Pressure-Induced Split of the Density Wave Transitions in La_3Ni_2O_7-δ

The unveiling of superconductivity in La_3Ni_2O_7-δ under pressure, following the suppression of a high-temperature density wave (DW) state, has attracted considerable attention. Notably, the nature of this competing DW order remains elusive, presenting a crucial question that demands further investigation. Here, we employ the muon-spin rotation/relaxation (μSR) technique combined with dipole-field numerical analysis to probe the magnetic response of La_3Ni_2O_7-δ as a function of hydrostatic pressure. At ambient pressure, μSR experiments reveal commensurate static magnetic order below T_ N≃ 151K. The comparison of the observed internal magnetic fields with dipole-field calculations reveals the magnetic structure's compatibility with a stripe-type arrangement of Ni moments (≃0.3-0.7μ_ B), characterized by alternating lines of magnetic moments and non-magnetic stripes. Experiments under pressure (up to p≃2.3 GPa) demonstrate an increase of the magnetic ordering temperature at a rate dT_ N/ dp≃ 2.8 K/GPa. This trend is opposite in sign and significantly smaller in magnitude compared to the changes observed in the DW order of unknown origin reported by Wang et al. [arXiv:2309.17378]. Our findings reveal that the ground state of the La_3Ni_2O_7-δ system is characterized by the coexistence of two distinct orders – the spin density wave and, most likely, charge density wave – with a notable pressure-induced separation between them.