Magnetic properties of pure, Sr- and Ca-doped La<SUB>2</SUB>NiO<SUB>4+&#948;</SUB> ceramics: onset of high-T<SUB>c</SUB> superconductivity

We present the results for the temperature and field dependence of the magnetic for ceramic materials of the composition La<SUB>2&#8722;x</SUB>M<SUB>x</SUB>NiO<SUB>4</SUB>, with M=Sr or Ca and 0&#8804;x&#8804;0.4. The onset of a strong diamagnetism has been observed at temperatures between 8 and 70 K, depending on sample composition, annealing conditions. and thermal cycling procedures. The results are similar to those obtained earlier for monocrystalline samples and are likewise interpreted as due to the onset of superconductivity in a minority phase. A comparison with the results for superconducting La<SUB>1.8</SUB>Sr<SUB>0.2</SUB>Cu<SUB>0.9</SUB>Ni<SUB>0.1</SUB>O<SUB>4</SUB> ceramics is also made; this illustrates some unique features of the nickelate systems, such as the high values of the critical fields H<SUB>c1</SUB> and H<SUB>c2</SUB>. The differences between monocrystalline and ceramic systems are also discussed