We have studied the magnetic properties of cobalt clusters in a beam as functions of cluster size (N=20–200), magnetic field, and cluster temperature. The measured magnetic moments per atom increase with applied magnetic field and cluster size but decrease with increasing temperature, consistent with superparamagnetic behavior. Rapid orientation fluctuations of the true magnetic moments of these isolated particles yield the smaller, time-averaged, observed values. The true magnetic moments are (2.08±0.20)${\mathrm{\mu }}_{\mathit{B}}$ per atom and exceed the bulk value. Magnetically resolved isomers are seen for N=55–66.

The basic magnetic properties of three-dimensional nanostructured materials can be drastically different from those of a continuous film. High-resolution magnetic force microscopy studies of magnetic submicrometer-sized cobalt dots with geometrical dimensions comparable to the width of magnetic domains reveal a variety of intricate domain patterns controlled by the details of the dot geometry. By changing the thickness of the dots, the width of the geometrically constrained magnetic domains can be tuned. Concentric rings and spirals with vortex configurations have been stabilized, with particular incidence in the magnetization reversal process as observed in the ensemble-averaged hysteresis loops.

We have analyzed the magnetic domain structure in high quality molecular beam epitaxy (MBE) grown (0001)-hcp cobalt films of thicknesses varying from 10 to 500 nm for which the magnetic domain width is of the order of the film thickness. As the thickness of the film is increased, the magnetization turns from fully in plane (10 nm) to fully out of plane (50 nm) over a relatively large thickness range. Above 50 nm, the local domain imaging by magnetic force microscopy (MFM) together with overall magnetization measurements confirm the characteristic multidomain structure with perpendicular orientation predicted by Kittel. The periodicity of domains is then described by a √d law (where d is the film thickness) that leads to the determination of domain wall energies. The occurrence of magnetic bubbles and periodic stripe patterns is visualized by variable field MFM. © 1996 The American Physical Society.

Low-temperature spin-polarized scanning tunneling microscopy is employed to study spin transport across single cobalt-phthalocyanine molecules adsorbed on well-characterized magnetic nanoleads. A spin-polarized electronic resonance is identified over the center of the molecule and exploited to spatially resolve stationary spin states. These states reflect two molecular spin orientations and, as established by density functional calculations, originate from a ferromagnetic molecule-lead exchange interaction.