BIQUADRATIC EXCHANGE INTERACTIONS IN THE EUROPIUM MONOCHALCOGENIDES

We have examined the biquadratic exchange interaction strengths in the Europium monochalcogenides EuO, EuS, EuSe and EuTe using magnetizatio n data of the paramagnetic phase and elaborate the consequences this a dditional interaction mechanism has on the magnetic phase diagrams of EuSe and EuTe. It is shown that the cubic susceptibility chi(3) obeys a Curie-Weiss law at sufficiently high temperatures and that the assoc iated Curie-Weiss temperature theta(3) is a measure for the biquadrati c interaction strength. For all these materials the biquadratic intera ction is ferromagnetic (theta(3) > 0). This leads to a conflicting sit uation in the case of EuTe for which theta(1) < 0. We attribute the pe culiar observation, that the MnO superstructure reflection intensities as observed with neutron scattering correspond only to 0.6 of that mo ment expected for perfect magnetic order, to the presence ni biquadrat ic interactions. The critical field B-c follows a T-2 law in the spin- wave regime (T < 0.8 K) for EuTe and EuSe but for these two materials with an antiferromagnetic ground state the cubic susceptibility chi(3) diverges at a temperature T which is 2.5 K and 1.2 K above the order ing temperature, respectively. In the temperature range T-c < T < T t he magnetization curves exhibit some weak but definite anomaly which m ight be interpreted as a field-induced transition into the ferromagnet ic state. A new multicritical point has been identified along the crit ical field curve B-c of EuSe.