Gigantic directional asymmetry of luminescence in multiferroic $\mathrm{CuB}{}_2\mathrm{O}{}_4$

In multiferroic materials, luminescence intensities can be direction dependent, i.e., different between the opposite propagating directions of emitted light. However, the effect has not been thought to be used for technological applications, since only small directional asymmetry has been reported so far. Here we show that the effect is robust in multiferroic ${\mathrm{CuB}}_2{\mathrm{O}}_4$. The luminescence intensity changes by about $70\%$ between the opposite directions of the emission, which is about 100 times larger than the previously reported values. We demonstrate that such a gigantic directional asymmetry of luminescence can be applied to the imaging of canted antiferromagnetic domains. The observation of the effect and its application to magnetic domain imaging are important for a deeper understanding of light-matter interactions as well as technological applications such as optical reading techniques for magnetic memory devices.

Figure 1

(a), (b) Schematic diagram of direction dependent luminescence in ${\mathrm{CuB}}_2{\mathrm{O}}_4$ (orange arrows). Red arrows show the magnetic moments of ${\mathrm{Cu}}^{2+}$ ions at $A$ sites in magnetic fields of (a) $B_{110}<0$ and (b) $B_{110}>0$. (c) Schematic illustration of the experimental setup. The magnetic field was applied along the [110] axis. The sample was excited by a 0.5 mW He-Ne laser at 633 nm (1.96 eV). The laser light was cut by a long-pass filter after the sample. The emitted light polarized along the [110] axis was selected by a polarizer and measured by a spectrometer. (d) Blue and red lines indicate optical spectra of absorption and PL for the light of $E^{\omega }\parallel \left[110\right]$ and $B^{\omega }\parallel \left[001\right]$ in zero magnetic field at $T=15$ K, respectively.

Figure 2

(a) Magnetic field dependence of the PL spectrum at $T=15$ K in a magnetic field $B\parallel \left[110\right]$ for the emitted light of $E^{\omega }\parallel \left[110\right]$ and $B^{\omega }\parallel \left[001\right]$. The inset shows the energy diagram of ${\mathrm{Cu}}^{2+}$ ions at $A$ sites. The transition process is attached. (b) The DDL spectra $\left(\mathrm{\Delta }I/I\right)$ in the Voigt configuration (red) and the Faraday configuration (black).

Figure 3

(a) Temperature dependence of the DDL signal $\mathrm{\Delta }I/I$ at each peak position in a magnetic field of $B=500$ Oe. (b) Magnetic-field dependence of the PL intensity at $T=15$ K in a magnetic field along the [110] axis. (c) Schematic illustration of the microscopic mechanism of the DDL. The sign of the interference term between the $E1$ and $M1$ transitions changes with the reversal of either the magnetic field or the propagating direction of light.

Figure 4

(a), (b) Magnetic domain images of a $({\mathrm{Cu}}_{0.95}{\mathrm{Ni}}_{0.05}){\mathrm{B}}_2{\mathrm{O}}_4$ crystal obtained at $T=10$ K by (a) PL at 1.3988 eV and (b) absorption at 1.406 eV. The exposure times of both images are 20 s. Bright and dark regions correspond to the magnetic domains with the magnetization in the $\left[110\right]$ and $\left[\overline{1}\overline{1}0\right]$ directions, respectively. The diameter of the hole of the copper plate is $600\mu \mathrm{m}$. There is no sample in the top dark (bright) region in (a) [(b)].