Antiferromagnetic resonance in multiferroic HoMnO₃

Multiferroic rare-earth manganites have attracted much attention because of the coexistence of ferroelectric and magnetic orders. Among hexagonal manganites, the largest magnetoelectric effect was found in HoMnO3, which is ferroelectric below Tc = 875 K and antiferromagnetically ordered below TNMn = 75 K (Mn subsystem). In addition, at TNHo ~ 5 K the Ho subsystem undergoes a transition into an antiferromagnetically ordered state. The coupling between the ferroelectric and antiferromagnetic order parameters in HoMnO3 has not been yet unambiguously explained, revealing the importance of geometrical frustrations and a complex interplay between Ho and Mn magnetic subsystems [1, 2]. Here, the magnetic excitation spectrum in high-quality single-crystalline samples of HoMnO3 was probed by means of X-band (9.3 GHz) electron spin resonance spectroscopy. A relatively strong absorption was found below 5 K, which corresponds to the temperature of the magnetic ordering of the Ho subsystem. The observed mode exhibits a very pronounced anisotropic behavior and can be interpreted as excitations within the antiferromagnetically ordered Ho subsystem.