In oxygen-rich GaAs the optical absorption of substitutional oxygen (OAs) in the near infrared was investigated by measuring its magnetic circular dichroism (MCDA). An MCDA was discovered which originates from a paramagnetic ground state. By investigating its thermal decay kinetics and its dependence on optically induced recharging processes involving EL2 defects and by comparison with analogous local vibrational mode measurements, it could be shown that the MCDA is due to the metastable B′ charge state of the OAs centers. The B′ state is thus identified as the paramagnetic charge state of the OAs center.

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GaAs layers grown by t h e molecular beam epitaxy (MBE) method at low temperatures (200 "C) and also MBE samples grown at 300 "C highly doped with Be or Si show high concentrations of A s antisite-related defects in the optical absorption and in t h e magnetic circular dichroism of the absorption (MCDA). With optical detection of EPR it is shown that these antisite-related defects have properties similar to those of the EL2 defects except for the EL2 bleaching characteristics and the so called zero phonon line. Their spin-lattice relaxation

Using the technique of neutron spin-echo spectroscopy, we have conducted a direct measurement of the superparamagnetic relaxation of nanoscale, magnetic monodomain iron particles in the time range between 0.01 and 160 nsec and for momentum transfers q between 0.035 and 0.15 Å 21 . Using a phenomenological model which includes the effect of a particle size distribution, we are able to determine the temperature T and q dependence of the longitudinal superspin relaxation time spanning 4 orders of magnitude. We find that for T & 40 K, interparticle correlations affect the longitudinal superspin fluctuations. [S0031-9007 (98)08356-2]