The physics of the growth mechanisms, characterization of epitaxial structures and device properties of GaAs and other compound semiconductors on Si are reviewed in this paper. The nontrivial problems associated with the heteroepitaxial growth schemes and methods that are generally applied in the growth of lattice mismatched and polar on nonpolar material systems are described in detail. The properties of devices fabricated in GaAs and other compound semiconductors grown on Si substrates are discussed in comparison with those grown on GaAs substrates. The advantages of GaAs and other compound semiconductors on Si, namely, the low cost, superior mechanical strength, and thermal conductivity, increased wafer area, and the possibility of monolithic integration of electronic and optical devices are also discussed.

The exchange bias (EB) effect was discovered 60 years ago by Meiklejohn and Bean. Meanwhile the EB effect has become an integral part of modern magnetism with implications for basic research and for numerous device applications. The EB effect was the first of its kind which relates to an interface effect between two different classes of materials, here between a ferromagnet and an antiferromagnet. Here we review fundamental aspects of the exchange bias effect. Contents I. Introduction 1 II. Stoner-Wohlfarth model 2 III. Discovery of the Exchange Bias effect 4 IV. Ideal model of the exchange bias: Phenomenology 5 V. The ideal Meiklejohn-Bean model: Quantitative Analysis 6 A. The sign of the exchange bias 7 B. The magnitude of the EB 8 C. The 1/t F dependence of the EB field 9 D. Coercivity and exchange bias 9 VI. Realistic Meiklejohn and Bean model 9 A. Analytical expression of the exchange bias field 11 B. Azimuthal dependence of the exchange bias field 11 C. Magnetization reversal 12 D. Rotational hysteresis 13 VII. Néel's AF domain wall -Weak coupling 13 VIII. Malozemoff Random Field Model 13 IX. Domain State Model 15 X. Mauri model 17 A. Analytical expression of exchange bias field 18 B. Azimuthal dependence of the exchange bias field 20 XI. Kim-Stamps Approach -Partial domain wall 21 XII. The Spin Glass model of exchange bias 22 A. Hysteresis loops as a function of the conversion factor f 24 B. Phase diagram of exchange bias and coercive field within the spin glass model 25 C. Azimuthal dependence of exchange bias and coercive field within the spin glass model. 25 D. Dependence of exchange bias field on the thickness of the antiferromagnetic layer 26 E. The blocking temperature for exchange bias 28

We have carried out detailed experimental studies of the exchange bias effect of a series of CoO/Co(111) textured bilayers with different Co layer thickness, using the magneto-optical Kerr effect, SQUID magnetometry, polarized neutron reflectivity, x-ray diffraction, and atomic force microscopy. All samples exhibit a pronounced asymmetry of the magnetic hysteresis at the first magnetization reversal as compared to the second reversal. Polarized neutron reflectivity measurements show that the first reversal occurs via nucleation and domain wall motion, while the second reversal is characterized by magnetization rotation. Off-specular diffuse spin-flip scattering indicates the existence of interfacial magnetic domains. All samples feature a small positive exchange bias just below the blocking temperature, followed by a dominating negative exchange bias field with decreasing temperature.

The spin density wave (SDW) magnetism of thin epitaxial Cr films has recently become the focus of interest because of its mediating role in exchange coupled superlattices. While the incommensurate SDW magnetism and the Néel temperature are well established for bulk Cr, the question arises of how these properties are altered in thin films and superlattices either due to dimensionality effects or due to proximity with the ferromagnetic or paramagnetic boundary layers. After a brief introduction to the basic properties of bulk Cr, this review provides an overview of the SDW magnetism in thin Cr films, starting with surface properties and continuing with the discussion of Cr films of various thickness. The emphasis is more on SDW order in different confined environments than on exchange coupling. The scaling of the Néel temperature with thickness, the critical thickness for the onset of SDW order, the orientation of the SDW wave vector in different environments and the enhancement of SDW order due to proximity effects are extensively discussed. Most important is the role of the interface roughness in case of contact with a ferromagnetic layer. Conflicting results obtained with different experimental techniques are critically reviewed and an interpretation of the SDW order depending on interface quality is proposed.

Employing magnetometry measurements, we have studied Co3O4 nanowires focusing on the core-shell behavior. We find two magnetic contributions, i.e., a regular antiferromagnetic and an additional irreversible one. The first contribution can be attributed to the antiferromagnetically ordered wire cores. The nature of the second one can be identified using thermoremanent and isothermoremanent magnetizaton curves as magnetic fingerprints of the irreversible magnetization. We conclude that the nanowire shell behaves like a two-dimensional diluted antiferromagnet in a field.

We have studied Josephson junctions with barriers prepared from the Heusler compound Cu2MnAl. In the as-prepared state the Cu2MnAl layers are non ferromagnetic and the critical Josephson current density jc decreases exponentially with the thickness of the Heusler layers dF . On annealing the junctions at 240 • C the Heusler layers develop ferromagnetic order and we observe a dependence jc(dF ) with jc strongly enhanced and weakly thickness dependent in the thickness range 7.0 nm < dF < 10.6 nm. We attribute this feature to a triplet component in the superconducting pairing function generated by the specific magnetization profile inside thin Cu2MnAl layers.

We report on measurements of structural, superconducting, and magnetic properties of trilayer and bilayer systems combined of superconducting Pb and ferromagnetic Fe. The Pb/Fe layers can be grown on Al 2 O 3 with reasonably flat interfaces, there is no alloying of the components at the interface and Fe is found to be ferromagnetic down to the monolayer range. This is a favorable situation for an S/F proximity system, since it corresponds closely to the situation treated in theoretical models. We find an oscillation of the superconducting transition temperature when plotted versus the thickness of the ferromagnetic layer, which we regard as a clear indication of an unconventional, propagating superconducting pair wave function in the Pb/Fe system. We fit our results using recent theoretical model calculations and find evidence for a strongly reduced transparency of the Pb/Fe interface. We regard this as an essential feature of the proximity effect in Pb/Fe and discuss its physical origin.

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) display an asymmetry in the first magnetization reversal process and training in the subsequent reversal processes. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in-plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.