Picosecond Spin Current Generation from Vicinal Metal-Antiferromagnetic Insulator Interfaces

We report the picosecond spin current generation from the interface between a heavy metal and a vicinal antiferromagnet insulator ${\mathrm{Cr}}_2{\mathrm{O}}_3$ by laser pulses at room temperature and zero magnetic field. It is converted into a detectable terahertz emission in the heavy metal via the inverse spin Hall effect. The vicinal interfaces are apparently the source of the picosecond spin current, as evidenced by the proportional terahertz signals to the vicinal angle. We attribute the origin of the spin current to the transient magnetic moment generated by an interfacial nonlinear magnetic-dipole difference-frequency generation. We propose a model based on the in-plane inversion symmetry breaking to quantitatively explain the terahertz intensity with respect to the angles of the laser polarization and the film azimuth. Our work opens new opportunities in antiferromagnetic and ultrafast spintronics by considering symmetry breaking.

Figure 1

AFM image and height profile marked by the white line of (a) ${\mathrm{Al}}_2{\mathrm{O}}_3(0001)$ substrate and (b) 15-nm-thick ${\mathrm{Cr}}_2{\mathrm{O}}_3(0001)$ film with $\alpha =0.5°$. (c) The schematic of the time-domain THz spectroscopy measurement setup. (d) The relation between crystal and laboratory coordinates. The polarization of the laser $E_{\text{laser}}$ is defined by the angle $\theta$ with respect to $x^{\prime }$. The sample surface is set in the $x^{\prime }{y}^{\prime }$ plane and the azimuth angle is defined by $\phi$ with respect to $x^{\prime }$.

Figure 2

(a) The time domain spectroscopy of $E_{x^{\prime }}^{\mathrm{THz}}$ measured on a series of $\mathrm{Pt}(4)/{\mathrm{Cr}}_2{\mathrm{O}}_3(15)$ bilayers grown on the ${\mathrm{Al}}_2{\mathrm{O}}_3(0001)$ substrates with different $\alpha$ at $\theta =\phi =0°$. (b) The $\alpha$ dependence of the peak values of $E_{x^{\prime }}^{\mathrm{THz}}$.

Figure 3

(a) $E_{x^{\prime }}^{\mathrm{THz}}$ of $\mathrm{Al}(5)/{\mathrm{Cr}}_2{\mathrm{O}}_3(15)$, Pt(4), ${\mathrm{Cr}}_2{\mathrm{O}}_3(15)$, $\mathrm{Pt}(4)/{\mathrm{Cr}}_2{\mathrm{O}}_3(15)$, and $\mathrm{W}(4)/{\mathrm{Cr}}_2{\mathrm{O}}_3(15)$ on ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrates with $\alpha =2°$ at $\theta =\phi =0°$. (b) Response of $E_{x^{\prime }}^{\mathrm{THz}}$ to external fields of $\mathrm{Pt}(4)/{\mathrm{Cr}}_2{\mathrm{O}}_3(15)$ with $\alpha =2°$ at $\theta =\phi =0°$. (c) The time domain spectroscopy of $E_{x^{\prime }}^{\mathrm{THz}}$ measured on $\mathrm{Pt}(4)/{\mathrm{Cr}}_2{\mathrm{O}}_3(15)$ with different pump laser fluence with $\alpha =2°$ at $\theta =\phi =0°$. (d) The peak values of $E_{x^{\prime }}^{\mathrm{THz}}$ as a function of the pump fluence, the solid blue line is a linear fit.

Figure 4

The THz waveforms of the $\mathrm{Pt}(4)/{\mathrm{Cr}}_2{\mathrm{O}}_3(15)$ bilayers with $\alpha =2°$ for (a) various $\phi$ at $\theta =0$ and (c) various $\theta$ at $\phi =0°$. The corresponding peak values of $E_{x^{\prime }}^{\mathrm{THz}}$ as functions of (b) $\phi$ and (d) $\theta$. The red dots are the raw data, and the blue lines represent the fitted curves with Eqs. (4) and (5), respectively.