Magnetic phase transitions close to the bicritical concentration inEuSySe1−y

The critical behavior of the magnetic phase transitions for concentrations very close to the bicritical point in the solid-solution system ${\mathrm{EuS}}_{\mathrm{y}}$ ${\mathrm{Se}}_{1\mathrm{-}\mathrm{y}}$ are analyzed by specific-heat, susceptibility, and magnetization measurements. At the ferromagnetic side of the bicritical point one observes nonuniversal critical behavior which asymptotically crosses over towards three-dimensional Heisenberg behavior. At the antiferromagnetic side of the bicritical point an order–improper-disorder transition with an antiferromagnetically ordered state below the ordering temperature and purely ferromagnetic fluctuations above the ordering temperature is observed.