Magnetic excitation spectrum and hierarchy of magnetic interactions in ErFeO3

Abstract

We report a comprehensive investigation of the excitation spectrum of ErFeO${}_3$ orthoferrite by means of time-of-flight neutron spectroscopy. The spectrum consists of two distinct components: strongly dispersive spin wave excitations of the Fe${}^{3+}$ sublattice spanning $\approx$~9 - 65 meV, and crystal electric field (CEF) excitations of Er${}^{3+}$ ions below 36 meV. The observed spin wave dispersions and spectral weight are well captured within linear spin wave theory, enabling extraction of the key Fe-Fe exchange parameters. Low-energy incident neutrons with their enhanced energy resolution, further revealed the dispersive character and splitting of Kramers-degenerate CEF levels. We show that the dispersion is caused by the exchange coupling between Er${}^{3+}$ ions, while the degeneracy is lifted by interactions between the Er${}^{3+}$ and Fe${}^{3+}$ sublattices. We further explore the influence of dipolar and antisymmetric exchange interactions with the focus on the magnetic ground state of ErFeO${}_3$, with particular attention to the low-temperature spin arrangement. Taken together, our results provide a detailed account of the spin dynamics in ErFeO${}_3$ and reveal a hierarchy of interactions scales characteristic for orthoferrites.

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