Switching of an antiferromagnet controlled by spin canting in a laser-induced hidden phase

TL;DR

Ultrafast spin-reorientation in a canted antiferromagnet Fe$_3$BO$_6$ is driven by a laser-induced modification of the Dzyaloshinskii-Moriya interaction, which changes inter-sublattice canting and can create a hidden phase. The study combines fs pump-probe MOKE experiments with a phenomenological Landau-type theory to separate nonthermal canting effects from subsequent thermally activated heating. The key findings are that a hidden phase $\Gamma_2'$ broadens the coexistence range of SR states and determines the latency and partial switching via the magnetization mismatch $\Delta_M$; full switching requires reaching $\Delta_M=0$ (e.g., at $F=0.6$ J cm$^{-2}$). This work demonstrates a mechanism to dynamically tune first-order spin switching via a laser-driven hidden state, with implications for ultrafast magnetic control and neuromorphic applications.

Abstract

During laser-induced phase transitions, fast transformations of electronic, atomic, and spin configurations often involve emergence of hidden and metastable phases. Being inaccessible under any other stimuli, such phases are indispensable for unveiling mechanisms and controlling the transitions. We experimentally explore spin kinetics during ultrafast first-order 90$^{\circ}$ spin-reorientation (SR) transition in a canted antiferromagnet Fe$_3$BO$_6$, and reveal that the transition is controlled by the canting between the magnetic sublattices. Laser-induced perturbation of the Dzyaloshinskii-Moriya interaction results in a change of the intersublattice canting within first picoseconds, bringing Fe$_3$BO$_6$ to a hidden phase. Once this phase emerges, laser-induced heating activates precessional 90$^\circ$ spin switching. Combination of the spin canting and heating controls the final spin configuration comprising coexisting initial and switched phases. Extended phase coexistence range is in a striking contrast to the narrow SR transition in Fe$_3$BO$_6$ induced by conventional heating.

Figures (5)

• Figure 1: Equilibrium SR transition in Fe$_3$BO$_6$. (a) Crystal structure of Fe$_3$BO$_6$, and spins arrangement at T < $T_\mathrm{SR}$ and T > $T_\mathrm{SR}$. (b) Temperature dependence of MOKE $\uptheta^p_S$ and $\uptheta^l_S$ proportional to $M^a_S$ and $M^c_S$ below and above $T_\mathrm{SR}$, respectively. Switching of the magnetization by 90$^\circ$ and the change of the intersublattice angle at SR transition are shown in insets. (c) Experimental geometry for magneto-optical detection of the SR transition. (d) Equilibrium magneto-optical hysteresis loops at temperatures below and above $T_\mathrm{SR}$.
• Figure 2: Kinetics of the laser-driven SR transition. (a) Pump-probe signal $\Delta\uptheta(t)$ measured at $\upmu_0H~=~$200 mT, $T_0$ = 385 K, and $F$ = 0.5 J$\cdot$cm$^{-2}$ (symbols) and its fit (line). Inset zooms the time range of latency (see text for details). (b, d) MOKE hysteresis loops at the time delays (b) $t=3$ ps and (d) 200 ps after laser excitation (symbols), and at $t=-10$ ps (dashed line). Solid lines are the guides to an eye. (c) Field dependences of the precession frequencies $f_1$ (open symbols) and $f_2$ (closed symbols) obtained from $\Delta\uptheta(t,F)$$T_0$ = 385 K, and of the qFMR frequency (line) calculated at $T_0$ = 430 K.
• Figure 3: SR transition at various laser pulse fluences. (a) Pump-probe signals $\Delta\uptheta(t)$ (symbols) at $\upmu_0H~=~$200 mT, $T_0$ = 385 K at various pump fluences, and their fits (lines). (b) Fluence dependences of $\Updelta\uptheta_\mathrm{L}$ during the latency $\uptau_\mathrm{L}$ (red symbols) and $\Updelta\uptheta_\mathrm{F}$ at $t>100$ ps (blue symbols). Fluence dependences of (c) the fraction $N^\mathrm{SW}$ of the material switched to the $\Gamma_4$ phase, (d) the precession frequency $f_2$ (circles) collated with the temperature dependence of the qFMR frequency obtained from BLS (stars), (e) the latency $\uptau_\mathrm{L}$, and (f) the magnetization mismatch $\Updelta_\mathrm{M}$ [Eq. \ref{['eq:mismatch']}].
• Figure 4: Phase diagram of the laser-induced SR transition. Phase diagram $t-\Delta_\mathrm{M}$ of the laser-induced SR transition based on the fluence dependence at $T_0=385$ K with the color-coded switched fraction $N^\mathrm{SW}$. Open and closed symbols mark the start and the end ($t_\mathrm{SR}$) of the latency. Left-hand side illustrates the spins configuration at $t <$$t_\mathrm{SR}$. Right-hand side shows the thermodynamics potential resulting from the DMI change, and the precession at $t >$$t_\mathrm{SR}$.
• Figure 5: (a) Pump-probe signals $\Delta\uptheta(t)$ (symbols) at $\upmu_0H~=~$200 mT, $F$ = 0.5 J$\cdot$cm$^{-2}$ at various initial temperature, and their fits (lines). (b) The magnetization mismatch $\Delta_\mathrm{M}$ (closed circles), $\Delta\theta_\mathrm{L}$ (open circles), and the switched fraction $N^\mathrm{SW}$ (triangles) vs. initial and final temperatures as measured at the fixed fluence $F$ = 0.5 J$\cdot$cm$^{-2}$. (c) 2D plot of $N^\mathrm{SW}$ vs. barrier $\Updelta\Upphi$ and the final temperature $T_\mathrm{F}$ as obtained from the fluence dependences at $T_0$ = 385 (circles) and 365 K (triangles) and the temperature dependeces at $F$ = 0.5 (diamonds) and 0.35 J$\cdot$cm$^{-2}$ (pentagons).