O. Schulte

Oxidation of thin chromium nitride films: kinetics and morphology

Magnetron-sputtered, 150 - 500 nm thick chromium nitride films of CrN and stoichiometry were exposed to air at 500 - . Oxide scale growth and morphology and the phase transformations during oxidation were investigated by means of Rutherford backscattering spectrometry, resonant nuclear reaction analysis, scanning force microscopy (SFM) and x-ray diffraction. A combined logarithmic and parabolic scale growth was found and interpreted as due to inwards diffusion of oxygen via grain boundaries (logarithmic) and outwards lattice diffusion of Cr (parabolic). Pt-marker experiments confirmed this interpretation. The activation energies of the two processes were determined to be Q = 60(20) and . During oxidation of a transformation into CrN was observed. A linear increase in the surface roughness and surface grain size with the oxide layer thickness was found by SFM.

Characterization of magnetron-sputtered chromium and iron nitride films

Abstract Chromium and iron nitride layers (with thicknesses of 100–1000 nm) were deposited onto silicon substrates via reactive rf magnetron sputtering. Stoichiometry, phase formation and surface morphology were analyzed by a combination of different methods: Rutherford Backscattering Spectrometry, Resonant Nuclear Reaction Analysis, Conversion Electron Mossbauer Spectroscopy, Scanning Tunneling Microscopy and X-Ray Diffraction.

Magnetic and structural properties of cerium/iron multilayers

Abstract Ce/Fe multilayers with modulation lengths between 18 and 200 A were prepared by ion-beam sputtering in an UHV system and structurally characterized by X-ray diffraction at small angles and 57 Fe conversion-electron Mossbauer spectroscopy. Good periodicity and sharp concentration profiles at the interfaces are found. The magnetic properties are unusual. For Fe-layer thicknesses below 25 A, relatively independent of the Ce-layer thickness, the Curie temperature is reduced to values below 180 K. This is accompanied by a distinct decrease of the spontaneous magnetization and the appearance of hysteresis in the low-field susceptibility at low temperatures. Isothermal magnetization curves point to noncollinear magnetic order. Evidently, these phenomena are closely related to the transition in the Fe layers from the bcc crystalline to an amorphous structure occurring near 25 A according to X-ray diffraction at large angles and RHEED diagrams.

Interfacial magnetism of Ce/Fe and CeH∼2/Fe multilayers studied by 57Fe Mössbauer spectroscopy

Abstract The distribution of the magnetic hyperfine fields and the magnetic anisotropy of Ce/Fe and CeH ∼2 /Fe multilayers have been studied by Mossbauer spectroscopy between 4.2 and 300 K. The extension of the Ce-Fe interaction defining a ‘magnetic interface’ exceeds the structural extension of the interface and is more than two times larger at the Ce/Fe than at the CeH ∼2 /Fe interface. This is proposed to be a consequence of a strong hybridization between the Ce-5d and Fe-3d states that is suppressed by hydrogenation. From the relative intensity of the Mossbauer lines we show that for the CeH ∼2 /Fe system with sufficiently thin Fe layers the orientation of the spontaneous magnetization is perpendicular to the film plane at low temperatures and switches to a parallel alignment at higher temperatures in a sharp transition. In contrast, the Ce/Fe system is spontaneously magnetized in the film plane.

Influence of hybridization in the Magnetic Circular X-ray Dichroism at the Ce-M4,5 absorption edges of Ce-Fe systems

Abstract We have performed a Magnetic Circular X-ray Dichroism (XMCD) study at the Ce-M 4,5 absorption edges on some Ce-Fe systems. We find that the dichroism signal in these systems is very sensitive to the degree of hybridization of the 4 f electrons with the valence band. XMCD is able to demonstrate that the ground state of Ce in the more strongly hybridized systems results in a mixing of atomic J = 5 2 and J = 7 2 states.

Hydrogen-induced changes of the structural and magnetic properties of cerium/iron multilayers

Multilayers of Ce and Fe show a low Curie temperature and saturation magnetization below a critical thickness of the individual Fe layers where amorphous growth occurs. We have studied on a series of such multilayers with different modulation lengths prepared by ion-beam sputtering the impact of hydrogen absorption on their magnetic properties. Hydrogenation was performed during film growth either reactively by introducing hydrogen gas into the UHV chamber or by irradiation with a beam of low-energy hydrogen ions. Hydrogen is absorbed only in the Ce layers, with a concentration near CeH2. For suitable modulation lengths, the Curie temperature and saturation magnetization are considerably enhanced with respect to the non-hydrided multilayers. This is correlated with the changes in structure and the quality of the interfaces induced by hydrogenation: the irradiation process itself reduces the critical thickness for amorphous growth of Fe, and the chemical interaction of hydrogen causes a considerable sharpening of the interfaces.

Modifications of structure and magnetism of Ce/Fe multilayers induced by irradiation with low-energy Ar ions

Abstract We demonstrate the strong effect of low-energy ion irradiation during the growth process on the structural and magnetic properties of Ce/Fe multilayers prepared by ion-beam sputtering in an UHV system. The heterostructures show good periodicity and sharp concentration profiles at the interfaces. For the non-irradiated samples, Fe grows in an amorphous structure below a critical Fe-layer thickness of 24 A. In this low-thickness range, the spontaneous magnetization MS(0) and the ferromagnetic Curie temperature TC are anomalously small. Irradiation of the growing films reduces the critical thickness for amorphous growth of the Fe layers. The main consequence is the removal of the anomaly of MS(0) inherent to the amorphous Fe phase in the non-irradiated multilayers.

Structural and magnetic properties of La/Fe multilayers

The structural and magnetic properties of La/Fe multilayers were investigated by X-ray diffraction, RHEED, magnetometry and57Fe Mössbauer spectroscopy. Comparison is made with previous results obtained for Ce/Fe multilayers. Remarkably sharp interfaces are found, with roughness between 2 and 2.5 Å. The magnetic interface in the Fe sublayers resulting from the distribution of magnetic hyperfine fields distinctly exceeds the extension of the structural interface and points to a magnetic proximity effect. This is discussed in relation to a strong 3d-5d hybridization recently found in measurements of magnetic circular X-ray dichroism. Both the structural and magnetic La/Fe interface is less extended than the interface in Ce/Fe multilayers. Below a thickness of about 25 Å, the individual Fe layers grow in an amorphous structure on the La layers. In this case, Curie temperatures are below 200 K and the Fe-layer saturation magnetization is reduced up to 50%, and there is evidence of a non-collinear spin structure. It is argued that this mainly reflects the properties of pure amorphous Fe.

The fragile magnetic structures of Fe/CeH2−δ multilayers

Fe/CeH{sub 2-{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 CeH{sub 2-{delta}} layer thickness. However, when both layers have thickness larger than {approx}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.

Non-collinear spin structures in CeH2/Fe

Abstract In the present study we have investigated the interlayer coupling in CeH2/Fe multilayers by neutron reflectometry. For a wide range of interlayer thicknesses we observe an exchange coupling of the iron layers through the nonmagnetic CeH2 spacer. In addition to a ferro- and antiferromagnetic alignment of the moments the neutron spectra give evidence of a long-periodic magnetic structure which is identified to be an antiferromagnetic fan-like arrangement of the magnetization vectors in adjacent Fe layers.