Thermodynamics of spin reorientations in TmFeO3 ceramics observed with terahertz time domain spectroscopy

•The terahertz spectra of TmFeO3 ceramics at various temperatures were measured.•We investigated the spin reorientations of TmFeO3 using terahertz resonances.•An anisotropic energy mechanism was introduced to explain the thermodynamics.•The changes in magnetic properties verified the process of spin reorientations.

Continuous transitions Γ4→Γ24→Γ2 have been shown in TmFeO3 orthoferrite. The spin reorientations of TmFeO3 ceramics are investigated using terahertz time domain spectroscopy through ferromagnetic and antiferromagnetic resonances at decreased temperature. An anisotropic energy mechanism is introduced to research the thermodynamics of spin reorientations. It is derived analytically from changes in ferromagnetic resonant frequencies that anisotropic energy Kac decreases and reverses its sign in the vicinity of spin reorientations, and increased antiferromagnetic resonant frequencies originate from increased Kab. Testing M–H (moment–magnetic field) and M–T (moment–temperature) curves, we found changes in macroscopic magnetic properties. Further, thermodynamics and process of spin reorientations are verified.