111 research outputs found
Tuning the4fstate occupancy of Ce in highly correlatedCeSi∕Femultilayers: An x-ray absorption spectroscopy study
No full textSpectra of x-ray absorption were measured at the L-2,L-3(2p) and M-4,M-5(3d) edges of Ce in multilayers [Ce1-xSix/Fe]xn (x between 0.1 and 0.65), with single-phase amorphous Ce1-xSix sublayers. The study uncovers the highly correlated nature of this layered system; an alpha-phase-like electronic configuration of Ce is observed, which indicates considerable hybridization between the 4f and conduction-band states. This is at variance with single alloy films Ce1-xSix which show a gamma-phase-like Ce configuration already at x=0.1. X-ray magnetic circular dichroism measured at the L-2,L-3 edges of Ce in the multilayers reveals magnetic order on the 5d electrons, induced by the interaction with Fe at the interfaces. With increasing the Si content, the strength of the 4f-conduction-band hybridization is reduced, which is reflected in a growing occupation of the Ce 4f states. Variations of the line shape and intensity of the L-2,L-3-edge dichroism spectra are very complex. The spectra not only are related to the magnetic 5d polarization in the ground state but are largely controlled by the exchange interaction between the photoexcited 5d electron and the 4f electron, which generates a spin-dependent enhancement of the radial parts of the 2p-to-5d matrix element. The strength of the 4f-5d exchange interaction can be controlled by varying the composition of the Ce1-xSix sublayers. At high Si concentration and low temperature, it induces a change in sign of the dichroic signal. We present a detailed discussion within a simple phenomenological model
Hydrogen-controlled interlayer exchange coupling in Fe/LaHx multilayers
No full textMagneto-optic Kerr magnetometry and neutron reflectometry reveal that Fe layers exhibit magnetic exchange coupling through LaHx spacer layers. Ferromagnetic and antiferromagnetic coupling is observed on multilayers of these materials depending on the thickness of the hydride layers, but without oscillatory behavior. Starting from metallic La dihydride spacer layers the effect of dissolving increasingly more hydrogen was examined. Sign and value of the coupling depend crucially on the hydrogen content x. The coupling can be inverted from antiferromagnetic to ferromagnetic and vice versa. These alterations are due to modifications of the electronic structure of the hydride. When the hydrogen absorption saturates the hydride layers become insulating and the exchange coupling is likely to disappear. In this final state the multilayers are always characterized by a very soft ferromagnetic rectangular hysteresis curve. Upon removal of the hydrogen to the initial concentration the original magnetic structure is restored. (C) 2001 Elsevier Science B.V. All rights reserved
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No full textSpin reorientation transition in Fe/CeH2 multilayers probed by soft X-ray resonant magnetic scattering
No full textThe magnetic domain configurations of Fe 3d spins in Fe/CeH2 multilayers were measured by soft X-ray resonant magnetic scattering. The interface region could be probed by setting up X-ray standing waves due to the multilayer periodicity. By resolving first- and second-order magnetic scattering contributions, we show that the latter probe directly the magneto-crystalline anisotropy which is dominated by the Fe interface layers causing a spin reorientation transition when the temperature is lowered
Thin layers of Fe, Co and Ni onV(2)O(3) (11(2)over-bar0) and V2O3 (0001): A comparison of the interfacial magnetic interactions
No full textMagnetization reversal of thin Fe, Co and Ni layers deposited on single-crystalline layers of V2O3 is shown to be governed by various contributions to the magnetic anisotropy of the ferromagnetic layers, which reflect the sensitivity of the systems to the crystallographic and magnetic moment orientation or the structural morphology of the interfaces. The complex behaviour observed is related to the rich phase diagram of the oxide, which undergoes a transition from a paramagnetic metallic to an insulating antiferromagnetic phase as the temperature is decreased to below similar to 150 K. Focussing on the anisotropy contributions that result from the exchange interaction across the interface with anti ferromagnetic V2O3, we have found a remarkable behaviour of the Fe overlayers which grow in a textured BCC(110) structure. The exchange anisotropy, probed as a bias of the ferromagnetic hysteresis loops for field-cooled samples, is absent for the interface with the (11 (2) over bar0) surface of antiferromagnetic V2O3, but exists for the interface with the V2O3 (0001) surface. This points to a very different electronic interaction at the two interfaces. The behaviour is at variance with what is observed for the Co and Ni overlayers which show an exchange anisotropy for both surfaces of the oxide. Measurements of X-ray magnetic circular dichroism reveal the presence of a magnetically dead layer in Fe at the interface with V2O3 (11 (2) over bar0). It is likely to disrupt the exchange coupling, and thus may be at the origin of the missing exchange anisotropy of Fe on this surface. The different evolution of the spin magnetic moment of Fe with the overlayer thickness suggests a different growth morphology on the two surfaces of the oxide. As a result of the interaction with Fe, the V2O3 layers acquire a small magnetic polarization at the interface at room temperature for both orientations. (C) 2005 Elsevier B.V. All rights reserved
Interface magnetism and magnetic structure of GdN∕Fe multilayers studied by x-ray magnetic circular dichroism
No full textWe have used the element specificity of x-ray magnetic circular dichroism to separate the contributions of the component layers to the magnetization and local magnetic structure of artificial nanoscale ferrimagnetic multilayers GdN/Fe. The Fe layers, by strong antiferromagnetic interlayer exchange coupling, not only magnetize a narrow interface region in paramagnetic GdN but induce long-range magnetic order in the volume of the GdN layers at temperatures considerably above the Curie temperature of the bare layers (T-C(GdN) approximate to 60 K), in support of a theoretical mean- field prediction. We propose that the effect may be related to the special electronic band structure of GdN, which shows a transition from narrow-gap semiconducting to metallic conduction at T-C(GdN). In an elevated external magnetic field the GdN- and Fe-sublayer magnetic moments adopt a canted configuration. In this state the local magnetization in each GdN layer is highly nonuniform at low temperature. The interior turns its moment into the field direction almost abruptly from one atomic layer to the next in a single block. This indicates weak magnetic coupling between the interfacial and volume magnetizations in GdN. The results are at variance with the much studied "model" multilayer system Gd/Fe, where magnetic order in the Gd-layer volume appears intrinsically only significantly below the Curie temperature of bulk Gd. Furthermore, magnetization reversal in the Gd layers in a magnetic field occurs gradually on a considerably larger length scale, only if they are sufficiently thick. (c) 2006 American Institute of Physics
Isolating the interface magnetocrystalline anisotropy contributions in magnetic multilayers
No full textThe interface magnetocrystalline anisotropy energy (MAE) in Fe/CeH2 multilayers has been site and element-specifically isolated by combining soft x-ray resonant magnetic scattering (SXRMS) with soft x-ray standing waves. Using the different temperature evolutions of the Fe and Ce SXRMS contributions, following an in-plane to out-of-plane spin reorientation, the interface Fe 3d MAE and Ce 4f single-ion anisotropy have been separated. The results demonstrate that the transition metal interface MAE dominates the spin reorientation while the rare-earth contribution becomes significant only at much lower temperatures
The fragile magnetic structures of Fe/CeH2−δ multilayers
No full textFe/CeH2-delta multilayers exhibit at room temperature evidence of interlayer exchange coupling. Subsequent Fe layers are either parallel or antiparallel to each other, depending on the Fe and CeH2-delta layer thickness. However, when both layers have thickness larger than similar to 15 Angstrom, the antiferromagnetic structure becomes fragmented into domains laterally limited to a few microns, and the magnetic structures become very fragile. Small magnetic fields of a few Oersteds acting on the samples during growth induce helimagnetic configurations which coexist with antiferromagnetic coupling. The magnetic structures can be permanently destroyed by applying magnetic fields larger than 150 Oe. (C) 2000 Elsevier Science B.V. All rights reserved
GdN thin films: Bulk and local electronic and magnetic properties
No full textA study of high-quality thin films of the ferromagnet gadolinium nitride, GdN, is reported. The films, prepared by reactive ion-beam sputtering, show good stoichiometry and the lattice parameter, Curie temperature T-C, and saturation magnetization of the bulk material. The electrical conductivity is thermally activated down to the onset of magnetic ordering where there is evidence of a transition to metallic behavior. The transition can be tuned by a magnetic field, as reflected by a giant negative magnetoresistance. The ordered 4f moment extracted from the spectra of x-ray magnetic circular dichroism at the gadolinium M-4,M-5 edges is consistent with the S-8(7/2) configuration of Gd3+; it varies with temperature as the macroscopic magnetization of the GdN layers. The experimental K-edge photoabsorption spectra of nitrogen in this compound indicate the presence of N p character of the low-lying unoccupied conduction-band states, pointing to hybridization of the N 2p and Gd (5d,6s) states. However, a comparison of the spectra with the theoretical partial density of vacant N p states shows considerable disparities that are not well understood. The exchange field generated by the Gd f electrons in the ferromagnetic phase of GdN induces a magnetic polarization of the N p band states, as can be concluded from the observation of strong magnetic circular dichroism at the K edge of nitrogen. It indicates the presence of an important spin-orbit interaction in the final N p states
multilayers across the metal-insulator transition in the hydrides: An x-ray magnetic circular dichroism study
No full textRare-earth hydrides RHx show a metal-to-insulator transition for x between 2 and 3. In an ionic picture strong Coulomb interactions between the electrons on H sites are responsible for opening up a gap of similar to2 eV between the valence bands derived from RH and H-H hybridization and a set of bands of predominantly R-metal d character. We have studied the magnetic polarization near the interfaces of Fe/RHx multilayers (R=La, Ce) across the metal-to-insulator transition in the hydrides by measurements of x-ray magnetic circular dichroism (XMCD) at the R L-2,L-3 edges. The mean Fe-induced magnetic polarization of the R 5d states is considerably reduced in the insulating phase but remains finite. We attribute this to the presence of 5d states induced into the energy gap of the insulator sublayers by Fe, as they result from recent calculations of the electronic structure of ferromagnet/insulator interfaces. Variation of the RHx sublayer thickness reveals that the 5d polarization decays exponentially away from the interface, on a length scale of about 10 A into the volume of the RHx sublayers, both in the metallic and insulating phase. To our knowledge this is the first experimental observation that metal-induced gap states evanescent into the interior of an insulator may be spin polarized. The identical decay length in both RHx phases, independent of the R element, is remarkable. The R L-2,L-3 XMCD spectra themselves reveal the complex interplay between the magnetic polarization by Fe 3d and R 5d hybridizations, and the R 4f magnetic moment. In fact, they are not only related to the magnetic 5d polarization in the ground state, but are largely controlled by the exchange interaction between the 2p core level and the spin polarized 5d band and, in the case of Ce, by the difference between the radial parts of the 2p-to-5d matrix element for the 5d majority and minority spin channels, resulting from the 4f-5d exchange interaction. It induces a drastic modification of the line shape and even a change in sign when the samples are cooled to low temperature or oriented under different angles with respect to the beam. We present a detailed discussion within a simple phenomenological model
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