Intrinsic spin Nernst effect of magnons in a noncollinear antiferromagnet

We investigate the intrinsic magnon spin current in a noncollinear antiferromagnetic insulator. We introduce a definition of the magnon spin current in a noncollinear antiferromagnet and find that it is in general nonconserved, but for certain symmetries and spin polarizations the averaged effect of nonconserving terms can vanish. We formulate a general linear response theory for magnons in noncollinear antiferromagnets subject to a temperature gradient and analyze the effect of symmetries on the response tensor. We apply this theory to single-layer potassium iron jarosite KFe3(OH)6(SO4)2 and predict a measurable spin current response

Stabilization and control of Majorana bound states with elongated skyrmions

We show that elongated magnetic skyrmions can host Majorana bound states in a proximity-coupled two-dimensional electron gas sandwiched between a chiral magnet and an $s$-wave superconductor. Our proposal requires stable skyrmions with unit topological charge, which can be realized in a wide range of multilayer magnets, and allows quantum information transfer by using standard methods in spintronics via skyrmion motion. We also show how braiding operations can be realized in our proposal

Skyrmions and Antiskyrmions in Quasi-Two-Dimensional Magnets

A stable skyrmion, representing the smallest realizablemagnetic texture, could be an ideal element for ultra-dense magnetic memories. Here, we review recent progress in the field of skyrmionics, which is concerned with studies of tiny whirls of magnetic configurations for novel memory and logic applications, with a particular emphasis on antiskyrmions. Magnetic antiskyrmions represent analogs of skyrmions with opposite topological charge. Just like skyrmions, antiskyrmions can be stabilized by the Dzyaloshinskii-Moriya interaction, as has been demonstrated in a recent experiment. Here, we emphasize differences between skyrmions and antiskyrmions, e.g., in the context of the topological Hall effect, skyrmion Hall effect, as well as nucleation and stability. Recent progress suggests that antiskyrmions can be potentially useful for many device applications. Antiskyrmions offer advantages over skyrmions as they can be driven without the Hall-like motion, offer increased stability due to dipolar interactions, and can be realized above room temperature

Magnetoelectric control of topological phases in graphene

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which involves the topological electronic states coupled to the AFM order parameter known as the Néel vector. The control of these states is envisioned through manipulation of the Néel vector by spin-orbit torques driven by electric currents. Here we propose a different approach favorable for low-power AFM spintronics, where the control of the topological states in a two-dimensional material, such as graphene, is performed via the proximity effect by the voltage induced switching of the Néel vector in an adjacent magnetoelectric AFM insulator, such as chromia. Mediated by the symmetry protected boundary magnetization and the induced Rashba-type spin-orbit coupling at the interface between graphene and chromia, the emergent topological phases in graphene can be controlled by the Néel vector. Using density functional theory and tight-binding Hamiltonian approaches, we model a graphene/Cr2O3 (0001) interface and demonstrate nontrivial band gap openings in the graphene Dirac bands asymmetric between the K and K′ valleys. This gives rise to an unconventional quantum anomalous Hall effect (QAHE) with a quantized value of 2e^2/h and an additional steplike feature at a value close to e^2/2h, and the emergence of the spin-polarized valley Hall effect (VHE). Furthermore, depending on the Néel vector orientation, we predict the appearance and transformation of different topological phases in graphene across the 180° AFM domain wall, involving the QAHE, the valley-polarized QAHE, and the quantum VHE, and the emergence of the chiral edge states along the domain wall. These topological properties are controlled by voltage through magnetoelectric switching of the AFM insulator with no need for spin-orbit torques

Intrinsic spin Nernst effect of magnons in a noncollinear antiferromagnet

We investigate the intrinsic magnon spin current in a noncollinear antiferromagnetic insulator. We introduce a definition of the magnon spin current in a noncollinear antiferromagnet and find that it is in general nonconserved, but for certain symmetries and spin polarizations the averaged effect of nonconserving terms can vanish. We formulate a general linear response theory for magnons in noncollinear antiferromagnets subject to a temperature gradient and analyze the effect of symmetries on the response tensor. We apply this theory to single-layer potassium iron jarosite KFe3(OH)6(SO4)2 and predict a measurable spin current response

Magnetoelectric control of topological phases in graphene

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which involves the topological electronic states coupled to the AFM order parameter known as the Néel vector. The control of these states is envisioned through manipulation of the Néel vector by spin-orbit torques driven by electric currents. Here we propose a different approach favorable for low-power AFM spintronics, where the control of the topological states in a two-dimensional material, such as graphene, is performed via the proximity effect by the voltage induced switching of the Néel vector in an adjacent magnetoelectric AFM insulator, such as chromia. Mediated by the symmetry protected boundary magnetization and the induced Rashba-type spin-orbit coupling at the interface between graphene and chromia, the emergent topological phases in graphene can be controlled by the Néel vector. Using density functional theory and tight-binding Hamiltonian approaches, we model a graphene/Cr2O3 (0001) interface and demonstrate nontrivial band gap openings in the graphene Dirac bands asymmetric between the K and K′ valleys. This gives rise to an unconventional quantum anomalous Hall effect (QAHE) with a quantized value of 2e^2/h and an additional steplike feature at a value close to e^2/2h, and the emergence of the spin-polarized valley Hall effect (VHE). Furthermore, depending on the Néel vector orientation, we predict the appearance and transformation of different topological phases in graphene across the 180° AFM domain wall, involving the QAHE, the valley-polarized QAHE, and the quantum VHE, and the emergence of the chiral edge states along the domain wall. These topological properties are controlled by voltage through magnetoelectric switching of the AFM insulator with no need for spin-orbit torques

Skyrmions and Antiskyrmions in Quasi-Two-Dimensional Magnets

A stable skyrmion, representing the smallest realizable magnetic texture, could be an ideal element for ultra-dense magnetic memories. Here, we review recent progress in the field of skyrmionics, which is concerned with studies of tiny whirls of magnetic configurations for novel memory and logic applications, with a particular emphasis on antiskyrmions. Magnetic antiskyrmions represent analogs of skyrmions with opposite topological charge. Just like skyrmions, antiskyrmions can be stabilized by the Dzyaloshinskii-Moriya interaction, as has been demonstrated in a recent experiment. Here, we emphasize differences between skyrmions and antiskyrmions, e.g., in the context of the topological Hall effect, skyrmion Hall effect, as well as nucleation and stability. Recent progress suggests that antiskyrmions can be potentially useful for many device applications. Antiskyrmions offer advantages over skyrmions as they can be driven without the Hall-like motion, offer increased stability due to dipolar interactions, and can be realized above room temperature

Controlling Topological Effects in Magnetic Systems

The importance of topology in condensed matter physics has expanded in recent years as it has become a key piece of understanding the physics in various phenomena. In this thesis, we examine several examples of the power of topological concepts in defining and understanding certain systems and effects. We consider magnetic skyrmions, whirls of magnetic texture, the topology of which is signified by the topological charge, a quantity dependent on the configuration of the magnetic moments in real space. The stability granted by this topology makes skyrmions of interest for computing applications. For elongated skyrmions coupled to a superconductor, the existence of an energy gap can indicate that the region around the skyrmion is in a topological phase. This makes it possible for Majorana bound states to exist at the edges of the skyrmion. Majorana bound states provide great promise for quantum computing, as they behave as non-abelian anyons. We also study Berry curvature, for both electrons and magnons in momentum space, which leads to various versions of the Hall effect in both cases. These Hall effects, which can give rise to transverse currents, including charge and spin currents, could also be used for devices. In the electron case, we show how in a system of graphene atop a chromia substrate, the Chern number represents the topological nature of the system and signifies which effects will be present, such as the quantum anomalous Hall effect and valley-polarized quantum anomalous Hall effect. For an insulating noncollinear antiferromagnet, we calculate the magnon spin Nernst response for potassium iron jarosite KFe3(OH)6(SO4)2