Anisotropy of Antiferromagnetic 180° Domains in ${\mathrm{LiCoPO}}_4$ and ${\mathrm{LiNiPO}}_4$

Unexpected three-dimensional distributions of antiferromagnetic 180° domains are observed in ${\mathrm{LiCoPO}}_4$ and ${\mathrm{LiNiPO}}_4$ by optical second harmonic generation. Domains in ${\mathrm{LiCoPO}}_4$ are isotropic in spite of the quasi-two-dimensional magnetic structure whereas domains in ${\mathrm{LiNiPO}}_4$ are distinctly anisotropic, but in contrast to the anisotropy of the magnetic structure. The diversity reveals a potential for fine-tuning magnetic properties determined by the distribution of domains or domain walls and the urgent need for an improved understanding of spatial correlations in antiferromagnets.

Figure 1

Spectral and temperature dependence of SHG in ${\mathrm{LiCoPO}}_4$ (a)–(c) and ${\mathrm{LiNiPO}}_4$ (d)–(f). Bias of all insets is zero. The inset in (a) and panel (a) use the same absolute scale; all other intensities are arbitrary scales. Spectra were taken at 10 K. Temperature dependence was measured at 2.50 eV.

Figure 2

SHG images of ${\mathrm{LiCoPO}}_4$ and ${\mathrm{LiNiPO}}_4$. Temperature was 10 K. White bars correspond to 0.3 mm.

Figure 3

Three-dimensional domain topology of ${\mathrm{LiCoPO}}_4$ (010). (a) SHG image at 10 K and $2\hslash \omega =2.40\mathrm{eV}$. (b) SHG image at 10 K and $2\hslash \omega =2.65\mathrm{eV}$. (c) SHG intensity obtained from Eq. (2) for two domains stacked perpendicular to the surface in dependence of the thickness $d$ of the “$-$” domain. Values used for the simulation: sample thickness $L=103\mu \mathrm{m}$, absorption coefficient (at 2.65 eV) $\alpha =185{\mathrm{cm}}^{-1}$ (from Ref. 15), phase mismatch $\Delta k=1015{\mathrm{cm}}^{-1}$ [from fit of minimum and maximum SHG intensities in (b)]. (d) Domain structure perpendicular to the surface for the section in (b) derived graphically from (b) and (c). Horizontal lines are taken from the maxima and minima in (c). Vertical lines correspond to the position of the maxima and minima in the section.