Ni doping effect on the magnetic properties of polycrystalline Y-type hexaferrite Ba_0.5Sr_1.5Zn₂Fe_12−xNi _xO₂₂

The microstructure and magnetic properties of multiferrioc polycrystalline Y-type hexaferrite Ba_0.5Sr_1.5Zn₂Fe_12−xNi_xO₂₂ (BSZFNO, x = 0, 0.05, 0.1, 0.2, 0.35 and 0.5) synthesized by the solid-state reaction method are investigated. It is found that the shrink of BSZFNO lattice constant and the increase of grain size appear after substituting Fe ion with Ni ion, and Ni doping depresses the formation of Z-type and M-type impurity phases. BSZFNO undergoes two main magnetic phase transitions. Near room temperature, the temperature of phase transition from proper screw spin phase to collinear ferromagnetic phase increases with Ni amount increasing, suggesting the enhancement of exchange interaction of Fe(4)–O–Fe(5). The magnetic phase transition at lower temperature is attributed to the spin transition from a longitudinal conical spin order to a proper screw spin order, whose transition temperature varies due to different magnetocrystalline anisotropy with introducing Ni into BSZFO lattices. Furthermore, Ni doping leads to the expansion of the range of field-driven magnetic phases, especially intermediate-II and intermediate-III helical spin phase. The saturation magnetization at 5 K increases with Ni doping and arrives at a maximum value with Ni content x = 0.2, finally decreases with x. The increasing of the pitch of spin helix in initial Ni-doping leads to the enhancement of the magnetization. However, the conical spin phase eventually collapses in higher-concentration Ni-doping.