Optomagnonic logic based on optical nonthermal magnetization switching in near-compensated iron-garnets

TL;DR

This work addresses heating-limited all-optical magnetization switching by proposing heating-free optomagnonic control in nearly compensated ferrimagnets near the magnetization compensation point. The authors leverage the inverse Faraday effect to induce a fast, nonthermal effective field and switch between two degenerate noncollinear equilibrium states, with switching thresholds that depend on the external field and pulse helicity. They demonstrate a suite of optomagnonic logic elements (NOT, XNOR, NOR, NAND) that are reconfigurable via $H_{\mathrm{ext}}$, enabling deterministic bit writing and flexible logic operations using optical inputs encoded in pulse amplitude and helicity. The approach promises energy-efficient, reconfigurable optomagnonic memory and logic devices, with bistable bits stabilized by Néel relaxation on practical timescales and readout achievable through ultrafast magneto-optical techniques.

Abstract

We propose a set of optomagnonic logic elements based on the effect of optical magnetization switching via the non-thermal inverse Faraday effect induced by femtosecond laser pulses in nearly compensated iron-garnet film with uniaxial anisotropy. Two equilibrium states in such a film are separated by a potential barrier that might be overpassed if the femtosecond pulse fluence exceeds a threshold value, so that magnetization is reversed after the pulse action. The switching threshold depends strongly on the value of applied in-plane external magnetic field, and is different for the two initial magnetization states and two opposite optical pulse helicites. This makes it possible to perform optomagnonic non-thermal deterministic writing of a magnetic bit. Such switching mechanism can be used for realizing reconfigurable optomagnonic logic elements without thermal assistance. Logical operations are implemented by encoding inputs in the amplitude and helicity of the optical pulses, while outputs are written as the magnetization state. The study demonstrates a pathway towards heating-free optomagnonic logic and memory devices.

Figures (6)

• Figure 1: Ferrimagnetic film with uniaxial magnetic anisotropy in the external magnetic field $H_{\mathrm{ext}}$ along $x$ axis. The easy axis of the film is along $z$ axis. The $\theta$ angle is between $\mathbf{L}$ and XOY plane, $\phi$ is the angle between the projection $\mathbf{L}$ on the XOY plane and $x$ axis.
• Figure 2: (a): 2D plot showing the effective potential $U_{\mathrm{eff}}$ as a color map, with the angle $\theta$ on the x-axis and the angle $\phi$ on the y-axis. The inset depicts the profile $U_{\mathrm{eff}(\theta)}$ for $\phi=0$, the introduced notations of $\Delta U_{\mathrm{eff}}$ and $\theta_0$ are presented. (b): Dependence of the initial angle $\theta_0$ and the height of the potential barrier $\Delta U_{\mathrm{eff}}$ on the external magnetic field $H_{\mathrm{ext}}$.
• Figure 3: Dynamics of magnetization and effective potential landscape. Temporal evolution of $\theta(t)$ and $\phi(t)$ for different values of $H_{\mathrm{IFE}} = 500, 787, 790, 880$ Oe acting on the system, (a) and (b), respectively. Insets display the same $\theta(t)$ and $\phi(t)$ but in a different time range. (c) 3D representation of the effective potential $U_{\mathrm{eff}}(\theta,\phi)$ with superimposed dynamical trajectories $\phi(t)$ and $\theta(t)$. The parameters of numerical calculation: $T=296$ K, $H_{\mathrm{ext}} = 2527$ Oe, $K = 2660$ erg/cm$^3$, $\chi$$= 9.4 \times 10^{-5}$, $\alpha = 10^{-4}$, $m = 2.33$ emu/cm$^3$, $(M_{Fe}-M_R)/(M_{Fe}+M_R) = 6\cdot 10^{-3}$
• Figure 4: Dependence of the minimal effective inverse Faraday effect field $H_{\mathrm{IFE}}$ (left axis) and optical pump fluence (right axis) required for the magnetization switching on the applied in-plane external magnetic field $H_{\mathrm{ext}}$. Two curves are shown: one corresponding to the case with positive initial angular velocity $\frac{d\theta}{dt}|_{t=0} < 0$ (right circular polarization $\sigma_{+}$), and another for $\frac{d\theta}{dt}|_{t=0} > 0$ (left circular polarization $\sigma_{-}$), where switching is less efficient.
• Figure 5: The realization of XNOR gate with $H_{\mathrm{ext}}=2.53$ kOe, and the input bits "$0_\sigma$" and "$1_\sigma$", encoded by $H_{\mathrm{IFE}}=\pm0.7$ kOe, respectively. The form of potential $U_{\mathrm{eff}}(\phi=0, \theta)$ is illustrated by gray solid lines, the initial state corresponds to "$0_w$" (green dot), the output bits "$0_w$" and "$1_w$" are depicted by orange dots.