C. Bellouard

High tunnel magnetoresistance in epitaxial Fe/MgO/Fe tunnel junctions

We report on spin-polarized tunneling in fully epitaxial Fe/MgO/Fe/Co tunnel junctions. By increasing the thickness of the insulating layer (tMgO), we have strongly enhanced the tunnel magnetoresistance. Values up to ∼100% at 80 K (∼67% at room temperature) have been observed with tMgO=2.5 nm. This tunnel magnetoresistance ratio, which is much larger than the one predicted by the Julliere’s model, can be understood in the framework of ab initio calculations.

Epitaxial MgO layer for low-resistance and coupling-free magnetic tunnel junctions

Epitaxially grown magnetic tunnel junctions MgO(100)/Fe/MgO/Fe/Co/Pd have been elaborated by molecular beam epitaxy, with insulating layer thickness down to 0.8 nm. The continuity of this layer was checked at different spatial scales by means of morphological (high resolution transmission electronic microscopy), electric (local impedance), and magnetic (magnetoresistance and hysteresis loop) measurements. These junctions show a low resistance (4 kΩ μm2), tunnel magnetoresistance up to 17%, and a very small interlayer magnetic coupling.

Static and dynamic aspects of spin tunnelling in crystalline magnetic tunnel junctions

Single-crystal magnetic tunnel junctions employing bcc (100) Fe electrodes and MgO(100) insulating barrier are elaborated by molecular beam epitaxy. The magneto-transport properties are investigated in two extreme regimes. First, for extremely small MgO thickness, we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunnelling of the minority spin interfacial resonance state. Second, for large MgO barrier thickness, the tunnel transport validates specific spin filtering effects in terms of symmetry of the electronic Bloch function and symmetry-dependent wavefunction attenuation in the single-crystal barrier. Within this framework, we present giant tunnel magnetoresistive effects at room temperature (125–160%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the spin filtering. We point out imperfect filtering effects and a strong implication of the minority surface state of Fe on the low voltage variation of tunnel magnetoresistance. The insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance and activates a resonant tunnelling mechanism via the interfacial resonance state.

Dual Spin Dynamics of Small Fe Particles

Inelastic neutron scattering has been used to study the dynamics of a dispersion of fine Fe particles (diameter ~ 20 A) on the time scale of 10-10-10-11 s. It gives the first experimental evidence of a dual dynamics in zero field corresponding to a relaxation time (longitudinal) and to a fluctuation one (semi-transverse) at low temperature, merging in an isotropic regime at high temperature. In this isotropic regime, the fluctuation time appears limited to a temperature-independent value which could be related to the anisotropy energy of the particle.

Oxygen incorporation effects in annealed epitaxial La(1−x)SrxMnO3 thin films

This paper presents our results regarding oxygen incorporation effects in epitaxial La(1-x)SrxMnO3 thin films, deposited on SrTiO3 (001) single crystal substrates, by annealing in different gas mixtures of argon and oxygen. A particular emphasis is placed on the correlation of structural properties with the magnetic properties of the films, Curie temperature, and coercive field. In this sense, we demonstrate that the evolution of the diffuse part of the ω-scans performed on the films are due to oxygen excess in the film lattice, which creates cationic vacancies within the films. Also, we show that two regimes of oxygen incorporation in the films exist, one in which the films evolve toward a single phase and oxygen stoichiometry is recovered, and a second one dominated by oxygen over-doping effects. In order to support our study, XPS measurements were performed, from which we have evaluated the Mn3+/Mn4+ ionic ratio.

Control of the magnetic anisotropy of GdFe thin films

Abstract GdFe amorphous layers have been prepared with well defined in-plane magnetic anisotropy. The best results were obtained when the sample was deposited on glass substrate kept at 90 K during the deposition process. When the substrate was kept at room temperature the anisotropy is less well defined and a perpendicular anisotropy component appears as the thickness of the layer increases. The effect of the position of the sample during the preparation is shown.

Reduced magnetic moment per atom in small Ni and Co clusters embedded in AlN

Nanoparticles of cobalt and nickel embedded in AlN matrix have been prepared by ion implantation or sputter deposition with average diameters ranging from 0.8 to 9 nm. The average magnetic moment per atom versus the average cluster diameter was measured either by x-ray magnetic circular dichroism and/or by SQUID magnetometry. The resulting values are always smaller than the bulk ones. An interpretation is given in terms of an interaction between the atoms located at the cluster surface and the AlN matrix, leading to a magnetically dead layer about 0.1 nm thick.

Spin-orbit coupling effect by minority interface resonance states in single-crystal magnetic tunnel junctions

Symmetry dependent scattering effect by minority interface resonance states (IRS) has been evidenced in full-epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs). Two types of samples with and without carbon doped bottom Fe/MgO interface were fabricated to represent two different types of IRS in the minority channel in the vicinity of the Fermi level. By analysis of the first- principles calculated local density of states (LDOS) and the temperature dependence of conductance in parallel configuration at low bias, we show that the IRS in the carbon free sample is dominated by the delta5 symmetry. This has a major contribution on the majority deltai to delta5 channel scattering and explains the enhancement of the delta5 conductance in the parallel configuration at low temperature. Furthermore, the spectral composition of the IRS in the carbon doped interface is found to be dominated by the delta1 symmetry, which is responsible for the suppression of delta5 channel in the parallel conductance.

Giant magnetoresistance study of granular Co-Fe/Ag thin films: correlation with magnetization

We have studied the giant magnetoresistance of evaporated Co65Fe35/Ag alloys and we have correlated the effect with magnetization. We have observed a maximum effect of 92% at 4.2 K under a field of 60 kOe. The room-temperature effect can be roughly evaluated to 35%. The absolute decrease rho (0)- rho (H) in resistivity plotted versus the square of magnetization shows clearly two regimes: under low fields, it presents a linear variation independent of the temperature; under high fields (M/Ms>0.7), it deviates from it with an increasing slope. This behaviour is discussed in the framework of a recent model developed for granular materials.

Antiferromagnetic phase diagram of YBa2(Cu1−xFex)3Oy

Neutron‐diffraction measurements have been performed in the YBa2(Cu1−xFex)3Oy system with 0.01≤x≤0.12 and y≂6.2. At low iron content, two magnetic transitions are observed with decreasing temperature and a noncollinear phase (AF1+AF2) at low temperature. This results from the occurrence of a very small magnetization μCu1 on the oxygen‐deficient layers. Around x=0.03, the AF1 phase disappears, μCu1 remaining very small. At higher iron content, in the AF2 phase an increase of μCu1 with increasing x is observed, together with a spin‐glass‐like freezing of the iron spins. The results suggest that charge‐transfer effects play a crucial role in the change of magnetic phases at low x values, whereas the magnetic effects of iron substitution dominate at higher concentrations.