Complex borides with a ferromagnetic component: Development of the problemof magnetic superconductors

Superconductors with a ferromagnet as one of the principal components are c onsidered. A group of new stable superconductors (of the Y-Ni-B system) con taining a ferromagnetic component up to 50% at. is found out as well. The Y Ni1.9B1.2 phase is revealed to have the highest critical parameters among t he known superconductors with a ferromagnetic component (T-c similar to 13, 5 K, DeltaT(c) similar to 0,6 K, J(c) - 5.10(4) A/cm(2), H-c2(0) up to 30 k Oe). The model of its crystal structure is proposed. The YNi1.9B1.2 superco nductor is characterized by an anomalous position of the H-c2(T) curve (the positive convexity within 9.5-13.5 K). In a normal state, it is characteri sed by an anomalous dependence of magnetic susceptibility on the temperatur e and magnetic field, i.e. magnetic susceptibility chi shows no changes wit h increasing temperature (within 15-273 K) in a fixed magnetic field and de creases with increasing magnetic field from 0.214 to 4.28 kOe at a fixed te mperature. The anomalous position of the curve can be described within the WHH theory in the Fulde and Maki approximation, the positive convexity of t he H-c2(T) curve being attributed to anomalous magnetic properties of the Y Ni1.9B1.2 compound in a normal state. The coexistence of super conductivity with antiferromagnetism is assumed in the analyzed compound.