Evidence of spin density waves in La$_3$Ni$_2$O$_{7-δ}$

Abstract: The recently discovered superconductivity with critical temperature $T_c$ up to 80 K in the double-layer Nickelate La$3$Ni$_2$O${7-\delta}$ under pressure has drawn great attention. Here we report the positive muon spin relaxation ($\mu^+$SR) study of polycrystalline La$3$Ni$_2$O${6.92}$ under ambient pressure. Zero-field $\mu^+$SR experiments reveal the existence of magnetic order in La$3$Ni$_2$O${6.92}$ with $T_{N}=154\ \rm{K}$. The weak transverse field $\mu^+$SR measurements confirms the bulk nature of magnetism. In addition, a small quantity of oxygen deficiencies can greatly broaden the internal magnetic field distribution sensed by muons.