Session K41: Chiral Magnetism and Structures I

Rise of Chiral Spintronics

Chirality is one of the fundamental asymmetries in nature. Recently chiral nature of magnetic and nonmagnetic structures have been enormously highlighted and arisen to be very useful for potential application to not only spintronics but biology and pharmaceutical industries For example, it has been reported that the combination with spin-orbit interaction such as spin Hall effect can be very efficient in manipulation of magnetic elements [1], and spin currents in chiral molecules induced by chirality interacts with magnetic structures [2]. I will present recent remarkable progress in emergent novel phenomena associated with chiral properties from not only emergent magnetic nanostructures but chiral molecules: spin-orbit torques from perpendicularly magnetized ultrathin films [2], exchange coupling torque [3] and entirely new phase of chiral exchange drag [4] from synthetic antiferromagnets, and separation of chiral molecules by magnetic structure: enantiomer [5]. In the end I will discuss potential applications and promising outlooks from these new findings.

[1] S. S. P. Parkin, and S.-H. Yang, Nat. Nano. (2015).
[2] O. B. Dor, S. Yochelis, A. Radko, K. Vankayala, E. Capua, A. Capua, S.-H. Yang, L. T. Baczewski, S. S. P. Parkin, R. Naaman, Y. Paltiel, Nat. Comms 8, 14567 (2017).
[3] K.-S. Ryu, L. Thomas, S.-H. Yang, and S. S. P. Parkin, Nat. Nano. 8, 527 (2013).
[4] S.-H. Yang, K.-S. Ryu, and S. S. P. Parkin, Nat. Nano. 10, 221 (2015).
[5] S.-H. Yang, C. Garg, and S. S. P. Parkin, “Chiral Exchange Drag and Chirality Oscillations in Synthetic Antiferromagnets”, Nature Physics (2018) under review.
[6] K. Banerjee-Ghosh, O.B. Dor, F. Tassinari, E. Capua, S. Yochelis, A. Capua, S.-H. Yang, S.S.P. Parkin, S. Sarkar, L. Kronik, L. T. Baczewski, R. Naaman, and Y. Paltiel, Science 360, 1331 (2018).   

Unit cell thick ferrimagnetic Mn3Z Heusler Domain Wall motion

Antiferromagnets and ferrimagnets are of particular interest for spintronic devices. In this talk, we present the Domain Wall (DW) motion of Mn₃Z (Z=Ge, Sn, Sb) Heuslers, using the chemical templating layer (CTL) concept. Unit cell thick, ferrimagnetic, binary Heusler compounds, have low magnetization and high perpendicular magnetic anisotropy in their tetragonally distorted forms, can sustain current driven DW motion in nanowire racetracks of -129 ms^-1 with the lowest current density to initiate motion of 2.8 10⁶ A cm^-2. The direction of the DW motion is determined by the bulk spin polarization of the Heusler. Moreover, the DW motion shows contribution by chiral spin orbit torques when studied under in-plane fields. We discuss the complex mechanism of the DW motion in Heusler films and identify a bulk Dzyaloshinskii-Moriya interaction and a spin Hall effect deriving from the CTLs.
One example of the extraordinary abilities of the Mn₃Z Heusler family, is shown by systematically tuning by an order of magnitude the bulk DMI field strength, by varying the composition of the Heusler alloy. These results are the first demonstration of DW motion in ultra-thin Heusler alloys with complex and tunable magnetic properties and is an important step to enable Heusler spintronics applications.
  

Binding a Hopfion in Chiral Magnet Nanodisk

Hopfions are three-dimensional topological textures characterized by the integer Hopf invariant $Q_H$. Here, we present the realization of a zero-field, stable hopfion spin texture in a magnetic system consisting of a chiral magnet nanodisk sandwiched by two films with perpendicular magnetic anisotropy. The preimages of the spin texture and numerical calculations of $Q_H$ show that the hopfion has $Q_H=1$. Furthermore, another non-trivial state that includes a monopole--antimonopole pair (MAP) is also stabilized in this system. By applying an external magnetic field, hopfion and MAP states with the same polarization can be switched between each other. The topological transition between the hopfion and the MAP state involves a creation (annihilation) of the MAP and twist of the preimages. Our work paves the way to study non-trivial 3D topological spin textures and stimulates more investigations in the field of 3D spintronics.   

Visualizing Chiral Ferrimagnetism in Amorphous GdCo Films

Inversion symmetry breaking has become a thriving research in modern magnetism with phenomena including Rashba effect, spin Hall effect and Dzyaloshinskii-Moriya interaction (DMI). The latter one may stabilize chiral spin textures with topologically non-trivial properties, such as Skyrmions. So far, chiral spin textures have mainly been studied in helimagnets and thin ferromagnets with heavy-element capping.

Here, we show, on the example of chiral ferrimagnetism in amorphous GdCo, that the concept of chirality driven by interfacial DMI can be generalized to complex multicomponent systems. Utilizing Lorentz microscopy and X-ray magnetic circular dichroism spectroscopy, and tailoring thickness, capping and rare-earth composition, we find that a 2nm-thick GdCo film preserves ferrimagnetism and stabilizes chiral domain walls. The type of chiral domain walls depends on the rare-earth composition/saturation magnetization, enabling a possible temperature control of the intrinsic properties of ferrimagnetic domain walls.

Reference: R. Streubel et al., Adv. Mater. 30, 1800199 (2018).
  

Beyond Baby Skyrmions: principle chiral model in noncollinear magnets

We discuss noncollinear magnetic phenomena whose local order parameter is characterized by more than one spin vector. By focusing on the simple cases of 2D triangular and 3D pyrochlore lattices, we demonstrate that their low-energy theories can be described by a one-parametric class of sigma models continuously interpolating between the classical Heisenberg model and the principal chiral model. A natural consequence of our derivation is that stable Skyrmion of type π₃(SO(3)) can be generically induced in noncollinear magnets.   

Boundary twists, instabilities, and creation of skyrmions and antiskyrmions

We formulate and study the general boundary conditions dictating the magnetization profile in the vicinity of an interface between magnets with dissimilar properties. We show that in general case the boundary conditions lead to the magnetization profile corresponding to the N\'eel, Bloch, or intermediate twist due to Dzyaloshinskii-Moriya interactions. We explore how such twists can be utilized for creation of skyrmions and antiskyrmions, e.g., in a view of magnetic memory applications. To this end, we study various scenarios how skyrmions and antiskyrmions can be created from interface magnetization twists due to local instabilities. We also show that a judicious choice of Dzyaloshinskii-Moriya tensor (hence a carefully designed material) can lead to local instabilities generating certain types of skyrmions or antiskyrmions. The local instabilities are shown to appear in solutions of the Bogoliubov-de-Gennes equations describing ellipticity of magnon modes bound to interfaces. In one considered scenario, a skyrmion-antiskyrmion pair can be created due to instabilities at an interface between materials with properly engineered Dzyaloshinskii-Moriya interactions. We use micromagnetics simulations to confirm our analytical predictions.   

Transport signatures of interfacial exchange coupling and chiral spin textures in magnetic insulator thin films

Ferrimagnetic insulators (FMIs) attract tremendous interest for spintronic applications due to high characteristic frequency, low Gilbert damping, and absence of Ohmic loss. First, we demonstrate the critical role of dimensionality on the SOT efficiency by systematically studying the FMI layer thickness dependent SOT efficiency in tungsten/thulium iron garnet (W/TmIG) bilayers. Current-induced switching in FMI thin films is demonstrated with a thickness up to 15 nm [1]. Second, we use temperature dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in various heavy metal/ferrimagnetic insulator heterostructures. This approach enables detection of both the magnetic proximity effect onset temperature and magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. At last, we show the transport signatures of chiral spin textures, topological Hall effect, in the platinum/ferrimagnetic insulator bilayer. [1] Q. Shao, et al., Nat. Commun., 9, 3612 (2018)   

Z2 topological invariant for magnon spin Hall systems

The classification and characterization of different phases of matter based on the topology of band structures has recently attracted much attention. It is known that some of the topological phenomena in electron systems are carried over to bosonic systems [1]. However, the topological invariant of bosonic systems with time-reversal symmetry has not yet been identified, as Kramers' theorem cannot be applied to them. To address this issue, we first introduce the pseudo-time-reversal operator which ensures the existence of ``Kramers pairs'' of bosons. Then, we define the Z₂ topological invariant for magnon spin Hall systems [2] using the Berry connection and curvature for bosons, which are different from those of electrons [3]. Furthermore, we propose two magnetic models with magnon bands carrying the nontrivial Z₂ topological invariant. We also demonstrate that the presence (absence) of helical edge states corresponds to the nontrivial (trivial) value of the Z₂ topological invariant.

[1] H. Katsura, N. Nagaosa, and P. A. Lee, Phys. Rev. Lett. 104, 066403 (2010).
[2] H. Kondo, Y. Akagi, and H. Katsura, arXiv:1808.09149.
[3] R. Matsumoto, R. Shindou, and S. Murakami, Phys. Rev. B 89,054420 (2014).   

Observation of long-wavelength modulation in chiral Mn1/3NbS2

We have investigated the magnetic properties of Mn_1/3NbS₂, through neutron diffraction and magnetic susceptibility measurements. Mn_1/3NbS₂ crystallizes in the non-centrosymmetric hexagonal space group P6₃22 and is built from NbS₂ layers intercalated by Mn ions. The Mn atoms are situated in the octahedral holes between trigonal prismatic layers of 2H-NbS₂. The magnetic susceptibility and magnetization indicate a magnetic transition at T_N ~ 45 K with an anisotropy that is similar to that observed in Cr_1/3NbS₂. Neutron diffraction reveals increased scattering near the structural Bragg peaks. However, our data indicate that the magnetic scattering had a wider q-dependence than the nuclear Bragg peaks suggesting a long wavelength modulation of the magnetic ordering along the c-axis.   

Investigating the stability of incommensurate spin textures in GaV4S8

The polar noncentrosymmetric GaV₄S₈ hosts a Néel skyrmion lattice (SkL) below T_C ~ 13 K. The Néel SkL extends over a broad range of temperature and magnetic field relative to the SkL in the chiral B20 helimagnets. It has been proposed that the modulated phases evolve into a ferromagnetic ground state at temperatures below ~ 5 K. By exploiting the magnetocaloric effect, we analyze the temperature and field evolution of the magnetic entropy change to gain insight into the stabilization mechanism of the magnetic phases close to T_C, which are thought to be governed by thermal fluctuations. We also investigate the behavior of the magnetic entropy change at low temperatures approaching the magnetic ground state.   

Non-trivial topology in 2D magnetic systems without skyrmions

The magnetic skyrmion is a kind of two-dimensional(2D) topological non-trivial spin textures which can be identified by an integer number, called topological charge. However, our study on 2D chiral magnets, however, showed that one topological charge does not necessarily correspond to one skyrmion and the non-trivial topology can be found in a disordered phase. Here, we further investigate non-trivial topology in various systems where no-skyrmion is expected in low magnetic field region. In 2D spin-frustrated systems, such thermally driven topology is dominated in a wide range of magnetic fields; In the antiferromagnetic system with staggered Dzyaloshinskii-Moriya interactions, giant response of topological charge is found in a canted spin-ordering phase. Both two systems are related to real systems so that we expect the topological properties can be observed by thermal magnon and/or anomalous Hall effect.   

Direct imaging of topological phase discontinuities in chiral spin textures via electron ptychography beyond diffraction-limited resolution

Real-space imaging of magnetic structures can provide crucial information for understanding many magnetic phenomena. Here, utilizing a new electron microscope detector, EMPAD, we resolve the internal fine structure of skyrmions in single crystal FeGe using Lorentz STEM and electron ptychography. We have unambiguously uncovered the topological phase discontinuities near the skyrmion lattice dislocation and boundaries. We also demonstrate a significant resolution improvement using electron ptychography, allowing magnetic imaging beyond the diffraction limit imposed by the electron optics. Magnetic domain images from ultrathin metallic multilayer films show the outstanding detection limit of our new imaging method. This new direct imaging technique can be applied to determine complex chiral spin structures in a wide variety of materials.   

Multiple magnetic phases in chiral Mn$_{1/3}$NbS$_2$

The chirality in materials always leads to the intriguing functionalities. In a magnetic system, the spin texture can be chiral due to the competition of Dzyaloshinskii-Moriya (DM) interaction and ferromagnetic coupling. The unique magnetic structure could be modulated into a particlelike order by applying field such as a magnetic soliton. In this talk, we present the systematic magneto-transport study on the chiral magnetic Mn1/3NbS2single crystal, a proposed candidate of chiral magnetic solation semimetal. With tuning temperature and applied field, the system undergoes several magnetic states. An anisotropic magnetic phase diagram is revealed in this the layered materials.