Surface Magnetic Microstructural Analysis using Scanning Electron Microscopy with Polarization Analysis (SEMPA)

Abstract

High resolution imaging of magnetic microstructure has important ramifications for both fundamental studies of magnetism and the technology surrounding the magnetic recording industry. In SEMPA, a focused beam of electrons excites secondary electrons on a ferromagnet's surface. The secondaries leave the solid with an electron spin polarization which is characteristic of the net spin density in the ferromagnet. This is related directly to the sample magnetization. By scanning the beam and analyzing the secondary electron spin polarization at each point, a magnetization map of the ferromagnet's surface is generated.As SEMPA is a surface sensitive, magnetic microstructural analysis technique, the environment local to the specimen must be ultra-high vacuum. A schematic of our SEMPA instrument is shown in figure 1. The probe forming electron optical column of a SEMPA system must produce small probes with high currents (> 1 nA) at long working distances ( > 10 mm). The SEMPA system may be equipped with a single, or multiple spin detectors as in figure 1. Two detectors are used for the acquisition of all three orthogonal components of the polarization vector signal. The inefficiency of all polarimeters makes SEMPA time consuming when compared to conventional SEM. The polarized secondary electrons must be extracted efficiently without introducing any deleterious effects into the focused incident electron beam. This imposes limits on incident beam energies and extraction voltages for the transport optics. Additionally, the transport optics must map the scanned spot of the secondary electrons produced under the incident rastered beam, to a (stationary) position on the spin detector such that undesirable instrumental asymmetries are not introduced.