How it works
Experimental Techniques to Discover Magnetization Dynamics
Magneto-Optical Kerr Effect (MOKE)
The Magneto-Optical Kerr Effect (MOKE) describes the change in polarization of light when it reflects off a magnetized material. Depending on the orientation of magnetization relative to the light’s incidence and polarization, MOKE is classified into longitudinal, transverse, and polar geometries. The Kerr rotation (θₖ) and Kerr ellipticity (εₖ) are the measurable parameters and are typically proportional to the magnetization M of the sample.
The MOKE technique allows for non-contact, surface-sensitive detection of magnetic properties with high spatial resolution.
To determine the magnetization state of a ferromagnetic or ferrimagnetic material, we utilize the Magneto-Optical Kerr Effect (MOKE). First observed in 1877 by John Kerr, this effect describes how the polarization plane of linearly polarized light rotates slightly when reflected from a magnetized surface. Specifically, Kerr noted that when light reflects perpendicularly off the polar surface of an iron electromagnet, the polarization plane is rotated in the opposite direction of the magnetizing current.
Polar- (a), transverse- (b) and longitudinal- (c) MOKE configuration. The grey shaded area indicates the plane of incidence of the linearly polarized light, indicated by the blue arrows.
Time-resolved MOKE (TR-MOKE)
Time-resolved magneto-optical Kerr effect (time-resolved MOKE, often abbreviated as TR-MOKE or TR MOKE) measures how the magnetization of a sample evolves in time by monitoring changes in the polarization of reflected light. In many CRI²SPIN setups, the primary excitation is provided by microwave currents in on-chip antennas, coplanar waveguides or striplines that generate a dynamic magnetic field at GHz frequencies.
By synchronizing the laser probe with the microwave drive and varying the phase between them, time-resolved MOKE (TR-MOKE) measurements become stroboscopic snapshots of magnetization dynamics. This allows you to study ferromagnetic resonance, spin-wave propagation, precessional motion and damping processes over a wide frequency range, from the low-GHz regime up to the onset of THz dynamics.
In addition to microwave-based on-chip excitation, CRI²SPIN systems can also be configured for optical pump–probe experiments. Here, an ultrafast pump pulse excites the sample while a time-delayed probe pulse detects the Kerr rotation and ellipticity, enabling access to ultrafast demagnetization and other processes on sub-picosecond timescales.
Time-resolved MOKE can be combined with scanning microscopy – either via galvo–galvo beam steering or precision XYZ stages – as well as with cryogenic sample environments. This enables time-resolved MOKE (TR-MOKE) microscopy on thin films, patterned devices and nanostructures from room temperature down to cryogenic conditions.
TR-MOKE – Frequently asked questions
What is TR-MOKE (time-resolved MOKE)?
TR-MOKE (time-resolved magneto-optical Kerr effect, sometimes written as time-resolved MOKE, TR-MOKE or TR MOKE) is a time-resolved magnetometry technique that uses polarized light to probe magnetization dynamics. In many experiments the sample is excited by microwave fields generated in on-chip antennas or coplanar waveguides, and the laser probe is synchronized to this excitation to record the Kerr signal as a function of phase or delay.
Do I need an optical pump–probe setup for TR-MOKE?
No. Many time-resolved MOKE experiments use purely microwave-based on-chip excitation and a synchronized probe beam. However, TR-MOKE can also be implemented in an optical pump–probe scheme, where an ultrafast pump pulse excites the sample and a delayed probe pulse measures the Kerr response. CRI²SPIN systems support both microwave-driven and optical pump–probe configurations, depending on the requirements of your experiment.
TR-MOKE working principle
The TR-MOKE technique relies on stroboscopic measurement, enabling the detection of only coherent and repeatable magnetic phenomena. To probe spin waves in the nanosecond (GHz) time or frequency range, excitation events faster than this timescale are required. Our system supports multiple laser sources to investigate magnetization dynamics with high temporal resolution.
The figure presents a schematic illustration of a typical TR-MOKE setup: Femtosecond laser pulses enable stroboscopic measurements of coherently excited spin waves. A galvo-galvo scanner directs the laser beam across the sample surface (indicated by the dashed rectangle), or alternatively, the sample itself can be moved using a high-precision XYZ piezo stage to scan relative to the laser focus. Both scanning methods are offered.
https://nbn-resolving.org/urn:nbn:de:bvb:91-diss-20230324-1693644-1-1
Galvo-Galvo Scanning system
Schematic overview of a Galvo-Galvo scanning system used in one of CRI²SPIN’s TR-MOKE setups: Two independent mirror galvanometers (M1 and M2) steer the incoming laser beam in two dimensions. A scan lens and tube lens—arranged in a Kepler-like telescope configuration—focus the deflected beam into the objective’s focal plane. This optical setup maintains normal incidence to the sample surface, ensuring that the incoming and reflected beam paths are identical for precise alignment and measurement.
https://nbn-resolving.org/urn:nbn:de:bvb:91-diss-20230324-1693644-1-1
Cryogenic Microscopy
Our systems support cryogenic microscopy, enabling high-resolution optical and magneto-optical measurements at low temperatures. This capability is essential for studying temperature-dependent magnetic phase transitions, quantum materials and spin coherence in solid-state systems. We offer customizable sample environments with precise thermal control down to cryogenic temperatures, compatible with both time-resolved MOKE (TR-MOKE) and fluorescence detection.
Cryogenic time-resolved magneto-optical Kerr effect (cryo time-resolved MOKE, often abbreviated as cryo TR-MOKE) builds on the same principles as our room-temperature TR-MOKE systems. By synchronizing the laser probe with microwave currents in coplanar waveguides or striplines on the sample, cryo TR-MOKE measurements reveal magnetization dynamics, spin-wave propagation and damping in low-temperature and quantum regimes.
In addition to cryo TR-MOKE, the same cryogenic platforms can be configured for ferromagnetic resonance (cryo FMR). Broadband rf excitation and flexible magnetic field configurations enable quantitative studies of magnetic anisotropy, damping and spin-transport phenomena at low temperatures, and can be combined with optical access for correlated TR-MOKE and FMR measurements on the same device.
Together, these capabilities make CRI²SPIN cryogenic setups a turnkey solution for experiments on quantum and strongly correlated materials, superconducting and topological systems, as well as for spintronic and microwave device architectures based on on-chip structures.
Ferromagnetic Resonance (FMR)
FMR is a technique for probing the resonant precession of magnetization in a material under an applied RF field and static magnetic bias. The magnetization vector M precesses around the effective magnetic field when the RF frequency matches the material’s natural resonance condition, governed by the Kittel equation:
\displaystyle f=\frac{\gamma}{2\pi}\sqrt{B\bigl(B+\mu_0 M_{\text{eff}}\bigr)}where f is the resonance frequency, B is the external magnetic field, and M_{eff} is the effective magnetization. FMR provides quantitative information on magnetic anisotropy, damping, and exchange interactions.
In CRI²SPIN setups, ferromagnetic resonance is typically implemented using broadband coplanar waveguides (CPWs) and on-chip microwave structures, allowing in-plane and out-of-plane geometries over a wide frequency range. The same approach can be extended to cryogenic conditions (cryo FMR), using dedicated inserts that combine rf lines, magnets and low-temperature sample environments, and can be combined with time-resolved MOKE (TR-MOKE) and ISHE measurements on the same device.
Inverse Spin Hall Effect (ISHE)
The ISHE is a key phenomenon in spintronics where a spin current is converted into a transverse charge voltage in a non-magnetic material with strong spin-orbit coupling. This effect enables the electrical detection of spin currents generated by spin pumping from a ferromagnetic material during FMR:
\displaystyle \mathbf{J}_{\mathrm{c}} \propto \mathbf{J}_{\mathrm{s}} \times \boldsymbol{\sigma}\,.
Here, \mathbf{J}_s is the spin current, \mathbf{\sigma} is the spin polarization direction, and \mathbf{J}_c is the induced charge current. ISHE serves as a powerful method for detecting spin transfer and quantifying spin Hall angles.
Cryogenic FMR and ISHE – Coming soon
CRI²SPIN is developing a FMR and ISHE insert for variable temperature inserts (VTI). The covered temperature range is room temperature to 4 K (details depending on the details [CR1] of the VTI used). We will offer inserts with a minimum diameter of 28 mm equipped with coplanar waveguides (CPW) for in-plane and out-of-plane FMR and ISHE measurements in the frequency range 2-40 GHz (higher frequencies and ST-FMR upon request). We use two distinct CPWs for in-plane or out-of-plane measurements which can be mounted in the same insert (in-plane to out-of-plane rotation available upon request) which are connected to two high frequency lines.
User-friendly software
Our state-of-the-art software complements all CRI²SPIN instruments, providing an intuitive interface for easy operation. The software features a graphical user interface (GUI) that simplifies the data acquisition process, making it accessible for both novice and experienced researchers. Automatic data storage ensures you never lose your valuable results, while advanced analysis tools help you interpret your findings effectively. Designed to work seamlessly with our instruments, our software enhances your research experience by streamlining workflows and improving data accuracy. Elevate your research capabilities with our comprehensive software solution.
