L. Calmels

Experimental application of sum rules for electron energy loss magnetic chiral dichroism

We present a derivation of the orbital and spin sum rules for magnetic circular dichroic spectra measured by electron energy loss spectroscopy in a transmission electron microscope. These sum rules are obtained from the differential cross section calculated for symmetric positions in the diffraction pattern. Orbital and spin magnetic moments are expressed explicitly in terms of experimental spectra and dynamical diffraction coefficients. We estimate the ratio of spin to orbital magnetic moments and discuss first experimental results for the Fe L2,3 edge.

Half-metallicity, magnetic moments, and gap states in oxygen-deficient magnetite for spintronic applications

The electronic structure near oxygen vacancies in half-metallic magnetite has been calculated using first principles methods. Oxygen vacancies are responsible for the existence of gap states occupied by majority and minority spin electrons. We discuss whether these defects modify the spin magnetic moments, the magnetization, the magnetic coupling between Fe ions, and the half-metallic behaviour of magnetite. These results, which contribute to remove stumbling blocks to magnetite-based spintronic devices, could be useful to analyze the conductivity, the magnetotransport and magnetic properties, the electron and optical spectra of actual magnetite electrodes.

Atomic site sensitivity of the energy loss magnetic chiral dichroic spectra of complex oxides

The quantitative analysis of magnetic oxide core level spectra can become complicated when the magnetic atoms are located at several nonequivalent atomic sites in the crystal. This is, for instance, the case for Fe atoms in magnetite, which are located in tetrahedral and octahedral atomic sites; in this case, the x-ray magnetic circular dichroic (XMCD) spectra recorded at the L2,3 edge of Fe contain contributions from the different nonequivalent atomic sites, which unfortunately cannot be separated. Energy loss magnetic chiral dichroic (EMCD) spectra are the transmission electron microscope analogies of the XMCD spectra. One of the important differences between these two techniques of magnetic analysis is that EMCD uses a fast electron beam instead of polarized light. The fast electrons behave like Bloch states in the sample, and the fine structure of the EMCD spectra is strongly influenced by channeling and dynamical diffraction effects. These effects can be adjusted by changing the experimental configur...

Calculated and experimental electron energy-loss spectra of La2O3, La(OH)3, and LaOF nanophases in high permittivity lanthanum-based oxide layers

Using first principles methods, the Ou2009K energy-loss near-edge structure of cubic and hexagonal La2O3, La(OH)3, and LaOF phases have been calculated. These calculations support the identification of nanocrystalline phases evidenced experimentally by electron energy-loss spectroscopy (EELS) performed in a transmission electron microscope. The nanocrystals have been observed in atomic layer deposited La2O3 thin films developed for advanced metal-oxide-semiconductor field effect transistor applications. The presence of the nanophases can be explained by the hygroscopicity and the particular reactivity with fluorine of La2O3. These calculations provide a set of EELS fingerprints which will be useful for local phase identification in La2O3-based materials.

Magnetic properties of FeCo alloys measured by energy-loss magnetic chiral dichroism

The energy loss magnetic chiral dichroism (EMCD) in a transmission electron microscope to study magnetic devices of reduced dimensions is a young and very promising technique. It relies on the study of electron energy loss spectra acquired in specific locations of the diffraction pattern. The sensitivity of the method to the magnetic properties is investigated on a series of FexCo1−x alloys. The spectra can reflect modifications of magnetic or structural properties and the iron-cobalt alloys present the advantage of having a crystal structure which remains body centered cubic on a wide composition range. A significant variation of the EMCD signal as a function of the composition of the alloy has been detected and attributed to changes in the magnetic moment.

Electron states and magnetic moments in Co/Ni(111) multilayers and overlayers

Single crystal Co/Ni(111) superlattices with perpendicular magnetic anisotropy could give access to interesting magnetic memories switched with a rather small spin-torque current. The magnetic behavior of Co/Ni(111) superlattices is not perfectly understood in terms of the electronic structure. We report the existence of minority spin quantum-well and surface states in Ni/Co/Ni(111) sandwiches and Co/Ni(111) overlayers, and we show that the number of these states depends on the Co layer thickness. In particular, we find quantum-well states with dxzu2009+u2009dyz symmetry above Fermi level that can be measured by inverse photoemission spectroscopy and states with dxyu2009+u2009dx2−y2 symmetry below the Fermi level. The spin magnetic moment increases at the interfaces or at the surface.

Modification of the Δ1 and Δ5 electron states induced by alloying effects in Fe-based alloys for magnetic tunnel junctions

We have studied the modification of the electron states with Δ1 and Δ5 symmetries, induced by alloying effects in bcc Fe-based alloys. The corresponding density of states has been calculated for disordered alloys with the code SPR-KKR, which is based on the density-functional theory and uses the Korringa–Kohn–Rostoker method and the coherent potential approximation for the description of the chemical disorder. We have studied the modification of the Δ1 and Δ5 densities of states of Fe1−xMx alloys as a function of the nature of the alloy metal M (M=V, Cr, and Co) and as a function of its chemical content x.