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Micro-focus BLS

 

This subsystem is designed for investigations of magnetization dynamics in metallic magnetic nano-structures. The main principles of the micro-focus BLS setup are similar to those of the standard BLS setup. The significant difference consists in the focusing optics and the realization of the sample positioning system. In contrast to the dielectric magnetic films, metallic layers are not transparent for light. Therefore for BLS probing of the dynamic magnetization the so-called backward light scattering geometry is used. This means that the light is focused onto the sample by means of a microscope objective, is reflected from the sample surface, and then collected by the same objective lens.

The measurement attachment used for the micro focusing is shown in the following figure. It comprises a three-axis piezo-electric positioning stage, which allows a precise positioning of a sample in the two in-plane directions and, additionally, the movement of the sample in the vertical direction for accurate focusing of the probing light. The full travel of the stage accounts 100 micrometers in the in-plane directions and 20 micrometers in the vertical direction. The positioning accuracy for all the directions is better than 10 nanometers.

Fig1

The probing light is focused onto the surface of the sample by means of a long working distance microscope objective with the magnification of 100 and the numerical aperture of 0.75 and working distance of 4 mm. In order to reach the theoretically achievable focusing of the light limited by diffraction the laser beam is additionally collimated in order to decrease its divergence. As a result, a focal spot as small as about 250 nanometers is realized for the probing light with the wavelength of 514 nanometers.

The micro-focus BLS setup allows the same type of measurements as described in the section standard BLS setup but with the ultimate spatial resolution of 250 nanometers. The layout of the micro-focus setup is shown in the following figure. Using this setup we were able, for example, to study eigenmodes of submicrometer magnetic squares with Landau domain structure and to investigate spin-wave radiation by spin-valve magnetic elements to be used in future integrated magnetic memory circuits MRAM. Recently the setup has also allowed the first direct experimental observation of spin-wave amplification by spin-polarized electric current.

Fig2


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