Probing Co-doped ZnO DMS properties by transport and XMCD measurements

The ability to externally control the properties of magnetic materials would be highly desirable both from fundamental and technological point of views. In this respect, dilute magnetic semiconductor (DMS), in which a fraction of atoms of the nonmagnetic semiconductor host is replaced by magnetic ions, have recently attracted broad interest for their potential application in spintronics. In this work, we focused on transition metal Co doped Zinc oxide because room temperature ferromagnetism was both theoretically predicted [1] and experimentally observed [2]. However, the origin of such ferromagnetism, in particular whether it is a signature of a true DMS behaviour (long range magnetic interaction between the doping ions) or it arises from the formation of secondary phases, segregation or clustering is still under debate [3]. Measuring the dependence of the magnetic properties on the carrier concentration can clarify the underlying physics. The insulating-gate field-effect transistor structures are realized in ZnO/Strontium Titanate (SrTiO3) heterostructures [4] by pulsed laser deposition. These devices offer the capability to modulate the carrier density of a probe accessible (light, AFM tip, ...) channel, by more than 5 orders of magnitude (from ?1015 to ?1020 e-/cm3, estimated by Hall effect measurements under FE). The samples were characterized by resistivity, Hall effect, magnetoresistance, Seebeck effect, synchrotron X-ray adsorption spectra (XAS) and magnetic dichroism (XMD) while modulating the carrier density by electric field. Photo Emission Electron Microscopy (XPEEM) and magnetic force microscopy (MFM) showed that the samples are homogeneous and without clustering of the Co ions. A large effect of the magnetic ions, strongly dependent on the carrier concentration, was observed on the transport properties under magnetic field and anomalous Hall effect was observed at low temperature but neither ferromagnetic or antiferromagnetic signal was detectable in the XMD spectra. These results indicate that the diluted magnetic ions strongly affect the carrier scattering process. However, long range magnetic order among the ions, spontaneous or mediated by the delocalized carriers, has not been observed. [1] T. Dietl et al, Science 287 1019 (2000); K. Sato et al., Semicond. Sci. Technol. 17, 367 (2002); M.H.F. Sluiter et al., Phys. Rev. Lett. 94 187204 (2005) [2] S.W.Jung, et al. APL 80, 4561 (2002); X.C. Liu et al, APL 88 252503 (2006) [3] T. Fukumura, et al. APL 78, 958 (2001); K. Ueda, et al. APL 79, 988 (2001); S. Yin et al PRB 73 224408 (2006) [4] E. Bellingeri et al, APL, 86 012109 (2005); E. Bellingeri et al, Thin Solid Films 486, 186 (2005); E. Bellingeri et al, Superlattices and Microstructures 38, 446 (2005); E. Bellingeri et al, Proc. SPIE 6474, 64741R (2007) [5] Q.Xu, et al, Phys. Rev. B 73, 205342 (2006); Q.Xu, et al J. Appl. Phys 063918 (2007).