Thin films of HfO 2 produced by pulsed-laser deposition on sapphire, yttria-stabilized zirconia, or silicon substrates show ferromagnetic magnetization curves with little hysteresis and extrapolated Curie temperatures far in excess of 400 K. The moment does not scale with film thickness, but in terms of substrate area it is typically in the range 150-400 B nm −2 . The magnetization exhibits a remarkable anisotropy, which depends on texture and substrate orientation. Pure HfO 2 powder develops a weak magnetic moment on heating in vacuum, which is eliminated on annealing in oxygen. Lattice defects are the likely source of the magnetism.

Thin films of SnO 2 prepared by pulsed-laser deposition on R-cut sapphire substrates exhibit ferromagnetic properties at room temperature when they are doped with Cr, Mn, Fe, Co, or Ni, but not with other 3d cations. Extrapolated Curie temperatures are generally in excess of 500 K. The moment of the films is roughly independent of doping level, from 0.1-15 at. %, with a value per unit substrate area of 200± 100 B nm −2 . When magnetization is expressed as a moment per 3d dopant ion, it varies from more than the spin-only value at low concentrations to less than 0.2 B /ion near the percolation threshold. Greatest values are found for iron. The magnetization of the films is highly anisotropic with values when the field is applied perpendicular to the substrate more than double the in-plane values. There is little hysteresis except at high doping levels. The oxides are degenerate n-type semiconductors with a Hall mobility of 100 cm 2 V −1 s −1 and 1.4ϫ 10 19 carriers cm −3 in a one-band model, but no anomalous Hall effect or magnetoresistance was observed at room temperature. The data are discussed in relation to ͑a͒ the donor impurity-band model of ferromagnetism in semiconductors and ͑b͒ the magnetic defect model.

We have analyzed the electron transport processes in Al/AlOx/Al junctions. The samples were produced by glow-discharge-assisted oxidation of the bottom electrode. The nonlinear I–V curves of 17 samples were measured at room temperature, being very well fitted using the Simmons’ equation with the insulating barrier thickness, barrier height, and the junction area as free parameters. An exponential growth of the area normalized electrical resistance with thickness is obtained, using just values from I–V curve simulations. The effective tunneling area corresponding to the “hot spots” can be quantified and is five orders of magnitude smaller than the physical area in the studied samples.

The electronic properties of cobalt-doped ZnO were investigated through site-selective and element-sensitive x-ray-absorption spectroscopy in the vicinity of the Co L 2,3 edge, the oxygen K edge, and at the Zn L 3 edge. The spectroscopic measurements of the ferromagnetic cobalt-doped ZnO films appear to have additional components in the O K edge x-ray-absorption spectrum not observed in the undoped films. The observed features may derive from both hybridization with unoccupied Co 3d states and also from lattice defects such as oxygen vacancies. Only minor changes in the Zn L 3 edge spectra were observed. These observations are consistent with a polaron percolation model in which the ferromagnetic coupling is mediated by shallow donor electrons trapped in oxygen vacancies and couples the Co atoms substituted on Zn sites in the hexagonal wurtzite ZnO structure. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2165916͔ Dilute magnetic semiconductors ͑DMS͒ are an especially interesting area of research because of their potential applications in spin electronics and magneto-optics. DMS based on the wide-band-gap semiconductor ZnO doped with a transition metal have been predicted theoretically to be good candidates for room-temperature ferromagnetism. 1 Subsequently Ueda et al. 2 reported ferromagnetism in Codoped ZnO. Following their work many conflicting reports have attributed the origin of ferromagnetic behavior as being due to substitution by Co atoms in the ZnO 2-6 or due to clustering of Co atoms in secondary phases that are ferromagnetic; 7 while others report no ferromagnetic behavior even though Co occupies the substitutional sites. 8 Three different mechanisms have been proposed to explain the origin of ferromagnetism in transition-metal-doped ZnO: ͑I͒ A model of ferromagnetism where there is an exchange interaction mediated by carriers in the valence or conduction band and the localized moment of the ion, 1 ͑II͒ a double exchange mechanism in which hopping of 3d electrons from one ion to the next results in ferromagnetic behavior. 9 , and ͑III͒ an impurity band model 10 where localized ionic moments create magnetic polarons in a defect-related donor impurity band. Clearly, it is desirable to find a way to distinguish between these models and to decide whether any of them is the correct model for doped ZnO systems. Hence, it is important to verify that the transition-metal dopants occupy the Zn site and that ferromagnetism can result. In light of earlier reports such as that of Wi et al. 8 substitutional cobalt may be a necessary but not sufficient condition for ferromagnetism. This realization may reconcile some of the apparently conflicting experimental reports.In this report, element specific soft-x-ray-absorption techniques were used to help address the origin of roomtemperature ferromagnetism in Co-doped ZnO grown by pulsed laser deposition ͑PLD͒. The power of x-rayabsorption spectroscopy ͑XAS͒ at the Co L 2,3 edge allows the identification of the oxidation state and site symmetry of the cobalt i...