Circular and magnetic circular dichroism of some simple antiferromagnetic fluorides

We present results of an extensive experimental investigation of the circular-dichroic (CD) and magnetic-circular-dichroic (MCD) properties of the antiferromagnetic compounds MnF₂, CoF₂, and FeF₂. The effective magnetic splitting factors for a number of transitions, including pure electronic and magnon sidebands, have been measured through their MCD properties and are used to illustrate the sensitivity attainable using modulation techniques. The manifolds studied entailed the ⁶A1g→⁴T1g and ⁴A1g, ⁴Eg absorptions in MnF₂, the ⁵T₂ to ³T₁ and ³E₁ transitions in FeF₂, and the ²T1g state of CoF₂. A number of weak transitions in the ⁴T1g state of MnF₂ have been identified as have a number of transitions which are obscured by overlapping absorptions which are nondichroic. Results are also presented on the observation of the explicit behavior of the phase of the MCD signal as a function of the applied field; this property allowed us to determine the existence of Davydov coupling in a transition of CoF₂. For MnF₂ and CoF₂, finite CD signals are observed at zero field; uniaxial stress applied along the base diagonals, [110] and [1{}10], allowed us to identify the source of the signal as a basalplane distortion arising when these compounds are in their ordered state. The nature of the mechanism responsible for this small distortion is not understood at the present moment, though several mechanisms may be ruled out, exclusive of imperfections in crystals, in view of our results.