Antiferromagnetic ordering in superconducting and oxygen-deficient nonsuperconducting R Ba sub 2 Cu sub 3 O sub 7 minus. delta. compounds ( R =Nd and Sm)

Low-temperature (0.5--4 K) specific-heat measurements have been made on oxygen-deficient ({delta}{approx}0.5) nonsuperconducting {ital R}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} ({ital R}=Nd and Sm) compounds and compared to measurements previously performed on their high-critical-temperature {ital T}{sub {ital c}}{approx}92 K superconducting counterparts ({delta}{approx}0.1). We show that the specific-heat anomaly due to magnetic ordering of the Nd{sup 3+} and Sm{sup 3+} ions in the {ital T}{sub {ital c}}{approx}92 K superconducting {ital R}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} ({ital R}=Nd and Sm) compounds can be well described by a two-dimensional anisotropic antiferromagnetic Ising model with exchange-interaction parameters {ital E}{sub 1} and {ital E}{sub 2} in the {ital a}-{ital b} plane; the respective values of the Neel temperature {ital T}{sub {ital N}} and the ratio {ital E}{sub 1}/E{sub 2} are 0.5 K and 50 for Nd and 0.61 K and 11 for Sm.