Observation of Single-Ion Transitions in the Ordered States of FeBr_2

Abstract

By utilizing a far infrared laser operating in the THz range, we have observed new electron spin resonance (ESR) lines in the ordered states of a layered antiferromagnet with strong uniaxial anisotropy, FeBr₂. We interpret these ESR lines as arising from transitions within the lowest triplet state of single Fe²⁺ ions. Good agreement between theory and experiment is obtained without any adjustable parameters.