Magnetic behavior of single crystal epilayers of the diluted magnetic semiconductor Zn1-xCoxSe

SQUID magnetometry (6-100 K, 4 Tesla) and 9.3-GHz EPR studies have been carried out on thin single-crystal films as a function of Co concentration for 0.0076⩽x⩽0.094. The samples were grown by molecular beam epitaxy on GaAs(001) substrates and were characterized by RHEED. X-ray fluorescence, and several X-ray diffraction techniques. At low temperatures the magnetic behavior is dominated by the isolated paramagnetic Co²⁺ ions (S=3/2) in Zn sites. The reduction of the measured magnetization from the calculated magnetization shows that nn (and perhaps nnn) Co²⁺ pairs are antiferromagnetically coupled with an exchange constant exceeding 20 K. For higher x samples, EPR at 5 K gives an isotropic g=2.27 Co²⁺ line with an x-dependent linewidth. For x=0.0076, strong angularly dependent EPR lines are seen that are thought to arise from Co²⁺ ions in low-symmetry sites, probably in the neighborhood of dislocations. Their contribution to the magnetization also is reduced relative to isolated Co²⁺ ions