Magnetic properties and signatures of moment ordering in the triangular lattice antiferromagnet ${\mathrm{KCeO}}_2$

The magnetic ground state and the crystalline electric field level scheme of the triangular lattice antiferromagnet ${\mathrm{KCeO}}_2$ are investigated. Below $T_N=300$ mK, ${\mathrm{KCeO}}_2$ develops signatures of magnetic order in specific heat measurements and low energy inelastic neutron scattering data. Trivalent ${\mathrm{Ce}}^{3+}$ ions in the $D_{3d}$ local environment of this compound exhibit large splittings among the lowest three $4f^1$ Kramers doublets defining for the free ion the $J=5/2$ sextet and a ground state doublet with dipole character, consistent with recent theoretical predictions by Eldeeb et al. [Phys. Rev. Materials 4, 124001 (2020)]. An unexplained, additional local mode appears, and potential origins of this anomalous mode are discussed.

Figure 1

(a) Experimentally determined lattice parameters and $R\overline{3}m$ crystal structure of ${\mathrm{KCeO}}_2$ at 300 mK. Layers of K ions (purple) separate triangular sheets of ${\mathrm{CeO}}_6$ octahedra. (b) The ${\mathrm{CeO}}_6$ octahedra are trigonally compressed along the crystallographic $c$ axis, resulting in a $D_{3d}$ symmetry local environment about the Ce ions.

Figure 2

(a) Powder neutron diffraction data of ${\mathrm{KCeO}}_2$ collected at 300 mK on the BT-1 diffractometer. No new magnetic reflections are resolved at 300 mK. Solid line shows the results of Rietveld refinement of the data and the lower panel shows the difference between the model and the data. (b) 90 mK diffraction data with high temperature (5 K) background subtracted and collected in a dilution refrigerator. The solid green line shows a simulated diffraction pattern for q=(1/3, 1/3, 0) ${120}^{\circ }$ type order with 0.6 ${\mu }_B$ per Ce site. It is meant to provide a visual benchmark for the detection limit of magnetic order.

Figure 3

(a) Specific heat $C_p\left(T\right)$ of ${\mathrm{KCeO}}_2$ (cyan) measured in zero magnetic field and plotted with a Debye fit (orange) modeling lattice contributions. The magnetic specific heat peaks at 300 mK, suggesting ${\mathrm{KCeO}}_2$ enters an ordered state. The integrated magnetic entropy is overplotted as a black line and approaches $R\ln \left(2\right)$ at high temperature (indicated by the horizontal dashed black line). (b) $C_p\left(T\right)$ of ${\mathrm{KCeO}}_2$ collected under fields of ${\mu }_{0}H=0,9,$ and 14 T.

Figure 4

(a) Inverse magnetic susceptibility of ${\mathrm{KCeO}}_2$ collected under 0.5 T and overplotted with a Curie-Weiss fit to the data. (b) Field dependence of isothermal magnetization at 2, 10, 100, and 300 K. (c) EPR data collected at X-band and 4 K fit to a highly anisotropic $g$ factor of $g_{\perp }=$ 1.9927 and $g_{//}=0.3202$.$1\mathrm{emu}{\mathrm{mol}}^{-1}{\mathrm{Oe}}^{-1}=4\pi {10}^{-6}{\mathrm{m}}^3{\mathrm{mol}}^{-1}$.

Figure 5

(a) Momentum and energy map of neutron scattering intensities collected on the ARCS spectrometer. (b) $\mathbf{Q}$-averaged energy cut of inelastic neutron scattering (INS) spectrum $I(\mathbf{Q},\hslash \omega )$ collected at 5 K and $E_i=300$ meV on the ARCS spectrometer with full width at half maximum (FWHM) energy resolution at the elastic line of 12.8 meV. A linear background is subtracted in order to determine integrated intensity ratios. (c) $\mathbf{Q}$-averaged energy cut of $E_i=600$ meV INS data showing the anomalous $E_3$ mode and the onset of transitions into the $J=7/2$ manifold of states that begins at 280 meV. Uncertainties in energy values represent the FWHM energy resolution at each energy transfer, and error bars (often within the symbols) are uncertainty levels of 1-sigma.

Figure 6

Low energy inelastic neutron scattering (INS) spectra $I(\mathbf{Q},\hslash \omega )$ collected on the CNCS spectrometer at (a) ${\mu }_{0}H=0$ T and 40 mK, (b) ${\mu }_{0}H=8$ T and 40 mK, (c) ${\mu }_{0}H=0$ T and 12 K. Magnetic intensity suggestive of static order originates from $\mathbf{Q}=1.4$ Å, close to the crystallographic $K$ point. (d) Subtracting ${\mu }_{0}H=0$ T and 12 K data from the data in panel (a) reveals spin-wave-like excitations in ${\mathrm{KCeO}}_2$. Two distinct modes appear, the first with a bandwidth up to 1 meV from the structural $K$ point and the second flatter mode with its spectral weight anchored close to 1.25 meV. (e) Under a field of ${\mu }_{0}H=8$ T, the powder-averaged modes begin to diffuse.