Magnetism and superconductivity in a iodine doped bismuth-2234 HTSC: properties and computer simulation

Full text: We research the phase interaction and phase transformation in Bi1.6Pb0.4Sr2Ca3Cu4O12+y - BiOI system and modification of superconducting and magnetic properties of these compounds by method of the 'anion(oxygen) - anion(iodine)' substitution. The synthesis of the 2234 phase take place in the narrow temperature interval 840 - 850 C in presents of Pb ions, BiOI powder and with Ca/Sr>1. Introduction of halogen into Bi1.6Pb0.4Sr2Ca3Cu4O12-y results in a change of melting character into quasi congruent one. Significant increasing in microhardness of samples and decreasing in their porosity, which was observed, has the positive influence on superconductive parameters of these compounds. We observed two superconducting transitions in the temperature region 60 -110 K with Tc1 108 K ('high- Tc' phase) and Tc2 = 65K ('low- Tc' phase) for Bi1.6Pb0.4Sr2Ca3Cu4O12-zIz samples with z > 0.025, respectively. In the external magnetic field with magnitude higher as 2.5 T two magnetic phase transitions are observed: the first short- range antiferromagnetic ordering with TN=67.8 K and teta1 = -31 K; and second long range ferromagnetic(or metamagnetic) ordering with TC= 14.625 K and teta2 1.5 K. The existence of competing ferromagnetic and antiferromagnetic interaction is not very surprising, because there exist no direct Cu-O-Cu superexange path in the Bi1.6Pb0.4Sr2Ca3Cu4O12-zIz structure, respectively for the antiferromagnetic ferromagnetic interaction (depending on the spin and charge state of cooper ions: d(z2) or d(x2-y2) of copper). The mechanisms of appearing and coexisting of the magnetic ordering and superconductivity in these compounds are determined. A mathematical model and the computer simulation process are proposed. (authors)