Abstract. Room-temperature ferromagnetism is observed in (110) oriented ZnO films containing 5 at % of Sc, Ti, V, Fe, Co or Ni, but not Cr, Mn or Cu ions. There are large moments, 1.9 and 0.5µ B /atom for Co-and Ti-substituted oxides, respectively. Scsubstituted ZnO shows also a moment of 0.3 µ B /Sc. Magnetization is very anisotropic, with variations of up to a factor three depending on the orientation of the applied field relative to the R-cut sapphire substrates. Results are interpreted in terms of a spin-split donor impurity band model, which can account for ferromagnetism in insulating or conducting high-k oxides with concentrations of magnetic ions that lie far below the percolation threshold. The variation of the ferromagnetism with oxygen pressure used during film growth is evidence of a link between ferromagnetism and defect concentration.PACS Numbers: 75.50.Pp; 75.30.Hx;75.30.Gw;75.70 [2][3][4][5][6] or another transition element [7][8][9][10]. The results are sensitive to the form of the sample and preparation method. Other studies found lower magnetic ordering temperatures [11][12][13][14], or no ferromagnetism at all above 3 K for any 3d dopant [15]. In the absence of an exchange mechanism which could account for a high Curie temperature at doping levels far below the percolation threshold, these reports have been received with skepticism, and the belief that the ferromagnetism must somehow be associated with clustering or incipient formation of secondary phases. But there is spectroscopic evidence that divalent cobalt does indeed substitute on the tetrahedral sites of the wurtzite structure [1,11,17], with a wide solid solubility range [15]. A search by Rode et al [4] revealed no evidence for phase segregation in Co-doped ZnO films.Nevertheless, until a clear connection between the magnetic properties and electronic structure can be shown, doubts that doped zinc oxide is truly a magnetic semiconductor will persist.We recently proposed a model for high-temperature ferromagnetism in dilute n-type

Articles you may be interested inEffect of metal-ion doping on the optical properties of nanocrystalline ZnO thin films

Conical refraction was produced by a transparent biaxial crystal of KGd(WO 4 ) 2 illuminated by a laser beam. The ring patterns at different distances from the crystal were magnified and projected onto a screen, giving rings whose diameter was 265 mm. Comparison with theory revealed all predicted geometrical and diffraction features: close to the crystal, there are two bright rings of internal conical refraction, separated by the Poggendorff dark ring; secondary diffraction rings decorate the inner bright ring; as the distance from the crystal increases, the inner bright ring condenses onto an axial spot surrounded by diffraction rings. The scales of these features were measured and agreed well with paraxial theory; this involves a single dimensionless parameter ρ 0 , defined as the radius of the rings emerging from the crystal divided by the width of the incident beam. The different features emerge clearly in the asymptotic limit ρ 0 ≫1; in these experiments, ρ 0 =60.

Articles you may be interested inEffect of structural and magnetic exchange coupling on the electronic transport of NdNiO3 films intercalated with La0.7Sr0.3MnO3 thin layersThe magnetic, transport, and structural properties of (La 0.1 Sr 0.3 ͒MnO 3 films deposited on MgO ͑001͒ are reported as a function of thickness and substrate temperature. The substrate temperature is fundamental in determining the structural properties and a deviation from the optimum temperature ͑680°C͒ leads to grain boundaries and an imperfect ͑001͒ texture. Films with different thicknesses were deposited at the optimum deposition temperature. Magnetization and resistivity measurements on these films are interpreted in terms of magnetic and electric ''dead'' layers. The electric dead layer is an insulating layer approximately 4 nm thick while the magnetic dead layer is a region of weakly coupled noncollinear spins approximately 10 nm thick at each interface.

The angular distribution of electron temperature and density in a laser-ablation plume has been studied for the first time. The electron temperature ranges from 0.1 to 0.5 eV and is only weakly dependent on the angle in the low-intensity range studied here. In contrast, the typical ion energy is about 2 orders of magnitude larger, and its angular distribution is more peaked about the target normal. The derived values of the electron density are in agreement with the measured values of ion density.PACS numbers: 79.20. Ds, 81.15.Fg Laser ablation of solids with nanosecond pulses of high intensity leads to complicated interactions of the laser beam with both the solid and the ablated material. Some of the fundamental physical features, such as the nature of the laser absorption in the vaporized material and acceleration mechanism for the ions, are not yet fully understood [1][2][3][4]. Nevertheless, the processing of solids by intense laser light and the production of thin films by pulsed laser deposition (PLD) are widely used techniques for a variety of materials [5,6].When a nanosecond laser pulse strikes a solid surface the rapid rise in temperature leads to intense evaporation of atoms and molecules from the solid. Even at relatively low intensities near the threshold for ablation ͑0.2 1.0 GW cm 22 ͒, it is observed that the ablated material is significantly ionized [7,8], and the ions in the plasma plume have energies ranging up to several hundred eV [9]. At the end of the nanosecond laser pulse, the ablated material exists as a thin layer of plasma on the target surface. Initially the expansion of the plume is primarily driven by the plasma pressure gradients [2], but there may be an additional contribution from Coulomb repulsion between the ions if there is significant net loss of the more mobile electrons [3,10]. In any case, when the plume has propagated more than a few hundred mm from the target surface, the major part of the initial thermal energy in the plasma is converted to the directed kinetic energy of the ions, which are much more massive than the electrons [11,12].The energy distribution of the ions has been measured using time-of-flight (TOF) optical spectroscopy [13,14] and ion probes [4,[7][8][9][15][16][17]. Mostly these ion probe measurements have been for the plasma flow close to the normal of the target surface. Recently, we measured the ion energy distribution for angles up to more than 60 ± , and it was observed that both the number and the average energy of the ions are strongly peaked about the target normal [8,9]. The electron component of the laser-induced plasma plume has been less widely studied, and to our knowledge only the electron temperature and density in the plasma flow perpendicular to the target surface have been reported [17][18][19][20]. This is somewhat surprising, since in PLD the temperature and density of the electrons in the plasma will have a major influence on both the gas phase chemistry, between the target and the substrate, and the surface chemistry on the gr...

For laser ablation plumes that are significantly ionized, Langmuir probes have proved to be a useful tool for measuring the plume shape, ion energy distribution, and electron temperature. Typically in laser ablation plasmas the flow velocity is supersonic, which complicates the interpretation of the current-voltage probe characteristic. In this paper we describe some recent developments on the application of Langmuir probes for laser ablation plume diagnosis. We have investigated the behavior of the probe when it is orientated perpendicular, and parallel, to the plasma flow, and show how an analytical model developed for plasma immersion ion implantation, can quantitatively describe the variation of the ion current with probe bias for the case when the plasma flow is along the probe surface. The ion signal recorded by a probe in the parallel position is proportional to the ion density and the square root of the bias voltage. It is shown that the current varies as m i −1/2 so that by comparing the ion signals from the parallel and perpendicular positions it is possible to estimate the mass of the ions detected. We have also determined the temporal variation of electron temperature. A planar probe oriented parallel to the plasma flow, where the ion current due to the plasma flow is eliminated, gives a more reliable measurement of T e ͑Ͻ0.6 eV͒. The measured T e is consistent with the measured ion current, which is dependent on T e when the time taken for an element of plasma to traverse the probe is longer than the time taken for the matrix ion sheath extraction phase.

Abstract:The manipulation of a Gaussian laser beam using conical diffraction in a high optical quality biaxial crystal of KGd(WO 4 ) 2 has been examined in detail with emphasis on the experimental techniques involved and intuitive explanations of the notable features. Two different optical arrangements were used to form the Pogendorff double-ring light pattern in the focal image plane. The formation of both diverging and non-diverging zeroth and first order Bessel beams was investigated. The various intensity distributions and polarization properties were measured and compared with the predictions of well-established theory. 187-197 (1999). ©2009 Optical Society of America

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...