Unusual thermal properties of the nonsuperconducting PrBCO cuprates at low temperatures

Abstract The unusual thermal properties of two ceramic samples of nonsuperconducting PrBa 2 Cu 3 O 6+ x (or PrBCO) cuprates in two states of oxygenation ( x = 0.44 and 0.95), are analyzed using experiments based on magnetization, M and differential magnetic susceptibility, χ d . The spontaneous magnetization, M S and χ d show similar 1/ T variation with a fast decay above the Pr antiferromagnetic (AFM) ordering temperature T N (Pr) = 9 and at 14 K, respectively. Only a tiny singular-point occurs in M S - T curve, close to T N (Pr), whereas a visible anomaly at the low-critical point T cr = 4-5 K can be seen in χ d - T curve which presents an almost temperature-independent variation before a shape change when crossing T N (Pr). The Pr magnetic contribution to the linear thermal expansion, α and specific heat, C p shows a substantial increase which continues by a well-defined broad anomaly at T N (Pr) when T decreases, and provides evidence for the existence of the spin reorientation phase transition at T 2 ~ 6-7 and 9-10 K, respectively for x = 0.44 and 0.95 while the T cr -point remains at 4-5 K. All these features observed below and well above T N (Pr), cannot be described suitably simply in terms of the magnetoelastic coupling of the crystal field states with the lattice strains or Schottky anomaly. The weak ferromagnetic component due to the significant Pr-Cu(2) coupling appears as the key for the anomalous thermal and magnetic properties of the PrBCO compounds whatever the oxygen doping.