Junior Research Group Magnonics: Methods
Brillouin-Light-Scattering microscopy (µBLS)
Brillouin light scattering (BLS) is an optical measurement technique for the investigation of microscopic dynamics of solids. It is used to investigate the frequency of magnetization dynamics, e.g. propagating spin waves. The basic principle reposes on the inelastic scattering of photons with spin waves. This can be understood as the creation and annihilation of them. The result is energy win or loss of the scattered laser light and therefore a change in frequency, which is analyzed. The change in frequency of the laser light is equal to the frequency of the spin waves. For higher spatial resolution the light is focused by a microscope objective in BLS microscopy experiments (µBLS). Due to the distribution of the angles of incidence, the information about the wave vector of the spin waves becomes uncertain. The loss of this information can be recovered by phase resolved µBLS measurements.
In this special case the spin waves are excited and the laser light is modulated with the frequency of the same source. If the modulated light interacts with excited spin waves in the sample like a magnonic wave guide, then it comes to the interference of the inelastic scattered light and the modulated part. By scanning along the wave guide the interference conditions vary due to the changed path of the spin waves. Typical minima and maxima can be measured and allow for the reconstruction of the wavelength of the spin waves and therefore the correlation to the exciting frequency for the mapping of the spin wave dispersion.
An additional measurement technique is the time resolved µBLS. Here the magnetization dynamics can be investigated in the time domain. The detected photons are assigned to the measured time of flight and the interferometer properties, more precisely the mirror distance which allows for the determination of the photon frequency. Due to these possibilities it is a promising tool to investigate spin waves in time dependent fields or nonlinear processes of spin waves.
Time-resolved magneto-optical Kerr microscopy (TR-µMOKE)
The magneto-optical Kerr effect (MOKE) describes the magnetization induced changes into the reflected light by a magnetic material. It is a well-established technique, which allows to qualitatively study the magnetic properties of a system. The effect consists in the rotation of the polarization plane of the light once reflected by a magnetic material, rotation that is directly proportional to the magnetization state of the sample. With MOKE it is therefore possible to study quasi-static magnetization reversal properties of a system.
In our group two different MOKE experimental setups are currently running using the same principle. The light of a laser source is s-polarized through a thin-film polarizer and then focused onto the sample surface. After this, a Wollaston prism divides the focused reflected light into two orthogonally polarized beams, which are measured by a home-built detector unit equipped with two photodiodes. The polarization of the reflected light is analyzed by measuring the normalized differential signal, which is directly proportional to the Kerr rotation.
Two experimental setups are currently available in our group:
- Longitudinal MOKE: In this geometry the magnetization vector is parallel to both the reflection surface and the plane of incidence. Laser wavelength of 406 nm, magnetic field up to ± 75 mT, automated sample positioning and rotation.
- Polar MOKE: In this geometry the magnetization vector is perpendicular to both the reflection surface and the plane of incidence. Laser wavelength of 488 nm, magnetic field up to ± 2 T.
This setup is currently used to realize All Optical Switching and Laser induced Skyrmion Bubbles.
The laser pulses are provided by a Ti:Sa femtosecond laser with a wavelength of 800 nm and a repetition rate of 5.2 MHz. The pulse duration is in the range of 50 fs. The laser can provide pulse energies up to 400 nJ. It is possible to create linear polarized, as well as circular polarized laser pulses at the samples surface. We also included a Kerr-Microscope to the setup, which can be used to measure the out of plane magnetic contrast while illuminating the sample by the laser pulses.
It is also possible to create the second harmonic of the 800 nm pulses by using a BBO-Crystal. By doing this, we can realize Pump-Probe experiments by detecting the 400 nm pulses, which were reflected at the samples surface. At the same time, it is possible to move a delay stage to change the path length of the 800 nm pulses to make a delay between the Pump (800 nm) and the Probe (400 nm) pulses at the sample.
For experiments, we can apply either an in plane field up to 1.5 Tesla or an out of plane field up to 2.0 T.
The micromagnetic model has shown to be accurate compared with experiments. Using micromagnetic simulations which are based on solving the Landau-Lifshitz-Gilbert equation of motion the dynamics of spin waves as well as the inner structure of magnetic textures such as domain walls and vortices can be resolved at the submicron scale. This approach is used to supplement the interpretation of experimental data or even for standalone work. In this group, mostly the GPU-based software MuMax3 is used.
AC/DC transport system
- two-dimensional Kerr microscopy
- electrical detection via inverse Spin-Hall-Effect (iSHE)
- broadband ferromagnetic resonance for spin pumping