Cooperative spin-magnon coupling in the temperature- and magnetic field-dependent Raman spectra of ErFeO₃

The recent report[1] of Dicke cooperativity in the magnetic interactions of erbium orthoferrite ErFeO₃, as observed in terahertz (THz) spectra, stimulates a complementary Raman spectroscopic study. Bulk ErFeO₃ forms an orthorhombic perovskite crystal structure, space group Pbnm, and demonstrates antiferromagnet ordering of the Fe³⁺ spins below the Néel temperature T_N ~ 650 K, with additional magnetic phases occurring below room temperature. A novel, magneto-Raman microscope system affords measurement of low-frequency (down to ~10 cm^-1) Raman-active lattice (phonons) and magnetic (magnons) excitations as a function of polarization, temperature (2 to 300)K, and magnetic field (0 to 9)T. We discuss the dependence of the observed Raman-active phonons and magnons in b-cut ErFeO₃ on polarization, temperature, and magnetic field.  At low temperature (T ~ 1.6 K) and magnetic field H || c, the Er³⁺ electron paramagnetic resonance couples with an Fe³⁺ quasi-ferromagnetic magnon demonstrating anti-crossing behavior.  Additionally, we compare our Raman spectra with THz measurements to further elucidate the nature of magnetic interactions in this system.

[1] ]X. Li et al., Science 361, 794 (2018).