Critical spin dynamics in the antiferromagnet La4Ni3O8 from 139La nuclear magnetic resonance

We report La-139 nuclear magnetic resonance data in La4Ni3O8 that reveal the presence of antiferromagnetic order below T-N similar to 105 K. This remarkable compound contains two-dimensional (2D) layers of NiO2 with unusual Ni valences. La4Ni3O8, with antiferromagnetic order of S = 1/2 Ni1+ spins, is analogous to the parent compounds of the cuprate superconductors. Our data clearly reveal dramatic spectral changes and low-energy antiferromagnetic correlations associated with the onset of long-range order below T-N. These critical spin fluctuations, however, do not diverge at the second-order phase transition but rather appear to be associated with a T = 0 transition characteristic of 2D spin fluctuations.