Magnetic/semiconductor hybrid 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 this talk we will report the growth, interface magnetism and magneto-transport of several important magnetic/semiconductor hybrid spintronic structures, in particular, with half metallic magnetic oxides and Heusler alloys.  The hybrid spintronic structures integrating half-metallic magnetic oxides and Heusler alloys are particularly exciting for the second generation spintronics as a 100% spin polarisation is expected for high efficient spin injection [1]. In the metal based system, while our previous XMCD study [2]. demonstrated that the Fe atoms on GaAs (100) are ferromagnetic, our 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 for the first time single crystal half metallic Fe₃O₄ on GaAs [3], and further found a moderate Schottky barrier in this system ideal for spin-injection [4]. It is also encouraging that the ultrathin Fe₃O₄ films of 3-4nm have a large magnetic moments close to that of the bulk value [5].  Our recent pump probing measurements show that the half metallic Fe₃O₄ has a much larger damping constant than that of the metallic magnetic films. Very recently, we have shown the growth of half magnetic oxides on Si and with high quality epitaxial MgO [6]. 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. For device applications, we have fabricated a novel vertical magnetic/semiconductor spintronic device with a GaAs membrane, and a change of MR was observed, showing unambiguously the room temperature electric spin injection and detection in the hybrid spintronic structures.

1. 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
2. J. S. Claydon, Y.B. Xu, M. Tselepi, J.A.C. Bland, G. van der Laan, Phys. Rev. Lett., 93 JUL 16, 037206-037209, 2004.
3. 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
4. Hassan SSA, Xu YB, Ahmad E, Lu YX,  IEEE Trans on Magn.43 (6): 2875-2877 JUN 2007 
5.  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)
6. Hassan SSA and Xu YB, reported at Intermag 2009, Sacramento, CA, May 2009.

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