Virtual Bicritical Point in CsMnF3

The ordering temperature $T_c$ of the easy-plane antiferromagnet CsMn${\mathrm{F}}_3$ was measured as a function of magnetic field $H$. when H is perpendicular to the easy plane, $T_c$ decreases monotonically with increasing $H$, but the decrease is not proportional to $H^2$. The latter behavior is explained in terms of a virtual bicritical point which exists mathematically at a negative value of $H^2$.