Confined dipole and exchange spin waves in a bulk chiral magnet with Dzyaloshinskii-Moriya interaction

The Dzyaloshinskii-Moriya interaction (DMI) has an impact on excited spin waves in the chiral magnet Cu2OSeO3 by means of introducing asymmetry in their dispersion relations. The confined eigenmodes of a chiral magnet are hence no longer the conventional standing spin waves. Here we report a combined experimental and micromagnetic modeling study by broadband microwave spectroscopy, and we observe confined spin waves up to eleventh order in bulk Cu2OSeO3 in the field-polarized state. In micromagnetic simulations we find similarly rich spectra. They indicate the simultaneous excitation of both dipole- and exchange-dominated spin waves with wavelengths down to (47.2±0.05) nm attributed to the exchange interaction modulation. Our results suggest the DMI to be effective in creating exchange spin waves in a bulk sample without the challenging nanofabrication and thereby in exploring their scattering with noncollinear spin textures.

Magnetic states and spin-wave modes in single ferromagnetic nanotubes

Daniel Rüffer

In this thesis the electrical properties, magnetic states and spin wave resonances of individual magnetically hollow ferromagnetic nanotubes have been studied. They were prepared from the different materials Nickel (Ni), Permalloy (Py) and Cobalt-Iron-Boron (CoFeB), deposited as shells onto non-magnetic Gallium-Arsenide (GaAs) semiconductor nanowires via Atomic Layer Deposition (ALD), thermal evaporation and magnetron sputtering, respectively. The resulting nanotubes had lengths between 10 to 20 μm, diameters of 150 to 400 nm and tube walls (shells) which were 20 to 40nm thick. Structural analysis of the tubes by Transmission Electron Microscopy revealed a poly(nano)crystalline (Ni, Py) and amorphous (CoFeB) structure. Electrical transport experiments as a function of temperature revealed different transport mechanisms for each of the materials. Electron-phonon scattering dominated the temperature dependence of the resistivity in Ni, while a clear evidence for electron magnon scattering was observed in Py. Electron-electron interaction in granular and amorphous media was identified as the major contribution to the temperature dependence in CoFeB. The Anisotropic Magnetoresistance (AMR) ratios have been determined for all tubes and different temperatures. Ni nanotubes exhibited a large relative AMR effect of 1.4% at room temperature. The AMR measurements provided information about the magnetic configurations as well as the magnetization reversal mechanism. Indications for the formation of vortex segments in Ni tubes were found for the magnetization reversal when the magnetic field was perpendicular to the nanotube axis. In cooperation with the Poggio group in Basel, cantilever magnetometry has been used for the further characterization of the nanotube magnetization. The magnetization curves were compared to the AMR measurements and finite element method (FEM) micromagnetic simulations. The comparison between the experimental results and the simulations suggested that the roughness of Ni tubes gave rise to segmented magnetic switching. An almost perfect axial alignment of the remanent magnetization has been observed in Py and CoFeB nanotubes. The influence of the inhomogeneous internal field in transverse magnetic fields was investigated by simulation. The segment-wise alignment of spins with the field direction is argued to provoke characteristic kinks in the hysteresis curve and measured AMR effect. Magnetothermal spatial mapping experiments using the anomalous Nernst effect (ANE) complemented the magnetotransport experiments in cooperation with the group of Prof. Grundler in Munich. Here, first evidence of end-vortices entering the nanotube before reversal could be found. Electrically detected spin wave resonance experiments have been performed in cooperation with the group of Prof. Grundler on individual nanotubes. The detected voltage, generated by the spin rectification effect, revealed multiple resonances in the GHz frequency. The experimentally observed resonances were compared to calculated ones extracted from dynamic simulations. With this comparison, the signatures could be attributed to azimuthally confined spin-wave modes. The deduced dispersion relation suggested the quantization of exchange dominated spin waves in that resonance frequencies follow roughly a quadratic dependence on the wave vector.

EPFL2014

Neutron spectroscopy in the layered quantum magnet SrCu₂(BO₃)₂ and in transition metal phosphorus trisulfides (MPS₃)

Diane Virginie Marie Lançon

Collective magnetic excitations are a fascinating aspect of condensed matter physics, where neutron scattering can provide valuable insight into the magnetic properties of physical realisations of model systems. This thesis focuses on the excitation spectra of layered quantum magnets in the case of the frustrated quantum magnet SrCu

_2

(BO

_3

)

_2

and the family of quasi-2D antiferromagnets MPS

_3

, with M a transition metal. \begin{itemize}[topsep=2pt,itemsep=-0.7ex] \item SrCu

_2

(BO

_3

)

_2

is a physical realisation of the two-dimensional Shastry-Sutherland theoretical model, constructed as orthogonal dimers with the product of singlets on the strong antiferromagnetic

J

bond as an exact ground state. The spin interactions for such a particular geometry induces strong frustration which leads to unconventional magnetism and exotic phases of matter. This work is concerned with a series of aspects of the magnetic excitations in this compound. The excitation spectra as a function of field, temperature and pressure are measured using neutron time-of-flight spectroscopy. The experimental results show that correlations, bound magnons and finite temperature properties are highly unconventional and these results are compared with existing theories on frustrated model systems. In addition, predicted topological properties of SrCu

_2

(BO

_3

)

_2

in an applied field are confirmed experimentally. \item The transition metal phosphorus trisulfides (MPS

_3

) are a family of quasi two-dimensional materials on a honeycomb lattice with weakly bound magnetic planes. This work focuses mainly on the exchange interactions and critical properties of FePS

_3

, which is largely anisotropic with the S=2 Fe

^{2+}

moments pointing normal to the

(a,b)

plane. Inelastic neutron scattering on single crystals is used to measure the spin wave dispersion, providing new insight on the strength of the coupling interactions and anisotropies and showing that FePS

_3

is a good two-dimensional model antiferromagnet. Similar experiments on powdered samples of NiPS

_3

show low-Q dispersive spin waves with a small spin-gap. Critical properties of FePS

_3

close to the Néel temperature are further discussed, as the magnetic nature of the measured quasi-elastic scattering is confirmed. Based on magnetization measurements in high pulsed fields, a possible tricritical point in the 40-50T range is proposed. \end{itemize} The work presented in this thesis has been carried out in a collaboration between the Institut Laue Langevin in Grenoble and the Laboratory for Quantum Magnetism of the EPFL.

EPFL2017

Propagation of Spin Waves Excited in a Permalloy Film by a Finite-Ground Coplanar Waveguide: A Combined Phase-Sensitive Micro-Focused Brillouin Light Scattering and Micromagnetic Study

Dirk Grundler

The excitation of spin waves by a microwave current injected into a coplanar waveguide with finite-width ground lines on a continuous Permalloy film is investigated both experimentally and numerically. Phase sensitive micro-focused Brillouin light scattering has been employed to reveal the spatial profile of the propagating spin waves in the magnetostatic surface wave geometry. The experimental results have been satisfactorily reproduced by means of micromagnetic simulations. The exciting microwave field used in this simulation has the spatial profile defined by the coplanar waveguide and user-defined periodic boundary conditions were employed in order to simulate the extended system. The resulting space and time dependent evolution of the magnetization has been analyzed by means of one and two dimensional fast Fourier transform algorithm in order to obtain the spatial profile and the frequency spectrum of the excited spin waves as well as their dispersion relations. Evidence is given to asymmetric emission from the two sides of the coplanar waveguide due to the symmetry breaking related to the sense of precession of the dynamical magnetization, as well as to the near-field effects of the extended spin wave emitter.

2013