  Recent discoveries of superconductivity in hydrogen-rich compounds stabilized
by high pressures have shown that a critical temperature of superconductivity
Tc can reach near room temperature values. The current studies focus on the
search for new superconductors with higher Tcs to understand the ultimate limit
of conventional superconductivity, and high-Tc superconductors at lower
pressures. However, high pressure conditions put serious limitations on
experimental studies. Electrical transport measurements have long been the
primary experimental technique for reliable detection of superconductivity at
high pressures, whereas magnetic susceptibility measurements continue to be a
challenge. Here, we dramatically changed the protocol of magnetic measurements
to probe the trapped magnetic flux. We tested this technique on samples of H3S
and LaH10 and propose it for a routine examination of superconductivity at high
pressures. Using the temperature- and magnetic field dependences of the trapped
flux we estimated Tc ~195 K, a lower critical field ~0.36 T, an irreversibility
field ~1.7 T, a full penetration field ~8.8 T, the London penetration depth ~37
nm, and critical current densities jc(T) in H3S at ~155 GPa in a wide
temperature range. The trapped flux method is found to be sensitive to a
particular phase in a mixture of superconducting phases: concomitant phases of
sulfur and hydrogen-depleted LaHx were detected. In contrast to the Meissner
state occurring at low external magnetic fields, a magnetic response from the
trapped flux at zero applied magnetic field is in tens times stronger and is
not contaminated by the magnetic background of a bulky diamond anvil cell. This
technique can be a powerful tool for the screening of new superconducting
materials, the study of multiphase, contaminated samples, or samples with a low
superconducting fraction at ambient pressure too.
