  In our study, we determine the alignment of magnetic domains in a CoFeB layer
using THz radiation. We generate THz-pulses by fs-laser-pulses in magnetized
CoFeB/Pt heterostructures, based on spin currents. An LT-GaAs Auston switch
detects the radiation phase-sensitively and allows to determine the
magnetization alignment. Our scanning technique with motorized stages with step
sizes in the sub-micrometer range, allows to image two dimensional magnetic
structures. Theoretically the resolution is restricted to half of the
wavelength if focusing optics in the far-field limit are used. By applying
near-field imaging, the spatial resolution is enhanced to the single digit
micrometer range. For this purpose, spintronic emitters in diverse geometric
shapes, e.g. circles, triangles, squares, and sizes are prepared to observe the
formation of magnetization patterns. The alignment of the emitted THz radiation
can be influenced by applying unidirectional external magnetic fields. We
demonstrate how magnetic domains with opposite alignment and different shapes
divided by domain walls are created by demagnetizing the patterns using minor
loops and imaged using phase sensitive THz radiation detection. For analysis,
the data is compared to Kerr microscope images. The possibility to combine this
method with THz range spectroscopic information of magnetic texture or
antiferromagnets in direct vicinity to the spintronic emitter, makes this
detection method interesting for much wider applications probing THz excitation
in spin systems with high resolution beyond the Abbe diffraction limit, limited
solely by the laser excitation area.
