Are there Optical Magnons?

U. Köbler Research Centre Jülich, Institute PGI, 52425 Jülich, Germany

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Optical magnons should occur in magnets containing two in-equivalent magnetic species only. However, Heisenberg interactions between in-equivalent magnetic atoms can be expected to be weak. This is because free exchange of electrons between chemically different magnetic atoms appears not generally possible. To the best of our knowledge optical magnons have never been identified unambiguously experimentally. Confusion is provided by the fact that two magnon branches commonly occur in antiferromagnets with ferromagnetically ordered crystallographic planes and opposite spin orientations from plane to plane. This applies to MnO, EuTe, CoCl2, Fe2O3, K2FeF4. Associated with the ferromagnetic planes is a particular low-energy magnon branch. The high-energy magnon branch is the antiferromagnetic branch and not an optical magnon. In Fe3O4 (magnetite), weak interactions between the Fe2+ moments and the Fe3+ moments are evidenced by the fact that the order parameters of the FeO and of the Fe2O3 subsystem have different temperature dependencies. The observed two magnon branches can be attributed to the Fe2O3 and to the FeO subsystem, respectively. This applies equally to the two observed magnon branches in mixed crystals such as Rb2Mn0.5Ni0.5F4, KCo0.71Mn0.29F3 or Mn0.3Co0.7F2 that can be understood as modified dispersions of the constituent materials.

DOI: 10.12693/APhysPolA.133.459 PACS numbers: 75.30.Ds, 75.30.Et