Spintronics: Magneto-Optical Kerr Effect (MOKE)
The Magneto-Optical Kerr Effect (MOKE) and the Faraday effect describe the change in polarization of incident light as it is reflected (or transmitted) by a magnetic material. These effects can be used for modulating the amplitude of light and form the basis of optical isolators and optical circulators that are integral to optical telecommunications networks and various laser applications. MOKE was widely used as an optical readout technique for logic state of magnetic storage media (hard disk drives), and the MOKE technique offers promise for real-time readout of logic states in new magnetic memory technologies such as MRAM.
MOKE is most commonly used in research settings to characterize the electronic and magnetic properties of materials such as the magnetic domain structure, spin density of states, and magnetic phase transition dynamics. Recent experimental progress on high quality nanostructures and 2D materials (e.g. transition metal dichalcogenides, graphene, topological insulators) promises to harness these magneto-optic effects for enhanced control of light at the nanoscale for integrated photonic or spintronic devices.
Montana Instruments has developed a cryogenic platform to meet the demanding needs of the magneto-optical and spintronic research community. The system provides an easy-to-use, turnkey solution so that you can begin making magneto-optical measurements immediately. Below you will find some of the common experimental challenges faced by the magneto-optical research community and how Montana Instruments has provided versatile solutions with minimal user input required.
- The magneto-optical Kerr effect is generally a weak signal. Therefore, larger incident laser powers are required to generate sufficient signal for detection.
- The high incident laser power can locally heat the device
- Magnetic phase transition temperatures and interesting magnetic phenomena often occur at cryogenic temperatures. Examples include the formation of magnetic textures such as vortices in high temperature superconductors and skyrmion phase formation in helimagnets.
- Thermal and mechanical excitation reduce spin lifetimes and generates additional noise in the MOKE signal. Therefore, an ultra-stable cryogenic environment is required to investigate novel magnetic phenomena.
Keys for Optimizing a Magneto-Optical Kerr Effect (MOKE) Experiment
MOKE experiments require versatile optical and electrical access with an integral magnetic field. The sample must be in an ultra-stable cryogenic environment while also being re-configurable for multiple experimental geometries.
|Focus Area||Why It's Important||The Cryostation Difference|
|Magnetic Field||A magnetic field can be used to magnetize the sample and then swept to determine coercivities.||Longitudinal and polar MOKE configurations are available with the Cryostation Magneto-Optic Module. Fields of up to 0.7T can be achieved for probing spin dynamics. The bipolar power supply provides continuous sweeping through zero-field. External magnets can also be seamlessly integrated. Inquire with our applications staff for details on higher magnetic field setups.|
|Low Temperatures (<4K)||Cryogenic temperatures reduce spin dephasing effects and improve signal to noise ratio which allows higher incident laser power.||Cryogenic temperatures < 4K are accessed seamlessly. Dial in your experimental temperature using the automated software and your sample will be there shortly.|
|Low Vibrations||Low vibrations provide a stable sample platform which results in minimum noise added to the MOKE signal for maximum signal collection efficiency.||Minimal system vibrations (< 5nm) provide an ultra-stable cryogenic platform with maximum resolving power for small MOKE signals. Minimal sample drift when cooling from 300K to 4K allows you to easily track a small magnetic domain or small sample across a wide temperature range.|
|Sample & Optical Access||The sample needs to be optically accessible and reconfigurable for multiple magnetic field orientations to fully elucidate spin physics.||The Cryostation provides easy access to the sample space with unparalleled optical access and additional options for low working distance. The sample can be easily re-oriented with respect to the applied magnetic field. Optical windows have been optimized to reduce background Faraday effects.|
|Electrical Access||Electrical excitation of the sample can be used for magneto-transport studies and other spin based electrical measurements.||The base panels of the Cryostation may be used to add many low frequency/DC wires in addition to coaxial wires for low loss, higher frequency signal (up to ~20 GHz). The sample space is kept uncluttered through the use of specially designed low thermal heat load cryogenic ribbon cables.|
|Usability||Temperature sweeping and magnetic field sweeping are required to map the magnetic phase diagram and elucidate spin physics.||Operating without the use of liquid helium, one never has to stop data collection in the middle of an important experiment. The temperature and magnetic field can be swept easily with the Cryostation Magneto-Optic software.|
related techniques & CONFIGURATIONS
The Cryostation Base Platforms offer multiple solutions for MOKE and spintronics related research. A robust family of configurable options and accessories can be combined to meet the needs of various experimental techniques, including:
- Magneto-Optical Kerr Effect
- Optical Magnetometry
- Photoluminescence, Polarization Resolved
- Spin Transport and Dynamics
- Magnetic Domain Wall Motion
- Electrical and RF Measurements
- Transmission Experiments
|Experimental Technique||Recommended Configuration||Research Spotlight|
|Longitudinal MOKE||Magneto-Optic Module||Kawakami Lab|
|Polar and Longitudinal MOKE||
|Time-Resolved (Transient) MOKE||Magneto-Optic Module||Kawakami Lab|
|High Magnetic Field (>1.0T) MOKE||High NA Narrow Castle for External Magnet Integration||Xu & Cobden Labs|
Experimental Configurations for Cryogenic (4K) Magneto-Optical Effects
Macroscopic Magnetic Domain Imaging with Longitudinal MOKE Geometry
In the longitudinal MOKE geometry a magnetic field is applied in the plane of the sample. The magnetic domains in the sample will tend to orient along the magnetic field direction. A laser light source (typically around 650nm) is passed through a polarizer to impart a pre-defined polarization. The light then passes through an objective lens where it is focused onto the sample region of interest. The incident polarized light reflects off the sample surface. The reflected signal is rotated by the interaction between the polarized probe beam and the magnetic domain structure of the sample. The magnitude of the rotation is proportional to the local magnetization. The reflected signal passes through an analyzer so that the Kerr rotation signal can be measured. The degree of Kerr rotation can be used to determine the orientation and magnitude of the local magnetic domain.
Macroscopic Magnetic Domain Imaging with Polar MOKE Geometry
In the Polar MOKE geometry, a magnetic field is applied out-of-plane with respect to the sample plane (easy-axis). This generates a magnetization that is transverse to the sample plane. To collect the maximum signal, the incident laser excitation is aligned perpendicular to the sample plane (shown to the right). As in the longitudinal geometry, the polarization of incident laser light is rotated by a small degree when it reflects off the surface of the magnetic sample. The magnitude of the Kerr rotation is related to the strength and orientation of the local magnetic domain. In the Cryostation setup, the sample is rotated by 90 degrees compared to the longitudinal geometry and a small mirror is introduced between the poles of the magnet to direct the light parallel to the applied magnetic field.
Optical Detection of Current-Driven Domain Wall Motion
Dynamics of magnetic domain wall motion driven by magnetic field pulses or current pulses can be studied using time-resolved or transient MOKE spectroscopy. For example, consider the case of magnetic domain wall motion in a magnetic nanowire structure such as those being considered for racetrack memory concepts. A domain wall is injected at a predefined location using either a current pulse or a magnetic field pulse. The MOKE signal probes the magnetization of a local spot on the nanowire with a spatial resolution of <1µm and a temporal resolution of ~150fs. If the time, t = 0, corresponds to the domain wall injection then the time delay of the probe pulse is scanned at a fixed location along the nanowire until a change in the MOKE signal is observed. The change in MOKE signal corresponds to magnetization reversal due to the movement of the magnetic domain wall. By measuring the signal onset at various locations along the nanowire racetrack, the domain wall velocity can be calculated.
Imaging Spin Dynamics and Population Lifetimes
Time resolved MOKE measurements can also be used to study lifetimes of spin populations. A pump pulse with a given polarization is used to prepare a spin population in a material of interest. The probe pulse delay time is then scanned and the strength of the MOKE signal can be used to calculate the spin population density. The lifetime of the spins can then be calculated by examining the time dependence of the spin population.
The spin dynamics of a transition metal dichalcogenide (TMD), WS2, was studied at cryogenic (< 6K) temperatures using time resolved Kerr rotation (TRKR). Comparing the TRKR signal to micro-photoluminescence the researchers demonstrated an unexpected anticorrelation between strong exciton luminescence and high spin density. This work provided new insights on the transfer of spin angular momentum from short-lived excitons to long-lived spin states of resident conduction electrons. Additional details of this work can be read here: Studying the Spin Dynamics of Monolayer WS2
- Durham Magneto Optics Ltd & Beguivin, A. Characterization of the Montana Instruments Cryostation C2 for low temperature Magneto-Optical Kerr Effect measurements using the NanoMOKE 3.
- Bushong, E. J. et al. Imaging Spin Dynamics in Monolayer WS2 by Time-Resolved Kerr Rotation Microscopy. arXiv:1602.03568 [cond-mat] (2016).
- Aivazian, G. et al. Magnetic Control of Valley Pseudospin in Monolayer WSe2. Nature Physics 11, 148–152 (2015).
- Henn, T. et al. Ultrafast supercontinuum fiber-laser based pump-probe scanning MOKE microscope for the investigation of electron spin dynamics in semiconductors at cryogenic temperatures with picosecond time and micrometer spatial resolution. Review of Scientific Instruments 84, 123903 (2013).