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

Abstract 

Superconductors with a ferromagnet as one of the principal components are considered. A group of new stable superconductors (of the Y-Ni-B system) containing a ferromagnetic component up to 50% at. is found out as well. The YNi_1.9 B_1.2 phase is revealed to have the highest critical parameters among the known superconductors with a ferromagnetic component (T_c∼13,5 K , ΔT_c∼ 0,6 K, J_c∼ 5 ^.10⁴ A/cm², H_c2(0) up to 30 kOe) . The model of its crystal structure is proposed. The YNi_1.9B_1.2 superconductor 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 characterised by an anomalous dependence of magnetic susceptibility on the temperature and magnetic field, i.e. magnetic susceptibilityχ shows no changes with increasing temperature (within 15–273 K) in a fixed magnetic field and decreases with increasing magnetic field from 0.214 to 4.28 kOe at a fixed temperature. The anomalous position of the curve can be described within the WHH theory in the Fulde and Maki approximation, the positive convexity of the H_c2(T) curve being attributed to anomalous magnetic properties of the YNi_1.9B_1.2 compound in a normal state. The coexistence of superconductivity with antiferromagnetism is assumed in the analyzed compound.