R. Desfeux

Microstructure and magnetic properties of strained La0.7Sr0.3MnO3 thin films

The lattice deformation of dense strained La0.7Sr0.3MnO3 (LSMO) films is shown to control the easy direction of the magnetization. Optimized pulsed laser deposited conditions allow the fabrication of dense LSMO thin films which present an exceptional flatness with a peak–valley roughness (Rp–v) of 1 A, associated to epitaxial grains as large as 1 μm. Electron microscopy coupled with x-ray diffraction have been used to study the unit cell distortion of both tensile and compressive dense LSMO films as a function of the thickness. No relaxation of the lattice distortion imposed by substrate has been observed in the thickness range 10–60 nm. The Curie temperature is not significantly affected by the nature of the substrate: a TC of 350 K is observed for both SrTiO3 (STO) and LaAlO3 (LAO) substrates, i.e., close to the bulk material (369 K). In contrast, the easy direction of magnetization depends on the substrate. For tensile films deposited on the STO substrate, the unit cell is elongated along the film’s pl...

Substrate effect on the magnetic microstructure of La0.7Sr0.3MnO3 thin films studied by magnetic force microscopy

Colossal magnetoresistive La0.7Sr0.3MnO3 thin films have been grown under tensile strains on (100)-SrTiO3 substrates and compressive strains on (100)-LaAlO3 and (110)-NdGaO3 substrates by pulsed laser deposition. Using magnetic force microscopy (MFM), a “feather-like” magnetic pattern, characteristic of films with an in-plane magnetization, is observed for films deposited on both SrTiO3 and NdGaO3 while a “bubble” magnetic pattern, typical of films with an out-of-plane magnetization, is recorded for LaAlO3. We show that the shape of the magnetic pattern imaged by MFM is fully correlated to the easy direction of the magnetization in the film.

Crystallinity, surface morphology, and magnetic properties of La0.7Sr0.3MnO3 thin films: An approach based on the laser ablation plume range models

We report on a systematic investigation of the influence of pulsed laser deposition (PLD) parameters upon the microstructure, surface morphology, and magnetic properties of La0.7Sr0.3MnO3 (LSMO) thin films grown on (100) SrTiO3 substrates. The optimization of the physical properties requires a careful exploration of the main parameters such as the oxygen pressure P and the target-to-substrate distance D. We show that there is a strong correlation between both these parameters and an optimal distance (D=L0), which can be calculated from a PD3 scaling law (in accordance with a shock wave model). This particular value L0 corresponds to the distance for which all species are thermalized in the plume. In the D–P diagram, L0 defines two distinct regions for the morphology and the microstructure: (i) when D 1 μm) morphology. These films are perfectly epitaxial on the substrate, single-phase and not relaxed. (ii) When D≫L0, the films are colum...

Growth of CaCuO2 and SrxCa1 –xCuO2 epitaxial films on NdGaO3 substrates by pulsed laser deposition

High quality epitaxial films of the ‘infinite layer’(IL) compound SrxCa1 –xCuO2(x= 0–1) have been grown by pulsed laser deposition on NdGaO3(110) substrates, thus including the last member of this series CaCuO2, for the first time, to our knowledge. The quality of films varied with the composition: higher quality films were obtained for x= 0 and 0.1. The experimental results have been explained by taking into account the lattice match between film and substrate: a better match results in a better film quality. A very good match is obtained on NdGaO3 substrates for IL films with small or zero Sr content. Attempts to grow IL films were also carried out on SrTiO3 and LaAlO3 substrates. On SrTiO3 substrates it was not possible to grow IL films in the absence of Sr, while on LaAlO3 substrates no IL films could be grown at all. In the case of SrTiO3, the lattice match is good only for Sr-rich films, while in the case of LaAlO3, no good match can be obtained for any composition. Such substrate effects are not unexpected due to the importance of the substrate for the pseudomorphic stabilization of the otherwise unstable IL structure.

NANOSCALE INVESTIGATIONS OF ELECTRICAL PROPERTIES IN RELAXOR Pb(Mg1/3Nb2/3)O3-PbTiO3 THIN FILMS DEPOSITED ON PLATINUM AND LaNiO3 ELECTRODES BY MEANS OF LOCAL PIEZOELECTRIC RESPONSE

ABSTRACT At very low temperature (450°C), (111)-oriented and polycrystalline 0.7Pb(Mg1/3Nb2/3) O3-0.3PbTiO3 (PMN-PT) thin films have been grown on platinum (Pt) and lanthium niobate (LaNiO3) bottom electrodes respectively. Macroscopic measurements reveal lower coercive fields for PMN-PT grown on LaNiO3 compared to on platinum, while the piezoelectric coefficient d 33 is greater. At the nanometer scale, local piezoelectric hysteresis loops show that the voltages required for domain switching and piezoelectric response are the highest for PMN-PT deposited on LaNiO3. The electrical results can be explained by taking into account the effects induced by both electrodes on the surface morphology and structural properties of the films.

PULSED LASER DEPOSITION OF SRCUO2/CACUO2 SUPERLATTICES

We have shown that the pulsed laser deposition (PLD) technique, with no in situ diagnostic, can be used to grow good quality superlattices based on the infinite layer compounds SrCuO2 and CaCuO2. We demonstrated that the major source of disorder in these superlattices, grown at temperatures lower than 600 °C, comes from random discrete thickness fluctuations. However the random discrete thickness fluctuations affect essentially only one unit cell. This makes the PLD technique very attractive for the growth of new artificial high‐Tc structures based on the infinite layers compounds.

Constrained ferroelectric domain orientation in (BiFeO3)m(SrTiO3)n superlattice

Ferroelectric domains were investigated using piezoresponse force microscopy in superlattices composed of multiferroic BiFeO3 and SrTiO3 layers. Compared to single BiFeO3 thin films, a reduction in the domains size and a suppression of the in-plane orientation of domains are observed in a superlattice of (BiFeO3)4(SrTiO3)8, suggesting a constrained ferroelectric domain orientation along the out-of-plane ⟨001⟩ direction. Such modification of domain size and orientation in BiFeO3-based heterostructures could play a vital role on engineering the domains and domain wall mediated functional properties necessary for device applications.

Nanoscale investigations of switching properties and piezoelectric activity in ferroelectric thin films using piezoresponse force microscopy

With respect to nanoscale ferroelectric thin films research, piezoresponse force microscopy (PFM) for domain imaging and local piezoelectric spectroscopy for switching properties and piezoelectric activity measurements require the control of experimental conditions associated to the experimental set-up. To avoid any misinterpretation of the results, stiffness of the cantilever, or frequency and amplitude of the driving AC voltage have to be carefully investigated. In this paper, optimal working conditions determined with our experimental set-up are described in order to evidence the architecture of domains, to measure the coercive voltage and to evaluate the piezoelectric activity of Pb(Zr, Ti)O3 thin films. Illustrations are carried out on highly oriented, unetched and ion beam etched tetragonal films; explanations of the behaviour of the ferroelectric material at the nanoscale level are given.

Superconducting fast microbolometers operating below their critical temperature

The bolometric nature of the optical sensitivity of YBCO (YBa2Cu3O7−δ) thin‐film microstrips in the superconducting state is demonstrated. Below the critical temperature Tc the critical current temperature dependence of the device provides a suitable temperature sensor. A theoretical thermal model that gives reliable forecasts for the sensor time constant and sensitivity over the 10–90 K temperature range has been developed. Sensitivity measurements on high‐quality YBCO films deposited on MgO are in quite good agreement with the model, showing that the observed signals are entirely bolometric. The effective heat capacity of the sensor is that of the film in the irradiated area. Owing to the high thermal conductivity of MgO, the substrate stays at the heat sink temperature and the main resistance to heat flow is the film/substrate interface. The heat capacity of the irradiated area of the film, strongly coupled to the heat sink through the thermal boundary resistance gives high‐speed bolometers behaving as...

Ferroelectric Control of Organic/Ferromagnetic Spinterface

Organic multiferroic tunnel junctions based on La0.6 Sr0.4 MnO3 /poly(vinylidene fluoride) (PVDF)/Co structures are fabricated. The tunneling magneto-resistance sign can be changed by electrically switching the ferroelectric polarization of PVDF barrier. It is demonstrated that the spin-polarization of the PVDF/Co spinterface can be actively controlled by tuning the ferroelectric polarization of PVDF. This study opens new functionality in controlling the injection of spin polarization into organic materials via the ferroelectric polarization of the barrier.