Generation of terahertz transients from ${\mathrm{Co}}_2{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$-Heusler-alloy/normal-metal nanobilayers excited by femtosecond optical pulses

We generated pulses of electromagnetic radiation in the terahertz (THz) frequency range by optical excitation of ${\mathrm{Co}}_2{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ (CFMS)/normal-metal (NM) bilayer structures. The CFMS is a Heusler alloy showing a band gap in one spin channel and is therefore a half metal. We compared the THz emission efficiency in a systematic manner for four different CFMS/NM bilayers, where NM was either Pt, Ta, Cr, or Al. Our measurements show that the THz intensity is highest for a Pt capping. We also demonstrate the tunability of the THz amplitude by varying the magnetic field for all four bilayers. We attribute the THz generation to the inverse spin Hall effect. In order to investigate the role of the interface in THz generation, we measured the spin mixing conductance for each CFMS/NM bilayer using a ferromagnetic resonance method. We found that the spin-orbit coupling cannot completely describe the THz generation in the bilayers and that the spin transmission efficiency of the CFMS/NM interface and the spin diffusion length, as well as the oxidation of the NM layer, play crucial roles in the THz emission process.

Figure 1

Schematics of the experimental setup. (a) Laser beam paths for the generation and detection of THz radiation. (b) CFMS/NM bilayers are illuminated from the NM side and the THz signal on the MgO side is detected; the external in-plane magnetic field points along the $y$ axis. (c) The CPW transmission line with the sample placed in the middle of the structure. The external field $B$ is parallel and the radiofrequency (rf) field $h_{\mathrm{rf}}$ perpendicular to the CPW transmission line below the sample.

Figure 2

Measured THz transients for CFMS/NM bilayers with NM standing for Pt, Ta, Cr, and Al (a) and corresponding FFT spectra (b). Note in (a) the different ordinate scaling for Pt (left) and Ta, Cr, Al (right). The samples were illuminated by laser pulses with a wavelength of $\lambda =800\mathrm{nm}$, pulse duration $\tau =100\mathrm{fs}$, and laser fluence of $6\mu \mathrm{J}/{\mathrm{cm}}^2$. The laser pulse was impinging at the capping layer under an angle of approximately 5 ° with respect to the surface normal. The transients are all measured in an in-plane magnetic field of 16 kA/m applied in the same direction.

Figure 3

Comparison of the magnetization (black symbols) and THz peak amplitude (color symbols) hystereses under variation of the external magnetic field for CFMS/NM bilayers with different NM capping materials. The diameter of the laser-beam focus spot for the THz measurements was 200 μm ($1/e^2$ beam waist).

Figure 4

Schematic sketch of spin pumping via FMR. The rf field induces precession of the localized spins in the CFMS layer around the magnetization direction $M$. These, in turn, are able to polarize the delocalized electrons, which leads to a spin accumulation $\delta m$ (red curve) inside the CFMS layer. The gradient of $\delta m$ into the Pt layer causes a spin current $J_s$ across the interface [43].

Figure 5

(a) Frequency sweeps from 0.5 to 20 GHz with typical resonant absorption dips for several external magnetic field values. For reasons of clarity, not all measured absorption curves are shown. (b) Extracted magnetic field linewidths $\mathrm{\Delta }H$ calculated using Eqs. (2) and (3) vs the resonance frequencies $f_{\mathrm{res}}$ for Pt capping. The inset shows the increasing resonance frequency vs the external applied magnetic field for all capping layers.

Figure 6

Normalized THz amplitude $A^{\mathrm{THz}\left(\mathrm{norm}\right)}$ (green circles), $Z$ dependence of SOC $Z^{4\left(\mathrm{norm}\right)}$ (yellow triangles and line), and effective spin mixing conductance $g_{\mathrm{eff}}^{\uparrow \downarrow \left(\mathrm{norm}\right)}$ (blue diamonds) as a function of atomic number $Z$ for all four NM materials used as capping layers. $Z^4$ dependence of SOC is weighted with the factors $({\lambda }_{\mathrm{sf}}/t_{\mathrm{FM}/\mathrm{NM}})\tanh \left[t_{\mathrm{NM}}/\left(2{\lambda }_{\mathrm{sf}}\right)\right]$ and $g_{\mathrm{eff}}^{\uparrow \downarrow \left(\mathrm{norm}\right)}$ (purple stars and line) to take into account the spin transmission efficiency of the CFMS/NM interfaces, the effective (nonoxidized) thickness of the NM layer, and the spin diffusion length of the different capping layers, respectively. All values of $A^{\mathrm{THz}\left(\mathrm{norm}\right)}$, $Z^{4\left(\mathrm{norm}\right)}$, and $g_{\mathrm{eff}}^{\uparrow \downarrow \left(\mathrm{norm}\right)}$ as well as the weighted $Z^4$ dependence are normalized to the corresponding values for the Pt capping.

Figure 7

(a) THz transients for CFMS/Pt taken at opposed in-plane magnetic field directions. (b) THz peak amplitude vs the fluence at the sample position for the CFMS/Pt bilayer.

Figure 8

(a) Hysteresis loops $A^{\mathrm{THz}}\left(H\right)$ of the peak amplitude of THz transients measured with the external field applied at 0 ° and 45 ° for CFMS/Pt. (b) Magnetic hysteresis loops $M\left(H\right)$ measured by VSM with the external field applied at 0 ° and 45 ° also for CFMS/Pt.

Figure 9

FMR field linewidths $\mathrm{\Delta }H$ vs the resonance frequencies $f_{\mathrm{res}}$ for all capping materials. The solid lines are fits with Eq. (4) of the main text.

Figure 10

XRR results (red) and fits (blue) for (a) CFMS/Pt, (b) CFMS/Ta-TaO, and (c) CFMS/Al-AlO layer systems.

Figure 11

XANES (top row) and XMCD (bottom row) spectra for the element-sensitive investigation of structural and magnetic properties of the CFMS close to the interfaces with the Al, Cr, Ta, and Pt capping layers (dark to light color line).