A. Nefedov

Diffractometer for soft x-ray resonant magnetic scattering

We report on the design and construction of a new diffractometer for soft x-ray resonant magnetic scattering which has been built at the Ruhr-University Bochum. The ultrahigh vacuum-compatible diffractometer comprises a two-circle goniometer and works in horizontal scattering geometry. Rotation of the detector and sample is realized by two differentially pumped rotating platforms with motors and gears external to the vacuum. The sample environment consists of a closed-cycle He cryostat that allows heating up to 600 K and applying a magnetic field of ±2.5 kOe. All functions of the experimental setup have been extensively tested at the BESSY II Synchrotron (beamline PM3). The experimental results demonstrate the performance of the instrument.

Atmospheric pressure oxidation of Pt(111)

The oxidation of the platinum (111) single crystal surface and the formation of platinum dioxide have been studied by in situ surface x-ray diffraction and x-ray reflectivity. At an oxygen partial pressure of 500 mbar and at temperatures from 520 to 910 K, the experiments disclose the growth of two atomic layers of a bulk-like, strongly distorted α-PtO2. The epitaxial Pt oxide layer is oriented hexagon on hexagon with respect to the Pt(111) surface leading to a (8 × 8) superstructure surface unit cell. In a second set of experiments, a 100 A thick epitaxial Pt(111) film on sapphire was exposed to oxygen. At 670 K and near atmospheric oxygen pressures we find the formation of a several angstrom thick oxide layer. After annealing the sample at 720 K in a vacuum the oxide layer desorbs, recovering the 100 A thick Pt film. Subsequent oxidation at 720 K and reduction cycles lead to a slight increase in surface roughness and the formation of macroscopically visible holes in the Pt film. These results point to a Pt re-dispersion which sets in during chemical reactions at atmospheric pressures.

The new diffractometer for surface X-ray diffraction at beamline BL9 of DELTA.

The experimental endstation of the hard X-ray beamline BL9 of the Dortmund Electron Accelerator is equipped with a Huber six-circle diffractometer. It is dedicated to grazing-incidence X-ray diffraction and X-ray reflectivity experiments on solid surfaces and thin films as well as to powder diffraction measurements. A new set-up for grazing-incidence X-ray scattering of liquids has been built up using a silicon mirror to reflect the incident X-ray to the liquid surface at angles of incidence around the critical angle of total reflection of the sample. X-ray reflectivity measurements of a polymer film and grazing-incidence X-ray diffraction measurements of an epitaxically grown Gd40Y60 film, an oxidized surface of Fe-15at.%Al alloy and aqueous salt solutions are presented and discussed.

Dose dependence of ferromagnetism in Co-implanted ZnO

We have studied the structural, magnetic, and electronic properties of Co-implanted ZnO(0001) films grown on Al2O3 (112¯0) substrates for different implantation doses and over a wide temperature range. Strong room temperature ferromagnetism is observed with magnetic parameters depending on the cobalt implantation dose. A detailed analysis of the structural and magnetic properties indicates that there are two magnetic phases in Co-implanted ZnO films. One is a ferromagnetic phase due to the formation of long range ferromagnetic ordering between implanted magnetic cobalt ions in the ZnO layer and the second one is a superparamagnetic phase, which occurs due to the formation of metallic cobalt clusters in the Al2O3 substrate. Using x-ray resonant magnetic scattering, the element specific magnetization of cobalt, oxygen, and Zn was investigated. Magnetic dichroism was observed at the Co L2,3 edges as well as at the O K edge. In addition, the anomalous Hall effect is also observed, supporting the intrinsic nat...

Intrinsic room temperature ferromagnetism in Co-implanted ZnO

We report on the structural and magnetic properties of a cobalt-implanted ZnO film grown on a sapphire substrate. X-ray diffraction and transmission electron microscopy reveal the presence of a( 1 0 ¯ 1 0)-oriented hexagonal Co phase in the Al2O3 sapphire substrate, but not in the ZnO film. Co clusters, with a diameter of about 5–6 nm, form a Co rich layer in the substrate close to the ZnO/Al2O3 interface. Magnetization measurements indicate that there exist two different magnetic phases in the implanted region. One originates from the Co clusters in Al2O3, the other one belongs to a homogeneous ferromagnetic phase with a ferromagnetic Curie temperature far above room temperature. In the latter case, the ferromagnetism can be attributed to Co substitution on Zn sites in the ZnO layer. We have observed magnetic dichroism at the Co L2,3 and O K edges at room temperature as well as the multiplet structure in x-ray absorption spectra around the Co L3 edge, supporting the intrinsic nature of the observed ferromagnetism in a Co-implanted ZnO film. The magnetic moment per substituted cobalt is found to be about 2.81 µB, which is very close to the theoretical expected value of 3 µB/Co for Co 2+ in its high spin state.

High-temperature ferromagnetism in Co-implanted TiO2 rutile

We report on structural, magnetic and electronic properties of Co-implanted TiO2(1 0 0) rutile single crystals for different implantation doses. Strong ferromagnetism at room temperature and above is observed in TiO2 rutile plates after cobalt ion implantation, with magnetic parameters depending on the cobalt implantation dose. While the structural data indicate the presence of metallic cobalt clusters, the multiplet structure of the Co L3 edge in the XAS spectra provides evidence that a sizeable portion of the dopants occupy substitutional Co 2+ sites. The detailed analysis of the structural and magnetic properties indicates that there are two magnetic phases in Co-implanted TiO2 plates. One is a ferromagnetic phase due to the formation of long range ferromagnetic ordering between implanted magnetic cobalt ions in the rutile phase, and the second one is a superparamagnetic phase which originates from the formation of metallic cobalt clusters in the implanted region. Using x-ray resonant magnetic scattering, the element specific magnetizations of cobalt, oxygen and titanium in Co-implanted TiO2 single crystals are investigated. Magnetic dichroism was observed at the Co L2,3 edges as well as at the O K edge. Anomalous Hall effect measurement indicates n-type carriers in Co-implanted TiO2 rutile. The interaction mechanism, which leads to ferromagnetic ordering of substituted cobalt ions in the host matrix, is also discussed.

Spin polarization of oxygen atoms in ferromagnetic Co-doped rutile TiO2

Of central interest in the research of dilute magnetic semiconductors is the coupling mechanism leading to a ferromagnetic ground state. Using x-ray resonant magnetic scattering, we have analyzed the element specific magnetic hysteresis curves of Co, Ti, and oxygen in Co-doped TiO2 synthesized by ion implantation. Magnetic dichroism was observed at the Co L2,3 edges, as well as at the O K edge, indicative of a spin polarization of oxygen atoms in the TiO2 host matrix. The hysteretic shapes and the coercive field values measured at the Co L3 and O K edges are identical (1.9kOe at 30K).

X-ray resonant magnetic scattering study of [Co2MnGe/Au]n and [Co2MnGe/V]n multilayers

We study the structure and magnetism of the ferromagnetic half metallic Heusler compound Co2MnGe in high-quality [Co2MnGe/Au]n- and [Co2MnGe/V]n-multilayers by hard x-ray scattering and soft x-ray resonant magnetic scattering. The diffuse hard x-ray scattering reveals that in [Co2MnGe/Au]n at the interfaces correlated roughness dominates and interdiffusion is negligible, whereas in [Co2MnGe/V]n the roughness is uncorrelated and dominated by interdiffusion. An analysis of energy-dependent scans in the soft x-ray regime at the superlattice Bragg peaks allows a determination of the element-specific magnetic moment density profile within the Co2MnGe layers. We find that the magnetic moment density profile determined for Co and Mn is definitely different; moreover, it is narrower than the chemical density profile and asymmetric with respect to the growth direction. For [Co2MnGe/Au]n at room temperature a non-ferromagnetic interface layer exists with a thickness of about 0.6 nm at the bottom and 0.45 nm at the top of the Co2MnGe layers; for [Co2MnGe/V]n at the bottom and the top the corresponding thicknesses are 0.5 nm and 0.35 nm, respectively.

Structural and magnetic properties of stoichiometric epitaxial CoO ∕ Fe exchange-bias bilayers

We present a detailed study of the magnetic and structural properties of the CoO/Fe bilayers by using a combination of x-ray diffraction (crystalline structure), Rutherford backscattering (chemical composition), and SQUID magnetometry (magnetic characterization) measurements. We prepared stoichiometric and single crystalline CoO thin films by a post-deposition annealing process in ultrahigh-vacuum conditions from sputter deposited hyperstoichiometric, polycrystalline CoO films. A simultaneous increase of the CoO crystalline quality and chemical phase purity has been achieved. Subsequently, the annealed CoO layers served as a template for the growth of epitaxial Fe films. The epitaxial relation between CoO and Fe was found to be of the Nishiyama-Wassermann type, where the Fe[001] direction is parallel aligned to the CoO[1-10] direction. The magnetic properties of CoO/Fe bilayers are strongly influenced by the stoichiometry of the antiferromagnetic CoO layer. Especially the blocking temperature is sensitive to the oxygen concentration within the CoO layer. While the blocking temperature of stoichiometric, epitaxial CoO/Fe exchange-bias bilayers almost equals the Neel temperature of bulk CoO, it is strongly reduced for superstoichiometric, polycrystalline CoO.

Fine-tuning of the spin-density-wave state in Cr/V heterostructures via hydrogen uptake

We show that the magnetic state in rather thick Cr films can be finely tuned via hydrogen uptake into adjacent vanadium layers at rather low hydrogen pressures. By changing the hydrogen concentration and, hence, the electronic structure in the V layers, it is possible to affect the global properties of spin-density waves (SDWs) in Cr layers, including the SDW period and the Néel temperature. We provide direct experimental evidence that hydrogen uptake into V layers can be used to switch between incommensurate and commensurate SDW states in a reproducible way.