Terahertz Generation via Picosecond Spin-to-Charge Conversion in ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ Heterojunction

Experimental investigations of ultrafast electro-optical properties in magnetic materials manifest their great potential for emerging spintronic optoelectronic devices. Here, using time-resolved terahertz emission spectroscopy, we construct a spintronic terahertz emitter consisting of an ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ heterojunction. A femtosecond spin current pulse is generated in the thin film of the $\mathrm{Ni}-\mathrm{Fe}$ layer when it absorbs a femtosecond laser pulse, and then the spin current is converted into a transient charge current by the metallic ${\mathrm{Ir}\mathrm{Mn}}_3$ layer on picosecond timescales. We timely record the terahertz emission associated with this ultrafast conversion process by means of electro-optic sampling. Besides, the spin-to-charge conversion efficiency of the ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ heterojunction is determined via quantitative analysis of the spin torque ferromagnetic resonance results. We have both optically verified and electrically studied the spin-to-charge conversion of the ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ heterojunction. Our results enlarge the material choice range of spintronic terahertz emitters, which may promote further investigations of ultrafast spin-to-charge conversion in different heterojunction materials.

Figure 1

(a) Schematic illustration of ultrafast spin injection and conversion in ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ heterojunction. $\theta$ is the polarization angle of the external magnetic field with respect to the $y$ axis. (b) The magnetization moment $M$ of ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ heterojunction as a function of magnetic field $H_x$, where $\theta$ is set as $0^{\circ }$.

Figure 2

Sketch of terahertz emission experimental measurement setup.

Figure 3

Ultrafast spin-current-induced terahertz emission from ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ heterojunction. (a) Typical terahertz emission in the time domain emitted from ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ heterojunction and pure $\mathrm{Ni}-\mathrm{Fe}$ film. The azimuthal angle of the external magnetic field is $0^{\circ }$ and the pump power of the incident laser is 100 mW. (b) The corresponding Fourier transforms for the terahertz emission in the frequency domain. (c) The terahertz emission in the time domain with various pump powers. (d) The peak-to-peak magnitude of terahertz radiation with respect to the pump power. The solid line is a fit to the data.

Figure 4

(a) Terahertz emission in the time domain from ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ heterojunction for two typical azimuthal angles of external magnetic field $\theta$. (b) The corresponding Fourier transforms for the terahertz emission in the time domain. (c) The dependence of peak-to-peak magnitude of terahertz radiation on the azimuthal angle of external magnetic field $\theta$ from $0^{\circ }$ to ${360}^{\circ }$. The solid curve is a fit to the data of the complete period. (d) The peak-to-peak magnitude of terahertz radiation measured in ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ heterojunction as a function of azimuthal angle of external magnetic field $\theta$ and time delay.

Figure 5

STFMR measurements. (a) A typical STFMR signal (symbols) of an ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ device at 6 GHz with fits (lines), where the violet and red lines represent the symmetric and antisymmetric Lorentzian components, respectively. (b) The STFMR signal of the ${\mathrm{Ir}\mathrm{Mn}}_3/\mathrm{Ni}-\mathrm{Fe}$ device at 6 GHz with different input powers. (c) Resonance frequency $f$ as a function of the resonant field $H_0$. The solid curve represents a fit to the Kittel formula. (d) Resonance linewidth $\mathrm{\Delta }H$ as a function of the resonant frequency $f$.