Using a low-temperature molecular-beam epitaxy growth procedure, Ga 1Ϫx Mn x As -a III-V diluted magnetic semiconductor -is obtained with Mn concentrations up to xϳ9%. At a critical temperature T c ͑T c Ϸ50 K for xϭ0.03-0.05͒, a paramagnetic to ferromagnetic phase transition occurs as the result of the interaction between Mn-h complexes. Hole transport in these compounds is strongly affected by the antiferromagnetic exchange interaction between holes and Mn 3d spins. A model for the transport behavior both above and below T c is given. Above T c , all materials exhibit transport behavior which is characteristic for systems near the metal-insulator transition. Below T c , due to the rising spontaneous magnetization, spin-disorder scattering decreases and the relative position of the Fermi level towards the mobility edge changes. When the magnetization has reached its saturation value ͑below ϳ10 K͒ variable-range hopping is the main conduction mechanism. The negative magnetoresistance is the result of the expansion of the hole wave functions in an applied magnetic field. ͓S0163-1829͑97͒04044-7͔

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Coils for pulsed magnetic fields were developed using layer by layer reinforcement with fibre composite material. The thickness of each individual layer of the reinforcement was optimized using a simple 'finite element' calculation method. A machine was designed and built that allows the winding of a layer of fibre rovings of precisely controlled thickness between the layers of wire. So far, a field of 58 T has been obtained in a 20 mm bore; in a 12 mm bore a maximum field of 66.9 T resulted in destruction of the coil.

Magnetophonon resonance is studied in the resistivity of high mobility GaAs-GaA1As heterojunctions. In contrast to previous studies which show an exponentially damped series of oscillations, we find this only at low magnetic fields.

In the development of conductors for high-field coils one has to find a compromise between high strength (to i withstand the Lorentz force) and high conductivity (longer pulse time, minimal power, energy and volume required). Th e design of coils is a non-linear process that is partly based on educated guesses guided by trial and error. Based on the cor~cept of the "current carrying capability" a simple scheme for optimal choice of wire for the relatively simple cases of energy limited coils (or the innermost parts of power limited coils) is developed. On basis of this criterion, the optimal direction of research on new wires or materials is determined.