A Procedure to Protect Granular and Porous Samples From Oxygen Contamination in High-Resolution Magnetization Measurements: Application to Granular High- $T_{c}$ Cuprate Superconductors

Due to the paraantiferromagnetic transition of oxygen at 45 K, the contamination with this element may deeply affect measurements performed with high-resolution magnetometers around this temperature. We have studied this spurious effect by measuring with a commercial SQUID-based magnetometer (Quantum Design, model MPMS-XL) the magnetization of granular La 1.85 Sr 0.15 CuO 4 (LSCO) superconductors by using different sample holders. Our results demonstrate the crucial role played by the O 2 physisorption in the extended surface of the LSCO grains, an effect that may be deeply mitigated by encapsulating the grains into an epoxy resin (EPO-TEK 301). This procedure may be also useful for high-precision magnetization measurements at low temperatures in other granular or porous materials.