Christophe Lefevre

One pot synthesis of monodisperse water soluble iron oxide nanocrystals with high values of the specific absorption rate

We report a highly reproducible route to synthesize iron oxide nanoparticles (IONPs) with control over size and shape and with size dispersions around 10%. By tuning the relative ratio of squalane to dibenzyl ether, which were used as solvents in the synthesis, the size of the particles could be varied from 14 to around 100 nm, while their shape evolved from cubic (for size ranges up to 35 nm) to truncated octahedra and octahedra (for sizes from 40 nm up to 100 nm). Fine tuning of the size within each of these ranges could be achieved by varying the heating ramp and the iron precursor to decanoic acid ratio. We also demonstrate direct water transfer of the as-synthesized IONPs via in situ ligand exchange with gallol polyethylene glycol molecules, the latter simply added to the crude nanocrystal mixture at 70 °C. The specific absorption rate (SAR) values measured on the water transferred IONPs, at frequencies and applied magnetic fields that are considered safe for patients, confirmed their high heating performance. Finally, this method allows the transfer of 35 nm nanocubes as individually coated and stable particles to the water phase. For the first time, the heating performance of such large IONPs has been studied. This work uncovers the possibility of using large IONPs for magnetic hyperthermia in tumor therapy.

Raman scattering of magnetoelectric gallium ferrite thin films

Gallium ferrite, Ga(2-x)Fe(x)O(3) (GFO), is a promising magnetoelectric material as it exhibits both magnetic and electric orders close to room temperature. Here, we report a temperature-dependent investigation of GFO thin films with x = 1.0 and 1.4 by using Raman scattering. Our investigation suggests the absence of a structural phase transition of both films in the investigated 90-500 K temperature range, which is similar to earlier observations on bulk samples. We note, however, the occurrence of weak anomalies in the temperature-dependent band position of some phonons, which we attribute to spin-phonon coupling as the anomalies occur close to the Néel temperature of the materials.

Optical Transitions in Magnetoelectric Ga0.6Fe1.4O3 from 0.73 to 6.45 eV

The optical properties of polycrystalline Ga0.6Fe1.4O3 bulk are determined by spectroscopic ellipsometry from 0.73 to 6.45 eV. Complex dielectric function ɛ = ɛ1 + iɛ2 spectra are obtained from the multilayer analysis. The ellipsometric data exhibit numerous optical structures, and the transition energies are accurately obtained by analyzing the second-energy derivatives of the data. The origins of the optical structures are explained in terms of Fe3+ ligand field transitions and ligand-to-metal charge transfer transitions.

Tuning the conductivity type in a room temperature magnetic oxide: Ni-doped Ga0.6Fe1.4O3 thin films

Ni-Doped thin films of the room temperature ferrimagnetic oxide Ga0.6Fe1.4O3 were deposited by pulsed laser deposition and their electronic transport and structural and magnetic properties were studied. The actual insertion of the Ni cations within the Ga0.6Fe1.4O3 structure has been checked by resonant X-ray scattering. A clear extremum is noticed for all properties for the 2% Ni doping: extrema in the crystallographic cell parameters of the films, maximum in the Curie temperature, and maximum in the electric resistivity. We also observed a change of conductivity type for this dopant concentration, from n-type for Ni contents below 2% to p-type for Ni contents above 2%. We explain this behavior by the existence of oxygen vacancies in the pulsed laser deposited Ga0.6Fe1.4O3 thin films, which results in the reduction of some of the Fe3+ into Fe2+ cations, and n-type conduction via a hopping mechanism. The insertion of Ni2+ cations first deals with the presence of oxygen vacancies and reduces the number of n-type carriers in the films, in a compensation-like mechanism. When the number of introduced Ni2+ cations dominates the number of oxygen vacancies, conductivity becomes p-type and starts to increase again. We believe that the tunability of the conduction type and magnitude in thin films of a room temperature ferrimagnetic material paves the way towards new all oxide electronic devices.

Effects of iron concentration and cationic site disorder on the optical properties of magnetoelectric gallium ferrite thin films

Room-temperature dielectric function e = e1 + ie2 spectra of magnetoelectric Ga2−xFexO3 (x = 0.9, 1.0, and 1.4) thin films are determined by spectroscopic ellipsometry (SE) as a function of Fe concentration x. The SE data are analysed by a multilayer model with a series of Tauc-Lorentz oscillators. While the threshold energies slightly decrease as x increases, the oscillator strength shows a strong composition-dependence for the major optical structure at ∼3.5 eV. The experimental data are compared to the e spectra obtained by density functional theory (DFT) calculations. Even though the overall shape of e spectra is consistent, the experimental data and calculated spectra show a clear discrepancy in the oscillators strength ratio of the two optical structures at ∼3.5 and ∼6.0 eV. The DFT calculations suggest that a significant disordering in the cationic (Ga and Fe) sites in Ga2−xFexO3 is present in thin films, which influences their optical properties. This work demonstrates a successful application of optical characterization for determining the cationic sites occupation in thin films, which in turn improves our understanding of Physics and Chemistry in Ga2−xFexO3 thin films and paves a pathway to the development of new multifunctional devices.

Determination of the cationic distribution in oxidic thin films by resonant X-ray diffraction: the magnetoelectric compound Ga2−xFexO3

The cationic distribution is decisive for both the magnetic and electric properties of complex oxides. While it can be easily determined in bulk materials using classical methods such as X-ray or neutron diffraction, difficulties arise for thin films owing to the relatively small amount of material to probe. It is shown here that a full determination of the cationic site distribution in thin films is possible through an optimized processing of resonant elastic X-ray scattering experiments. The method is illustrated using gallium ferrite Ga2−xFexO3 samples which have been the focus of an increasing number of studies this past decade. They indeed represent an alternative to the, to date, only room-temperature magnetoelectric compound BiFeO3. The methodology can be applied to determine the element distribution over the various crystallographic sites in any crystallized system.

A magnetization study of ErMn6Sn6−xGax single crystals (0.11 ≤ x ≤ 1.20)

Dc and ac magnetic measurements have been performed on single crystals of HfFe6Ge6 type ErMn6Sn6−xGax compounds prepared by the flux method. At low temperature all the compounds display easy-axis magnetization and undergo a spin reorientation transition to easy-plane magnetization below room temperature. The complete ferrimagnetic order (μsat = 4 μB fu−1) is reached at 10 K only in the easy direction. From low field magnetization measurements at 10 K, a first order magnetic process (FOMP) has been detected for all the compounds and ascribed to the competition between the higher order magnetocrystalline terms. An additional transition detected from the imaginary part of the ac susceptibility for the x = 0.85 and 1.20 compositions was explained in terms of displacement of Bloch walls. The anisotropy constants, evaluated, following the Sucksmith and Thompson method, for some compositions at room temperature, decrease at increasing Ga content.