  High transverse magnetic field and zero field muon spin rotation and
relaxation measurements have been carried out in a lightly oxygen-doped high-Tc
parent compound La2CuO4 in a temperature range from 2 K to 300 K. As in the
stoichiometric compound, muon spin rotation spectra reveal, along with the
antiferromagnetic local field, the presence of an additional source of magnetic
field at the muon. The results indicate that this second magnetic order is
driven by the antiferromagnetism at low temperature but the two magnetic orders
decouple at higher temperature. The ability of muSR to detect this additional
magnetism deteriorates with doping, thus rendering the technique impotent to
reveal time-reversal symmetry breaking in superconductors.
