The discovery of superconductivity in La₃Ni₂O₇ with a critical temperature nearly 80 K above 14 GPa[1] has highlighted bilayer nickelates as promising candidates for high-Tc superconductors. While a number of transport studies have reported resistance drops, magnetic signals as large as expected for bulk superconductivity have not been clearly observed. As a result, the superconductivity in La₃Ni₂O₇ reported so far is regarded as a filamentary superconductivity. However, quantitative evaluation of the superconducting volume fraction has remained incomplete, mainly because magnetization measurements above 15 GPa are technically challenging[2, 3].

In this work, we performed magnetization measurements on bilayer nickelates, including La₃Ni₂O₇ and La_3-xPr_xNi₂O₇, using a newly developed pressure cell fitting to a commercial SQUID magnetometer. Our cell enables measurements above 20 GPa with sufficient sensitivity to detect filamentary superconductivity corresponding to a volume fraction of an order of 1%. Compared with existing high-pressure ac susceptibility or other magnetometry techniques, the new method provides a platform for efficient screening and quantitative evaluation of superconducting samples.

In La₃Ni₂O₇ samples, we observed a diamagnetic signal corresponding to filamentary superconductivity with a volume fraction of a few percent. Notably, a Meissner signal was also detected in the field-cooled measurement, providing magnetic evidence for superconductivity in this regime.