Biquadratic exchange and critical behaviour in the diluted antiferromagnet EuxSr1−xTe

Magnetization, specific heat, magnetocaloric effect, and neutron diffraction measurements have been performed in order to better understand the magnetic interactions and phase transitions in the diluted antiferromagnetic system EuxSr1−xTe. Due to the low Néel temperature ofT N =9.8 K and the associated small critical field ofB C (T=0)=7.5 T, EuTe provides the opportunity to change the angle between the antiferromagnetic moments continuously between 180° and 0° by applying a magnetic field and allows thus to probe the type of the magnetic interaction. It is found that in addition to the common bilinear Heisenberg exchange mechanism there is evidence for a small biquadratic exchange term, which has important consequences on the critical behaviour of EuxSr1−xTe. In the case of EuTe the biquadratic exchange supports the antiferromagnetic order, such that virtually no anisotropy exists and this restricts the anti-Herromagnetic (AF) to spin-flop (SF) transition toB 0=0. On dilution with strontium, the biquadratic interaction decreases strongly and changes its sign at x≈0.83 thus favouring the perpendicular moment orientation. This acts like an anisotropy on the dominating antiferromagnetic bilinear interaction and shifts the AF-SF phase line to finite fields which reach values of 0.3 T forT→0. No bicritical behaviour is observed, but instead, the SF phase is the state of lowest energy in a small temperature interval belowT N . The critical exponent α(x) of the specific heat decreases in a non-linear way with dilution starting from a large positive value for EuTe. α(x) is close to the Heisenberg value −0.12 nearx≈0.83 where the biquadratic interaction is zero. The same applies for the exponent β of the sublattice magnetization.