Laurent Ranno

Strain-induced magnetic anisotropy in epitaxial manganite films

Ferromagnetic manganite films (La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 (LCMO)) have been deposited on lattice-mismatched perovskite single-crystalline substrates. Strained epitaxial growth is obtained using the pulsed laser deposition (PLD) technique. A pseudomorphic growth is observed up to a critical relaxation thickness, the value of which depends on the lattice mismatch. Above this critical thickness, the films do not relax homogeneously. Characterisation of the fully strained films has been carried out and the modifications of their magnetic and transport properties are discussed in relation with changes in the MnO6 octahedra geometry.

Ultrathin epitaxial cobalt films on graphene for spintronic investigations and applications

Graphene is an attractive candidate in spintronics for a number of reasons, among which are its electric-field-controlled conductivity, its expected long spin lifetime and its two-dimensional nature. A number of recent proposals call for the development of high-quality ferromagnetic thin films in contact with graphene, whereas only thick polycrystalline or three-dimensional (nanoclusters) morphologies have been demonstrated so far. We report on the growth of flat, epitaxial ultrathin Co films on graphene using pulsed laser deposition. These display perpendicular magnetic anisotropy (PMA) in the thickness range 0.5–1 nm, in agreement with our first-principles calculations. PMA, epitaxy and ultra-small thickness bring new perspectives on graphene-based spintronic devices making use of the zero-field control of an arbitrary magnetization direction, band matching between electrodes and graphene, and interface phenomena such as the Rashba effect and electric field control of magnetism.

Magnetic behavior of Fe:Al2O3 nanocomposite films produced by pulsed laser deposition

Structured nanocomposite films consisting of five Fe layers embedded in an amorphous Al2O3 matrix (Fe:Al2O3) have been grown by sequential pulsed laser deposition. The formation of well isolated quasispherical nanocrystals is observed for samples with Fe content per layer close to 6.5×1015 atoms/cm2. Increasing the Fe content leads first to the formation of elongated nanocrystals and then to quasicontinuous layers. The evolution in the shape and size of the nanocrystals is reflected in the magnetic behavior of these systems. A crossover from a low temperature ferromagnetic regime to a high temperature superparamagnetic regime is observed at a temperature of 23 K in the samples containing isolated quasi-spherical nanocrystals. In this case, a reduced moment per Fe atom (1.4 μB/atom) with respect to the value for α-Fe (2.2 μB/atom) is estimated. This behavior is attributed to the presence of a Fe-oxide surface shell on the nanocrystals. The large values of the estimated effective magnetic anisotropy (1.4×10...

Electric-field control of domain wall nucleation and pinning in a metallic ferromagnet

The electric (E)-field control of magnetic properties opens the prospects of an alternative to magnetic field or electric current activation to control magnetization. Multilayers with perpendicular magnetic anisotropy have proven to be particularly sensitive to the influence of an E-field due to the interfacial origin of their anisotropy. In these systems, E-field effects have been recently applied to assist magnetization switching and control domain wall (DW) velocity. Here we report on two new applications of the E-field in a similar material: controlling domain wall nucleation and stopping DW propagation at the edge of the electrode.

The Skyrmion Switch: Turning Magnetic Skyrmion Bubbles on and off with an Electric Field

Nanoscale magnetic skyrmions are considered as potential information carriers for future spintronics memory and logic devices. Such applications will require the control of their local creation and annihilation, which involves so far solutions that are either energy consuming or difficult to integrate. Here we demonstrate the control of skyrmion bubbles nucleation and annihilation using electric field gating, an easily integrable and potentially energetically efficient solution. We present a detailed stability diagram of the skyrmion bubbles in a Pt/Co/oxide trilayer and show that their stability can be controlled via an applied electric field. An analytical bubble model with the Dzyaloshinskii-Moriya interaction imbedded in the domain wall energy accounts for the observed electrical skyrmion switching effect. This allows us to unveil the origin of the electrical control of skyrmions stability and to show that both magnetic dipolar interaction and the Dzyaloshinskii-Moriya interaction play an important role in the skyrmion bubble stabilization.

Magnetic properties of HfO2 thin films

We report on the magnetic and transport studies of hafnium oxide thin films grown by pulsed-laser deposition on sapphire substrates under different oxygen pressures, ranging from 10−7 to 10−1 mbar. Some physical properties of these thin films appear to depend on the oxygen pressure during growth: the film grown at low oxygen pressure (P≈10−7 mbar) has a metallic aspect and is conducting, with a positive Hall signal, while those grown under higher oxygen pressures (7 × 10−5≤P≤0.4 mbar) are insulating. However, no intrinsic ferromagnetic signal could be attributed to the HfO2 films, irrespective of the oxygen pressure during the deposition.

The influence of magnetic and electronic inhomogeneities on magnetotransmission and magnetoresistance of La0.67Sr0.33MnO3 films

The optical (absorption of light and magnetotransmission of IR radiation), magnetic, and transport properties of the epitaxial La0.67Sr0.33MnO3 films of different thickness grown by laser ablation on the (100) SrTiO3 and LaAlO3 substrates were investigated. The effect of magnetotransmission reaches 6% at the temperature of 350K while magnetoresistance reaches 7.6% at 354K in a magnetic field of 8kOe. The factors, which influence the values of magnetotransmission and magnetoresistance of manganite films, are discussed.

Electric-field effect on coercivity distributions in FePt magneto-electric devices

We have investigated the contribution of stochastic thermally activated processes to the electric-field effects on coercivity in FePt. Coercive field distributions were measured under different gate voltages in solid-state field-effect structures. For low voltages, a shift in the coercive field distribution can be observed; however, it is not larger than the width of the distribution. Higher voltages are needed to obtain the splitting from the negative (zero) voltage distribution allowing for the unambiguous characterization of the electric-field effect. A virtual unipolarity in the electric-field effect has been identified as a feature introduced by the dielectric layer that disappears upon annealing.

New model for the magnetic structure of the marokite-type oxide CaMn2O4

Abstract This paper reports a neutron powder diffraction (NPD) study of polycrystalline marokite-type oxide CaMn 2 O 4 as a function of temperature in the range 1.5–280 K, together with magnetization measurements up to 8 T and 800 K. The orthorhombic room-temperature structure (space group Pbcm ) is confirmed. Manganese atoms are located in distorted octahedral sites with four short and two long distances at respectively 1.90–1.95 and 2.35–2.44 A, in agreement with the Jahn–Teller character of Mn 3+ ions. Below T N =220 K, this compound orders antiferromagnetically. The magnetic unit cell requires a doubling of the crystallographic a -axis. The magnetic structure has been determined by Rietveld refinement and group theory considerations. The most probable spin arrangement is found to belong to the Γ 2 irreducible representation in the Pbca space group.

Emissivity measurements with an atomic force microscope

We show that functionalized micromechanical bilayer levers can be used as sensitive probes to accurately measure radiative heat flux in vacuum between two materials at the micro scale. By means of calibration to one material these measurements can be made quantitative for radiative heat flux or for either temperature or material emissivity. We discuss issues and opportunities for our method and provide ample technical details regarding its implementation and demonstrate good correspondence with the Stefan Boltzmann law. We use this system to probe the phase transition of VO2 and find that radiative heat transfer in farfield between VO2 and glass can be reversibly modulated by a factor of 5.