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

We describe the design and construction of a sensitive dc torque magnetometer suitable for liquid as well as solid state samples. It can be used from room temperature down to very low temperatures (mK range) in magnetic fields produced either by superconducting or resistive magnets, in particular in the high fields (B≈30 T) produced by Bitter and hybrid magnets. The highest resolution that we have attained so far is about 0.3 pNm, which is mainly the result of a symmetric design. In typical laboratory conditions (at B=10 T) this corresponds to a magnetization resolution of about 3×10−11 emu. Other features include modularity, linearity, feedback and forward bias capabilities, and an in situ calibration of the signal.

Measurements of the nuclear magnetic spin-lattice relaxation time T 1 of liquid 3 He containing 0.5% 4 He and of 3 He-4 He mixtures as a function of magnetic field up to 22 T and at temperatures down to 40 mK show a new surface relaxation mechanism for liquid 3 He, which is proportional to the square of the magnetic field and can be described by classical relaxation theory. The intrinsic relaxation time of liquid 3 He, obtained from T 1 measurements by eliminating the surface relaxation contribution, is in good agreement with existing Fermi liquid theory. [S0031-9007(97)03546-1]

Magnetoresistance measurements of highly underdoped superconducting La 2−x Sr x CuO 4 films with x = 0.051 and x = 0.048, performed in dc magnetic fields up to 20 T and at temperatures down to 40 mK, reveal a magnetic-field induced transition from weak to strong localization in the normal state. The normal-state conductances per CuO 2 -plane, measured at different fields in a single specimen, are found to collapse to one curve with the use of a single scaling parameter that is inversely proportional to the localization length.The scaling parameter extrapolates to zero near zero field and possibly at a finite field, suggesting that in the zero-field limit the electronic states may be extended.Typeset using REVT E X 1

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