arXiv subjects

Yongjun Liu

Publications and source records attributed to Yongjun Liu.

7 recordsLinked to original sources

arXiv2024

Spin-wave Goos-H\"{a}nchen effect induced by 360 degree domain walls in magnetic heterostructures

In this work, lateral displacements of transmitted and reflected spin waves at a 360 degree domain wall (360DW), which is referred to as the spin-wave Goos-H\"{a}nchen effect (SWGHE), are systematically investigated in magnetic heterostructures with perpendicular easy/hard axis and wall-extension direction. Similar to the counterpart at heterochiral interfaces, the interfacial Dzyaloshinskii-Moriya interactions (IDMI) originating from a heavy-metal substrate is important for the emergence of SWGHE. More interestingly, the SWGHE can even survive in ferromagnets with biaxial anisotropy (either intrinsic or caused by shape anisotropy) in the absence of IDMI due to the unique 360DW-induced potentials which are distinct to the well-known P\"{o}schl-Teller ones. Numerics shows that these lateral displacements are generally fractions of the spin-wave wavelength. They can be further enhanced by an array of well-separated 360DWs thus provide a large variety for spin-wave manipulation.

cond-mat.mes-hall↗

arXiv2024

Hyperbolic Fractional Chern insulators

Fractional Chern insulators (FCIs) have attracted intensive attention for the realization of fractional quantum Hall states in the absence of an external magnetic field. Most of FCIs have been proposed on two-dimensional (2D) Euclidean lattice models with various boundary conditions. In this work, we investigate hyperbolic FCIs which are constructed in hyperbolic geometry with constant negative curvature. Through the studies on hyperbolic analogs of kagome lattices with hard-core bosons loaded into topological flat bands, we find convincing numerical evidences of two types of $\nu=1/2$ FCI states, {\emph {i.e.}}, the conventional and unconventional FCIs. Multiple branches of edge excitations and geometry-dependent wave functions for both conventional and unconventional $\nu=1/2$ FCI states are revealed, however, the geometric degree of freedom in these FCIs plays various roles. Additionally, a center-localized orbital plays a crucial role in the unconventional FCI state.

cond-mat.str-el↗

arXiv2023

Slonczewski-spin-current driven dynamics of 180$^{\circ}$ domain walls in spin valves with interfacial Dzyaloshinskii-Moriya interaction

Steady-flow dynamics of ferromagnetic 180$^{\circ}$ domain walls (180DWs) in long and narrow spin valves (LNSVs) with interfacial Dzyaloshinskii-Moriya interaction (IDMI) under spin currents with Slonczewski $g-$factor are examined. Depending on the magnetization orientation of polarizers (pinned layers of LNSVs), dynamics of 180DWs in free layers of LNSVs are subtly manipulated: (i) For parallel polarizers, stronger spin polarization leads to higher Walker limit thus ensures the longevity of faster steady flows. Meantime, IDMI induces both the stable-region flapping and its width enlargement. (ii) For perpendicular polarizers, a wandering of 180DWs between bi- and tri-stability persists with the criticality adjusted by the IDMI. (iii) For planar-transverse polarizers, IDMI makes the stable region of steady flows completely asymmetric and further imparts a high saturation wall velocity under large current density. Under the last two polarizers, the ultrahigh differential mobility of 180DWs survives. The combination of Slonczewski spin current and IDMI provides rich possibilities of fine controlling on 180DW dynamics, hence opens avenues for magnetic nanodevices with rich functionality and high robustness.

cond-mat.mes-hall↗

arXiv2022

Ground states and magnonics in orthogonally-coupled symmetric all-antiferromagnetic junctions

In this work, the rich ground-state structure of orthogonally-coupled symmetric all-antiferromagnetic junctions with easy-plane anisotropy is reported. Spin reorientation process rather than the traditional spin flop (SF) occurs, resulting in a novel phase in which Néel vectors preserve the mirror-reflection symmetry (termed as ``MRS phase"). The phase transitions between SF and MRS phases can be either the first- or second-order. After disturbed by external stimuli, magnons with different parities emerge. For in-plane dc fields, no couplings between magnons occur. When dc fields become oblique, coherent couplings between magnons with opposite parity emerge, leading to anticrossings in resonance frequencies. However, self-hybridization among magnons with the same parity never happens. More interestingly, spin waves based on MRS phase are linearly polarized and their polarization directions can be fine controlled.

cond-mat.mes-hall↗

arXiv2022

Topological spin textures in chiral magnets on the honeycomb lattice with magnetic fields

Topological spin textures, like skyrmions, have significant potential for spintronics applications. The main purpose of this work is to study further the topological spin textures on the discrete lattice with magnetic fields. In this work, we study a classical rotated Heisenberg model with Dzyaloshinskii-Moriya interaction, bond-dependent anisotropy and easy-axis anisotropy on the honeycomb lattice via Monte Carlo simulations. We mainly focus on phase diagrams with magnetic fields, especially on the non-trivial phases only existing with fields. The results demonstrate the emergence of field-induced incommensurate skyrmion superlattice including mixed skyrmion-bimeron states, ferromagnetic star, vortex and z-vortex superlattice. We systematically analyze structures of these topological spin textures through spin configurations, structure factors, topological charge and vector chirality. We hope that our results could be useful for the identification of topological phases experimentally.

cond-mat.str-el↗

arXiv2021

Chiral domain wall dynamics in magnetic heterostructures with bulk Dzyaloshinskii-Moriya interactions

In this work, dynamics of chiral domain walls in long and narrow magnetic heterostructures based on non-centrosymmetric chiral magnets with bulk Dzyaloshinskii-Moriya interactions (DMI) and perpendicular magnetic anisotropy is systematically investigated. The driving forces can be out-of-plane magnetic fields and in-plane currents, correspondingly both steady and precessional flows are considered. Their dividing points (the Walker critical field and current density) are obtained as functions of bulk DMI strength ($D_{\mathrm{b}}$) and the ratio ($κ$) of total (crystalline plus shape) anisotropy in the hard axis over that in the easy one. When far beyond Walker breakdown, the dependence curve of wall velocity on external in-plane bias field takes parabolic shape around the compensation point where the total in-plane field disappears. The center shift is determined by $D_{\mathrm{b}}$, $κ$, and the wall's topological charge, thus can be used to measure the bulk DMI strength in chiral magnets.

cond-mat.mes-hall↗

arXiv2019

Robust OFDM integrated radar and communications waveform design based on information theory

An integrated radar and communications system (IRCS) where a monostatic radar transceiver is employed for target classification while simultaneously used as a communications transmitter is considered. The radar combined propagation-target response (joint response of the radar propagation channel and target) and communications channel response are generally frequency selective but the corresponding frequency response functions are not exactly known. In particular, these frequency response functions are only known to lie in an uncertainty class. To ensure the IRCS simultaneously provides acceptable target classification performance and communications rate, a robust orthogonal frequency division multiplexing (OFDM) integrated radar and communications waveform (IRCW) design method is proposed. The approach finds a waveform that simultaneously provides a sufficiently large weighted sum of the communications data information rate (DIR) and the conditional mutual information (MI) between the observed signal and the radar target over the entire uncertainty class. First, the conditional MI and DIR based on the integrated OFDM radar and communications waveform are derived. Then, a robust OFDM IRCW optimization problem based on the minimax design philosophy is developed such that closed-form solution is derived. Finally, several numerical results are presented to demonstrate the effectiveness of the proposed method.

eess.SP↗