K. Westerholt

Interfacial domain formation during magnetization reversal in exchange-biased CoO/CO bilayers

We have carried out detailed experimental studies of the exchange bias effect of a series of CoO/Co(111) textured bilayers with different Co layer thickness, using the magneto-optical Kerr effect, SQUID magnetometry, polarized neutron reflectivity, x-ray diffraction, and atomic force microscopy. All samples exhibit a pronounced asymmetry of the magnetic hysteresis at the first magnetization reversal as compared to the second reversal. Polarized neutron reflectivity measurements show that the first reversal occurs via nucleation and domain wall motion, while the second reversal is characterized by magnetization rotation. Off-specular diffuse spin-flip scattering indicates the existence of interfacial magnetic domains. All samples feature a small positive exchange bias just below the blocking temperature, followed by a dominating negative exchange bias field with decreasing temperature. For very thin Co-films the coexistence of ferromagnetic domains with parallel and perpendicular magnetization directions leads to a peculiar shape of the hysteresis with an extended plateau like region of almost zero magnetization.

Asymmetric magnetization reversal on exchange biased CoO/Co bilayers

We study magnetic hysteresis loops after field cooling of a CoO/Co bilayer by MOKE and polarized neutron reflectivity. The neutron scattering reveals that the first magnetization reversal after field cooling is dominated by domain wall movement, whereas all subsequent reversals proceed essentially by rotation of the magnetization. In addition, off-specular diffuse scattering indicates that the first magnetization reversal induces an irreversible change of the domain state in the antiferromagnet.

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...

Electrical detection of photoinduced spins both at room temperature and in remanence

We demonstrate a photodetector with ferromagnetic contacts which can electrically detect the polarization degree of incoming light using spin filtering of photoinduced spin-polarized electron currents. Our structure is a pin diode with a single GaAs quantum well as active region and a Fe∕Tb multilayer on top of a MgO tunnel barrier as n-contact where the spin-polarized electron current is filtered. The photocurrent depends on the magnetization of the contacts and on the polarization of the injected light. We prove that even in remanence and at room temperature the degree of circular polarization of the incident light can be unambiguously determined by the photocurrent intensity.

Room temperature electrical spin injection in remanence

We demonstrate electrical spin injection from ferromagnetic Fe/Tb multilayer structures with remanent perpendicular magnetization into GaAs-based light-emitting diodes at room temperature. Using a reverse-biased Schottky contact and a MgO tunnel contact, respectively, we achieve spin injection at remanence. The maximum degree of circular polarization of the emitted light is 3% at room temperature.

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.

Residual resistivity and oxygen stoichiometry in Pr2−xCexCuO4+δ single crystals

Abstract Using an improved high-temperature annealing procedure we homogeneously vary the oxygen content of Pr 2− x Ce x CuO 4+δ single crystals and change the superconducting transition temperature in small steps. Minute changes of the oxygen concentration δ give rise to a strong increase of the residual resistivity but leave the Hall and the Seebeck coefficient nearly unchanged. A strong correlation of the change of the residual resistivity and the change of T c suggests that the point-defect scattering rate on oxygen interstitials or oxygen vacancies is an important parameter determining T c in the electron-doped high- T c systems.

Pressure dependence of the superconducting transition temperature of Bi- and Tl-based high-Tc superconductors

Abstract We report low field magnetization measurements of a single crystal Bi 2 Sr 2 Ca 1 Cu 2 O 8 and polycrystalline samples of the Bi 2 Sr 2 Ca 2 Cu 3 O 10 and the Tl 2 Ba 2 Ca 2 Cu 3 O 10 -phase under applied hydrostatic pressure up to 10 kbar. The pressure shift of T c for all compounds is positive with an initial slope d T c /d p = +0.19 K/kbar for the Bi(2212) and the d T c /d p = +0.16 K/kbar for the Bi(2223)-phase. For the Tl(2223)-phase we find a very strong shift of T c with pressure with a slope of d T c /d p = +0.50 K/kbar. A comparison of the pressure shift of T c for the different Y-, Bi- and Tl-based high- T c superconductors indicates that the shift is not determined by parameters of the CuO-layers alone but depends on details of the crystal structure. We discuss several mechanisms which are important for determining the absolute value of d T c /d p .

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.

Magneto-optical Kerr effects of ferromagnetic Ni-gratings

We report measurements of the magneto-optical Kerr effect (MOKE) on an optical grating of Ni-stripes on Si(111) and a grating of Al-stripes on a Ni thin film. The analysis of the MOKE signal at different orders of diffraction n reveals a change of the Kerr signal amplitude, which is periodic in n having the same periodicity as the total intensity of the diffracted light. For the grating of Ni-stripes we observe a definite change of the shape of the magnetic hysteresis loops also varying systematically with the order of diffraction n.