Z.Q. Li

Role of point defects in room-temperature ferromagnetism of Cr-doped ZnO

Polycrystalline CrxZn1−xO films and powders are prepared by cosputtering and sol-gel method, respectively. While room-temperature ferromagnetism is found in as-deposited films, the powders exhibit paramagnetism. Comparison of the structural and magnetic properties of the as-deposited, annealed, and powdered samples indicates that the interstitial zinc, together with Cr doping, plays an important role in the ferromagnetic origin of Cr:ZnO. The ferromagnetism in films can be described by bound magnetic polaron models with respect to defect-bound carriers.

Role of structural defects on ferromagnetism in amorphous Cr-doped TiO2 films

Amorphous CrxTi1−xO2 films with different Cr concentrations of 0⩽x⩽0.16 were prepared by cosputtering method at room temperature. All as-deposited samples show hysteresis behavior from 2to340K and the Curie temperatures are well above 390K. The saturation magnetization is about 3.21×10−1μB∕Cr for x=0.05 at 340K and decreases with increasing Cr dopant. After annealing at temperature above 300°C, the films crystallized into anatase structure and lost their ferromagnetic property. The results indicate that the ferromagnetism in amorphous Cr-doped TiO2 films is intrinsic and the structural defects play an important role in the ferromagnetism of Cr:TiO2 system.

Structure and magnetic properties of facing-target sputtered Co–C granular films

We studied the structure and magnetic properties of as-deposited and subsequently annealed CoxC100−x granular films fabricated by a DC facing-target magnetron sputtering system at room temperature using atomic force microscopy, x-ray diffraction (XRD), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy and a vibrating sample magnetometer. The average roughness, Rα, of the as-deposited CoxC100−x granular films is smaller than that of the Si(100) substrates. XRD and TEM analyses indicate that the as-deposited CoxC100 − x granular films are composed of ~2 nm amorphous cobalt grains dispersed in an amorphous carbon matrix, and their morphology is composition independent. The high resolution TEM image of the as-deposited Co30C70 film shows that cobalt and carbon have already separated during the deposition, even if the aggregation of cobalt is not complete. Annealing at 300–450°C causes the crystallization of amorphous cobalt followed by an increase in grain size and the graphitization of the amorphous carbon matrix. The constriction arising from the structural environment results in the coexistence of the hcp and fcc Co phases at temperatures higher than the phase transformation point of 425°C. Magnetic measurements reveal that the coercivity of the as-deposited CoxC100−x granular films decreases with the increase in cobalt concentration, and increases with decrease in film thickness. The enhanced coercivity can be attributed to the weakened intergrain interaction because of the increased percolation threshold and/or the destruction of long-range domain structures caused by the reduction in film thickness.

Structures, optical properties, and electrical transport processes of SnO2 films with oxygen deficiencies

The structures, optical and electrical transport properties of SnO2 films, fabricated by rf sputtering method at different oxygen partial pressures, were systematically investigated. It has been found that preferred growth orientation of SnO2 film is strongly related to the oxygen partial pressure during deposition, which provides an effective way to tune the surface texture of SnO2 film. All films reveal relatively high transparency in the visible range, and both the transmittance and optical band gap increase with increasing oxygen partial pressure. The temperature dependence of resisitivities was measured from 380 K down to liquid helium temperatures. At temperature above K, besides the nearest-neighbor-hopping process, thermal activation processes related to two donor levels ( and 100 meV below the conduction band minimum) of oxygen vacancies are responsible for the charge transport properties. Below K, Mott variable-range hopping conduction process governs the charge transport properties at higher temperatures, while Efros–Shklovskii (ES) variable-range-hopping conduction process dominates the transport properties at lower temperatures. Distinct crossover from Mott type to ES type variable-range-hopping conduction process at several to a few tens kelvin are observed for all SnO2 films.

Influence of Coulomb interaction on the electrical transport properties of ultrathin Al:ZnO films

We have measured the Hall coefficient RH and the electrical conductivity σ of a series of ultrathin (∼55 nm) Al:ZnO films. These films reveal mesoscopic characteristics in electrical transport properties, i.e., both RH and σ obey lnT law in a wide temperature range (from liquid-helium temperatures to ∼100 K for RH and from liquid-helium temperatures up to several tens K for σ). These observations not only disclose the fundamental physical properties of the ultrathin Al:ZnO films but also provide strong evidences for the validity of the current theories concerning the electron-electron interaction in granular metals.

Influence of oxygen partial pressure on the ferromagnetic properties of polycrystalline Cr-doped ZnO films

Polycrystalline Cr-doped ZnO films are prepared by the co-sputtering method. Diamagnetism is observed in the conductive samples deposited in pure Ar. However, ferromagnetism is found in films with the same Cr dopant prepared under different oxygen partial pressures. The magnetization shows a strong dependence on the Cr concentration and, especially, on oxygen pressure. It is found that native point defects, which can be adjusted by the oxygen partial pressure during deposition, play a crucial role in the observed magnetic behaviors. The obtained ferromagnetism can be described by the dopant-donor/acceptor hybridization model, which associates exchange interaction with shallow-bound carriers. These results may help to understand the wide range of experimentally determined magnetic moments and its changes with different metal types and concentrations prepared by different groups and methods.

Zero-Field-Cooled Magnetization and Coercivity of Itinerant Ferromagnet SrRuO3

Itinerant ferromagnetic SrRuO 3 with a Curie temperature Tc =160.7 K was prepared by a conventional solid reaction method. Its zero-field-cooled (ZFC) magnetization and field-cooled (FC) magnetization were measured over a temperature interval from 5-180 K, in a sequence of applied fields up to coercivity. The major hysteresis loops were measured over a range of temperatures, which span the ordered phase. An empirical model was proposed to analyze the ZFC behaviors by means of coercivities at different temperatures. The calculated ZFC magnetization is consistent with the measured ZFC magnetization. Different ZFC magnetization behaviors were explained by using this model. The results indicate that the various ZFC behaviors originate from the comparison between the temperature dependence of FC magnetization and that of coercivity.

Interdiffusion in low-temperature annealed amorphous CoMoN/CN compound soft-x-ray optical multilayer mirrors

Interfacial interdiffusion in dual-facing-target sputtered amorphous CoMoN/CN soft-x-ray optical multilayer mirrors has been investigated quantitatively by monitoring the enhancement of the first-order modulation peak on annealing in the temperature range of 498–548 K. Smaller negative interdiffusivity is quite noticeable by comparing with those of Co/C and CoN/CN systems, signalling that relatively stable interfaces were formed by incorporation of molybdenum. Thermodynamic calculation reveals that the increase of binding enthalpy is the main factor leading to a larger activation energy and hence the smaller interdiffusion coefficient. The results imply that it is possible to further improve the thermal stability of CoN/CN multilayers by doping with refractory metals such as molybdenum.

Antiferromagnetic-coupling-induced magnetoresistance enhancement in Fex(TiO2)1−x films

Fe-incorporated amorphous TiO2 films with different Fe volume fractions of 0.46⩽x⩽0.76 were deposited by cosputtering iron and Ti targets in an Ar+O2 mixture. X-ray diffraction and x-ray photoelectron spectroscopy analyses give a structure of nanosized Fe particles embedded in amorphous TiO2 matrix for the Fex(TiO2)1−x films. Magnetic measurements show antiferromagnetic coupling between nanoscaled Fe granules when x<0.60. The magnetoresistance of Fe0.46(TiO2)0.54 is about −7.6% at room temperature, which increases dramatically with decreasing temperature below ∼100K and reaches −29.3% at 3K. This significant enhancement of magnetoresistance can be qualitatively explained by antiferromagnetic coupling between Fe granules.

Simultaneous structural, magnetic, and electronic transition and electronic structure in La0.85Na0.15MnO3

Abstract The lattice parameters, magnetization and electronic resistivity of La 0.85 Na 0.15 MnO 3 as a function of temperature were investigated. A simultaneous structural, magnetic, and electronic transition was observed. We argued that this is the result of strong electron–lattice coupling and may be a trace of Jahn–Teller distortion. X-ray photoelectron valence-band spectra measurement at room temperature is also carried out and its results suggest the mobile charge carries had localized below T C .