven by finite amplitude EGAM, with energetic particles (EPs) playing a dominant role in the nonlinear couplings through finite orbit width effects. For ZFZF, the effects of EPs on EGAM nonlinear self-coupling dominate that of the thermal plasmas which are also present; while the second harmonic generation is only possible via finite amplitude coupling though EPs. Our findings may improve the understanding of stabilizing zonal modes, and consequently, drift wave turbulence.

physics.plasm-ph↗

arXiv2017

Magnon-polaron transport in magnetic insulators

We theoretically study the effects of strong magnetoelastic coupling on the transport properties of magnetic insulators. We develop a Boltzmann transport theory for the mixed magnon-phonon modes (magnon polarons) and determine transport coefficients and spin diffusion length. Magnon-polaron formation causes anomalous features in the magnetic field and temperature dependence of the spin Seebeck effect when the disorder scattering in the magnetic and elastic subsystems is sufficiently different. Experimental data by Kikkawa et al. [PRL 117, 207203 (2016)] on yttrium iron garnet films can be explained by an acoustic quality that is much better than the magnetic quality of the material. We predict similar anomalous features in the spin and heat conductivity and non-local spin transport experiments.

cond-mat.mes-hall↗

arXiv2016

Nonlinear excitation of fine-radial-scale zonal structures by toroidal Alfven eigenmode

The set of equations describing nonlinear evolution of a single toroidal Alfven eigenmode are derived, including both zero frequency zonal structure (ZFZS) generation and wave-particle phase space nonlinearities. The simplified case of neglecting wave-particle phase space nonlinearity is then investigated to focus on different roles of energetic particles and bulk plasmas on ZFZS generation. It is shown that energetic particles and bulk plasma play dominant roles in ZFZS generation in different nonlinear stages, and the corresponding processes are qualitatively different. Several properties of ZFZS generation, e.g., fine- vs. meso-scale, forced driven vs. spontaneous excitation, are clarified by the present analysis.

physics.plasm-ph↗

arXiv2016

Magnon Polarons in the Spin Seebeck Effect

Sharp structures in magnetic field-dependent spin Seebeck effect (SSE) voltages of Pt/Y$_{3}$Fe$_{5}$O$_{12}$ (YIG) at low temperatures are attributed to the magnon-phonon interaction. Experimental results are well reproduced by a Boltzmann theory that includes the magnetoelastic coupling (MEC). The SSE anomalies coincide with magnetic fields tuned to the threshold of magnon-polaron formation. The effect gives insight into the relative quality of the lattice and magnetization dynamics.

cond-mat.mtrl-sci↗