A. Fouchet

Oxide-diluted magnetic semiconductors: a review of the experimental status

Oxide-diluted magnetic semiconductors (O-DMS) have attracted a great deal of interest in recent years due to the possibility of inducing room temperature ferromagnetism. These materials are of particular interest for spintronic devices such as spin valves. This review describes the experimental status of the O-DMS including the recent results on ZnO- and TiO2-based systems.

Structural and magnetic properties of a series of low-doped Zn1−xCoxO thin films deposited from Zn and Co metal targets on (0001) Al2O3 substrates

We report on the synthesis of low doping Zn1−xCoxO (0<x<0.1) thin films on (0001)-Al2O3 substrates. The films were prepared in an oxidizing atmosphere, using the pulsed-laser deposition technique starting from Zn and Co metallic targets. We first studied the influence of the strains of ZnO and their structural properties. Second, we investigated the structural and the magnetic properties of the Zn1−xCoxO films. We show that at low doping, the lattice parameters and the magnetization of the Zn1−xCoxO films depend strongly on the Co concentration.

Investigation of laser-ablated ZnO thin films grown with Zn metal target: A structural study

High quality ZnO thin films were grown using the pulsed laser deposition technique on (0001) Al2O3 substrates in an oxidizing atmosphere, using a Zn metallic target. We varied the growth conditions such as the deposition temperature and the oxygen pressure. First, using a battery of techniques such as x-ray diffraction, Rutherford-backscattering spectroscopy, and atomic force microscopy, we evaluated the structural quality, the stress, and the degree of epitaxy of the films. Second, the relations between the deposition conditions and the structural properties that are directly related to the nature of the thin films are discussed qualitatively. Finally, a number of issues on how to get good-quality ZnO films are addressed.

Magnetic properties of the magnetophotonic crystal based on bismuth iron garnet

This article reports on the magnetism of continuous and patterned bismuth iron garnet (Bi3Fe5O12 or BIG) thin films for magnetophotonic crystal (MPC) applications. The exact knowledge of the magnetic properties is crucial for the design of fully functional MPC. BIG thin films were grown on several types of isostructural substrates by pulsed laser deposition. The growth conditions and bismuth transfer were optimized to obtain good quality magneto-optically active films compatible with nanostructuring process. MPC were successfully fabricated from BIG/GGG(001) films with low roughness and high Faraday rotation. Magnetic characteristics (magnetization, anisotropy, magnetic domains, magnetization reversal) of the continuous BIG films and MPC were extensively studied and compared to the results of the micromagnetic simulations performed for MPC with different anisotropy. The present study shows that the fabrication of the MPC structure lowers the magnetocrystalline and uniaxial in-plane anisotropies and induce...

Influence of the microstructure on the magnetism of Co-doped ZnO thin films

The prediction of ferromagnetism at room temperature in Co–ZnO thin films has generated a large interest in the community due to the possible applications. However, the results are controversial, going from ferromagnetism to nonferromagnetism, leading to a large debate about its origin (secondary phase, Co clusters or not). By carefully studying the microstructure of various Co–ZnO films, we show that the Co2+ partly substitutes the ZnO wurtzite matrix without forming Co clusters. Surprisingly, the ferromagnetism nature of the films disappears as the Co content increases. In addition, our results suggest that the observed ferromagnetism is likely associated to a large amount of defects—close to the interface and strongly depending on the growth temperature—which may explain the spreading of the results.

Structural, optical, and magnetic properties of the ferromagnetic semiconductor hematite-ilmenite Fe2-xTixO3-δ thin films on SrTiO3(001) prepared by pulsed laser deposition

Structural, electrical, and magnetic properties of epitaxial thin films of Fe1.5Ti0.5O3−δ grown by pulsed laser deposition on SrTiO3(001) have been studied. For all the investigated range of preparation conditions, the thin films are single phase. The films prepared using oxygen pressure PO2 above 3×10−7 Torr presents a R(3¯)c symmetry structure. A single phase with the R(3¯) symmetry is obtained for the substrate temperature Ts=730 °C and PO2=2.6×10−7 Torr. This sample showed a ferrimagnetic behavior with Curie temperature Tc above room temperature, with a high magnetic moment of 0.6 μB/formula at 300 K. The film containing the disordered R(3¯)c phase is weakly ferromagnetic, with a spin glass-like signature. Optical properties seem to be governed more by the oxygen vacancies induced during the growth, than by the atomic ordering of titanium.

Microstructure and self-exchange coupling in a YFeO3 film

The microstructure of a thick film of YFeO3 grown on (100)-SrTiO3 has been investigated by transmission electron microscopy. The film is divided into two different growth regions with the bottom part characterized by strong contrast variations in the high resolution transmission electron microscopy images indicating the presence of high strain fields. The top part of the film between 50 nm and the film surface shows a preferential growth of (001) YFeO3 ∥ (001) SrTiO3 columns with relaxation into a strain free state. The mosaicity of the film and the grain boundaries determine the exchange coupling between columns and results in a strong perpendicular magnetic anisotropy.

In situ optical characterization of metal-insulator transition in LaNiO3 and SrTiO3 perovskites in pulsed laser deposition chamber

In situ optical characterizations of metal-insulator (MI) transition have been developed in a pulsed laser deposition (PLD) chamber. It consists of associating a spectroscopic ellipsometer and an infrared camera. The first one is sensitive to modifications of the optical indexes and the second one to the emissivity changes. We focus onto the metallic LaNiO3 and the insulating SrTiO3, which present opposite electrical properties as a function of their oxygen content. Using a combinatorial synthesis approach during fabrication by PLD onto LaAlO3 substrate, we observe the evolution of the apparent temperature for different area corresponding to single LaNiO3 or SrTiO3 layers and heterostructures during reduction and reoxygenation process at constant temperature. Thus, the possibility to grow off-stochiometric samples by PLD or to reduce them by heating under high vacuum, associated to the ability of injecting controlled quantity of oxygen by pulsed valve, provide us an original tool in order to study the (MI) transition.

Interface chemical and electronic properties of LaAlO3/SrVO3 heterostructures

We have studied the chemical and electronic properties of LaAlO3/SrVO3 ultrathin films by combining hard x-ray photoemission spectroscopy and transport measurements. We compare single SrVO3 (SVO) ultrathin films and SrVO3 buried below a polar LaAlO3 (LAO) thin layer, both epitaxially grown on SrTiO3. While ultrathin films (4 unit cells) of SVO do show insulating behavior over the entire temperature range, the LAO/SVO interface has a resistivity minimum at 250 K. When increasing the SVO layer thickness, the minimum is observed to shift to higher temperatures, but the resistivity stays always smaller than that of comparable SVO single films. Hard x-ray photoemission spectroscopy reveals a surface or interface related V5+ component in the V 2p spectra for SVO films and LAO/SVO heterostructures, respectively, attributed to a strongly oxidized component. This chemical reconstruction is weaker in LAO/SVO heterostructures compared to single SVO films. We show that this dead layer in SVO ultrathin films has to be considered when the film thickness reaches the few unit-cells limit and propose solutions on how to prevent this detrimental effect.