Seminars Archive

Magnetic properties of nanostructured systems studied by soft X-rays absorption and scattering techniques

Abstract
The control of magnetisation in low dimensional systems has become a crucial topic since the spintronics scenario has set up in last two decades. This seminar faces two different cases within this exciting framework. First, the long debated issue on the presence of a magnetically dead layer in ferromagnetic/semiconductor (FM/SC) interfaces, potentially deteriorating the capability of efficiently injecting spin-polarized currents in the SC, is tackled by means of a new method for the intriguing case of epitaxial Fe deposited on GaAs(001). The method consists in the use of Co impurities as a magnetic marker to be accurately dispersed at n MLs from the substrate, within a 6 MLs even Fe film: with n varying from 0 to 6, the complete profile of the magnetization state can be monitored by means of x-ray magnetic circular dichroism (XMCD), which is an element-selectivity magnetometry resolving both spin and orbital momenta. The careful preparation and characterization of such sample is performed, under UHV conditions, by means of surface science techniques like LEED and XPS, and MOKE was used to check in situ the magnetisation. The presence of a ‘dead layer’ has been finally ruled out, and the layer-by-layer change of the spin and orbital magnetic moments is discussed as a function of the reduced symmetry at interface and surface, and of the measured segregation of As to the surface.[1] As a second scientific case, a magnetic array of rectangular permalloy submicrometric dots has been replicated with varying thickness (10-120 nm) by means of a X-ray lithography at LILIT beamline, and has been investigated with X-rays magnetic scattering at the L3 resonant edge of Fe, a technique somewhat twin to the magnetic dichroism in absorption. The structural analysis, typical of X-ray diffraction, has been exploited jointly to the resonantly enhanced magneto-optic effects, to record the field dependent scattered intensities at different geometries, so probing the collective magnetic behaviour during reversal, as well as the possible magnetic correlations between pairs of neighbouring dots.[2] Micromagnetic simulations (OOMMF free code) have been run to corroborate these results and an interpretative procedure for the different magnetic hysteresis recorded at higher order Bragg peaks has been traced in analogy with diffracted MOKE experiments. Finally, as a tail of the seminar, I will briefly report on the activity of the INSTM laboratory at ELETTRA, whose goal is the construction of STM instrumentations for spectroscopic investigations of magnetic-related properties of isolated molecular systems. [1] L. Giovanelli et al., Phys. Rev. B 72, 45221 (2005). [2] C. Spezzani et al., Phys. Rev. B 69, 224412 (2004).