We report measurements of the oxygen-isotope effect (OIE) on the in-plane penetration depth lambda(ab)(0) in underdoped La2-xSrxCuO4 single crystals. A highly sensitive magnetic torque sensor with a resolution of Deltatau approximately 10(-12) N m was used for the magnetic measurements on microcrystals with a mass of approximately 10 &mgr;g. The OIE on lambda(-2)(ab)(0) is found to be -10(2)% for x = 0.080 and -8(1)% for x = 0.086. It arises mainly from the oxygen-mass dependence of the in-plane effective mass m(*)(ab). The present results suggest that lattice vibrations are important for the occurrence of high temperature superconductivity.

Angular-dependent magnetic torque measurements performed near the critical temperature on single crystals of HgBa2CuO4+y, La2−xSrxCuO4, and YBa2Cu3O6.93 are scaled, following the 3D XY model, in order to determine the scaling function dG ± (z)/dz which describes the universal critical properties near Tc. A systematic shift of the scaling function with increasing effective mass anisotropy γ = m * ab /m * c is observed, which may be understood in terms of a 3D-2D crossover. Further evidence for a 3D-2D crossover is found from temperature-dependent torque measurements carried out in different magnetic fields at different field orientations δ, which show a quasi 2D "crossing region" (M * , T * ). The occurrence of this "crossing phenomenon" is explained in a phenomenological way from the weak z dependence of the scaling function around a value z = z * . The "crossing" temperature T * is found to be angular-dependent. Torque measurements above Tc reveal that fluctuations are strongly enhanced in the underdoped regime where the anisotropy is large, whereas they are less important in the overdoped regime.

An extremely sensitive miniature torque magnetometer using Si p-doped piezoresistive cantilevers is described. The magnetization m↘ of very small magnetic or superconducting samples (≤1 μg) deposited on the cantilever can be measured via the torque τ↘=m↘×B↘ produced on them by an applied field B. The high resolution in the lever deflection of the order of 0.1 Å corresponds to a torque sensitivity of the order of Δτ≂10−14 Nm. In a homogeneous field of 1 T this device allows magnetic moments as small as Δm≂10−14 Am2 to be measured, a value far smaller than that measurable by the best commercial superconducting quantum interference device magnetometers. Measurements performed on microcrystals of the high-Tc superconductors Bi2Sr2Ca1Cu2O8 and Hg1Ba2Ca3Cu4O10 in the static and dynamic modes demonstrate the excellent performance of this device at low temperature and in magnetic fields ranging between a few mT and 5 T.

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