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

In this contribution, first synthesis of semiconducting ZnSiN 2 and ZnGeN 2 from solution is reported with supercritical ammonia as solvent and KNH 2 as ammonobasic mineralizer.T he reactions were conducted in custom-built highpressure autoclaves made of nickel-based superalloy.T he nitrides were characterized by powder X-ray diffractiona nd their crystal structures were refinedb yt he Rietveld method. ZnSiN 2 (a = 5.24637(4), b = 6.28025(5), c = 5.02228(4) , Z = 4, R wp = 0.0556) and isotypic ZnGeN 2 (a = 5.46677(10), b = 6.44640(12), c = 5.19080(10) , Z = 4, R wp = 0.0494) crystallize in the orthorhombic space group Pna2 1 (no. 33). The morphology and elemental composition of the nitrides were examined by electron microscopy and energy-dispersive X-ray spectroscopy (EDX). Well-definedsingle crystalswith adiameter up to 7 mmw ere grownb ya mmonothermal synthesis at temperatures between 870 and 1070 Ka nd pressures up to 230 MPa. Optical properties have been analyzed with diffuse reflectance measurements. The band gaps of ZnSiN 2 and ZnGeN 2 were determined to be 3.7 and 3.2 eV at room temperature, respectively.I ns itu X-ray measurements were performed to exemplarily investigate the crystallization mechanism of ZnGeN 2 .D issolution in ammonobasic supercritical ammonia between 570 and6 70 Kw as observed which is quite promising for the crystal growth of ternary nitrides under ammonothermalconditions.

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The thermal expansion coefficients of the hexagonal SiC polytypes 4H and 6H and with Al and N dopants have been determined for temperatures between 300 and 1770 K. Further, a set of the room temperature lattice parameters in dependence on doping with N, Al, and B has been obtained. Data for the thermal expansion were taken on a triple axis diffractometer for high energy x rays with a photon energy of 60 keV, which allows the use of large single crystals with a volume of at least 6×6×6 mm3 without the need to consider absorption. The room temperature measurements for samples with different dopants have been performed on a four-circle diffractometer. The thermal expansion coefficients along the a- and c-directions, α11 and α33, increase from 3×10−6 K−1 at 300 K to 6×10−6 K−1 at 1750 K. It is found that α11 and α33 are isotropic within 107 K−1. At high temperatures both coefficients for doped samples are ∼0.2×10−6 and 0.3×10−6 K−1 lower than for the undoped material.

We report the formation of nanosize ferromagnetic MnAs crystallites imbedded in low-temperature grown GaAs using Mn+ ion implantation and subsequent annealing. The structural and magnetic properties of the crystallites have been characterized by transmission electron microscopy, electron beam induced x-ray fluorescence, and superconducting quantum interference device magnetometry. After an optimized thermal annealing at 750 °C, MnAs crystallites of 50 nm in size are formed. These nanomagnets show room temperature ferromagnetism.