Critical spin dynamics in the antiferromagnetLa4Ni3O8fromLa139nuclear magnetic resonance

Abstract

We report $^{139}\mathrm{La}$ nuclear magnetic resonance data in ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{8}$ that reveal the presence of antiferromagnetic order below ${T}_{N}~105$ K. This remarkable compound contains two-dimensional (2D) layers of ${\mathrm{NiO}}_{2}$ with unusual Ni valences. ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{8}$, with antiferromagnetic order of $S=1/2 {\mathrm{Ni}}^{1+}$ 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.