Ultrafast magnetization dynamics of insulating antiferromagnets without inversion symmetry

Abstract

Antiferromagnets (AF) as an alternative magnetic structure for ferromanget (F) has received much attention as a promising material for spintronic applications due to its ultrafast dynamics and the negligible magnetization. The simplest AF is a form of magnetic structure in which the magnetic moments in a material align antiparallel to each other. With the combination of low symmetry and spin-orbit couplings, giving rise to Dzyaloshinskii-Moriya (DM) interactions, chiral AFs give rise to favorable contributions to conventional magnetic solitons. In this thesis, we present the theoretical studies of chiral antiferromagnetic structures affected by antisymmetric DM interactions, together with several experimental results. The theoretical approach is based on the phenomenological model of chiral AF where equilibrium magnetic states are described as static parameters in the low temperature limit with a fixed magnitude of the magnetization. Target materials are electrically insulating AFs and time scale of spin dynamics are near the precessional region; whole processes complete within picoseconds and dynamic frequency is around terahertz frequency range. First, we characterize experimentally and numerically static property of magnetization of insulating and weakly canted antiferromagnet (CAF) YFeO3 through dynamic approach. YFeO3 is weakly CAFs due to bulk DM interaction. The broken inversion symmetry splits dynamic degeneracy where there are two distinct resonant motions such as quasi-ferromagnetic (F-mode, 0.3 THz) and antiferromagnetic motion (AF-mode, 0.52 THz). In F-mode, the weak magnetization behaviors like F although its dynamics are different from pure ferromagnetic motion. Here, two sublattices’ dynamics in simple AF without DM interaction give a hint to us about our first work; two sublattices have opposite helicities and its collective motion results in in-phase and out-of-phase precession due to the positive sign of exchange interaction. Here, the latter is canceled out in magnitude, which shows magnetization state of being zero in magnitude. By using time-domain terahertz spectroscopy, we can measure out-of-plane mode by changing magnetic domain states and thereby monitor magnetization states, which nicely is matched with magnetization state of YFeO3 measured by vibrating sample magnetometer measurement. Second, we investigate analytically and numerically weakly CAFs’ dynamics driven by spin current. Based on Landau Lifshitz Gilbert equation, we introduce 2-dimensional pendulum model on Néel order, l = (s1-s2)/2 that nicely describes dynamic motions of CAFs. Furthermore, weak magnetization m = (s1+s2)/2 is coupled with l, together with DMI and exchange energy, we found that close examination of m provides fundamental understanding of its dynamics in linear and nonlinear regimes where m is detectable quantity in experiment; from the ellipticity of the precessional trajectory of m, we propose a theoretical way to measure DM interaction. Finally, we discuss magnetization reversal, induced by a spin current pulse, as a function of DM interaction and anisotropy energy. Here, DM interaction and anisotropy are interpreted as potential barrier that Néel order should be go over to switch. Here, the possibility of control of DM interaction by electric field is mainly regarded as energy-efficient manipulation of magnetism in next subject. Third, we reinvestigate CAF dynamics driven by time-varying magnetic field of light from derived equation of motion. In conclusion, magnetization excitation or switching can be realized through phase matching between magnetic field torque, dhy/dt and Néel order, but cannot be explained energetically, which has been reported before. Furthermore, we experimentally measure the magnetic parameters of a CAF by using terahertz time-domain spectroscopy. Our numerical calculation using the above parameters explains the experimental data well. Fourth, we introduce alternative switching scenario for energy-efficient manipulation of simple AF. Although ultrafast precession has been utilized intensively to realize such nanoscale devices based on current induced AF switching, spin currents inevitably experience scattering or Joule heating as they move and therefore a high current density is generally required. However, electric field could be a promising source as an energy-efficient way of addressing AFs because it only needs charge that is sufficient to charge a capacitor. Therefore, in this chapter, we analyze DM torque-driven switching behavior of the AFs, triggered by coupling between the electric field and the DM interaction based on spin-orbit interaction. In the end, we discuss the possibility of detecting the Néel state of AFs without net magnetization. In this final work, we report energy-efficient control of simple AF by modulating potential modulation, which is away from torque-driven and charge-based mechanism and thereby different with precious work. In the development of spintronic industry, a particular challenge is the manipulation of the magnetic state with high speed and low power consumption. Although research has focused on the current-induced spin–orbit torque based on strong spin–orbit coupling, the charge-based and the torque-driven devices have fundamental limitations: Joule heating, phase mismatching and overshooting. Therefore, we investigate numerically and theoretically alternative switching scenario of antiferromagnetic insulator in confined geometry. As the electric field could induce DM interaction and pseudo-dipole anisotropy, the resulting spiral texture takes symmetric or antisymmetric configuration due to additional coupling with the crystalline anisotropy. Therefore, by competing two spiral states, we show that the magnetization reversal of AFs is realized, which is valid in ferromagnetic counterpart. Our finding provides promising opportunities to realize the rapid and energy-efficient electrical manipulation of magnetization for future spin-based electronic devices.