Interplay between $\text{Co}3d$ and $\text{Ce}4f$ magnetism in CeCoAsO

We have investigated the ground-state properties of polycrystalline CeCoAsO by means of magnetization, specific heat, and solid-state NMR. Susceptibility and specific-heat measurements suggest a ferromagnetic order at about, $T_C=75\text{K}$. No further transitions are found down to 0.5 K. At 6.5 K a complex Schottky type of anomaly shows up in the specific-heat results. The interplay between $\text{Ce}4f$ and $\text{Co}3d$ magnetism being responsible for that anomaly is discussed. Furthermore ${}^{75}\text{A}\text{s}$-NMR investigations have been performed to probe the magnetism on a microscopic scale. As-NMR spectra are analyzed in terms of first and second-order quadrupolar interaction. The anisotropic shift component $K_{\text{ab}}$ and $K_{\text{c}}$ could be derived from the ${}^{75}\text{A}\text{s}$ powder spectra. Towards lower temperature a strong shift anisotropy was found. Nonetheless $K_{\text{iso}}$ tracks the bulk susceptibility down to $T=50\text{K}$ very well. Furthermore the presence of weak correlations among the Ce ions in the ferromagnetic state is discussed. The observed increase in $C/T$ towards lower temperatures supports this interpretation.

Figure 1

(Color online) Temperature dependence of the susceptibility of CeCoAsO at different fields. The right inset shows the magnetization at 2 K and the left inset shows a comparison of the susceptibility for CeCoPO (data taken from Ref. 19), CeCoAsO, and LaCoAsO (data taken from Ref. 17) at 1 T.

Figure 2

(Color online) (Left panel) Temperature dependence of the specific heat of CeCoAsO. The right inset shows the specific heat plotted as $C/T$ on a logarithmic temperature scale. The left inset shows the behavior of transition after subtracting the background. Right panel shows the specific heat plotted as $C/T$ vs $T$ at different field in the temperature range 1.8–20 K (note that $0.1\text{J}/{\text{mol-K}}^2$ was added as an offset in the $C/T$ plot on the right).

Figure 3

(Color online) Temperature dependence of the ${}^{75}\text{A}\text{s}$ field sweep NMR spectra at 48 MHz down to 50 K (left panel). Dotted line indicates the Larmor field estimated from the reference compound GaAs with ${}^{75}\text{K}\simeq 0^{23}$. ${}^{75}\text{A}\text{s}$ spectra below 50 K (right, bottom). ${}^{75}\text{A}\text{s}$ spectra at 200 and 100 K together with the theoretical simulation (solid line) (right, top).

Figure 4

(Color online) Temperature dependence of the ${{}^{75}\text{K}}_{\text{ab}}$, ${{}^{75}\text{K}}_{\text{c}}$, and ${{}^{75}\text{K}}_{\text{iso}}$. Inset shows the plots of ${{}^{75}\text{K}}_{\text{iso}}$ against the bulk susceptibility $\chi$ measured at 5 T.