Local Spin Resonance and Spin-Peierls-like Phase Transition in a Geometrically Frustrated Antiferromagnet

Abstract

Inelastic magnetic neutron scattering reveals a localized spin resonance at 4.5 meV in the ordered phase of the geometrically frustrated cubic antiferromagnet ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$. The resonance develops abruptly from quantum critical fluctuations upon cooling through a first order transition to a co-planar antiferromagnet at $T_c=12.5\mathrm{}\left(5\right)\mathrm{K}$. We argue that this transition is a three dimensional analog of the spin-Peierls transition.