Spin-polarized scanning tunneling microscopy (SP-STM) allows imaging and spectroscopic characterization of nanostructures with unsurpassed spatial resolution. We present results by SP-STM, where we investigate the correlation between structural, electronic, and magnetic properties of individual nm small Co islands with several hundred to thousands of atoms. We use external magnetic fields of up to 4 T to tune the magnetic state of both tip and sample, and we extract the corresponding change of the differential conductance of the tunnel junction.

A recent example is our measurement of magnetic hysteresis loops ofindividual Co islands on Cu(111) at 8 K. We find switching fields of up to 2.5 T for islands with roughly 8,000 atoms. The analysis of the energy barrier of magnetization reversal from measurements of the switching field suggests a crossover of the magnetization reversal mode with increasing island size around 7500 atoms from coherent rotation to domain wall formation. [PRL 108, 107206 (2012)]
We also exploit the high spatial resolution of SP-STM in magnetic fields to measure maps of the differential conductance within a single nm-small Co island. In connection with density functional theory calculations we demonstrate for the first time that the spin polarization is not homogeneous but spatially modulated within the Co island. We ascribe the spatial modulation of the spin polarization to spin-dependent electron confinement effect within the Co island. [Science 327, 843 (2010)]