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

Abstract

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 Neel 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.