Imaging antiferromagnetic domains in LiCoPO$_4$ via the optical magnetoelectric effect

Abstract

Antiferromagnetic (AFM) materials are considered as promising building blocks of novel data storage devices, still, detecting and manipulating AFM domains have remained challenging. Here, we demonstrate that the two antiphase domains of the magnetoelectric antiferromagnet LiCoPO$_4$ can be distinguished by their light absorption difference. Using visible and infrared spectroscopy, we observed spontaneous non-reciprocal absorption, also termed as directional dichroism, at the crystal field excitations of Co$^{2+}$ ions coordinated by distorted oxygen octahedra. This absorption contrast is particularly pronounced near the telecommunication wavelength of 1550 nm. These findings allowed us to image the AFM domains in LiCoPO$_4$ using a simple transmission light microscopy setup. Our findings suggest that optical magnetoelectric effects offer promising routes for probing the AFM order parameter in non-centrosymmetric transition metal compounds.

Figures (4)

• Figure 1: (a) Crystal and magnetic structure of LiCoPO$_4$ viewed from the $x$ direction. Green arrows show the direction of magnetic moments, their arrangement corresponds to one of the antiferromagnetic domains. (b) Schematic illustration of the absorption contrast between the antiphase antiferromagnetic (AFM) domains observed on a thin $xy$ cut of LiCoPO$_4$.
• Figure 2: (a) Absorption spectra of LiCoPO$_4$ measured at 5 K, after magnetoelectric (ME) poling for light polarizations $E^{\omega}\parallel x$ (blue, green) and $E^{\omega}\parallel y$ (red, orange). An AFM domain is selected depending on the sign of ME free energy term $B_xE_y$, where $B_x$ and $E_y$ are the poling magnetic and electric fields. In the colored energy ranges the sample is opaque, i.e., the absorption is beyond the detection limit. On the top of the panel, the energy levels are reproduced from the crystal field calculations of Ref. Kornev1999. (b) Absorption difference of the two AFM domains for different polarizations. The insets show a magnified energy range between 0.06 and 0.18 eV.
• Figure 3: Transmission images of LiCoPO$_4$ recorded at (a) 300 K and (b), (c) below $T_{\mathrm{N}}$ at 5 K after zero-field cooling (ZFC). (b) Brighter and darker regions indicate different antiferromagnetic domains. (c) The domain walls are highlighted by black lines.
• Figure 4: Transmission images of LiCoPO$_4$ recorded at 5 K after cooling the sample only in a magnetic field. (a) and (b) corresponds to images measured after poling only with magnetic fields pointing along and opposite to $\mathbf{H}\parallel y$, respectively.