Specific heat and magnetic order in GdBa sub 2 minus x Sr sub x Cu sub 3 O sub 7

We report measurements of the specific heat of GdBa{sub 2{minus}{ital x}}Sr{sub {ital x}}Cu{sub 3}O{sub 7} for {ital x}=0, 0.25, and 0.5 in the temperature range 0.6--5 K. In addition, measurements in magnetic fields from 0 to 3 T for the {ital 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 Neel temperature, but drastically changes the shape of the specific-heat curve below the ordering temperature. The Neel 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 {ital c} axis and weak couplings in the {ital a}-{ital b} plane are responsible for the antiferromagnetic order.