Antiferromagnetic resonance in multiferroic YMnO₃ and LuMnO₃

Multiferroic rare-earth manganites have attracted much attention because of the coexistence of ferroelectric and magnetic order. Combining conventional far-infrared Fourier-transform and THz-range free electron laser electron spin resonance (ESR) techniques, magnetic excitations in the hexagonal multiferroic materials YMnO₃ and LuMnO₃ have been studied. In the antiferromagnetically (AFM) ordered phase the gap in the excitation spectrum (~ 42 and ~ 48 cm⁻¹ for YMnO₃ and LuMnO₃, respectively) was observed directly. Similar slope of the frequency-field dependences of the AFM resonance modes, ~ 0.5 cm⁻¹/T, was found for both compounds. A fine structure of the AFM resonance absorption has been revealed by means of high-resolution ESR techniques, which can be explained taking into account a finite interaction between the neighboring Mn³⁺ layers.