Topological Antiferromagnets Research Articles

Domain structure and domain wall dynamics in topological chiral antiferromagnets from the viewpoint of magnetic octupole

Spintronics has been evolving rapidly; it becomes next-generation electronics exploiting both spin and charge degrees of freedom and a primary research field directly linked to topology and nano-magnetism in solid-state physics. In this article, we put our focus on the domain structure and domain wall dynamics based on a cluster magnetic octupole in topological antiferromagnets. Many issues are still not clear in terms of magnetic octupole domains (MODs) and domain walls. We first discuss the current status of the study on the antiferromagnetic domains and walls and then the MODs and walls from detection and manipulation viewpoints.

Chiral Photocurrent in Parity-Violating Magnet and Enhanced Response in Topological Antiferromagnet

A new classification of photocurrent responses in light of symmetry violations in the presence of magnetic order unveils two new types of photocurrent that can be readily tuned and enhanced by topological electronic structure in solids.

Topological magneto-optical effects and their quantization in noncoplanar antiferromagnets

Reflecting the fundamental interactions of polarized light with magnetic matter, magneto-optical effects are well known since more than a century. The emergence of these phenomena is commonly attributed to the interplay between exchange splitting and spin-orbit coupling in the electronic structure of magnets. Using theoretical arguments, we demonstrate that topological magneto-optical effects can arise in noncoplanar antiferromagnets due to the finite scalar spin chirality, without any reference to exchange splitting or spin-orbit coupling. We propose spectral integrals of certain magneto-optical quantities that uncover the unique topological nature of the discovered effect. We also find that the Kerr and Faraday rotation angles can be quantized in insulating topological antiferromagnets in the low-frequency limit, owing to nontrivial global properties that manifest in quantum topological magneto-optical effects. Although the predicted topological and quantum topological magneto-optical effects are fundamentally distinct from conventional light-matter interactions, they can be measured by readily available experimental techniques.

Photo-induced Floquet Weyl magnons in noncollinear antiferromagnets

We study periodically driven insulating noncollinear stacked kagome antiferromagnets with a conventional symmetry-protected three-dimensional (3D) in-plane 120° spin structure, with either positive or negative vector chirality. We show that the symmetry protection of the in-plane 120° spin structure can be broken in the presence of an off-resonant circularly or linearly polarized electric field propagating parallel to the in-plane 120° spin structure (say along the x direction). Consequently, topological Floquet Weyl magnon nodes with opposite chirality are photoinduced along the kx momentum direction. They manifest as the monopoles of the photoinduced Berry curvature. We also show that the system exhibits a photoinduced magnon thermal Hall effect for circularly polarized electric field. Furthermore, we show that the photoinduced chiral spin structure is a canted 3D in-plane 120° spin structure, which was recently observed in the equilibrium noncollinear antiferromagnetic Weyl semimetals Mn3Sn/Ge. Our result not only paves the way towards the experimental realization of Weyl magnons and photoinduced thermal Hall effects, but also provides a powerful mechanism for manipulating the intrinsic properties of 3D topological antiferromagnets.

Magnetotransport properties of γ-FeMn thin films grown by high-temperature sputtering

γ-FeMn is a topological antiferromagnet which hosts a noncoplanar spin structure at room temperature, promising for application to antiferromagnetic spintronics. In this work, we have investigated magnetotransport properties of FeMn thin films grown by dc magnetron sputtering on Al2O3 (0001) substrates. γ-phase (fcc) FeMn thin films are successfully obtained with use of a Cu seed layer covering with the Al2O3 surface, while nonmagnetic α-phase (bcc) FeMn thin films are formed without a Cu seed layer. When the sputtering temperature is set at 500 °C, γ-FeMn films grown on Cu/Al2O3 are highly oriented along the (111) plane, but minor α-Fe phases are included owing to alloying with the Cu layer. Ferromagnetic transports of α-Fe phases are observed in Hall and Nernst effects at low magnetic fields. By contrast, the slope of Hall conductivity at high magnetic fields is found to be several times larger for the γ-FeMn phase than for the α-FeMn phase, which suggests that the antiferromagnetic spin structure of γ-FeMn contributes to the Hall effect.

Magnetic structure of GdBiPt: A candidate antiferromagnetic topological insulator

A topological insulator is a state of matter which does not break any symmetry and is characterized by topological invariants, the integer expectation values of non-local operators. Antiferromagnetism on the other hand is a broken symmetry state in which the translation symmetry is reduced and time reversal symmetry is broken. Can these two phenomena coexist in the same material? A proposal by Mong {\it et al.}\cite{Mong2010} asserts that the answer is yes. Moreover, it is theoretically possible that the onset of antiferromagnetism enables the non-trivial topology since it may create spin-orbit coupling effects which are absent in the non-magnetic phase. The current work examines a real system, half-Heusler GdBiPt, as a candidate for topological anti ferromagnetism. We find that the magnetic moments of the gadolinium atoms form ferromagnetic sheets which are stacked antiferromagnetically along the body diagonal. This magnetic structure may induce spin orbit coupling on band electrons as they hop perpendicular to the ferromagnetic sheets.

Correlation of dislocation and domain structure of Cr(001) investigated by spin-polarized scanning tunneling microscopy

The Cr(001) surface has been investigated by spin-polarized scanning tunneling microscopy to image various defects and their effect on the magnetic structure at the surface. The usual magnetic structure is determined by the antiferromagnetic ordering of Cr leading to the topological antiferromagnetism of the (001) surface. We found that screw dislocations result in the formation of domain walls with a width of 120--170 nm. The dependence of the domain-wall width on the distance from the screw dislocation is studied experimentally and compared to micromagnetic simulations. Our results show that the size and shape of the spin structure is determined by two parameters, the exchange stiffness and the effective anisotropy. Subsurface step dislocations lead to an s-like bending of step edges on the sample surface. In spite of the step bending the topological antiferromagnetic order is strictly maintained. In some rare cases large scale images show a change of the spin-polarized part of the tunneling current. It is explained by the fourfold symmetry of the Cr(001) surface which leads to a $90\ifmmode^\circ\else\textdegree\fi{}$ degeneracy and the formation of according domains.

Cr monolayer on vicinal Pd surfaces

We report a study of the magnetic behavior of Cr monolayer deposited on a Pd stepped substrate using a real space tight-binding approach. The magnetism is described in the Hartree–Fock approximation of the Hubbard Hamiltonian. The density of states is calculated within the recursion technique. Multiple magnetic configurations are obtained for the following vicinal surfaces: (1 0 1), (1 0 3), (1 0 5), (1 0 7), and (1 0 9). Ferrimagnetic configurations are always found for the ground state with sizeable magnetic moments on the Cr sites for all the considered surfaces. But because the module of the Cr magnetic moments are not equal a resulting magnetization remains present and this should differentiate drastically with the “perfect” c(2×2) magnetic configuration on Pd(0 0 1) in which the mean magnetic moment is zero so that neither X-ray magnetic circular dichroism nor surface magneto-optical Kerr effect will detect anything! This behavior is in contradiction with the expected topological antiferromagnetism.

Magnetic order at stepped Fe/Cr interfaces

The influence of surface atomic step density on the magnetic moment of Cr layers on Fe(001) has been studied in situ by Alternating Gradient Magnetometry (AGM). Contrary to previous results on flat surfaces where large Cr moments were observed coupled antiferromagnetically to their Fe nearest neighbours, the sample moment remains essentially unchanged if Cr is deposited on a strongly faceted Fe surface. During addition of a second Fe layer on a 6 ML Cr/Fe(001) film the sample moment stays constant up to an average Fe thickness of 5 ML and increases linearly with the nominal Fe thickness for thicker layers. This behaviour is interpreted as topological antiferromagnetism due to the specific step structure of the surface in agreement with recent tight binding calculations.

Spin flux and magnetic solitons in an interacting two-dimensional electron gas: Topology of two-valued wave functions.

It is suggested that an interacting many-electron system in a two-dimensional lattice may condense into a topological magnetic state distinct from any discussed previously. This condensate exhibits local spin-1/2 magnetic moments on the lattice sites but is composed of a Slater determinant of single-electron wave functions which exist in an orthogonal sector of the electronic Hilbert space from the sector describing traditional spin-density-wave or spiral magnetic states. These one-electron spinor wave functions have the distinguishing property that they are antiperiodic along a closed path encircling any elementary plaquette of the lattice. This corresponds to a 2\ensuremath{\pi} rotation of the internal coordinate frame of the electron as it encircles the plaquette. The possibility of spinor wave functions with spatial antiperiodicity is a direct consequence of the two-valuedness of the internal electronic wave function defined on the space of Euler angles describing its spin. This internal space is the topologically, doubly-connected, group manifold of SO(3). Formally, these antiperiodic wave functions may be described by passing a flux which couples to spin (rather than charge) through each of the elementary plaquettes of the lattice. When applied to the two-dimensional Hubbard model with one electron per site, this new topological magnetic state exhibits a relativistic spectrum for charged, quasiparticle excitations with a suppressed one-electron density of states at the Fermi level.For a topological antiferromagnet on a square lattice, with the standard Hartree-Fock, spin-density-wave decoupling of the on-site Hubbard interaction, there is an exact mapping of the low-energy one-electron excitation spectrum to a relativistic Dirac continuum field theory. In this field theory, the Dirac mass gap is precisely the Mott-Hubbard charge gap and the continuum field variable is an eight-component Dirac spinor describing the components of physical electron-spin amplitude on each of the four sites of the elementary plaquette in the original Hubbard model. Within this continuum model we derive explicitly the existence of hedgehog Skyrmion textures as local minima of the classical magnetic energy. These magnetic solitons carry a topological winding number \ensuremath{\mu} associated with the vortex rotation of the background magnetic moment field by a phase angle 2\ensuremath{\pi}\ensuremath{\mu} along a path encircling the soliton. Such solitons also carry a spin flux of \ensuremath{\mu}\ensuremath{\pi} through the plaquette on which they are centered. The \ensuremath{\mu}=1 hedgehog Skyrmion describes a local transition from the topological (antiperiodic) sector of the one-electron Hilbert space to the nontopological sector. We derive from first principles the existence of deep level localized electronic states within the Mott-Hubbard charge gap for the \ensuremath{\mu}=1 and 2 solitons. The spectrum of localized states is symmetric about E=0 and each subgap electronic level can be occupied by a pair of electrons in which one electron resides primarily on one sublattice and the second electron on the other sublattice. It is suggested that flux-carrying solitons and the subgap electronic structure which they induce are important in understanding the physical behavior of doped Mott insulators.

Topological antiferromagnetism at Cr surfaces and interfaces.

Topological antiferromagnetism at Cr surfaces and interfaces.

Observation of vacuum tunneling of spin-polarized electrons with the scanning tunneling microscope.

Vacuum tunneling of spin-polarized electrons from a ferromagnetic ${\mathrm{CrO}}_{2}$ tip into a Cr(001) single crystal has been observed by means of a scanning tunneling microscope (STM) operated in UHV. Topographic STM images of the Cr(001) surface using a tungsten tip confirm the model of topological antiferromagnetism between ferromagnetic terraces separated by monatomic steps of 0.144 nm height. With ${\mathrm{CrO}}_{2}$ tips, the measured step-height values alternate around the mean value of 0.144 nm due to an additional contribution from spin-polarized-electron tunneling.

Ferromagnetism versus antiferromagnetism of the Cr(001) surface.

Three different spin arrangements for the (001) surface of Cr are discussed on the basis of self-consistent total-energy calculations: ferromagnetism, inside-the-surface $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetism, and "topological" antiferromagnetism between ferromagnetic terraces separated by single steps. The $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic configuration is found to be energetically unfavorable. Instead, topological antiferromagnetism is compatible with both the absence of magnetization observed by spin-resolved photoemission and the existence of spin-split surface states detected by energy- and angular-resolved photoemission.