  Ferrimagnets (FIMs) can function as high-frequency antiferromagnets while
being easy to detect as ferromagnets, offering unique opportunities for
ultrafast device applications. While the physical behavior of FIMs near the
compensation point has been widely studied, there lacks a generic understanding
of FIMs where the ratio of sublattice spins can vary freely between the
ferromagnetic and antiferromagnetic limits. Here we investigate the physical
properties of a model two-sublattice FIM manipulated by static magnetic fields
and current-induced torques. By continuously varying the ratio of sublattice
spins, we clarify how the dynamical chiral modes in an FIM are intrinsically
connected to their ferro- and antiferromagnetic counterparts, which reveals
unique features not visible near the compensation point. In particular, we find
that current-induced torques can trigger spontaneous oscillation of the
terahertz exchange mode. Compared with its realization in antiferromagnets, a
spin-torque oscillator using FIMs not only has a reduced threshold current
density but also can be self-stabilized, obviating the need for dynamic
feedback.
