SQUID magnetometry and magneto-optics of epitaxial EuSe

The complicated (H,T)-magnetic phase diagram of EuSe is caused by the critical balance between nearest and next nearest neighbour exchange interaction (J(NN) = 0.119 K and J(NNN) = -0.1209 K) and leads to various spin arrangements NNSS.... NSN.... NNS, NNN... [NS denotes opposite ferromagnetic order in adjacent (111) planes]. Beside the subtle local exchange of 5d-t(2g) electrons and localized holes with neighbouring Eu-4f spins, obviously also the strain status influences the occurrence of these different phases. We investigate the magnetic ordering phenomenon in a strained 2.5 mum EuSe film on BaF2 substrate by SQUID magnetometry and magneto-optics like spectral Faraday- and Kerr-effect measurements for temperatures from 2 K up to 200 K and for magnetic fields up to 5 T. The magneto-optical probe monitors the local environment of the photoexcited electron-hole pair, called magnetic exciton, located within a ferromagnetic surrounding (photoinduced magnetic polaron), whereas the integral magnetization measured by SQUID is most sensitive to long-range magnetic ordering. In spite of the dissimilarity of measurement techniques we find an influence of the long-range magnetic order (e.g. of the NNS- or NNN-matrix) on the non-resonant Kerr reflection. The complementarity of SQUID and magneto-optical methods is stringent only in the (resonant) spectral regime, where magnetic polarons are formed.