arXiv subjects

X. R. Wang

Publications and source records attributed to X. R. Wang.

At least 73 records · Page 4Linked to original sources

arXiv2018

Eavesdropping on spin waves inside the domain-wall nanochannel via three-magnon processes

One recent breakthrough in the field of magnonics is the experimental realization of reconfigurable spin-wave nanochannels formed by magnetic domain wall with a width of $10-100$ nm [Wagner \emph{et al}., Nat. Nano. \textbf{11}, 432 (2016)]. This remarkable progress enables an energy-efficient spin-wave propagation with a well-defined wave vector along its propagating path inside the wall. In the mentioned experiment, a micro-focus Brillouin light scattering spectroscopy was taken in a line-scans manner to measure the frequency of the bounded spin wave. Due to their localization nature, the confined spin waves can hardly be detected from outside the wall channel, which guarantees the information security to some extent. In this work, we theoretically propose a scheme to detect/eavesdrop on the spin waves inside the domain-wall nanochannel via nonlinear three-magnon processes. We send a spin wave in one magnetic domain to interact with the bounded mode in the wall. Two kinds of three-magnon processes, i.e., confluence and splitting, are expected to occur. The confluence process is conventional. We predict a stimulated three-magnon splitting (or "magnon laser") effect: the presence of a bound magnon propagating along the domain wall channel assists the splitting of the incident wave into two modes, one of which is identical to the bound mode in the channel. Micromagnetic simulations confirm our theoretical analysis. These results demonstrate that one is able to uniquely infer the spectrum of the spin-wave in the domain-wall nanochannel once we know both the injection and the transmitted waves.

cond-mat.mes-hall↗

arXiv2018

Subnanosecond magnetization reversal of magnetic nanoparticle driven by chirp microwave field pulse

We investigate the magnetization reversal of single-domain magnetic nanoparticle driven by linear down-chirp microwave magnetic field pulse. Numerical simulations based on the Landau-Lifshitz-Gilbert equation reveal that solely down-chirp pulse is capable of inducing subnanosecond magnetization reversal. With a certain range of initial frequency and chirp rate, the required field amplitude is much smaller than that of constant-frequency microwave field. The fast reversal is because the down-chirp microwave field acts as an energy source and sink for the magnetic particle before and after crossing over the energy barrier, respectively. Applying a spin-polarized current additively to the system further reduces the microwave field amplitude. Our findings provide a new way to realize low-cost and fast magnetization reversal.

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

Anomalous magnon Nernst effect of topological magnonic materials

The magnon transport driven by thermal gradient in a perpendicularly magnetized honeycomb lattice is studied. The system with the nearest-neighbor pseudodipolar interaction and the next-nearest-neighbor Dzyaloshinskii-Moriya interaction (DMI) has various topologically nontrivial phases. When an in-plane thermal gradient is applied, a transverse in-plane magnon current is generated. This phenomenon is termed as the anomalous magnon Nernst effect that closely resembles the anomalous Nernst effect for an electronic system. The anomalous magnon Nernst coefficient and its sign are determined by the magnon Berry curvatures distribution in the momentum space and magnon populations in the magnon bands. We predict a temperature-induced sign reversal in anomalous magnon Nernst effect under certain conditions.

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

Reconfigurable topological spin wave beamsplitters and interferometers

Conventional magnonic devices use three classes of magnetostatic waves that require detailed manipulation of magnetization structure that makes the design and the device/circuitry scalability difficult tasks. Here, we demonstrate that devices based on topological exchange spin waves do not suffer from the problem with additional nice features of nano-scale wavelength and high frequency. Two results are reported. 1) A perpendicular ferromagnet on a honeycomb lattice is generically a topological magnetic material in the sense that topologically protected chiral edge spin waves exist in the band gap as long as spin-orbit induced nearest-neighbor pseudodipolar interaction (and/or next-nearest neighbor Dzyaloshinskii-Moriya interaction) is present. 2) As a proof of concept, spin wave beamsplitters and spin wave interferometers are designed by using domain walls to manipulate the propagation of topologically protected chiral spin waves. Since magnetic domain walls can be controlled by magnetic fields or electric current/fields, one can essentially draw, erase and redraw different spin wave devices and circuitry on the same magnetic plate so that the proposed devices are reconfigurable and tunable. Devices made from magnetic topological materials are robust against both internal and external perturbations such as the spin wave frequency variation and device geometry as well as defects.

cond-mat.mes-hall↗

arXiv2017

A Generic Phase between Disordered Weyl Semimetal and Diffusive Metal

Quantum phase transitions of three-dimensional (3D) Weyl semimetals (WSMs) subject to uncorrelated on-site disorder are investigated through quantum conductance calculations and finite-size scaling of localization length. Contrary to previous claims that a direct transition from a WSM to a diffusive metal (DM) occurs, an intermediate phase of Chern insulator (CI) between the two distinct metallic phases should exist due to internode scattering that is comparable to intranode scattering. The critical exponent of localization length is $ν\simeq 1.3$ for both the WSM-CI and CI-DM transitions, in the same universality class of 3D Gaussian unitary ensemble of the Anderson localization transition. The CI phase is confirmed by quantized nonzero Hall conductances in the bulk insulating phase established by localization length calculations. The disorder-induced various plateau-plateau transitions in both the WSM and CI phases are observed and explained by the self-consistent Born approximation. Furthermore, we clarify that the occurrence of zero density of states at Weyl nodes is not a good criterion for the disordered WSM, and there is no fundamental principle to support the hypothesis of divergence of localization length at the WSM-DM transition.

cond-mat.dis-nn↗

arXiv2017

Chiral anomaly of Weyl magnons in stacked honeycomb ferromagnets

Chiral anomaly of Weyl magnons (WMs), featured by nontrivial band crossings at paired Weyl nodes (WNs) of opposite chirality, is investigated. It is shown that WMs can be realized in stacked honeycomb ferromagnets. Using the Aharonov-Casher effect that is about the interaction between magnetic moments and electric fields, the magnon motion in honeycomb layers can be quantized into magnonic Landau levels (MLLs). The zeroth MLL is chiral so that unidirectional WMs propagate in the perpendicular (to the layer) direction for a given WN under a magnetic field gradient from one WN to the other and change their chiralities, resulting in the magnonic chiral anomaly (MCA). A net magnon current carrying spin and heat through the zeroth MLL depends linearly on the magnetic field gradient and the electric field gradient in the ballistic transport.

cond-mat.mes-hall↗

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

Magnonic Weyl semimetal in pyrochlore ferromagnets

Topological states of matter have been a subject of intensive studies in recent years because of their exotic properties such as the topologically protected edge and surface states. The initial studies were exclusively for electron systems. It is now known that topological states can also exist for other particles. Indeed, topologically protected edge states have already been found for phonons and photons. In spite of active searching for topological states in many fields, the studies in magnetism are relatively rare although topological states are apparently important and useful in magnonics. Here we show that the pyrochlore ferromagnets with the Dzyaloshinskii-Moriya interaction are intrinsic magnonic Weyl semimetals. Similar to the electronic Weyl semimetals, the magnon bands in a magnonic Weyl semimetal are nontrivially crossing in pairs at special points (called Weyl nodes) in momentum space. The equal energy contour around the Weyl nodes gives rise to the Fermi arcs on sample surfaces due to the topologically protected surface states between each pair of Weyl nodes. Additional Weyl nodes and Fermi arcs can be generated in lower energy magnon bands when an anisotropic exchange interaction is introduced.

cond-mat.mes-hall↗

arXiv2016

Topological Anderson Insulators in Systems without Time-Reversal Symmetry

Occurrence of topological Anderson insulator (TAI) in HgTe quantum well suggests that when time-reversal symmetry (TRS) is maintained, the pertinent topological phase transition, marked by re-entrant $2e^2/h$ quantized conductance contributed by helical edge states, is driven by disorder. Here we show that when TRS is broken, the physics of TAI becomes even richer. The pattern of longitudinal conductance and nonequilibrium local current %Unlike for conventional topological insulators that, in the %absence of an external magnetic field, support only a single quantized %conductance in the quantum anomalous Hall effect region or a single %re-entrant quantized conductance in TAI, %our model exhibits novel TAI distribution displays novel TAI phases characterized by nonzero Chern numbers, indicating the occurrence of multiple chiral edge modes. Tuning either disorder or Fermi energy (in both topologically trivial and nontrivial phases), drives transitions between these distinct TAI phases, characterized by jumps of the quantized conductance from $0$ to $e^2/h$ and from $e^2/h$ to $2e^2/h$. An effective medium theory based on the Born approximation yields an accurate description of different TAI phases in parameter space.

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↗

arXiv2016

Who pumps spin current into nonmagnetic-metal (NM) layer in YIG/NM multilayers at ferromagnetic resonance?

Spin pumping in Yttrium-iron-garnet (YIG)/nonmagnetic-metal (NM) layer systems under ferromagnetic resonance (FMR) conditions is a popular method of generating spin current in the NM layer. A good understanding of the spin current source is essential in extracting spin Hall angle of the NM and in potential spintronics applications. It is widely believed that spin current is pumped from precessing YIG magnetization into NM layer. Here, by combining microwave absorption and DC-voltage measurements on YIG/Pt and YIG/NM1/NM2 (NM1=Cu or Al, NM2=Pt or Ta), we unambiguously showed that spin current in NM came from the magnetized NM surface (in contact with YIG) due to the magnetic proximity effect (MPE), rather than the precessing YIG magnetization. This conclusion is reached through our unique detecting method where the FMR microwave absorption of the magnetized NM surface, hardly observed in the conventional FMR experiments, was greatly amplified when the electrical detection circuit was switched on.

cond-mat.mtrl-sci↗

arXiv2016

Thermal spin current and spin accumulation at ferromagnetic insulator/nonmagnetic metal interface

Spin current injection and spin accumulation near a ferromagnetic insulator (FI)/nonmagnetic metal (NM) bilayer film under a thermal gradient is investigated theoretically. Using the Fermi golden rule and the Boltzmann equations, we find that FI and NM can exchange spins via interfacial electron-magnon scattering because of the imbalance between magnon emission and absorption caused by either non-equilibrium distribution of magnons or non-equilibrium between magnons and electrons. A temperature gradient in FI and/or a temperature difference across the FI/NM interface generates a spin current which carries angular momenta parallel to the magnetization of FI from the hotter side to the colder one. Interestingly, the spin current induced by a temperature gradient in NM is negligibly small due to the nonmagnetic nature of the non-equilibrium electron distributions. The results agree well with all existing experiments.

cond-mat.mes-hall↗

arXiv2015

Domain Walls as Spin Wave Waveguides

We numerically demonstrate that domain walls can be used as spin wave waveguides. We show that gapless spin waves bounded inside a domain wall can be guided by the domain wall. For Bloch walls, we further show that the bound spin waves can pass through Bloch lines and corners without reflection. This finding makes domain-wall-based spin wave devices possible.

cond-mat.mes-hall↗