Christian Back

Functional Spin Systems

Technical University of Munich

Department of Physics

James-Franck-Str. 1

85748 Garching

Tel. +49 89 289 12401

christian.back[at]um.de

Group Webpage

Description

Research focus: magnetization dynamics, topological quantum materials, hybrid spin-orbit field controlled materials, nanotechnology

The research of our group is focused on the detailed understanding of magnetization dynamics in specially designed ultra thin magnetic layers, in topological materials and at interfaces inducing spin-orbit interaction. We tailor novel hybrid magnetic structures and investigate their static and dynamic magnetic properties. Among the subjects covered in our research are the dynamics in confined magnetic systems, magnonics, spinorbitronics, hybrid topological materials, high resolution magnetic microscopy as well as magnetic phase transitions in low dimensional systems.

In our group we use several techniques to examine magnetization dynamics, the propagation of spinwaves and the efficiency of charge to spin current conversion. At the heart of our research projects are various time and spatially resolved high resolution magnetic microscopy techniques in combination with microwave excitation and detection.

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Complex spin structures in topological materials

Hybrid topological materials comprising e.g. of 3D topological insulators (TIs) and ultra thin magnetic layers (ML) promise highly efficient spin to charge conversion due the perfect spin momentum locking of the 2D helical surface states. Consequently these materials are of high relevance for the spintronics community. The topologically protected surfaces states forming at the interface between the TI and a topologically trivial magnetic material (metallic or insulating) are protected from direct backscattering by time reversal symmetry. When a pure spin current is injected from the magnetic material e.g. by spin pumping, it is expected to be efficiently converted into a charge current. Ultimately, highly efficient switching of the magnetization is expected to be realized in hybrid ML/TI nanostructures excited by short electrical current pulses.

We design and fabricate hybrid ML/TI systems using molecular beam epitaxy and investigate spin momentum locking using spin pumping based techniques.

A second class of topological materials we are interested in are complex skyrmion hosting magnetic materials. Prominent examples are the B20 Silicides FexCo1-xSi and MnSi or the insulating material Cu2OSeO3. In these materials we investigate the decay processes of the topologically protected skyrmion tubes as well as magnon transport.


Selected publications

  • Entropy-limited topological protection of skyrmions, Science Advances 3, e1701704 (2017).
  • Time resolved measurements of the switching trajectory of Pt/Co elements induced by spin-orbit torques, Physics Review Letters 118, 257201 (2017).
  • Dynamical Defects in Rotating Magnetic Skyrmion Lattices, Physics Review Letters 118, 207205 (2017).
  • Snell’s Law for Spin Waves, Physics Review Letters 117, 037204 (2016).
  • Spin Hall effects, Review of Modern Physics 87, 1213 (2015).

Publications

All-electrical detection of skyrmion lattice state and chiral surface twists

A. Aqeel, M. Azhar, N. Vlietstra, A. Pozzi, J. Sahliger, H. Huebl, T.T.M. Palstra, C.H. Back, M. Mostovoy

Physical Review B 103 (10), L100410 (2021).

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We study the high-temperature phase diagram of the chiral magnetic insulator Cu2OSeO3 by measuring the spin-Hall magnetoresistance (SMR) in a thin Pt electrode. We find distinct changes in the phase and amplitude of the SMR signal at critical lines separating different magnetic phases of bulk Cu2OSeO3. The skyrmion lattice state appears as a strong dip in the SMR phase. A strong enhancement of the SMR amplitude is observed in the conical spiral state, which we explain by an additional symmetry-allowed contribution to the SMR present in noncollinear magnets. We demonstrate that the SMR can be used as an all-electrical probe of chiral surface twists and skyrmions in magnetic insulators.

DOI: 10.1103/PhysRevB.103.L100410

Microwave Spectroscopy of the Low-Temperature Skyrmion State in Cu2OSeO3

A. Aqeel, J. Sahliger, T. Taniguchi, S. Maendl, D. Mettus, H. Berger, A. Bauer, M. Garst, C. Pfleiderer, C.H. Back.

Physical Review Letters 126 (1), 017202 (2021).

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In the cubic chiral magnet Cu2OSeO3 a low-temperature skyrmion state (LTS) and a concomitant tilted conical state are observed for magnetic fields parallel to h100i. Here, we report on the dynamic resonances of these novel magnetic states. After promoting the nucleation of the LTS by means of field cycling, we apply broadband microwave spectroscopy in two experimental geometries that provide either predominantly in-plane or out-of-plane excitation. By comparing the results to linear spin-wave theory, we clearly identify resonant modes associated with the tilted conical state, the gyrational and breathing modes associated with the LTS, as well as the hybridization of the breathing mode with a dark octupole gyration mode mediated by the magnetocrystalline anisotropies. Most intriguingly, our findings suggest that under decreasing fields the hexagonal skyrmion lattice becomes unstable with respect to an oblique deformation, reflected in the formation of elongated skyrmions.

DOI: 10.1103/PhysRevLett.126.017202

Spin structure relation to phase contrast imaging of isolated magnetic Bloch and Neel skyrmions

S. Poellath, T. Lin, N. Lei, W. Zhao, J. Zweck, C.H. Back

Ultramicroscopy 212, 112973 (2020).

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Magnetic skyrmions are promising candidates for future storage devices with a large data density. A great variety of materials have been found that host skyrmions up to the room-temperature regime. Lorentz microscopy, usually performed in a transmission electron microscope (TEM), is one of the most important tools for characterizing skyrmion samples in real space. Using numerical calculations, this work relates the phase contrast in a TEM to the actual magnetization profile of an isolated Neel or Bloch skyrmion, the two most common skyrmion types. Within the framework of the used skyrmion model, the results are independent of skyrmion size and wall width and scale with sample thickness for purely magnetic specimens. Simple rules are provided to extract the actual skyrmion configuration of pure Bloch or Neel skyrmions without the need of simulations. Furthermore, first differential phase contrast (DPC) measurements on Neel skyrmions that meet experimental expectations are presented and showcase the described principles. The work is relevant for material sciences where it enables the engineering of skyrmion profiles via convenient characterization.

DOI: 10.1016/j.ultramic.2020.112973

Ferromagnetic Resonance with Magnetic Phase Selectivity by Means of Resonant Elastic X-Ray Scattering on a Chiral Magnet

S. Pollath, A. Aqeel, A. Bauer, C. Luo, H. Ryll, F. Radu, C. Pfleiderer, G. Woltersdorf, C.H. Back

Physical Review Letters 123 (16), 167201 (2019).

Show Abstract

Cubic chiral magnets, such as Cu2OSeO3, exhibit a variety of noncollinear spin textures, including a trigonal lattice of spin whirls, the so-called skyrmions. Using magnetic resonant elastic x-ray scattering (REXS) on a crystalline Bragg peak and its magnetic satellites while exciting the sample with magnetic fields at gigahertz frequencies, we probe the ferromagnetic resonance (FMR) modes of these spin textures by means of the scattered intensity. Most notably, the three eigenmodes of the skyrmion lattice are detected with large sensitivity. As this novel technique, which we label REXS FMR, is carried out at distinct positions in reciprocal space, it allows us to distinguish contributions originating from different magnetic states, providing information on the precise character, weight, and mode mixing as a prerequisite of tailored excitations for applications.

DOI: 10.1103/PhysRevLett.123.167201

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