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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 ofTN=9.8 K and the associated small critical field ofBC(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 toB0=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 belowTN. The critical exponent a(x) of the specific heat decreases in a non-linear way with dilution starting from a large positive value for EuTe. a(x) is close to the Heisenberg value -0.12 nearx˜0.83 where the biquadratic interaction is zero. The same applies for the exponent ß of the sublattice magnetization.

In this paper, we report on measurements of the specific heatC of single-crystalline EuxSr1-xTe at temperatures between 60 mK and 15 K and in magnetic fields up to 6 T. Pure antiferromagnetic EuTe shows unusual critical behavior in the vicinity of the Néel temperatureTN=9.8 K with a positive critical exponent instead of the 3d-Heisenberg exponent a=-0.12. Possible reasons for this discrepancy between theory and experiment include magnetic anisotropy effects due to magnetic dipole-dipole interactions, which may give rise to a cross-over of the critical behavior very close toTN. This anisotropy is also seen in the specific heat below 1 K where an exponential decay ofC is observed, and in the dependence of the magnetic susceptibility on the direction of the applied field. With increasing dilution of EuTe with nonmagnetic Sr, the critical behavior changes: a becomes negative and decreases continuously towards a ˜-1 atx˜xc. This concentration dependence of a was previously observed in the diluted ferromagnetic system EuxSr1-xS. Our data thus support that the apparent change in the critical behavior depends on the degree of disorder. Samples with concentrationx lower than the critical concentrationxc reveal spin-glass behavior in the specific heat. In addition, the dependence ofTN on magnetic fields is discussed. The data yield a normalized magnetic phase boundaryBc(T)/Bc(T=0) vs.TN(B)/TN(B=0) which is independent of concentration.

High-resolution measurements of the specific heat,C, near the ferromagnetic phase transition of the diluted ferromagnetic system EuxSr1-xO are reported. Samples with four different concentrations (x=1,x=0.9,x=0.7 andx=0.5) have been studied and the expected phase diagram, i.e. the linear decrease ofTc with decreasingx, is confirmed. Our specific-heat data of the pure EuO samples yield a critical exponent a=-0.12±0.02 in contrast to literature results, but in agreement with the value expected theoretically for this three-dimensional Heisenberg system. The origin of the discrepancies is traced down to differences in data analysis.