All-Optical Magnetic Recording with Circularly Polarized Light

We experimentally demonstrate that the magnetization can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field. This optically induced ultrafast magnetization reversal previously believed impossible is the combined result of femtosecond laser heating of the magnetic system to just below the Curie point and circularly polarized light simultaneously acting as a magnetic field. The direction of this opto-magnetic switching is determined only by the helicity of light. This finding reveals an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.

Figure 1

The effect of ultrashort polarized laser pulses on magnetic domains in ${\mathrm{Gd}}_{22}{\mathrm{Fe}}_{74.6}{\mathrm{Co}}_{3.4}$. (a) Magneto-optical image of the initial magnetic state of the sample before laser exposure. White and black areas correspond to up ($M^{+}$) and down ($M^{-}$) magnetic domains, respectively. (b) Domain pattern obtained by sweeping at low speed ($\sim 30\mu \mathrm{m}/\mathrm{s}$) linear ($L$), right-handed (${\sigma }^{+}$), and left-handed (${\sigma }^{-}$) circularly polarized beams across the surface of the sample, with a laser fluence of about $11.4\mathrm{mJ}/{\mathrm{cm}}^2$. The central area of the remaining spots at the end of each scan line consists of small magnetic domains, where the ratio of up to down magnetic domains is close to 1.

Figure 2

Images of the laser spot focused onto a domain wall during the 1 kHz pulsed laser excitation with linear ($L$), right- (${\sigma }^{+}$), and left-handed (${\sigma }^{-}$) circular polarization. The images were obtained for a pulse fluence of about $11.4\mathrm{mJ}/{\mathrm{cm}}^2$. In all three cases the central region of the optically excited area is demagnetized (gray color) due to extensive heating. For circularly polarized excitation (${\sigma }^{+}$ and ${\sigma }^{-}$) opto-magnetic switching takes place on the perimeter of the excited area, where the temperature is just below $T_{\mathbf{C}}$.

Figure 3

Images of the magnetic film after exposure to $\sim 20$ ultrashort left-handed ${\sigma }^{-}$ (top row) and right-handed ${\sigma }^{+}$ (bottom row) circularly polarized laser pulses at different pulse energies. Before each exposure, the sample was saturated in the up ($M^{+}$) state. The graphs on the right show the intensity profile of the laser beams used for excitation.

Figure 4

All-optical magnetic recording by femtosecond laser pulses. (a) The effect of single 40-fs circular polarized laser pulses on the magnetic domains in ${\mathrm{Gd}}_{22}{\mathrm{Fe}}_{74.6}{\mathrm{Co}}_{3.4}$. The domain pattern was obtained by sweeping at high-speed ($\sim 50\mathrm{mm}/\mathrm{s}$) circularly polarized beams across the surface so that every single laser pulse landed at a different spot. The laser fluence was about $2.9\mathrm{mJ}/{\mathrm{cm}}^2$. The small size variation of the written domains is caused by the pulse-to-pulse fluctuation of the laser intensity. (b) Demonstration of compact all-optical recording of magnetic bits. This was achieved by scanning a circularly polarized laser beam across the sample and simultaneously modulating the polarization of the beam between left and right circular.