When linearly polarised light is reflected from a magnetic film, its polarisation becomes elliptic (Kerr ellipticity) and the polarization principal axis is rotated (Kerr rotation). This rotation is proportional to a component (depending on the geometry of experiment) of the magnetisation of the film. For this reason the MOKE magnetometry is a very popular tool for the characterization of the magnetic properties of materials. With a suitable set-up the sensitivity of the MOKE apparata can be pushed to detect the signal coming from very thin films (ideally down to the atomic plane) making the technique suitable for the study of magnetic structure of thin films, interfaces and nanostructures.
One MOKE apparatus is present on the MBE Cluster growth system. In this setup it is possible to perform MOKE in the longitudinal configuration and to record hysteresis loop measurements both in vacuum and on air. In this system (figure) polarized light impinges on the sample located between the poles of the electromagnet. The reflected light passes through the analyzer optical lens, which is set close to extinction. The signal is then detected by a photodiode; the current signal finally passes through a lock in amplifier, giving the Kerr signal of the sample. The optical components are mounted on an optical table. The sample on the head of the UHV manipulator can be cooled down to 5K (He flow cryostat) and stabilized by feedback at any temperature between 5K and 500K. The azymuthal angle can be varied by 360 degrees (around an axis perpendicular to the surface) enabling the measure of in-plane magnetic anisotropy. The physical dimensions of the Ti-UHV insert chamber limits the electromagnet gap to an effective field of 0.58 T on the sample. Reducing the gap in air allows to measure off-situ samples with H fields up to 0.8 T. Two sets of lasers and detectors allow working at 635 (red) and 405 (blue) nm wavelengths.
Other MOKE systems are available also at the Surface and Nano Science Lab. Three different types of magnets are installed in the UHV chamber. One magnet is set so as the transversal MOKE measurements can be performed during the growth of epitaxial thin films with e-beam evaporators. Its maximum magnetic field is 60 mT. The second magnet is for the polar MOKE measurement, and the maximum available magnetic field is 0.3 T. The third magnet is composed of three orthogonally placed non-core coils, which allow magnetizing samples along any direction for MOKE measurements. For all configurations, the sample temperature can be varied from 200K to 650K. The laser beam is first polarized with a polarizer and then focused (100 µm) on the sample with a lens; the reflected beam intensity is then measured with a photodiode. For longitudinal and polar MOKE measurements, an analyzer is installed in front of the detector. No modulation technique is applied but a low pass filter in order to remove 100 Hz noise.
|MOKE1- MBE Cluster|
|Laser wavelength||635 nm (red) / 405 nm (blue)|
|Laser source||WorldStarTech laser diode modules|
|Sample temperature range (UHV)||from 500 K to 77 K (liquid N) or 5 K (liquid He)|
|Sample azymuthal rotation||360 degree|
|Maximum H Field (T)||0.58 (in UHV) / 0.8 (in air)|
|Minimum H-field step||0.1 mT|
|Pole face diameter||76 mm|
|Frequency filter||Mechanical chopper (up to 2 kHz) / Photo Elastic Modulator (50 kHz)|
|Detector||Thorlab PDA100A2 (red) / PDA25K-EC (blue)|
|Detector spectral range||320-1100 nm (red) / 150-550 nm (blue)|
|MOKE2- Surface Lab|
|Laser||diode laser 635nm 5mW|
|Polarizer and Analyzer (if necessary)||Glan-Thompson prism|
|Spot size||approximately 100µm|
|Sample temperature||200 K – 650 K|
|Max. Magnetic field||transversal MOKE 600 mT, polar MOKE 0.3 T, 3D MOKE 8 mT|
Piero Torelli is responsible of APE-HE endstation and MBE Cluster within CNR-IOM. He obtained his Ph.D. in physics from the University of Paris Sud (Paris XI) with a doctoral thesis on the magnetic properties of small systems. He worked as post-doc at the EU-STREP FOX Project at ESRF in Grenoble. His research activity is focused on the investigation of the magnetic properties of nanostructure and thin films. The class of material that I study comprises: metals, diluted magnetic semiconductors and oxides, multiferroics and new functional materials.
The main scientific interest is to study electronic, magnetic and geometrical structures of surface and interfaces by means of photoelectron spectroscopy, photoabsorption spectroscopy, x-ray magnetic circular dichroism, STM and STS. The electronic and geometrical structures of magnetic materials are of particular interest.
My research activities are essentially focused on multiferroic (ferroelectric/ferromagnetic) materials. From the MBE growths to characterization measurements, passing through micro or nano-patterning steps. My Ph.D consisted to observe dynamical effects in magnetic SAW devices and my post-doc will consist to observe static effects.
My research activity is mainly devoted to the study of the interplay between magnetic and electronic-structural properties in highly correlated systems on the Cluster Growth at APE beamline (buried interfaces, 2D electron gases, ferroelectric and magnetic oxides heterostructures). I do also local contact activity on APE-HE beamline.