Magnetizing altermagnets by ultrafast asymmetric spin dynamics

Abstract

Laser pulses are known to induce symmetric demagnetization: equal loss of magnetic moments in the identical sublattices of antiferromagnets and ferromagnets at ultrashort timescales. Using time-dependent density functional theory, we show that linearly polarized laser pulses can drive asymmetric demagnetization between otherwise identical sublattices in the $d$-wave compensated altermagnet (AM) RuO$_2$, resulting in a \textit{photo-induced ferrimagnetic state} with a strong net magnetization of $\sim$0.2 $μ_B$ per unit cell. The sign and magnitude of this metastable magnetization are highly controllable by laser polarization. We identify polarization-selective asymmetric optical intersite spin transfer (a-OISTR) as the primary mechanism generating the net moment, followed by asymmetric spin flips (a-SF) that further amplifies it. Both effects originate from the characteristic nodal spin band topology of \textit{d}-wave AMs. Moreover, we demonstrate that this laser-induced magnetization is universal across various $d$-wave AMs, including experimentally confirmed KV$_2$Se$_2$O and RbV$_2$Te$_2$O. We uncover a robust route to light-controlled magnetization in AMs on ultrafast timescales.

Figures (4)

• Figure 1: Plorization-dependent symmetric (s) and asymmetric (a) OISTR and SF mechanism in RuO$_2$. (a-c) 3D Brillouin zone of RuO$_2$. Red and blue regions highlight the alternating symmetry of spin polarization at the nodal plane $k_z$=0. The laser pulses are applied with a various polarization angle $\theta$. Here, the $\hat{\mathbf{e}}_{\theta}$ is set along the spin-polarized M$^{\prime}$--$\Gamma$--M path ($\theta = 45^\circ$), S$^{\prime}$--$\Gamma$--S path ($\theta = 135^\circ$) and spin-degenerate Y$^{\prime}$--$\Gamma$--Y path ($\theta = 0^\circ$), X$^{\prime}$--$\Gamma$--X path ($\theta = 90^\circ$), respectively. (d-f) LDOS of two Ru atoms without SOC along the M$^{\prime}$--$\Gamma$--M, S$^{\prime}$--$\Gamma$--S and X$^{\prime}$--$\Gamma$--X (Y$^{\prime}$--$\Gamma$--Y) paths. The a-OISTR and a-SF occur between spin-up (blue) and spin-down (red) channels at $\theta = 45^\circ$ and 135$^\circ$ while s-OISTR and s-SF occur at $\theta = 0^\circ/90^\circ$. (g-i) Corresponding schematic of dominant SF from spin-down to spin-up channel for Ru2 at $\theta = 45^\circ$ and dominant SF from spin-up to spin-down channel for Ru2 at $\theta = 135^\circ$. The SF in Ru1 and Ru2 at $\theta = 0^\circ/90^\circ$ are equal.
• Figure 2: Polarization-dependent magnetization dynamics in RuO$_2$. (a-d) Normalized Ru atom-resolved spin moment as a function of time at polarization angle $\theta = 0^\circ$, 45$^\circ$, 90$^\circ$ and 135$^\circ$, respectively. The vector potential of the laser pulse is shown in gray (central frequency 1.63 eV, full width at half maximum (FWHM) of $\sim$10 fs, and an incident fluence of 9.8 mJ/cm$^2$). The net magnetic moment (net M) is shown in purple. Corresponding relationship between the net M and $\theta$ at t=15.8 fs, 25.5 fs and 36.1 fs, respectively, is shown in (e). (f) and (g) Magnetization density of RuO$_2$ at $\theta = 45^\circ$ and 135$^\circ$, respectively after the laser dissipates (t=36 fs). Yellow and Cyan domains indicate the spin-up and spin-down density, respectively. The isosurface is set to 0.0045 e/Å$^3$.
• Figure 3: Polarization-dependent transient magnetization distribution in k-space. (a) 3D Brillouin zone of RuO$_2$. (b-e) Snapshots of magnetization distribution at $k_z$=0 and t=36 fs with $\theta = 0^\circ$, 45$^\circ$, 90$^\circ$ and 135$^\circ$, respectively. Color intensity indicates magnetization magnitude; Yellow dashed lines represent the $\hat{\mathbf{e}}_{\theta}$ direction.
• Figure 4: Contribution of OISTR and SF process ($M_{OISTR}$ and $M_{SF}$) on demagnetization of Ru atoms in RuO$_2$. (a-c) Differences in the time-resolved occupation function $\Delta$DOS(t) in the absence of SOC at $\theta = 0^\circ/90^\circ$, $\theta = 45^\circ$ and $\theta = 135^\circ$. The negative value signifies a loss of electrons, and a positive value signifies a gain of electrons. (d-f) The time-dependent $M_{OISTR}$ and $M_{SF}$ for two Ru atoms and change in net M at $\theta = 0^\circ/90^\circ$, $\theta = 45^\circ$ and $\theta = 135^\circ$, respectively. (g) Laser-induced maximum net moment ($M_{net}$) of six d-wave AMs including monolayer (ML) Cr$_2$Se$_2$O, ML-Fe$_2$MoSe$_4$, KV$_2$Se$_2$O, RbV$_2$Te$_2$O, RuO$_2$ and CoF$_2$ during the initial 36 fs. (h) Time evolution of the $M_{OISTR}$ and $M_{SF}$ of RuO$_2$ with different SOC strengths ($\alpha_{SO}$).