Specific heat of Eu x Sr1−x Te

In this paper, we report on measurements of the specific heatC of single-crystalline Eu x Sr1−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 α=−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: α becomes negative and decreases continuously towards α ≈−1 atx≈xc. This concentration dependence of α was previously observed in the diluted ferromagnetic system Eu x Sr1−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.