News and posts

Phoebe M. Tengdin

EPFL

Heusler compounds are exciting materials for future spintronics applications because they display a wide range of tunable electronic and magnetic interactions such as metallicity, superconductivity, and giant magneto-resistance. However, the ultimate speed at which spins can be manipulated in materials is still an open question. In this work, we use a femtosecond light pulse to directly transfer spin polarization from one element to another in a half-metallic Heusler material, Co2MnGe. This spin transfer initiates as soon as light is incident on the material, showing that we can spatially transfer angular momentum between neighboring atomic sites on timescales less than 10 fs. Using ultrafast high harmonic pulses to simultaneously and independently probe the magnetic state of two elements during laser excitation, we find that the magnetization of Co is enhanced, while that of Mn rapidly quenches. By comparing our measurements to density functional theory, we show that the optical excitation directly transfers spin from one magnetic sub-lattice to another, via preferred spin-polarized excitation pathways. This direct manipulation of spins via light provides a path towards spintronic logic devices such as switches that can operate on few femtosecond or even faster timescales.

P. Perna

IMDEA NANOSCIENCE, Campus de Cantoblanco, Madrid, Spain

The development of room temperature magnetic devices exploiting Spin Orbit effects is at the forefront of actual research. A major challenge for future spintronics is to develop suitable spin transport channels with superior properties such as long spin lifetime and propagation length. Graphene can meet these requirements, even at room temperature [1]. However, the development of all-graphene spintronic devices requires that, in addition to its passive capability to transmit spins over long distances, other active properties are incorporated to graphene. The generation of long range magnetic order and spin filtering in graphene have been recently achieved by molecular functionalization [2,3] as well as by the introduction of giant spin-orbit coupling (SOC) in the electronic bands of graphene [4]. On the other side, taking advantage of the fast motion of perpendicular magnetic anisotropy (PMA) chiral spin textures, i.e., Néel-type domain walls (DWs) and magnetic skyrmions, can satisfy the demands for high-density data storage, low power consumption and high processing speed [5].
Here, I report on high quality, epitaxial graphene/Co(111)/Pt(111) stacks grown on (111)-oriented insulating oxide crystals, characterized by STM, LEED, STEM, Kerr Magnetometrry and Microscopy, XAS-XMCD, XMRS and SP-ARPES, which exhibit enhanced PMA for Co layers up to 4 nm thick and left-handed Néel-type chiral DWs stabilized by interfacial Dzyaloshinskii-Moriya interaction (DMI) localized at both graphene/Co and Co/Pt interfaces with opposite sign [6]. While the DMI at Co/Pt side is due to the intrinsic SOC [7], the sizeable DMI experimentally found at the Gr/Co interface has Rashba origin [6]. The active magnetic texture is protected by the graphene monolayer and stable at 300 K in air, and, since it is grown on an insulating substrate, amenable to transport measurements.

[1] W. Han, R.K. Kawakami, M. Gmitra and J. Fabian, Graphene Spintronics, Nat. Nanotech. 9, 794 (2014).
[2] M. Garnica et al., Long range magnetic order in a purely organic 2D layer adsorbed on epitaxial graphene, Nature Phys. 9, 368–374 (2013).
[3] D. Maccariello, et al., Spatially resolved, site-dependent charge transfer and induced magnetic moment in TCNQ adsorbed on graphene, Chemistry of Materials 26 (9), 2883-2890 (2014).
[4] F. Calleja et al., Spatial variation of a giant spin–orbit effect induces electron confinement in graphene on Pb islands, Nature Physics 11, 43–47 (2015).
[5] A. Fert, V. Cros and J. Sampaio, Skyrmions on the track, Nat. Nanotech. 8, 152–156 (2013).
[6] F. Ajejas, et al., Unravelling Dzyaloshinskii–Moriya interaction and chiral nature of Graphene/Cobalt interface, Nano Lett. 18(9), 5364-5372 (2018).
[7] F. Ajejas, et al., Tuning domain wall velocity with Dzyaloshinskii-Moriya interaction, Appl. Phys. Lett. 111, 202402 (2017).

Marta Brzezinska (1, 2)

(1) Department of Physics, University of Zurich,
Winterthurerstrasse 190, 8057 Zurich, Switzerland
(2) Department of Theoretical Physics, Wroc law University of Science and Technology,
Wybrze_ze Wyspianskiego 27, 50-370 Wroc law, Poland

Existing classications of topological phases are based on the presence (or absence) of
symmetries and the number of spatial dimensions being an integer. However, equipped with
a notion of locality and the possibility to take a thermodynamic limit, the classication
schemes can be extended in order to include quantum states on general graphs. In particular,
one can consider a class of self-similar geometries characterized by a fractional dimension.
In this talk, I will focus on two fractal lattices, Sierpinski carpet and gasket, exposed to an
external magnetic eld and described within tight-binding approximation. By investigating
spectral and localization properties, together with the real-space Chern number calculations
and level spacings analysis in the presence of disorder, I will show that these systems exhibit
features similar to quantum Hall states in almost two dimensions.

Maik Wagner-Reetz

Fraunhofer Institute for Photonic Microsystems, Group Spin-based Computing Königsbrücker Str. 178, 01099 Dresden, Germany

In semiconductor industry, the introduction and integration of unconventional materials, technologies and devices for new applications is a very challenging process with a lot of limitations and demands. Even if the expected benefit is tremendous, like it is foreseen for many spintronic applications, partially immense hurdles have to be mastered. First, the established contamination management of a Fab leads to a lot of restrictions and a limited availability with regard to approved elements from the periodic table. Furthermore, the Environmental, Health and Safety conditions from industry add various constraints, which lead to further limitations. Perhaps, the most important condition in semiconductor industry is the cost-of-ownership. The restrictions lead to a limited usability of known materials and difficulties to explore unconventional phases and compounds. Despite the restrictions, there is still plenty of room and a lot of new technologies were developed in recent years especially in the memory business.
Today, data is the life blood that is disrupting many industries. The vast majority of these data are stored in the form of non-volatile magnetic bits in hard disk drives, a technology developed more than half a century ago, that has reached fundamental scaling limits that impedes further increases in storage capacity. New approaches are needed. Based on very recent discoveries, spin-based implementations like e.g. Magnetic Random Access Memory (MRAM) or Racetrack Memory (RTM) are such approaches. The charge-to-spin-conversion and vice versa is a key element in spin-based computing systems and is addressed in recent research. Spin-Orbit-Coupling phenomena play a vital role in both, Spin-Orbit-Torque MRAM and RTM, where new materials with high Spin-Hall-Angles are needed. Therefore, several materials ranging from (heavy) metals to binary compounds are considered, like e.g. CoSi or TaP. A CoSi process sequence including wet chemical silicon oxide removal, Cobalt CVD deposition with annealing is available [1]. For TaP a thin film process is not yet available. Both CMOS-compatible compounds are considered to belong to the class of Weyl semimetals, which are theoretically proven to have high Spin Hall Angles [2,3]. Several open questions, like e.g. influences on the topological properties of interfaces or grain boundaries have to be addressed in order to pave the way for new unconventional approaches.

[1] V. S. Kuznetsova, S. V. Novikov, C. K. Nichenametla, J. Calvo, M. Wagner-Reetz. Structure and Thermoelectric Properties of CoSi-Based Film Composites. Semiconductors 53 (2019) 775-779.
[2] Hu, J.; Liu, J. Y.; Graf, D.; Radmanesh, S. M. A.; Adams, D. J.; Chuang, A. et al. π Berry phase and Zeeman splitting of Weyl semimetal TaP. Scientific reports 6 (2016) 18674.
[3] D.A. Pshenay-Severin, Y.V. Ivanov, A.A. Burkov, A.T. Burkov. Band structure and unconventional electronic topology of CoSi. J. Phys.: Condens. Matter 30 (2018) 135501.

Kevin GEISHENDORF

Leibniz Institute Dresden

Weyl semimetals exhibit interesting electronic properties due to their topological band structure.
In particular, large anomalous Hall and anomalous Nernst signals are often reported, which allow for
a detailed and quantitative study of subtle features. We pattern single crystals of the magnetic Weyl
semimetal Co3Sn2S2 into nanoribbon devices using focused ion beam cutting and optical lithography.
This approach enables a very precise study of the galvano- and thermomagnetic transport properties.
Indeed, we found interesting features in the temperature dependency of the anomalous Hall and
Nernst effects. We present an analysis of the data based on the Mott relation and identify in the
Nernst response signatures of magnetic fluctuations enhancing the anomalous Nernst conductivity
at the magnetic phase transition.

K. YAMAMOTO(1), G. C. Thiang(2), P. Pirro(3), K.-W. Kim(4), K. Everschor-Sitte(5) and E. Saitoh(6,7,1)

1 Advanced Science Research Center, Japan Atomic Energy Agency
2 School of Mathematical Sciences, University of Adelaide
3 Fachbereich Physik and Landesforschungszentrum OPTIMAS, Technische Universität Kaiserslautern
4 Center for Spintronics, Korea Institute of Science and Technology
5 Institut für Physik, Johannes Gutenberg-Universität Mainz
6 Department of Applied Physics, University of Tokyo
7 Institute for Materials Research, Tohoku University

Magnetostatic surface spin waves (a.k.a Damon-Eshbach mode) have long been known to have the largest decay lengths of all available modes and be robust against surface shapes and disorders [1-3]. Combined with their chiral and unidirectional propagation with respect to the direction of the ground state magnetisation, these features remind one of topologically protected edge states of quantum Hall systems. We present a topological characterisation of the dipolar spin wave Hamiltonian, which predicts, via the bulk-edge correspondence, the presence of robust surface spin wave modes without explicitly calculating eigenmodes of a system with boundaries [4].

While the characterisation is based on the symmetry class CI of electronic topological band theory, it is reformulated for the particular dynamical structure of classical Hamiltonian systems in which symplectic, rather than unitary, structure plays an essential role. By suitably identifying the symplectic structure with the chiral symmetry of class CI, assuming a preferred metric tensor in the space of canonical coordinates, we show that the surface spin waves appear not in a gap of bulk frequency spectrum, consistent with the magnetostatic surface spin waves.

[1] A. V. Chumak et al., Appl. Phys. Lett. 94, 172511 (2009).
[2] M. Mohseni et al., Phys. Rev. Lett. 122, 197201 (2019).
[3] T. Yu et al., Phys. Rev. B 99, 174402 (2019).
[4] K. Yamamoto et al., Phys. Rev. Lett. 122, 217201 (2019).

Jacob Gayles, Yan Sun, and Claudia Felser

Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany
(Dated: August 15, 2019)

Recently, the Heusler compounds Mn1:4PtSn and Mn1:4Pt0:9Pd0:1Sn were shown to stabilize an antiskyrmion
lattice above room temperature and with out an external magnetic field [1]. These Heusler compound forms in
a superstructure with the D2d symmetry, which allows for an anisotropic Dzyaloshinskii-Moriya interaction
(DMI) perpendicular to the tetragonal axis. Furthermore, many of these compounds show a spin reorientation
transition where the topological Hall effect is much larger below the transition than above in the known antiskyrmion
regime [2]. We use density functional theory calculations in combination with atomistic spin dynamic
calculation of MnxYZ compounds to extract the relevant exchange interactions that determine the rich phase
diagrams in these materials. The exchange interactions are between the large moments on the Mn atoms 4B,
which show magnetic states that are non-collinear ferrimagnetic up to the spin reorientation. The major role
of the spin-orbit driven DMI is due to the Z ion, either In, Ga,Sn or Sb where the Y ion (Ru,Rh,Pd,Ir, or Pt )
d-states lowered in energy due to the Jahn-Teller distortion. The content of Mn also plays a large role in the
stabilization of the magnetic textures. The Fermi level can be tuned by the Y ion, either In,Sn or Sb. We last
calculate the anomalous Hall effect and topological Hall effects in these regimes, to capture the influence of the
electronic structure on the Berry curvature.

Hikaru WATANABE

Kyoto University

The physics of multipole moment has been discussed in broad systems such as
strongly-correlated electron systems [1], multiferroic materials [2], and so on. Multipole
moments arise from the coupling between spin, orbital, sublattice degrees of freedom,
and lead to various phenomena in condensed matter: e.g. unconventional phases and
peculiar responses to external fields.
Furthermore, recent studies suggest that the uniform alignment of parityviolating
multipole moments, namely, odd-parity multipole order induces exotic
quantum phases and cross-correlated responses in metals [3]. An important feature of
odd-parity multipole ordered systems is unusual itinerant properties. In fact,
spontaneous emergence of spin-momentum coupling or asymmetric band structure is
closely related to intriguing quantum phenomenon.
In our work, we classified the even-/odd-parity multipole from the viewpoint of
point-group classification [4]. Our results systematically clarify the physical properties
of odd-parity multipole ordered systems and cross-correlated responses both in metals
and insulators, while previous studies have been limited to case studies [3]. We further
identified a novel transport phenomenon, magneto-piezoelectric effect, which is caused
by a coupling between elasticity and electricity in “conductors” [5]. This response may
suggest a topic in a recently developed field, antiferromagnetic spintronics [6], and will
promote functionalities of antiferromagnets.

[1] Y. Kuramoto, H. Kusunose, and A. Kiss, J. Phys. Soc. Japan 78, 072001 (2009); P.
Santini, S. Carretta, G. Amoretti, R. Caciuffo, N. Magnani, and G. H. Lander, Rev. Mod.
Phys. 81, 807 (2009).
[2] N. A. Spaldin, M. Fiebig, and M. Mostovoy, J. Phys. Condens. Matter 20, 434203
(2008).
[3] Y. Yanase, J. Phys. Soc. Japan 83 , 014703 ( 2014); S. Hayami, H. Kusunose, and Y.
Motome, Phys. Rev. B 90 , 024432 (2014); S. Sumita and Y. Yanase, Phys. Rev. B 93 ,
224507 (2016).
[4] H. Watanabe and Y. Yanase, Phys. Rev. B 98 , 2451 29 (201 8)
[5] H. Watanabe and Y. Yanase, Phys. Rev. B
96 , 0 64432 (2017) 2017); Y. Shiomi, H. Watanabe,
H. Masuda, H. Takahashi, Y. Yanase, and S. Ishiwata, Phys. Rev. Lett. 122 , 127207
(
[6] T. Jungwirth, X. Marti, P. Wadley, and J. Wunderlich, Nat. Nanotechnol. 11 , 231
( 2016); V. Baltz, A. Manchon, M. Tsoi, T. Moriyama, T. Ono, and Y. Tserkovnyak, Rev.
Mod. Phys. 90 , 015005 (

Haruki Watanabe

University of Tokyo

The interplay between symmetry and topology has been the central subject of condensed matter physics over decades. In particular, the systematic diagnosis of band topology enabled by the method of “symmetry indicators” underlies the recent advances in the search for new materials realizing topological crystalline insulators.
In this talk, we review the symmetry indicators for band insulators first and then discuss the extension of the formalism to topological superconductors.

Frank SCHINDLER

University of Zürich

Crystal defects in topological insulators are known to bind anomalous electronic states with two fewer dimensions than the bulk; the most commonly cited examples are the helical modes bound to screw dislocations in time-reversal invariant weak topological insulators. In my talk, I will explain how one can extend the classification of topological electronic defect states, in particular to time-reversal symmetry breaking magnetic systems. By mapping the Hamiltonians of planes in momentum space to the real-space surfaces between screw or edge dislocations with integer Burgers vectors, I show that crystalline defects can bind higher-order end states with fractional charge. I will present extensive numerical calculations that support these findings.