Possible Correlation-Enhanced Magnetic Ordering at Anomalously High Temperatures in Dy under Extreme Compression*

Most lanthanides order magnetically at temperatures $T_{\mathrm{o}}$ well below ambient, the highest being 292 K for Gd. The highly localized magnetic state of the heavy lanthanides should become unstable at sufficiently high pressure, leading to a competition between the RKKY interaction and Kondo physics. Most lanthanides undergo a volume collapse at a critical pressure $P_{\mathrm{vc}}$, the largest being 16{\%} in Ce at only 0.7 GPa but 6{\%} in Dy at 73 GPa, possibly a sign that the magnetic state has become unstable. Recent electrical resistivity measurements on Dy reveal a highly non-monotonic dependence of $T_{\mathrm{o}}$ on pressure. Immediately above $P_{\mathrm{vc}}$, $T_{\mathrm{o}}(P)$ in Dy shows a dramatic increase, extrapolating to values near 400 K at 160 GPa (1.6 Mbar). Interestingly, the pressure dependence of the magnetic spin-disorder resistivity $\rho _{\mathrm{sd}}(P)$ tracks that of $T_{\mathrm{o}}(P)$. The results of parallel experiments on Gd and further heavy lanthanides will also be presented.