Marie-Bernadette Lepetit

Large Increase of the Curie Temperature by Orbital Ordering Control

Using first principle calculations we showed that the Curie temperature of manganites thin films can be increased by far more than an order of magnitude by applying appropriate strains. Our main breakthrough is that the control of the orbital ordering responsible for the spectacular T{C} increase cannot be imposed by the substrate only. Indeed, the strains, first applied by the substrate, need to be maintained over the growth direction by the alternation of the manganite layers with another appropriate material. Following these theoretical findings, we synthesized such superlattices and verified our theoretical predictions.

Analysis of the multiferroicity in the hexagonal manganite YMnO3.

We performed magnetic and ferroelectric measurements, associated with Landau theory and symmetry analysis, in order to clarify the situation of the YMnO3 system, a classical example of type I multiferroics. We found that the only magnetic group compatible with all experimental data (neutron scattering, magnetization, polarization, dielectric constant, second harmonic generation) is the P6(3) group. In this group a small ferromagnetic component along c is induced by the Dzyaloshinskii-Moriya interaction, and observed here in magnetization measurements. We found that the ferromagnetic and antiferromagnetic components can only be switched simultaneously, while the magnetic orders are functions of the polarization square and therefore insensitive to its sign.

Magnetic coupling and long-range order in the spin–chain sulfide Ba2CoS3

In this paper, we report on the magnetic properties of Ba2CoS3, a spin–chain compound recently found to be the first Co2+-containing one-dimensional sulfide to show metallic-like conductivity and negative magnetoresistance. We carried out an in-depth experimental investigation of the local structure of the cobalt atoms, and ab initio calculations of the resulting electronic configuration of Co2+. From theoretical considerations, the intra-chain coupling was predicted to be antiferromagnetic. Experimentally, several estimates of this magnetic coupling were derived by analysing the temperature dependence of the magnetic susceptibility. Magnetic and heat capacity measurements also provided evidence of a three-dimensional antiferromagnetic ordering, a feature indicative of a noticeable inter-chain coupling in this quasi-1D system.

An ab initio study of magneto-electric coupling of YMnO3

This paper proposes the direct calculation of the microscopic contributions to the magneto-electric coupling, using ab initio methods. The electrostrictive and the Dzyaloshinskii-Moriya contributions were evaluated individually. For this purpose a specific method was designed, combining density functional theory calculations and embedded fragment, explicitly correlated, quantum chemical calculations. This method allowed us to calculate the evolution of the magnetic couplings as a function of an applied electric field. We found that in YMnO3 the Dzyaloshinskii-Moriya contribution to the magneto-electric effect is three orders of magnitude weaker than the electrostrictive contribution. Strictive effects are thus dominant in the magnetic exchange evolution under an applied electric field, and by extension in the magneto-electric effect. These effects however, remain quite small, and the modifications of the magnetic excitations under an applied electric field will be difficult to observe experimentally. Another important conclusion is that it can be shown that the linear magneto-electric tensor is null due to the inter-layer symmetry operations.

Importance of t 2 g − e g hybridization in transition metal oxides

We studied the influence of the trigonal distortion of the regular octahedron along the (111) direction, found in the

An ab initio evaluation of the local effective interactions in the NaxCoO2 family

rm CoO_2

Electronic structure of the Sr0.4Ca13.6Cu24O41 incommensurate compound

layers. Under such a distortion the

How to compute the magneto-electric tensor from ab-initio calculations?

t_{2g}

Real-space fluctuations of effective exchange integrals in high-Tc cuprates

orbitals split into one

Sr14Cu24O41: a complete model for the chain sub-system

a_{1g}