Specific heat and magnetic order in GdBa2-xSrxCu3O7
We report measurements of the specific heat of GdBa2-xSrxCu3O7 for x=0, 0.25, and 0.5 in the temperature range 0.65 K. In addition, measurements in magnetic fields from 0 to 3 T for the x=0 sample are reported. In each case a peak in the specific heat at the antiferromagnetic ordering temperature is recorded and analyzed. Strontium doping does not change the Néel temperature, but drastically changes the shape of the specific-heat curve below the ordering temperature. The Néel temperature is reduced proportional to the square of the applied magnetic field. A model calculation of the spin-wave contribution to the low-temperature specific heat shows that strong couplings along the c axis and weak couplings in the a-b plane are responsible for the antiferromagnetic order. © 1989 The American Physical Society.