Philippe Lecoeur

Strain-controlled magnetic domain wall propagation in hybrid piezoelectric/ferromagnetic structures

The control of magnetic order in nanoscale devices underpins many proposals for integrating spintronics concepts into conventional electronics. A key challenge lies in finding an energy-efficient means of control, as power dissipation remains an important factor limiting future miniaturization of integrated circuits. One promising approach involves magnetoelectric coupling in magnetostrictive/piezoelectric systems, where induced strains can bear directly on the magnetic anisotropy. While such processes have been demonstrated in several multiferroic heterostructures, the incorporation of such complex materials into practical geometries has been lacking. Here we demonstrate the possibility of generating sizeable anisotropy changes, through induced strains driven by applied electric fields, in hybrid piezoelectric/spin-valve nanowires. By combining magneto-optical Kerr effect and magnetoresistance measurements, we show that domain wall propagation fields can be doubled under locally applied strains. These results highlight the prospect of constructing low-power domain wall gates for magnetic logic devices.

Wavelength dependence of Pockels effect in strained silicon waveguides

We investigate the influence of the wavelength, within the 1.3μm-1.63μm range, on the second-order optical nonlinearity in silicon waveguides strained by a silicon nitride (Si₃N ₄) overlayer. The effective second-order optical susceptibility χxxy(2)¯ evolutions have been determined for 3 different waveguide widths 385 nm, 435 nm and 465 nm and it showed higher values for longer wavelengths and narrower waveguides. For wWG = 385 nm and λ = 1630 nm, we demonstrated χxxy(2)¯ as high as 336 ± 30 pm/V. An explanation based on the strain distribution within the waveguide and its overlap with optical mode is then given to justify the obtained results.

Low electrical resistivity in thin and ultrathin copper layers grown by high power impulse magnetron sputtering

Ultrathin copper (Cu) layers are in continuous demand in several areas, such as within microelectronics and space, as well as in instrumentation technology requiring an electrical resistivity as low as possible. However, the performance of modern copper connections is limited by the size-dependent value of the film resistivity, which is known to increase when the layer thickness is reduced to a few tens of nanometer. In this work, the authors have successfully deposited Cu thin films from 20 to 800 nm exhibiting reduced electrical resistivity by using a high power impulse magnetron sputtering (HiPIMS) process. The electrical and microstructural properties of such films were compared to samples deposited by conventional direct current magnetron sputtering (DCMS) within the same thickness range. For films as thin as 30 nm, the electrical resistivity was reduced by ∼30% when deposited by HiPIMS compared to DCMS, being only three times larger than the copper bulk value. The HiPIMS Cu films exhibit larger grai...

Influence of angular distribution on the deposition rate of species sputtered from a multicomponent target in different configurations: Applications to mixed valence copper oxides

This work deals with calculation of the thickness distribution of films deposited by sputtering in different configurations: ‘‘on axis’’ and ‘‘off axis’’ diodes, facing target and hollow cathode, and for different polar distributions of ejected species. We demonstrate that a difference in the polar distribution may induce deviations from stoichiometry especially in the diode configuration. In the facing target and hollow cathode configurations, the deviations are strongly reduced. The theoretical calculations fit well with the experimental results that were obtained with mixed valence copper oxide sputtered films.

Phase optimisation of PMN-PT thin films deposited by Pulsed Laser Deposition on MgO substrates and Pt-coated silicon

Stability and crystalline quality of PMN-PT (65/35) thin films deposited by Pulsed Laser Deposition on MgO <001> and MgO <111> are studied by X-Ray diffraction and Raman Scattering Spectroscopy. From these investigations, the stabilization of PMN-PT films on <111> Pt-coated silicon substrate is obtained by the integration of an oxide layer (La0.66Sr0.33MnO3) on the Pt surface prior to the PMN-PT deposition. Physical properties are found to be very robust with a polarization up to 40µC/cm2 for an electric field as high as 1MV/cm.

Spin electronic magnetic sensor based on functional oxides for medical imaging

To detect magnetic signals coming from the body, in particular those produced by the electrical activity of the heart or of the brain, the development of ultrasensitive sensors is required. In this regard, magnetoresistive sensors, stemming from spin electronics, are very promising devices. For example, tunnel magnetoresistance (TMR) junctions based on MgO tunnel barrier have a high sensitivity. Nevertheless, TMR also often have high level of noise. Full spin polarized materials like manganite La0.67Sr0.33MnO3 (LSMO) are attractive alternative candidates to develop such sensors because LSMO exhibits a very low 1/f noise when grown on single crystals, and a TMR response has been observed with values up to 2000%. This kind of tunnel junctions, when combined with a high Tc superconductor loop, opens up possibilities to develop full oxide structures working at liquid nitrogen temperature and suitable for medical imaging. In this work, we investigated on LSMO-based tunnel junctions the parameters controlling the overall system performances, including not only the TMR ratio, but also the pinning of the reference layer and the noise floor. We especially focused on studying the effects of the quality of the barrier, the interface and the electrode, by playing with materials and growth conditions.

Development of a microwave capacitive method for the spectroscopy of the complex permittivity

We describe a vector network analyzer-based method to study the electromagnetic properties of nanoscale dielectrics at microwave frequencies (1 MHz–40 GHz). The complex permittivity spectrum of a given dielectric can be determined by placing it in a capacitor accessed on its both electrodes by coplanar waveguides. However, inherent propagation delays along the signal paths together with frequency-dependent effective surface of the capacitor at microwave frequencies can lead to significant distortion in the measured permittivity, which in turn can give rise to artificial frequency variations of the complex permittivity. We detail a fully analytical rigorous correction sequence with neither recourse to extrinsic loss mechanisms nor to arbitrary parasitic signal paths. We illustrate our method on 3 emblematic dielectrics: ferroelectric morphotropic lead zirconate titanate, its paraelectric pyrochlore counterpart, and strontium titanate. Permittivity spectra taken at various points along the hysteresis loop help shedding light onto the nature of the different dielectric energy loss mechanisms. Thanks to the analytical character of our method, we can discuss routes to extend it to higher frequencies and we can identify unambiguously the sources of potential artifacts.

Fast magnetization switching in GaMnAs induced by electrical fields

We study the electrical field induced magnetization reversal in a GaMnAs thin film magnet at the nanosecond scale. Quasi-static electrical fields deplete partially the magnetic material, reducing its magneto-crystalline cubic anisotropy and affecting its transport properties. We demonstrate that electrical field pulses can trigger the nucleation of domains with reversed magnetization. Pulse durations of 2.5 ns are enough to induce the nucleation, indicating precessional effects in the dynamical magnetic response. Full reversal can be obtained with 10 ns and 3 V and the assistance of magnetic fields substantially lower than the coercivity of the material in the absence of gate voltage.

Magnetic Anisotropy of Strained La 0.7 Sr 0.3 MnO 3 Thin Films Studied by MOKE

The effects of the growth conditions and the lattice strains of pulsed laser deposited (PLD) La 0.7 Sr 0.3 MnO 3 (LSMO) thin films upon the magnetic behavior have been studied using magneto-optical Kerr effect (MOKE) at room temperature. First, the structural quality of the films was investigated by XRD and the surface morphology was studied using AFM. It is shown that both surface morphology and crystallinity are optimized when the target-to-substrate distance and the oxygen pressure are chosen in agreement with a PD 3 scaling law. Secondly, hysteresis loops have been recorded along the [100], [110] and [001] directions and the easy directions of magnetization have been determined for both stress states, i.e. tension on SrTiO 3 and compression on LaAlO 3 . In tensile films, the whole plane is found to be easy, whereas, in compressive films, the easy axis should be an intermediate direction between the films plane and its normal. Moreover, tensile films deposited under optimized growth conditions exhibit the largest anisotropy coefficient (K 1eff = -6.9×10 5 erg/cm 3 ).

New tailored cuprates grown by pulsed laser deposition

Abstract This paper reports on the use of pulsed laser deposition to grow tailored infinite-layer-derived structures. The goal is to modify the carrier by changing the nature of the charge reservoir intergrown with the infinite-layer structure. The challenges associated with the development of superlattices containing CaCuO 2 are focussed upon. Both structural and electrical properties of materials with various charge reservoirs are presented.