Magnetic circular dichroism of the 4A 1, 4E region in the antiferromagnetics MnF2 and K2MnF4

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Abstract
Liquid helium temperature magnetic circular dichroism (MCD) data are presented and analyzed in some detail for the 4A 1, 4E region of MnF2 and K2MnF4 with attention focused on the magnetic dipole origins. The MCD technique is ideal in this regard since it shows very prominent pseudo‐A terms for the π‐polarized origins while effectively ``filtering out'' almost all of the much more intense electric dipole absorption. In this experiment, the 4A 1 origin of MnF2 has been directly observed for the first time about 143 cm−1 to the blue of the 4Eθ origin, as predicted. The signs of the A terms provide an independent confirmation of the accepted order of the two states. The observed magnitudes of the MCD and absorption parameters can be rationalized on the basis of a simple octahedral model if substantial mixing (85%/15%) of the 4E θ and 4A 1 states is assumed. In K2MnF4 two very sharp ([inverted lazy s]1 cm−1) magnetic dipole origins are observed in the π and axial spectrum separated by 25.1 cm−1 which show very prominent A terms of opposite sign. Theoretical analysis of the MCD shows that the higher energy of these must correspond to the 4E ε origin. The lower energy origin is assigned to 4E θ, and a prominent A term about 350 cm−1 to the blue can be assigned with reasonable confidence to the 4A 1 origin, which is not evident in the absorption spectrum. The A terms that have been assigned to the three origins vanish above the spin‐flop field as expected. The absorption and MCD parameters for 4E ε are substantially smaller relative to 4E θ and 4A 1 than is predicted on the basis of a simple octahedral model.