We present a theoretical mapping to show that a ferromagnet with gain (loss) is equivalent to an antiferromagnet with an equal amount of loss (gain). Our finding indicates a novel first-order ferromagnet-antiferromagnet phase transition by tuning the gain-loss parameter. As an appealing application, we demonstrate the realization as well as the manipulation of the antiferromagnetic skyrmion, a stable topological quasiparticle not yet observed experimentally, in a chiral ferromagnetic thin film with gain. We also consider ferromagnetic bilayers with balanced gain and loss, and show that the antiferromagnetic skyrmion can be found only in the cases with broken parity-time symmetry phase. Our results pave a way for investigating the emerging antiferromagnetic spintronics and parity-time symmetric magnonics in ferromagnets.

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Higher-order topological insulator (HOTI) represents a new phase of matter, the characterization of which goes beyond the conventional bulk-boundary correspondence and is attracting significant attention by the broad community. Using a square-root operation, it has been suggested that a square-root HOTI may emerge in a hybrid honeycomb-kagome lattice.Here, we report the first experimental realization of the square-root HOTI in topological LC circuits. We show theoretically and experimentally that the square-root HOTI inherits the feature of wave function from its parent with corner states pinned to nonzero energies. The topological feature is fully characterized by the bulk polarization. To directly measure the finite-energy corner modes, we introduce extra grounded inductors to each node, which shifts corner states to zero-energy without affecting their spatial distributions. Our results experimentally substantiate the emerging square-root HOTI and pave the way to realizing exotic topological phases that are challenging to observe in condensed matter physics.

Magnetic skyrmions are chiral quasiparticles that show promise for future spintronic applications such as skyrmion racetrack memories and logic devices because of their topological stability, small size (typically ∼ 3 - 500 nm), and ultralow threshold force to drive their motion. On the other hand, the ability of light to carry and deliver orbital angular momentum (OAM) in the form of optical vortices has attracted a lot of interest. In this work, we predict a photonic OAM transfer effect, by studying the dynamics of magnetic skyrmions subject to Laguerre-Gaussian optical vortices, which manifests a rotational motion of the skyrmionic quasiparticle around the beam axis. The topological charge of the optical vortex determines both the magnitude and the handedness of the rotation velocity of skyrmions. In our proposal, the twisted light beam acts as an optical tweezer to enable us displacing skyrmions over large-scale defects in magnetic films to avoid being captured.

Magnetometers with exceptional sensitivity are highly demanded in solving a variety of physical and engineering problems, such as measuring Earth's weak magnetic fields and prospecting mineral deposits and geological structures. It has been shown that the non-Hermitian degeneracy at exceptional points (EPs) can provide a new route for that purpose, because of the nonlinear response to external perturbations. One recent work [H. Yang et al., Phys. Rev. Lett. 121, 197201 (2018)] has made the first step to realize the second-order magnonic EP in ferromangetic bilayers respecting the parity-time symmetry. In this paper, we generalize the idea to higher-order cases by considering ferromagnetic trilayers consisting of a gain, a neutral, and a (balanced-)loss layer. We observe both second-and third-order magnonic EPs by tuning the interlayer coupling strength, the external magnetic field, and the gain-loss parameter. We show that the magnetic sensitivity can be enhanced by 3 orders of magnitude comparing to the conventional magnetic tunneling junction based sensors. Our results pave the way for studying high-order EPs in purely magnetic system and for designing magnetic sensors with ultrahigh sensitivity. arXiv:2002.03085v1 [cond-mat.mtrl-sci]

Magnetic skyrmions are believed to be the promising candidate of information carriers in spintronics. However, the skyrmion Hall effect due to the nontrivial topology of skyrmions can induce a skyrmion accumulation or even annihilation at the edge of the devices, which hinders the real-world applications of skyrmions. In this work, we theoretically investigate the current-driven skyrmion motion on magnetic nanotubes which can be regarded as "edgeless" in the tangential direction. By performing micromagnetic simulations, we find that the skyrmion motion exhibits a helical trajectory on the nanotube, with its axial propagation velocity proportional to the current density. Interestingly, the skyrmion's annular speed increases with the increase of the thickness of the nanotube. A simple explanation is presented. Since the tube is edgeless for the tangential skyrmion motion, a stable skyrmion propagation can survive in the presence of a very large current density without any annihilation or accumulation. Our results provide a new route to overcome the edge effect in planar geometries.Ever since its experimental discovery, 1 the magnetic skyrmion, a chiral quasiparticle, 2,3 has been an active research area in condensed matter physics because of not only the potential for future spintronic applications such as skyrmion racetrack memories 4-6 and logic devices, 7,8 but also the fundamental interests. 9-13 In chiral magnets, skyrmions can be stabilized by the Dzyaloshinskii-Moriya interaction (DMI) of two types: 1,14-22 the bulk DMI and the interfacial one. The bulk DMI typically exists in noncentrosymmetric magnets, and can support the formation of Bloch-type (vortex-like) skyrmions, 1,15-18 while the latter one usually exists in inversion-symmetry breaking thin films, and can give rise to Néel-type (hedgehoglike) skyrmions. [19][20][21][22] Several methods have been proposed to drive the skyrmion motion, such as spin-polarized currents, 23 microwaves, 24 and thermal gradients, 25 to name a few. However, when the skyrmion is driven by an in-plane current via the spin transfer torque, the trajectory of its motion deviates from the current direction due to the intrinsic skyrmion Hall effect. 2,3,26-29 Furthermore, there exists a threshold current density above which skyrmions can annihilate at the film edge. 30 This edge effect strongly limits the speed of skyrmion propagation which is of vital importance for real applications. Several solutions have been proposed to overcome this problem. Zhang et al.proposed an antiferromagnetically exchange-coupled bilayer system, where the skyrmions move straightly along the current direction. 31 Upadhyaya et al. showed that the skyrmion can be guided in a desired trajectory by applying electric fields in a certain pattern. 32 More recently, Yang et al. discovered a novel twisted skyrmion state at the boundary of two antiparallel magnetic domains coupled antiferromagnetically, through which skyrmions with opposite polarities can transform mutually. 33 Under proper conditions, the doma...

In magnetic trilayer structures, a contribution to the Gilbert damping of ferromagnetic resonance arises from spin currents pumped from one layer to another. This contribution has been demonstrated for layers with weakly coupled, separated resonances, where magnetization dynamics are excited predominantly in one layer and the other layer acts as a spin sink. Here we show that trilayer structures in which magnetizations are excited simultaneously, antisymmetrically, show a spin-pumping effect roughly twice as large. The antisymmetric (optical) mode of antiferromagnetically coupled Ni 79 Fe 21 (8nm)/Ru/Ni 79 Fe 21 (8nm) trilayers shows a Gilbert damping constant greater than that of the symmetric (acoustic) mode by an amount as large as the intrinsic damping of Py (∆α 0.006). The effect is shown equally in field-normal and field-parallel to film plane geometries over 3-25 GHz. The results confirm a prediction of the spin pumping model and have implications for the use of synthetic antiferromagnets (SAF)-structures in GHz devices.Pumped spin currents 1,2 are widely understood to influence the magnetization dynamics of ultrathin films and heterostructures. These spin currents increase the Gilbert damping or decrease the relaxation time for thin ferromagnets at GHz frequencies. The size of the effect has been parametrized through the effective spin mixing conductance g ↑↓ r , which relates the spin current pumped out of the ferromagnet, transverse to its static (timeaveraged) magnetization, to its precessional amplitude and frequency. The spin mixing conductance is interesting also because it determines the transport of pure spin current across interfaces in quasistatic spin transport, manifested in e.g. the spin Hall effect.In the spin pumping effect, spin current is pumped away from a ferromagnet / normal metal (F 1 /N) interface, through precession of F 1 , and is absorbed elsewhere in the structure, causing angular momentum loss and damping of F 1 . The spin current can be absorbed through different processes in different materials. When injected into paramagnetic metals (Pt, Pd, Ru, and others), the spin current relaxes exponentially with paramagnetic layer thickness 3-5 . The relaxation process has been likened to spin-flip scattering as measured in CPP-GMR, where spin-flip events are localized to heavy-metal impurities 6 and the measurement reveals the spin diffusion length λ SD . When injected into other ferromagnets (F 2 in F 1 /N/F 2 ), the spin current is absorbed through its torque on magnetization 5,7 . A similar process appears to be relevant for antiferromagnets as well 8 .In F 1 /N/F 2 structures, only half of the total possible spin pumping effect has been detected up until now. For well-separated resonances of F 1 and F 2 , only one layer will precess with large amplitude at a given frequency ω, and spin current is pumped from a precessing F 1 into a static F 2 . If both layers precess symmetrically, with a) Electronic mail: Contact author. web54@columbia.edu the same amplitude and phase, equal and ...

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