  Heusler compounds exhibit a wide range of different electronic ground states
and are hence expected to be applicable as functional materials in novel
electronic and spintronic devices. Since the growth of large and defect-free
Heusler crystals is still challenging, single crystals of Fe2TiSn and Cu2MnAl
were grown by the optical floating zone technique. Two positron annihilation
techniques -Angular Correlation of Annihilation Radiation (ACAR) and Doppler
Broadening Spectroscopy (DBS)- were applied in order to study both, the
electronic structure and lattice defects. Recently, we succeeded to observe
clearly the anisotropy of the Fermi surface of Cu2MnAl, whereas the spectra of
Fe2TiSn were disturbed by foreign phases. In order to estimate the defect
concentration in different samples of Heusler compounds the positron diffusion
length was determined by DBS using a monoenergetic positron beam.
