M. J. Van Bael

Magnetization reversal in patterned ferromagnetic and exchange-biased nanostructures studied by neutron reflectivity (invited)

We have measured the off-specular polarized neutron reflectivity of periodic arrays of micron-sized rectangular polycrystalline ferromagnetic Co bars and exchange-biased Co∕CoO bars, which were prepared by a combination of electron-beam lithography and evaporation techniques. The intensity of the first-order off-specular neutron satellite reflection was monitored as function of the magnetic field parallel to the long edge of the bars, allowing analysis of the magnetization reversal process using the four spin-polarized scattering cross sections. The neutron data are compared with calculations based on a micromagnetic simulation. The influence of shape anisotropy on the reversal mechanism is demonstrated.

MAGNETIC PROPERTIES OF SUBMICRON CO ISLANDS AND THEIR USE AS ARTIFICIAL PINNING CENTERS

We report on the magnetic properties of elongated submicron magnetic islands and their influence on a superconducting film. The magnetic properties were studied by magnetization hysteresis loop measurements and scanning-force microscopy. In the as-grown state, the islands have a magnetic structure consisting of two antiparallel domains. This stable domain configuration has been directly visualized as a

Shapiro steps in a superconducting film with an antidot lattice

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Application of off-specular polarized neutron reflectometry to measurements on an array of mesoscopic ferromagnetic disks

-checkerboard pattern by magnetic-force microscopy. In the remanent state, after magnetic saturation along the easy axis, all islands have a single-domain structure with the magnetic moment oriented along the magnetizing field direction. Periodic lattices of these Co islands act as efficient artificial pinning arrays for the flux lines in a superconducting Pb film deposited on top of the Co islands. The influence of the magnetic state of the dots on their pinning efficiency is investigated in these films, before and after the Co dots are magnetized.

Magnetic-domain-controlled vortex pinning in a superconductor/ferromagnet bilayer

Shapiro voltage steps at voltages V_n=nV_0 (n integer) have been observed in the voltage-current characteristics of a superconducting film with a square lattice of perforating microholes (antidots)in the presence of radiofrequent radiation. These equidistant steps appear at the second matching field H_2 when the flow of the interstitial vortex lattice in the periodic potential created by the antidots and the vortices trapped by them, is in phase with the applied rf frequency. Therefore, the observation of Shapiro steps clearly reveals the presence of mobile intersitial vortices in superconducting films with regular pinning arrays. The interstitial vortices, moved by the driving current, coexist with immobile vortices strongly pinned at the antidots.

VORTEX CONFIGURATIONS IN A PB/CU MICRODOT WITH A 2 X 2 ANTIDOT CLUSTER

Using off-specular polarized neutron reflectometry with neutron spin analysis, we determined the magnetic properties of a large array of in-plane magnetized ferromagnetic Co disks. Resonant peaks are clearly observed in the off-specular reflectivity, due to the lateral periodicity of the disk array. Using polarized neutrons, the intensity of the resonant peak in the off-specular reflectivity is studied as a function of the magnetic field applied in the sample plane. Spin analysis of the reflected neutrons reveals the magnetization reversal and saturation within the disks.

Magnetization reversal in long chains of submicrometric Co dots

Vortex pinning in a type-II superconducting Pb film covering a Co/Pt multilayer with perpendicular magnetic anisotropy is investigated. Different stable magnetic-domain patterns like band and bubble domains can be created in the Co/Pt multilayer, clearly influencing the vortex pinning in the superconducting Pb layer. The most effective pinning is observed for the bubble domain state. We demonstrate that the pinning properties of the superconductor/ferromagnet bilayer can be controlled by tuning the size, density, and magnetization direction of the bubbles.

Surface enhancement of superconductivity in tin

We present a detailed study of the transport properties of a superconducting Pb/Cu microdot with a 2x2 antidot cluster. The superconducting-normal (S/N) phase boundary, critical currents and current-voltage characteristics of this structure have been measured. The S/N phase boundary as a function of field B (T_c(B)) reveals an oscillatory structure caused by the limited number of possible vortex configurations which can be realized in these small clusters of pinning centres (antidots). We have analyzed the stability of these configurations and discussed the possible dissipation mechanisms using the critical current (J_c(B)) and voltage-current (V(I)) characteristics data. A comparison of the experimental data of T_c(B) and J_c(B) with calculations in the London limit of the Ginzburg-Landau theory confirms that vortices can indeed be pinned by the antidots forming a cluster and that the ground-state configurations of the vortices are noticeably modified by sending current through the structure. The possibility of generating phase-slips as well as motion of the vortices in the 2x2 antidot cluster has also been discussed.

Roughness in Nb/Cu multilayers determined by x‐ray diffraction and atomic force microscopy

Long chains of 400 nm diam Co dots prepared by combined electron-beam lithography exhibit interesting magnetotransport properties. The magnetoresistance of the chains of dots is markedly different from single Co films, indicating a strongly modified magnetization reversal process. Magnetic force microscopy (MFM) shows that, after magnetic saturation, in the remanent state the single-domain dots are all oriented with their magnetic moment along the chain. A comparison of the magnetoresistance and the MFM reveals that the magnetization reversal occurs by coherent rotation of the magnetic moment in the single-domain dots forming the chain.

Practical limits for detection of ferromagnetism using highly sensitive magnetometry techniques

The possibility of surface enhancement of superconductivity is examined experimentally. It is shown that single crystal tin samples with cold-worked surfaces represent a superconductor with a surface-enhanced order parameter (or negative surface extrapolation length b), whose magnitude can be controlled.