Abstract
Electrical and magnetic properties of ( La 1− x Sr x ) 2 CuO 4+ δ annealed in high pressure oxygen gas up to 400 bar are studied. The superconducting La 2CuO 4+δ includes two separate phases: one is the superconducting phase with extra oxygen, and the other is the well known antiferromagnetic phase with Néel temperature T N at 240–300 K depending on the annealing atmosphere. The superconducting phase in the undoped sample becomes unstable by Sr doping. In the high pressure oxygen annealed La 2 CuO 4+ δ , a temperature dependent paramagnetism is observed even below T N, and spontaneous vortices are considered to be induced in the zero magnetic field cooling.
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