High-Field Magnetic Phase Transition in $\mathrm{Cu}{\left(\mathrm{N}{\mathrm{O}}_3\right)}_2·2\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$

Proton magnetic-resonance measurements are carried out on a single crystal of $\mathrm{Cu}{\left(\mathrm{N}{\mathrm{O}}_3\right)}_2·2\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$ in magnetic fields up to 50 kOe at temperatures below 1 K. The copper ions form pairs with an exchange interaction $-\frac{J}{k}$ of about 5 K. In magnetic fields around 36 kOe the much weaker interpair exchange $J^{\prime }$ leads to short- and long-range order below 0.35 and 0.16 K, respectively. The phase transition at 0.16 K is characterized by a spontaneous component of the time-averaged magnetic moment of the Cu ions perpendicular to the external field, and disappears below 29 and above 42 kOe.