In this talk, we will present our studies of the magnetic properties, in particularly, the interface magnetism in magnetic/semiconductor hybrid spintronic structures  using XMCD-based techniques. The hybrid spintronic structure is one of the most promising approaches for the development of the second generation spintronics controlling both spin and charge in a spin device. In the ferromagnetic metal based systems such as Fe/GaAs, Co/GaAs and Ni/GaAs, our XMCD studies  demonstrated that the Fe atoms on GaAs (100) are ferromagnetic with a bulk like spin moment down to submonolayer, and the orbital moments increase with the decreasing thickness to an enhancement of around 300%. Further measurements of Co and Ni show that the Co atoms are similar to that of Fe with bulk-like spin moments down to submonolayer, but the Ni atoms lost their magnetic moments on the GaAs surface. We have synthesized single crystal half metallic Fe3O4 on GaAs  and ferrimagnetic nature of the Fe3O4 has been confirmed by the XMCD measurements. We have further found a moderate Schottky barrier in this system ideal for spin-injection . It is also encouraging that the ultrathin Fe3O4 films of 3-4nm have a large magnetic moments close to that of the bulk value . Very recently, we have shown the growth of half magnetic oxides on Si and with high quality epitaxial MgO . We have also carried out detailed studies of the interface magnetic moments of various Heusler alloys on top of GaAs, where a large spin polarization is expected in the Heusler alloys. We have found that, however, the magnetic moments in the ultrathin Heusler alloy films are sharply reduced when the thicknesses were reduced down to 5-10nm. Using the elementary specific XMCD technique, we have found that the reduced spin moments of the Heusler alloy films on GaAs come from the Mn rather than the Co, but the orbital moments are not reduced.
 Y..B. Xu, E. Ahmad, J. S. Claydon, Y.X. Lu, S. S. A. Hassan, I. G. Will and B. Cantor, Journal of Magnetism and magnetic Materials, 304 (1): 69-74 SEP 1 2006, invited
 J. S. Claydon, Y.B. Xu, M. Tselepi, J.A.C. Bland, G. van der Laan, Phys. Rev. Lett., 93 (3) JUL 16, 037206-037209, 2004.
 Y. X. Lu, J. S. Claydon, Y. B. Xu, D. M. Schofield, S. M. Thompson, G. van der Laan, Phys. Rev. B, 70 (23), 233304-233307 DEC 2004
 Hassan SSA, Xu YB, Ahmad E, Lu YX, IEEE Trans on Magn.43 (6): 2875-2877 JUN 2007
 Z. C. Huang, Y. Zhai, Y. X. Lu, G. D. Li, P.K. J. Wong, Y. B. Xu, Y. X. Xu, and H. R. Zhai, Appl. Phys. Lett ., 92, (11), (2008)
 Hassan SSA and Xu YB, reported at Intermag 2009, Sacramento, CA, May 2009.
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