Field-derivative torque induced magnetization reversal in ferrimagnetic Gd$_{3/2}$Yb$_{1/2}$BiFe$_5$O$_{12}$

TL;DR

This work tackles how field-derivative torque (FDT) influences ultrafast magnetization switching in ferrimagnets driven by terahertz (THz) pulses. It employs a two-sublattice Landau-Lifshitz-Gilbert model that includes an FDT correction to the effective field and circular THz excitation to simulate switching in the ferrimagnetic garnet Gd3/2Yb1/2BiFe5O12. The key finding is that FDT substantially lowers the THz field threshold for switching and expands the switching region, compared with switching driven solely by Zeeman torque; linear polarization, by contrast, is less effective. The mechanism involves FDT-induced asymmetry between the Fe and RE sublattices under circular THz pumping, producing an early torque peak and a dip in the Néel vector that enables deterministic reversal without a transient ferromagnetic-like state. These insights guide material and pulse design toward faster, more energy-efficient THz spintronic memory devices.

Abstract

Understanding the mechanism of spin switching in ferrimagnets via the excitation of THz pulses holds promise for future-generation magnetic memory devices. Such spin switching can be accomplished by the Zeeman torque exerted by the THz pulses on the magnetic spins. Theoretical and experimental works have established that the field-derivative of a terahertz pulse also exerts a torque, field derivative torque (FDT). Here, we investigate the role of the FDT in the spin switching in ferrimagnetic Gd$_{3/2}$Yb$_{1/2}$BiFe$_5$O$_{12}$ using a computational approach. Our results foresee that the spin switching in the presence of the FDT requires less THz magnetic fields than the spin switching without the FDT. Without the FDT terms, the spin switching in the considered system requires an extremely high magnetic field. Furthermore, we compute the switching and non-switching contour diagrams to show that the FDT tremendously enhances the possibility of spin switching. These results not only shed light on the significance of the FDT in magnetization switching but also suggest materials where the switching effect is pronounced.

Figures (6)

• Figure 1: Schematic diagram for circularly polarized THz excitation of spins in $\rm Gd_{\frac{3}{2}}Yb_{\frac{1}{2}}BiFe_{5}O_{12}$. The rare-earth (Gd and Yb) spins are denoted in blue while the Fe spins are denoted in red. A small static magnetic field ${\bf B}_{\rm ext}$ is applied along the equilibrium direction.
• Figure 2: Simulation results of the magnetization dynamics without FDT (Left Panel) and with FDT (Right Panel) for incident THz magnetic fields of ${\rm B_0 }= 6.35$ T, $\sigma = 1$ ps and $\tau = 2.6$ ps. (a) and (f) show the $x$- and $y$-components of the circularly-polarized THz pulse, (b) and (g) show the $y$-component of the Néel vector, which gets switched in presence of FDT, and not switched in the absence of FDT. (c) and (h) show the $x$-component of the Néel vector while (d) and (i) show magnetization dynamics of the individual sublattices. The normalized $\vert {\bf l}\vert$ in the absence and presence of FDT are shown in (e) and (j), respectively.
• Figure 3: Magnetization reversal without the FDT at a field amplitude of B$_0 = 24.35$ T, $\sigma = 1$ ps and $\tau = 2.6$ ps. (a) The circularly polarized field components, (b) the Néel vector in the $y$-direction, (c) the Néel vector in the $x$-direction, (d) individual sublattice magnetization dynamics in the $y$-direction ("RE" and "Fe" sublattices) are shown.
• Figure 4: The value of normalized ${\rm l_y}$ after circular polarized THz excitation of $\rm Gd_{\frac{3}{2}}Yb_{\frac{1}{2}}BiFe_{5}O_{12}$. While the red areas signify norm. ${\rm l_y} = 1$, meaning the magnetization is not reversed, the blue areas denote norm. ${\rm l_y} = -1$ specifying the magnetization has reversed. The regions with other colors signify demagnetization. (a) Without FDT and (b) with FDT show the switching and non-switching areas within the field strength within B$_0 = 0 - 10$ T with pulse width range up to $\sigma = 4$ ps. (c) Without FDT and (d) with FDT show the same areas at a higher THz field strength within B$_0 = 20 - 30$ T.
• Figure 5: The switching diagram with linearly polarized THz pulse in the presence of FDT. The red areas signify norm. ${\rm l_y} = 1$, meaning the magnetization is not reversed.