Ultrafast control of lattice strain via magnetic circular dichroism

Using ultrafast x-ray diffraction, we directly monitor the lattice dynamics induced by femtosecond laser pulses in nanoscale thin films of bismuth iron garnet in external magnetic fields $H_{\mathrm{ext}}$. We control the ultrafast laser-induced lattice strain amplitude by changing the laser pulse helicity. The strength of $H_{\mathrm{ext}}$ is used as an external parameter to switch the helicity dependence on and off, respectively. Based on magneto-optical spectroscopic measurements, we explain these phenomena by magnetic circular dichroism. Our findings highlight an important approach for ultrafast manipulation of lattice strain in magnetic materials, in particular insulators, and open exciting perspectives towards ultrafast control of lattice strain and heat-induced magnetization switching and spin waves in bismuth substituted iron garnets using the polarization of light.

Figure 1

Experimental configuration and static x-ray diffraction results. (a) Sketch of the UXRD configuration with pump pulses and external magnetic field perpendicular to the film. (b) X-ray diffraction line profile recorded along the $q_z$ direction around the 004 Bragg reflections of the BIG layer and the GGG substrate. The arrows indicate the 004 Bragg reflection related to BIG and the GGG. The inset shows the reciprocal space map around the 004 Bragg reflections of BIG layer and GGG substrate.

Figure 2

Time-resolved lattice dynamics induced by right (${\sigma }^{+}$, squares) and left (${\sigma }^{-}$, solid circles) circular polarization pump beam with a pump fluence $F_{\mathrm{pump}}=3.5\mathrm{mJ}\mathrm{c}{\mathrm{m}}^{-2}$ for $H_{\mathrm{ext}}=450\mathrm{mT}$.

Figure 3

Strain amplitude $\eta$ measured as a function of orientation angle $\theta$ of the quarter wave plate for $F_{\mathrm{pump}}=3.5\mathrm{mJ}\mathrm{c}{\mathrm{m}}^{-2}$. The absolute strain $\eta \left(\theta \right)$ obtained for BIG at three different time delays of (a) $t=500\mathrm{ps}$, (b) $t=1000\mathrm{ps}$, and (c) $t=2000\mathrm{ps}$ for $H_{\mathrm{ext}}=450\mathrm{mT}$. (d) The relative strain $\left(\eta \left(\theta \right)-\overline{\eta }\right)/\overline{\eta }$ obtained for BIG and the nonmagnetic reference sample at $t=500\mathrm{ps}$ for $H_{\mathrm{ext}}=450\mathrm{mT}$. (e) The relative strain for BIG at $t=500\mathrm{ps}$ for $H_{\mathrm{ext}}=450\mathrm{mT}$ and $H_{\mathrm{ext}}=0\mathrm{mT}$. The arrows in (a) highlight the QWP angles corresponding to circular right (σ+), linear (π), and circular left (σ+) polarizations of the pump beam. The solid lines are guides to the eye. The error bars indicate the standard deviation of the time resolved data.

Figure 4

Static magneto-optical and magnetic properties of the BIG film measured in polar configuration. (a) Magnetic circular dichroism spectrum of the BIG film, including the spectrum of the pump beam used in the UXRD measurements (green line). (b) Magnetic circular dichroism hysteresis loop. The inset shows a magnified view of the region round 0 T.