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A MgB 2 powder was mechanically milled for different times. X-ray diffraction indicated the formation of an intermediate phase with an unknown structure before formation of amorphous phase. The intermediate structure was superconducting below 18 K, while the amorphous phase was nonsuperconducting till 4.2 K. After heat treatment at 700 C, all the mechanically milled samples formed back into the hexagonal MgB 2 structure with T c ¼ 39 K. X-ray and Raman investigations suggested that the intermediate structure was metastable with a lattice compression, which was probably associated with the reduction in transition temperature.

Laser annealing has been employed to decompose half-metallic CrO 2 into insulating Cr 2 O 3 in air ambient. While both Cr 2 O 3 and CrO 2 are present, the relative fraction of each phase was controlled by changing the duration and power of laser irradiation. Glancing angle x-ray diffraction and micro-Raman scattering were used to characterize samples before and after laser annealing. The laser-induced decomposition of CrO 2 into Cr 2 O 3 that leads to a threefold enhancement of the low-field magnetoresistance and the realization of phase control of the CrO 2 /Cr 2 O 3 system in selective microregions by laser irradiation implies: ͑i͒ optical lithography can be used as a potential method to directly control the magnetotransport properties which are strongly depended on the interface tunneling barrier and ͑ii͒ The CrO 2 polycrytalline phase could be much more attractive as a high-density magnetic storage medium.

Half-metallic CrO 2 powder compact with rod-shaped nanoparticles was studied by micro-Raman scattering in the presence of an external magnetic field at room temperature (300 K). In the low-field region (H 250 mT), the frequency and intensity of the E g mode, an internal phonon mode of CrO 2 , increase dramatically with increase in the magnetic field, while the corresponding linewidth decreases. The above parameters become constant when the CrO 2 powder enters the saturation state at higher magnetic field. The pronounced anomalies of the Raman phonon parameters under a low magnetic field are attributed to the spin-phonon coupling enhanced by the magnetic ordering, which is induced by the external magnetic field.