Giant Magnetic Fluctuations at the Critical Endpoint in Insulating HoMnO3

Although abundant research has focused recently on the quantum criticality of itinerant magnets, critical phenomena of insulating magnets in the vicinity of critical endpoints (CEP's) have rarely been revealed. Here we observe an emergent CEP at 2.05 T and 2.2 K with a suppressed thermal conductivity and concomitant strong critical fluctuations evident via a divergent magnetic susceptibility (e.g., chi''(2.05 T, 2.2 K)/chi''(3 T, 2.2 K)=23,500 %, comparable to the critical opalescence in water) in the hexagonal insulating antiferromagnet HoMnO3.