arXiv subjects

H. Y. Yuan

Publications and source records attributed to H. Y. Yuan.

At least 55 records · Page 3Linked to original sources

arXiv2019

Enhancement of antiferromagnetic magnon-magnon entanglement by cavity cooling

Magnon-photon coupling has been experimentally realized inside a cavity and the emerging field known as cavity spintronics has attracted significant attention for its potential docking with quantum information science. However, one seldom knows whether this coupling implies an entanglement state among magnons and photons or not, which is crucial for its usage in quantum information. Here we study the entanglement properties among magnons and photons in an antiferromagnet-light system and find that the entanglement between magnon and photon is nearly zero while the magnon-magnon entanglement is very strong and it can be even further enhanced through the coupling with the cavity photons. The maximum enhancement occurs when the antiferromagnet reaches resonant with the light. The essential physics can be well understood within the picture of cavity induced cooling of magnon-magnon state near its joint vacuum with stronger entanglement. Our results are significant to extend the cavity spintronics to quantum manipulation and further provide an alternate to manipulate the deep strong correlations of continuous variable entanglement with a generic stable condition and easy tunability.

cond-mat.mes-hall↗

arXiv2018

Atomic antiferromagnetic domain wall propagation beyond the relativistic limit

We theoretically investigate the dynamics of atomic domain walls (DWs) in antiferromagnets driven by a spin-orbit field. For a DW with the width of a few lattice constants, we identify a Peierls-like pinning effect, with the depinning field exponentially decaying with the DW width, so that a spin-orbit field moderately larger than the threshold can drive the propagation of an atomic DW in a step-wise manner. For a broad DW, the Peierls pinning is negligibly small. However, the external spin-orbit field can induce a fast DW propagation, accompanied by a significant shrinking of its width down to atomic scales. Before stepping into the pinning region, noticeable spin waves are emitted at the tail of the DW. The spin-wave emission event not only broadens the effective width of the DW, but also pushes the DW velocity over the magnonic barrier, which is generally believed to be the relativistic limit of the DW speed. While the existing dynamic theory based on the continuum approximation fails in the atomic scale, we develop an energy conversion theory to interpret the DW dynamics beyond the relativistic limit.

cond-mat.mes-hall↗

arXiv2018

Quantum carpets from Gaussian sum theory

In many closed quantum systems, an interesting phenomenon, called quantum carpet, can be observed, where the evolution of wave function exhibits a carpet-like pattern. The mechanism of quantum carpet is similar to the classical interference pattern of light. Although the origin of quantum carpets has been studied previously, there are still many interesting details worth exploring. Here, we presented a unified framework for a simultaneous analyzing on three features of quantum carpets, namely full revival, fractional revival and diagonal canal. For fractional revival, a complete formula is presented to explain its generation through "Gaussian sum theory", in which all the essential features, including the phases and amplitudes, of this phenomenon could be captured analytically. Moreover, we also revealed the relations between the interference terms of the diagonal canals and their geometric interpretations, providing a better understanding in the formation of diagonal canals.

quant-ph↗

arXiv2018

Wiggling skyrmion propagation under parametric pumping

We address the problem of how magnetic skyrmions can propagate along a guided direction by parametric pumping. As evidenced by our micromagnetic simulations, skyrmions can hardly be driven by either a static electric field or a static magnetic field alone. Although the magnetic anisotropy can be modified by an electric field, parametric pumping with an oscillating electric field can only excite the breathing modes. On the other hand, a static magnetic field can break rotational symmetry through the Zeeman interaction, but it cannot serve as an energy source for propelling a skyrmion. Here we found that the combination of a perpendicularly oscillating electric field and an in-plane static magnetic field can drive a skyrmion undergoing a wiggling motion along a well defined trajectory. The most efficient driving occurs when the frequency of the oscillating field is close to that of the breathing motion. The physics is revealed in a generalized Thiele equation where a net spin current excited by the parametric pumping can drive the skyrmion propagation through angular momentum transfer. Compared with other alternative proposals, our results open new possibilities for manipulating skyrmions in both metals and insulators with low-power consumption. The oscillating skyrmion motion can also be a microwave generator for future spintronic applications such as an nano-tool on a diamond Nitrogen-Vacancy center.

cond-mat.mes-hall↗

arXiv2018

Breaking the current density threshold in spin-orbit-torque magnetic random access memory

Spin-orbit-torque magnetic random access memory (SOT-MRAM) is a promising technology for the next generation of data storage devices. The main bottleneck of this technology is the high reversal current density threshold. This outstanding problem of SOT-MRAM is now solved by using a current density of constant magnitude and varying flow direction that reduces the reversal current density threshold by a factor of more than the Gilbert damping coefficient. The Euler-Lagrange equation for the fastest magnetization reversal path and the optimal current pulse are derived for an arbitrary magnetic cell. The theoretical limit of minimal reversal current density and current density for a GHz switching rate of the new reversal strategy for CoFeB/Ta SOT-MRAMs are respectively of the order of $10^5$ A/cm$^2$ and $10^6$ A/cm$^2$ far below $10^7$ A/cm$^2$ and $10^8$ A/cm$^2$ in the conventional strategy. Furthermore, no external magnetic field is needed for a deterministic reversal in the new strategy.

cond-mat.mes-hall↗

arXiv2018

Strong entanglement of spins inside a quantum domain wall

Magnetic domain walls (DWs) are widely regarded as classical objects in physics community, even though the concepts of electron spins and spin-spin exchange interaction are quantum mechanical in nature. One intriguing question is whether DWs can survive at the quantum level and acquire the quantum properties such as entanglement. Here we show that spins within a DW are highly entangled in their quantum description. The total magnetization of a magnetic DW is nonzero, which is a manifestation of the global entanglement of the collective spin state. These results significantly deepen our understanding of magnetic DWs and enable the application of DWs in quantum information science. The essential physics can be generalized to skyrmions so that they can also play a role in quantum information processing.

cond-mat.mes-hall↗

arXiv2018

A theory on skyrmion size and profile

A magnetic skyrmion is a topological object consisting of an inner domain, an outer domain, and a wall that separates the two domains. The skyrmion size and wall width are two fundamental quantities of a skyrmion that depend sensitively on material parameters such as exchange energy, magnetic anisotropy, Dzyaloshinskii-Moriya interaction, and magnetic field. However, there is no quantitative understanding of the two quantities so far. Here, we present general expressions for the skyrmion size and wall width obtained from energy considerations. The two formulas agree almost perfectly with simulations and experiments for a wide range of parameters, including all existing materials that support skyrmions. Furthermore, it is found that skyrmion profiles agree very well with the Walker-like 360\textdegree{} domain wall formula.

cond-mat.mes-hall↗

arXiv2017

Proper dissipative torques in antiferromagnetic dynamics

There is little doubt that the magnetization dynamics of ferromagnetic systems is governed by the Landau-Lifshitz-Gilbert equation or its generalization with various spin torques. In contrast, there are several sets of dynamic equations for two-sublattice antiferromagnets (AFMs) in literature that have different forms of dissipative torques and no proper dynamic equations for multi-sublattice AFMs and ferrimagnets in general. Here we introduce the general Rayleigh dissipation functional into the Lagrange equation and derive the proper form of the dissipative torques in the phenomenological equations for the AFMs with multiple sublattices. A new type of dissipative torque arising from inter-sublattice drag effect is discovered that has important influences on magnon lifetime and domain wall motion. In particular, our theory unifies different dynamic equations of AFMs in literature.

cond-mat.mes-hall↗

arXiv2017

Classification of Magnetic Forces on Antiferromagnetic Domain Wall

A major challenge in spintronics is to find an efficient means to manipulate antiferromagnet (AFM) states, which are inert relative to a uniform magnetic field, due to the vanishingly-small net magnetization. The question is, how does an AFM response to an inhomogeneous field? Here we address the problem through a complete classification of the magnetic forces on an AFM domain wall (DW), revealing the following physical properties: (i) the tiny net magnetization still responses to the field gradient. (ii) the Néel order is sensitive to the field difference between two sublattices. (iii) DW energy has a quadratic dependence on the magnetic field due to its noncollinear structure. Remarkably, the first two factors drive DW to the opposite directions in a nanowire, but the third effect tends to push the DW to the high field region. Consequently, the competition among these three forces can be applied to understand the seemingly-contradictory results on AFM motion in literature. Additionally, our results provide a new route for a speedy manipulating AFM DW; our numerical simulation indicated that for a synthetic antiferromagnet, the DW propagating speed can reach tens of kilometers per second, an order of magnitude higher than that driven by an electric current.

cond-mat.mes-hall↗

arXiv2017

Thermodynamic Entanglement of Magnonic Condensates

Over the last decade, significant progresses have been achieved to create Bose-Einstein condensates (BEC) of magnetic excitations, i.e., magnons, at the room temperature, which is a novel quantum many-body system with a strong spin-spin correlation, and contains potential applications in magnonic spintronics. For quantum information science, the magnonic condensates can become an attractive source of quantum entanglement, which plays a central role in most of the quantum information processing tasks. Here we theoretically study the entanglement properties of a magnon gas above and below the condensation temperature. We show that the thermodynamic entanglement of the magnons is a manifestation of the off-diagonal long-range order; the entanglement of the condensate does not vanish, even if the spins are separated by an infinitely large distance, which is fundamentally distinct from the normal magnetic ordering below the Curie temperature. In addition, the phase transition point occurs when the derivative of the entanglement changes abruptly. Furthermore, the spin-spin entanglement can be experimentally accessed with the current technology. These results provide a theoretical foundation for a future experimental investigation of the magnon BEC in terms of quantum entanglement.

cond-mat.mes-hall↗

arXiv2017

Mode-Dependent Damping in Metallic Antiferromagnets Due to Inter-Sublattice Spin Pumping

Damping in magnetization dynamics characterizes the dissipation of magnetic energy and is essential for improving the performance of spintronics-based devices. While the damping of ferromagnets has been well studied and can be artificially controlled in practice, the damping parameters of antiferromagnetic materials are nevertheless little known for their physical mechanisms or numerical values. Here we calculate the damping parameters in antiferromagnetic dynamics using the generalized scattering theory of magnetization dissipation combined with the first-principles transport computation. For the PtMn, IrMn, PdMn and FeMn metallic antiferromagnets, the damping coefficient associated with the motion of magnetization ($α_m$) is one to three orders of magnitude larger than the other damping coefficient associated with the variation of the Néel order ($α_n$), in sharp contrast to the assumptions made in the literature.

cond-mat.mtrl-sci↗

arXiv2017

Magnon-photon coupling in antiferromagnets

Magnon-photon coupling in antiferromagnets has many attractive features that do not exist in ferro- or ferrimagnets. We show quantum-mechanically that, in the absence of an external field, one of the two degenerated spin wave bands couples with photons while the other does not. The photon mode anticrosses with the coupled spin waves when their frequencies are close to each other. Similar to its ferromagnetic counterpart, the magnon-photon coupling strength is proportional to the square root of number of spins $\sqrt{N}$ in antiferromagnets. An external field removes the spin wave degeneracy and both spin wave bands couple to the photons, resulting in two anticrossings between the magnons and photons. Two transmission peaks were observed near the anticrossing frequency. The maximum damping that allows clear discrimination of the two transmission peaks is proportional to $\sqrt{N}$ and it's well below the damping of antiferromagnetic insulators. Therefore the strong magnon-photon coupling can be realized in antiferromagnets and the coherent information transfer between the photons and magnons is possible.

cond-mat.mes-hall↗

arXiv2016

Skyrmions and multi-sublattice helical states in a frustrated chiral magnet

We investigate the existence and stability of skyrmions in a frustrated chiral ferromagnet by considering the competition between ferromagnetic (FM) nearest-neighbour (NN) interaction ($J_1$) and antiferromagnetic (AFM) next-nearest-neighbour (NNN) interaction ($J_2$). Contrary to the general wisdom that long-range ferromagnetic order is not energy preferable under frustration, the skyrmion lattice not only exists but is even stable for a large field range when $J_2 \leq J_1$ compared with frustration-free systems. We defend that the enlargement of stability window of skyrmions is a consequence of the reduced effective exchange interaction caused by the frustration. A multi-sublattice helical state is found below the skyrmion phase, which results from the competition between AFM coupling that favors a two-sublattice Néel state and the chiral interaction that prefers a helix. As a byproduct, the hysteresis loop of the frustrated chiral system shrinks as the magnetization goes to zero and then opens up again, known as wasp-waist hysteresis loop. The critical field that separates the narrow and wide part of the wasp-waist loop depends exponentially on the strength of NNN coupling. By measuring the critical field, it is possible to determine the strength of NNN coupling.

cond-mat.mes-hall↗

arXiv2016

Influence of nonlocal damping on the field-driven domain wall motion

We derive the complete expression of nonlocal damping in noncollinear magnetization due to the nonuniform spin current pumped by precessional magnetization and incorporate it into a generalized Thiele equation to study its effects on the dynamics of the transverse and vortex domain walls (DWs) in ferromagnetic nanowires. We demonstrate that the transverse component of nonlocal damping slows down the field-driven DW propagation and increases the Walker breakdown field whereas it is neglected in many previous works in literature. The experimentally measured DW mobility variation with the damping tuned by doping with heavy rare-earth elements that had discrepancy from micromagnetic simulation are now well understood with the nonlocal damping. Our results suggest that the nonlocal damping should be properly included as a prerequisite for quantitative studies of current-induced torques in noncollinear magnetization.

cond-mat.mes-hall↗

arXiv2015

Dynamic magnetic susceptibility and electrical detection of ferromagnetic resonance

The dynamic magnetic susceptibility of magnetic materials near ferromagnetic resonance (FMR) is very important in interpreting dc-voltage in electrical detection of FMR. Based on the causality principle and the assumption that the usual microwave absorption lineshape around FMR is Lorentzian, general forms of dynamic susceptibility of an arbitrary sample and the corresponding dc-voltage lineshape are obtained. Our main findings are: 1) The dynamic susceptibility is not a Polder tensor for material with arbitrary anisotropy. Two off-diagonal elements are not in general opposite to each other. However, the linear response coefficient of magnetization to total rf field is a Polder tensor. This may explain why two off-diagonal elements are always assumed to be opposite to each other in analyses. 2) The frequency dependence of dynamic susceptibility near FMR is fully characterized by six numbers while its field dependence is fully characterized by seven numbers. 3) A recipe of how to determine these numbers by standard microwave absorption measurements for an arbitrary sample is proposed. Our results allow one to unambiguously separate the contribution of the anisotropic magnetoresistance to dc-voltage from that of the anomalous Hall effect. With these results, one can reliably extract the information of spin pumping and the inverse spin Hall effect, and determine the spin-Hall angle. 4) The field-dependence of susceptibility matrix at a fixed frequency may have several peaks when the effective field is not monotonic of the applied field. In contrast, the frequency-dependence of susceptibility matrix at a fixed field has only one peak. Furthermore, in the case that resonance frequency is not sensitive to the applied field, the field dependence of susceptibility matrix, as well as dc-voltage, may have another non-resonance broad peak. Thus, one should be careful in interpreting observed peaks.

cond-mat.mes-hall↗

arXiv2015

Boosting Domain Wall Propagation by Notches

We report a counter-intuitive finding that notches in an otherwise homogeneous magnetic nanowire can boost current-induced domain wall (DW) propagation. DW motion in notch-modulated wires can be classified into three phases: 1) A DW is pinned around a notch when the current density is below the depinning current density. 2) DW propagation velocity is boosted by notches above the depinning current density and when non-adiabatic spin-transfer torque strength $β$ is smaller than the Gilbert damping constant $α$. The boost can be manyfold. 3) DW propagation velocity is hindered when $β> α$. The results are explained by using the Thiele equation.

cond-mat.mes-hall↗

arXiv2015

Magnetic Vortex Guide

A concept of magnetic vortex guide is proposed and numerically studied. Similar to the waveguides of electromagnetic waves, a magnetic vortex guide allows a vortex domain wall to move along a nanostrip without annihilation at the strip edges. It is shown by micromagnetic simulations that a magnetic nanostrip of a properly designed superlattice structure or bilayered structure can serve as vortex guides.

cond-mat.mtrl-sci↗

arXiv2014

Current-Driven Domain Wall Depinning and Propagation in Notched Nanowires

Adiabatic spin transfer torque induced domain wall (DW) depinning from a notch and DW propagation in a nanowire with a series of notches is investigated. Surprisingly, notches help a current to depin a DW and make a DW easier to propagate along a wire. Following fascinating results on DW dynamics are found. 1) The depinning current density of a DW in a notch is substantially lower than the intrinsic threshold value below which a sustainable DW propagation doesn't exist in a homogeneous wire. 2) The DW displacement from a notch is insensitive to notch geometry and current density when it is between the depinning and the intrinsic threshold current density. 3) A current density below the intrinsic threshold value can induce a sustainable DW propagation along notched nanowires. These findings not only reveal interesting and complicated interaction between a current and a DW, but also have profound implications in our current understanding of current-driven DW dynamics as well as in the design of spintronic devices.

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