The blocking temperature T(B) has been determined as a function of the antiferromagnetic layer thickness in the Fe3O4/CoO exchange biased system. For CoO layers thinner than 50 A, T(B) is reduced below the Néel temperature T(N) of bulk CoO (291 K), independent of crystallographic orientation or film substrate ( alpha-Al2O3, SrTiO3, and MgO). Neutron diffraction studies show that T(B) does not track the CoO ordering temperature and, hence, that this reduction in T(B) does not arise from finite-size scaling. Instead, the ordering temperature of the CoO layers is enhanced above the bulk T(N) for layer thicknesses approximately less than or equal to 100 A due to the proximity of magnetic Fe3O4 layers.

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A photoluminescence study of an (A16a)As-GaAs quantum-well-wire array directly grown by molecular-beam epitaxy on a tilted substrate is described. A strong anisotropy was observed in the ratio of the electron-light-hole-exciton peak intensity to the electron-heavy-hole-exciton peak intensity. A theory incorporating the optical selection rule for two-dimensional quantum confinement is found to agree very well with the measured data. These results constitute the first evidence of two-dimensional quantum confinement in artificial wire structures having cross-sectional dimensions in the nanometer range. P ACS numbers: 78.6S.Fa, 73.20.Dx, 78.SS.Cr Low-dimensional structures' having quantum confinement (QC) of two or three dimensions such as quantum-well wires (QWW's) and quantum-well boxes (QWB's) have in the last few years attracted much attention not only for their potential in uncovering new phenomena in solid-state physics but also for their potential device applications. Extremely high electron mobility in QWW's' and high performance of QWW or QWB lasers and modulators are expected from theoretical predictions. Recent experiments in QWB resonanttunneling devices have in fact claimed to demonstrate new structures that are attributed to a zero-dimensional system. Transport measurements in narrow wires have also demonstrated a complete quenching of the Hall eA'ect associated with the one-dimensional quantum transport. Most of these low-dimensional structures have been made by fine lithographical methods such as electronbeam lithography, focused-ion-beam implantation, impurity-induced interdiff'usion, etc. However, the lateral dimensions in such structures have been much larger than the vertical dimensions and still in the submicron-meter range, leading to relatively small separations of subband energies. In most cases, the width broadening of energy levels have been larger than the energy separations, and these lithographical techniques seem to have intrinsic difhculties. However, a new approach to control the nucleation and growth kinetics available with molecular-beam epitaxy (MBE) on vicinal substrates has made possible the direct growth of QWW superlattices. Recently, a two-dimensional band-gap modulation of tilted superlattices (TSL) was successfully demonstrated by use of MBE and organometallicvapor-phase epitaxy. ' In those structures, the lateral dimensions are in the low-nanometer range.It is currently the only technique to make QWW arrays with these dimensions, and initial measurements, as reported here, suggest that it may be an extremely powerful technique for QWW device structures.In this paper, we report a photoluminescence (PL) study on a QWW array prepared directly by MBE, and the observation of a strong optical anisotropy in PL excitation (PLE) spectra: The ratio of electron-light-holeexciton peak intensity (I~,~h) to electron-heavy-holeexciton peak intensity (I~,hh) depends strongly on the polarization orientation of the incident light with respect to the QWW direction. Relative matrix elements of ...