  Recently, a signature of high-temperature superconductivity above the liquid
nitrogen temperature (77 K) was reported for La3Ni2O7 under pressure. This
finding immediately stimulates intense interest in the possible high-Tc
superconducting mechanism in double-layer nickelates. Interestingly, the
pressure-dependent phase diagram inferred from transport measurements indicates
that superconductivity under high pressure emerges from the suppression of a
density-wave-like transition at ambient pressure, which is similar to
high-temperature superconductors. Therefore, clarifying the exact nature of the
density-wave-like transition is important for determining the mechanism of
superconductivity in double-layer nickelates. Here, nuclear magnetic resonance
(NMR) spectroscopy of 139La nuclei was performed to study the density-wave-like
transition in a single crystal of La3Ni2O7. The temperature-dependent 139La NMR
spectrum and nuclear spin-lattice relaxation rate (1/T1) provide unambiguous
evidence for a spin-density-wave (SDW) transition with a transition temperature
TSDW of ~ 150 K. Furthermore, the anisotropic splitting of the NMR spectrum
suggests a possible double spin stripe with magnetic moments along the c axis.
In addition, the present NMR measurements also revealed spatial inhomogeneity
of magnetism due to inner apical oxygen vacancies. All these results will be
helpful for building a connection between superconductivity and magnetic
interactions in double-layer nickelates.
