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Research methods - MOKE
The magneto-optical Kerr effect (MOKE) provides for a convenient way to obtain the changes in magnetic moment at different fields and temperatures.
Principle
When linearly polarised light reflects from a magnetic film, its polarisation becomes elliptic (Kerr ellipticity) and the principal axis is rotated (Kerr rotation). The amount of rotation and ellipticity is proportional to a component of the magnetisation of the film. Which component is measured depends on the optical configuration used. The most commonly used configurations are:
1. Perpendicular incidence. In this case only the out-of-plane component of the magnetisation is detected (polar Kerr effect).
2. 'Grazing' incidence (typically 30 degrees) using s-polarized light. This adds a sensitivity to the component of M that lies in the film plane and in the plane of incidence (longitudinal Kerr effect). A considerable sensitivity to the out-of-plane component of the magnetisation remains. Visible light only penetrates some 20 nanometer in typical metals. Therefore, with MOKE only the magnetisation near the surface is probed. Since we are generally studying very thin films (typically tens of nm), this is not a real limitation.
Room-temperature MOKE
Longitudinal and polar configurations are used as routine tools for the magnetic characterisation of films. A dedicated room-temperature set-up provides for quick acquisition of in-plane hysteresis loops. These hysteresis loops provide information on the magnetic anisotropy present in the film, and in the case of multilayers on the coupling between the layers. In the figures below the set-up is shown, together with a typical hysteresis loop.
 is send through a polariser and reflects from a sample. The sample is fitted between magnet poles, allowing applied fields up to 0.8 T. The polarisation state of the reflected light is analysed using a combination of a Faraday rod and a polariser. The photo-diode detector is just visible in the bottom right.</p>');) |
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Room-temperature MOKE setup |
Hysterysis loops |
Low field and Susceptibility set-up
To study the temperature dependence of the magnetisation, two low-temperature set-ups are available. One of these is optimised for measurements in extremely low background fields (< 10 uT), allowing for a detailed study of magnetic phase transitions. The set-up can measure both the AC susceptibility dM/dH and (low-field) magnetisation loops as a function of temperature. The AC susceptibility is obtained by applying a small oscillating field (typically some tens of uT) to the sample, and phase-sensitively recording the magneto-optical response. This scheme results in an excellent sensitivity, allowing for measurements on few-monolayer thick magnetic films.
 is send through a polariser and reflects from a sample sitting inside an optical cryostat. The optical cryostat allows for accurate temperature control between 5 and 370 K. Also visible is a pair of Helmholtz coils, used to apply small (tens of uT) AC fields for the susceptibility measurements. With the same coils, also DC fields of up to 2 mT can be generated to measure hysterisis loops. The polarisation state of the reflected light is determined by a second, nearly crossed polariser and a photo-diode detector. This photo was taken while building the set-up. In its fully operational state, the sample is magnetically shielded by three seperate cylinders of mu-metal to minimise the effect of magnetic stray fields.</p>');) |
, the magnetization versus temperature. This plot shows M(T) for an iron-vanadium multilayer, consisting of repeats of two atoms-thick iron layer on top of a seven atoms-thick vanadium layer [Fe(2 ML)/V(7 ML)]x40. The remanent M(T) curve can be described by a power-law [(T-Tc)/T]^beta. Fitting the data, a Curie temperature Tc of 84.9 K and a critical exponent beta of 0.35 are found. The susceptibility is indirectly visible in this plot as the difference between the remanent and the 33 uT curve. Around Tc, the application of a small field induces a large change in the magnetization, indicating a high susceptibility. ');) |
Low temperature MOKE and susceptibility setup |
Magnetization vs. temperature at different fields |
Vector MOKE
Currently we are developing new instrumentation allowing measurements in a wide temperature range (2.2 - 500 K), different configurations (polar, longitudinal and transverse), different wavelengths of incoming light (spectroscopic) as well as relatively high field. This set-up will allow determination of the scaling of magnetisation with field in the vicinity of the ordering temperature as well as separation of the contribution from different elements in multilayered structures.
The new V-Moke set up is a powerful tool to perform magnetotometry and magnetooptic spectroscopy across a wide photon energy range at relatively high field ( maximum magnetic field of 600 mT) and low temperature (He cryostat-temperature dependent measurements in the range 4-500 K). It employs a lock-in technique and the modulation is performed with a photo-elastic modulator. In the center of the magnet the sample is mounted with its surface either parallel or perpendicular to the field. In such a way we are able to perform measurements in longitudinal, transverse and polar magnetooptic configuration. The simultaneously measured first harmonic (Kerr ellipticity) and second harmonic (Kerr rotation) of the detected intensity of light enable to study the magnetic behavior. It allows the determination of the scaling of magnetisation with field in the vicinity of the ordering temperature as well as separation of the contribution from different elements in multilayered structures.
The Vector Moke setup is financed by a generous grant from Knut and Alice Wallenberg Foundation.