Terahertz\nCrystal-Field Transitions and Quasi Ferromagnetic\nMagnon Excitations in a Noncollinear Magnet for Hybrid Spin-Wave Computation

The complexity of interactions between\nthe crystal-field and unusual\nnoncollinear spin arrangement in nontrivial magnets demands novel\ntools to unravel the mystery underneath. In this work, we study such\ninteraction dynamics of crystal-field excitations (CFE) and low-energy\nmagnetic excitations in orthochromite TmCrO<sub>3</sub> with controls\nof temperature and magnetic field using high-resolution magneto-terahertz\n(THz) time-domain spectroscopy. The THz energy spectrum spanning 0.5–10\nmeV possesses a low-frequency spin-excitation (magnon) mode and a\nmultitude of CFE modes at 10 K, all of which uniquely embody a range\nof phenomena. For the magnon mode, a temperature dependence of peak\nfrequency is induced by magnetic interactions between Tm and Cr subsystems.\nWhile a change from blue to red shift of peak frequency of this mode\nmarks the magnetization reversal transition, the spin-reorientation\ntemperature and change of magnetic anisotropy are depicted by different\nfeatures of field and temperature dependent peak frequency dynamics.\nThe modes corresponding to CFE are robust and laden with a multitude\nof submodes, which are attributes of nontrivial interactions across\ndifferent transitions. These modes are suppressed only upon substitution\nof Tb<sup>3+</sup> at the Tm<sup>3+</sup> site, which suggests a dominant\nrole of single-ion anisotropy in controlling entire THz excitation\nspectra. Overall, this remarkable range of phenomena seen through\nthe unique lens of all-optical THz tools provides deeper insights\ninto the origin of magnetic phases in systems with complex interactions\nbetween rare-earth and transition metal ions and provides a multitude\nof novel combinations of closely spaced modes for emerging hybrid\nspin-wave computation.