Session L41: Heterostructures and Thin Films for Skyrmions

Hybrid chiral domain walls and skyrmions in magnetic multilayers

Aiming at reducing the radius of individual magnetic skyrmions below 10 nm without making compromises between room-temperature stability and current-induced mobility¹, a multilayered magnetic system is often privileged². However, dipolar interactions can overcome the essential chiral properties of magnetic skyrmions stemming from the Dzyaloshinskii-Moriya interaction (DMI), for multilayers with too large magnetization or number of layers. Using circular dichroism in X-ray Resonant Magnetic Scattering, we demonstrate a reorientation of chirality along the vertical direction of magnetic domain-walls and skyrmions, leading to the formation of hybrid chiral skyrmions³. We next establish an extensive micromagnetic model of magnetic skyrmions, which allows obtaining their equilibrium profiles with an accurate and layer-resolved description of both size and chirality⁴. The resulting magnetization profiles allow predicting, within the Thiele formalism, the expected velocity of skyrmions under current-induced spin-orbit torques in different spin injection geometries. We find the requirements on the DMI magnitude to ensure an efficient motion, and observe a regime of compensation of the skyrmion Hall effect for hybrid chiral skyrmions. Beyond such issues related to the chirality of ferromagnetic skyrmions, we finally present our recent progresses in the study of multilayers hosting synthetic antiferromagnetic skyrmions, whose alternation of polarity enables a way to circumvent all issues related to dipolar interactions in skyrmion multilayers.
¹ F. Büttner et al, Sci Rep 8, 4464 (2018)
² C. Moreau-Luchaire et al, Nat Nano 11, 444 (2016); S. Woo et al, Nat Mater 15, 501 (2016)
³ W. Legrand et al, Sci Adv 4, eaat0415 (2018); Y. Dovzhenko et al, Nat Comm, 9, 2712 (2018)
⁴ W. Legrand et al, arXiv:1807.04935 (2018).   

Prediction and discovery of all-quantum skyrmions at room temperature

Room temperature skyrmions were recently observed in ferromagnetic multilayers [1–4]. Surprisingly, they were all at least several tens of nanometer in diameter. Their large size has a fundamental origin: these skyrmions are stabilized by a competition of classical long range fields, namely stray fields and externally applied fields. Based on accurate analytical modeling, we could show that this is a general property of ferromagnets due to their overwhelming stray fields [5].
In ferri- and antiferromagnetic materials, stray fields play no significant role. We predicted theoretically [5] and confirmed experimentally [6] that a different type of skyrmion can exist in these materials at room temperature, stabilized by all-quantum interactions. These skyrmions are insensitive to classical fields, exist even at zero external field, and can be as small as ~10 nm in diameter. Finally, they are also created by quantum spin-orbit torques [7].

[1] Büttner et al. Nat Phys 11, 225 (2015)
[2] Woo et al. Nat Mat 15, 501 (2016)
[3] Moreau-Luchaire et al. Nat Nano 11, 444 (2016)
[4] Boulle et al. Nat Nano 11, 449 (2016)
[5] Büttner et al. Sci Rep 8, 4464 (2018)
[6] Caretta, Mann, Büttner, et al. Nat Nano in press (2018). doi:10.1038/s41565-018-0255-3
[7] Büttner et al. Nat Nano 12, 1040 (2017)   

Structural Imprinting of Target Skyrmions in Magnetic Multilayers

We will report on a recent study of topological spin textures that were imprinted due to coupling between a 30nm thin permalloy (Py) nanodisk with diameters from 250-1000nm and a multilayer Ir/Co/Pt film with strong DMI. Using element-specific magnetic soft x-ray microscopy we were able to image the magnetic structure of the Py nanomagnets and the spin texture in the DMI film independently. We found a significant increase of the imprinted domain period (240nm) in the film under the disks compared to the free film (180nm), which can be traced back to a locally varying stray field energy. We stabilize extended target skyrmions in the film with up to four π rotations of the z component of magnetization. This is due to the reduced stray field energy and enforced radial symmetry caused by the Py disk. We confirm that these structures have a uniform chirality enforced by the DMI of the thin film by observing an asymmetric expansion of the domain walls as a function of applied magnetic field pulses. We also observe the overall structural stability of target skyrmions in an external magnetic field has no dependence on topological charge.   

Realization of magnetic skyrmions in thin films at ambient conditions

Magnetic skyrmions present interesting physics due to their topological nature and hold significant promise for future informational technologies. A key barrier to realizing skyrmion devices has been stabilizing these spin structures in ambient conditions. I will discuss how we exploited the tunable magnetic properties of amorphous Fe/Gd films to realize, for the first time, skyrmion lattices in continuous amorphous thin-films, stable at room temperature and zero magnetic field. These Bloch-type skyrmions are stabilized by dipolar interactions rather than traditional Dzyaloshinskii-Moriya interactions. Small angle neutron scattering was used in combination with soft x-ray microscopy providing a unique, multi-scale probe of the local and long-range order of these structures. Key to this work was to prepare an artificial stripe phase, which evolves into the hexagonally ordered skyrmion lattice structure under increasing magnetic field. These results identify a pathway to engineer controllable skyrmion phases in thin film geometries which are stable at ambient conditions.
  

SP-STM Studies of B20 MnGe

Materials in the B20 crystal structure have broken bulk inversion symmetry, which can lead to the formation of chiral magnetic textures. MnGe is an example with the smallest magnetic period (~3 nm) and largest topological Hall signals observed among B20 chiral magnets. Here, we present spin-polarized scanning tunneling microscopy (SP-STM) characterization of thin MnGe(111) films grown by molecular beam epitaxy. Atomic resolution images show a triangular lattice with a lattice parameter of 6.85 Å in good agreement with the bulk. Three of four possible surface terminations are observed in STM images of different terraces on the surface. Tunneling spectroscopy with a bulk Cr tip shows a pronounced dependence on magnetic field, which we compare to the spin-resolved local density of states calculated by DFT. Counter-intuitively, we observe the largest spin signal from one of the Ge-terminated surfaces, due to their antiferromagnetic coupling to the underlying Mn layer. We also examine magnetic field dependence in topography and spectroscopic maps to probe for surface magnetic structures.   

Stability of Skyrmions and Topological Hall Effect in Mn2CoAl thin films

Skyrmions are magnetic quasi-particles that have the potential to help resurrect Moore’s Law.^[1] Due to their topological protection, they could be used as non- volatile memory,^[2] while also allowing the design of logic gates^[3] that could be used in magnonic devices.
The presence of skyrmions can be detected by observing the topological contribution to the Hall resistance, due to the time-varying magnetic field experienced by an electron travelling through a material hosting skyrmions. We observe skyrmions in thin films of the ferromagnetic spin-gapless semiconductor Heusler alloy Mn₂CoAl capped by Pd.^[4] We can engineer our thin films to make the skyrmions stable from 3K to above room temperature. By careful control of the magnetic and thermal history of the sample, we can condition various forms of skyrmions in this material. In order to understand their stability, we have measured time dependent Hall resistance and magnetization in the skyrmion regions. We observe skyrmions that are stable for at least hours at room temperature in the absence of a magnetic field.

References:
1 D. Chandler, MIT News., 2017
2 A. Fert, V. Cros, and J. Sampaio, Nat. Nanotechnol., 2013, 8, 152–156 (2013).
3 S. Luo et al., Nano Lett., 2018, 18, 1180-1184
4 B. Ludbrook et al., Sci. Rep., 2017, 13620   

Determining Chirality of Non-Centrosymmetric FeGe and MnGe Thin Films via STM

Recent interest in the ‘B20’ phase of FeGe and MnGe stems from noncollinear magnetic states, such as magnetic Skyrmions. Here we present a joint STM and DFT study of FeGe and MnGe films grown by molecular beam epitaxy. An average surface lattice constant of ~6.8 Å (FeGe) and of ~6.9 Å (MnGe), in agreement with the bulk values, was observed via LEED, as well as in situ RHEED during the MBE growth. Atomic resolution images of each of the four possible chemical terminations in the FeGe films were characterized by distinct image contrast and corrugation, as well as local density of states in tunneling spectra. Likewise, three of the four possible chemical terminations of MnGe were identified in atomic resolution STM images. These assignments were confirmed by the good agreement between the STM images and DFT-simulated images using the Tersoff-Haman approximation. Having determined the surface terminations, STM images of the atomic layering order and registry across step edges allows us to uniquely determine the stacking order, and hence chirality of these films, which is difficult with conventional crystallography techniques.

  

Skyrmions in Monolayer van der Waals 2D Magnets

In this work, using Landau-Lifshitz Gilbert (LLG) model and first-principle calculations we demonstrate the occurrence of skyrmions in monolayer CrI₃. We considered a paramagnetic state and apply a range of magnetic fields from 0 T to 1.5 T. We found the system evolves under normal conditions (i.e., the time evolution under the influence of exchange and anisotropic interactions) to energetically favourable skyrmionic ground state. We derive the Heisenberg exchange (J), uniaxial magnetocrystalline anisotropy (K) and Dzyaloshinskii-Moriya interaction (DMI) parameters for monolayer CrI₃. Our calculations revealed that the isolated skyrmionic spin textures evolved in the order of a few picoseconds for a wide range of magnetic field. We found the chiral domain walls or helical states in the 2D monolayer CrI₃ along with the presence of skyrmions at low magnetic field strengths. However, with increasing strength of the magnetic field, the density of occurrence of chiral domains reduces converging and forming skyrmions. It is also observed that with the increase in the field strength the size of stable skyrmions reduces.   

Large terahertz resonance in MnGe thin films

Topological nature of matters often plays an important role in transport phenomena such as Hall effect. For example, the large anomalous Hall effect (AHE) and topological Hall effect (THE) have been observed in bulk MnGe [1]. In this study, we investigate the optical Hall conductivity by means of terahertz time domain spectroscopy in MnGe thin films. While the diagonal conductivity can be well described by the conventional Drude model, the Hall conductivity spectra show the large resonance structure at ~ 1.2 meV, which is significantly enhanced and dominates the DC Hall conductivity at low temperatures. We further analyze the Hall conductivity spectra based on the Karplus-Luttinger model, which indicates that the resonance structure consists of two interband transitions corresponding to AHE and THE.
[1] N. Kanazawa, et al., Phys. Rev. Lett. 106, 156603 (2011).   

Detecting Crystallographic Lattice Chirality using Resonant Inelastic X-ray Scattering

The control and detection of crystallographic chirality is an important and challenging scientific problem. Chirality has wide ranging implications from medical physics to cosmology including an intimate but subtle connection in magnetic systems, for example Mn_1−xFe_xSi. We demonstrate using theoretical calculations the feasibility of indirect K -edge bimagnon resonant inelastic X-ray scattering (RIXS) spectrum as a viable experimental technique to distinguish crystallographic handedness. We apply spin wave theory to the recently discovered √5 × √5 vacancy ordered chalcogenide Rb_0.89Fe_1.58Se₂ for realistic X-ray experimental set up parameters (incoming energy, polarization, and Bragg angle) to show that the computed RIXS spectrum is sensitive to the underlying handedness (right or left) of the lattice. A Flack parameter definition that incorporates the right- and left- chiral lattice RIXS response is introduced. It is shown that the RIXS response of the multiband magnon system RbFeSe arises both from inter- and intra- band scattering processes. The extinction or survival of these RIXS peaks are sensitive to the underlying chiral lattice orientation. This in turn allows for the identification of the two chiral lattice orientations.   

Numerical study of skyrmion-string dynamics

Magnetic skyrmion is a swirling emergent particle in two-dimensions characterized by the topological index called skyrmion number. The skyrmion in three dimensional chiral magnets forms a string, end points of which are monopole and antimonopole. This meandering one-dimensional object has many internal degrees of freedom including the rotation and breathing. The skyrmion string is driven by the current through the spin transfer torque effect, and the collision with the disorder induces a variety of phenomena. Here we show theoretically by the numerical simulation of a skyrmion string under current that the four distinct behaviors: (i) pinned string, (ii) depinned motion with the excitation of breathing mode, (iii) resonant excitation of breathing mode leading to the creation of monopole and antimonopole, and (iv) motional narrowing to recover the straight string, as the current density increases.   

Skyrmions Driven by Intrinsic Magnons

We study the dynamics of a Skyrmion in a magnetic insulating nanowire in the presence of time-dependent oscillating magnetic field gradients [1]. These ac fields act as a net driving force on the Skyrmion via its own intrinsic magnetic excitations. Making use the generalized micromagnetic equations of motion for the quantum propagation of a skyrmion in a magnetic insulator [2], we include the unavoidable coupling of the external field to the magnons, which gives rise to time-dependent dissipation for the Skyrmion. We demonstrate that the magnetic ac field induces a super-Ohmic to Ohmic crossover behavior for the Skyrmion dissipation kernels with time-dependent Ohmic terms. The ac driving of the magnon bath at resonance results in a unidirectional helical propagation of the Skyrmion in addition to the otherwise periodic bounded motion.

[1] Christina Psaroudaki and Daniel Loss, Skyrmions Driven by Intrinsic Magnons, Phys. Rev. Lett. 120, 237203 (2018).
[2] Christina Psaroudaki, Silas Hoffman, Jelena Klinovaja, and Daniel Loss, Quantum Dynamics of Skyrmions in Chiral Magnets, Phys. Rev. X 7, 041045 (2017).   

Relaxation dynamics in magnetic skyrmions with quenched disorder

Magnetic skyrmions are topologically protected spin textures of nanometer size found in certain chiral magnets. Skyrmions can be moved by very low current densities which makes them ideal for applications in spintronics such as data storage devices and logic gates. A thorough understanding of the relaxation processes for systems of interacting skyrmions far from equilibrium could prove invaluable in real world applications. We use a particle based model derived from Thiele's approach to study the relaxation dynamics of thin film skyrmions in the presence of randomly distributed defects. The particle model differs most notably from similar models which describe vortices in type-II superconductors by the addition of the Magnus force which always acts perpendicular to the forces in the plane. The interplay between the Magnus force, repulsive skyrmion-skyrmion interaction and Gaussian noise yields different regimes during non-equilibrium relaxation depending on the strength of the defects.