Single crystals of the oxypnictide superconductor SmFeAsO 0.8 F 0.2 with T c ≃ 45(1) K were investigated by torque magnetometry. The crystals of mass ≤ 0.1 µg were grown by a high-pressure cubic anvil technique. The use of a high-sensitive piezoresistive torque sensor made it possible to study the anisotropic magnetic properties of these tiny crystals. The anisotropy parameter γ was found to be field independent, but varies strongly with temperature ranging from γ ≃ 8 at T T c to γ ≃ 23 at T ≃ 0.4T c . This unusual behavior of γ signals unconventional superconductivity.

Magnetization and muon spin relaxation on MgB 2 were measured as a function of the applied magnetic field at 2 K. Both indicate an inverse-squared penetration depth strongly decreasing with increasing field H below about 1 T. Magnetization also suggests the anisotropy of the penetration depth increases with increasing H, interpolating between a low H c1 and a high H c2 anisotropy. Measurements of the torque as a function of the angle between the field and the c axis of the crystal are in agreement with this finding, while also ruling out drastic differences between the mixed state anisotropies of the two basic length scales penetration depth and coherence length.

Torque magnetometry is a powerful method to determine a sample's magnetic properties. In a torque magnetometer, the sample under interest is mounted on a suitable sensor, the core part of the instrument. Piezoresistive atomic force microscope tips are proven to be very useful torque sensors and have already been optimized for the specific needs of torque magnetometry. However, this type of sensor did not offer the possibility of measuring torque along two perpendicular directions simultaneously without sacrificing sensitivity. In this article we demonstrate a fundamentally new sensor design based on the piezoresistive readout technique, which offers a large variability of measurement modes. With a symmetric arrangement of the sensor legs, concurrent two-axis measurements are easily possible with two separate readout circuits. Furthermore, the variability can be exploited by fabricating a thermometer on the sample platform to directly measure the sample temperature. Test measurements are in good agreement with elasticity calculations.

The cuprate superconductors have highly anisotropic, layered structures. This causes the flux lattice penetrating them in the intermediate state to undergo dramatic changes when the direction of the applied magnetic field approaches the CuO 2 planes. We present highly sensitive torque measurements on tiny La 2−x Sr x CuO 4 single crystals showing these regimes of vortex structure and their influence on the determination of the magnetic anisotropy parameter ␥. Systematic measurements of torque as a function of magnetic field magnitude and orientation at various temperatures below T c made it possible to determine the conditions under which the structural changes occur, allowing a reliable determination of ␥. The values of ␥ obtained for La 2−x Sr x CuO 4 as a function of doping x, covering the whole doping regime ͑underdoped, optimally doped, and overdoped͒, are in good agreement with earlier results, indicating a flow toward quasi-two-dimensionality in the underdoped limit.

In order to simplify and optimize the operation of our home made torque magnetometer we created a new software system. The architecture is based on parallel, independently running instrument handlers communicating with a main control program. All programs are designed as command driven state machines which greatly simplifies their maintenance and expansion. Moreover, as the main program may receive commands not only from the user interface, but also from other parallel running programs, an easy way of automation is achieved. A program working through a text file containing a sequence of commands and sending them to the main program suffices to automatically have the system conduct a complex set of measurements. In this paper we describe the system's architecture and its implementation in LabVIEW.