Ferrimagnets (FiMs) persist in ultra-fast dynamics similar to antiferromagnets but are as easy to be electrically manipulated as ferromagnets, leading to a promising approach for achieving ultrafast devices with feasible tunability. Furthermore, spin-chain is of fundamental importance in condensed-matter physics, revealing unique properties such as quantum phase transition, edge effect, etc.1 Although the dynamics of FiM have been widely described by a two sublattices macro-spin model2, it hardly captures the sophisticated physical properties in multi-spin systems such as spin chains. Hence, we developed an atomistic spin model to investigate the spin dynamics in the FiM spin chain deployed by current-induced spin-transfer torque(STT). Our simulation results showed that in the finite FiM spin chain, first, the oscillation started from opposite-spin spatial region, defined as "oscillation core (OC)". Next, the oscillation spatial region would expand and finally only some specific spins formed stable oscillation (Fig.1). To further understand this phenomenon, we first introduce one OC into the spin chain. When excited by STT, the exchange mode can be further classified into non-flipped exchange mode (region I), critical exchange mode (region II), and flipped exchange mode (region III). In region I and III, the frequency increased linearly with different slops as the current density increased, yet in region II, the frequency slightly decreased, indicating the competition among exchange field (Hex), uniaxial anisotropy, and STT (Fig.2a). Next, when two OCs interacted, we find that smaller distance gives lager frequency, which is attributed to different Hex. Hence, proper choice of distance allows us to tune the Hex, achieving large-angle precession with THz frequency(Fig.2b). In summary, our work offers a novel understanding of FiM oscillation and proposes a strategy to construct energy-efficient spintronic oscillators with THz frequency. (This work at the National University of Singapore is supported by MOE-2019-T2-2-215 and FRC-A-8000194-01-00)
Fig.1 Schematic of FiM spin chain oscillation behavior evolved with time.
Fig.2 Numerical results of the oscillation in the FiM spin chain with (a)one OC and (b)two OCs.