Nanoparticle composites having structural intergrowths of hexaferrite and spinel ferrites prepared by gel-to-crystallite conversion and their magnetic properties

Nanoparticle composites of spinel (S) and hexaferrite (W or Y phase) in different ratios were prepared by a wetchemical process of gel-to-crystallite conversion. The compositions were selected on the line connecting W–S or Y–S in the $BaO–NiO–Fe_2O_3$ ternary phase diagram. High-resolution electron micrographs of these crystallites showcoherent intergrowth features involving hexaferrite and spinel ferrite structural blocks. Intergrown nickel ferrite blocks appear randomly, with different insertion widths (1–20 nm) with increase in the spinel ferrite content, within the hexaferrite matrix corresponding to M- or Y-type primitive repeat for the composites. Thermomagnetic curves clearly reveal the composite nature of the particle by way of two ferrite components. The specific magnetization of the composites show typical additive rule. With increasing spinel content, coercivity decreases continuously for the compositions on the W–S line, whereas it goes through a maximum on the Y–S line. This is explained on the basis of spin reorientation within the domains for intermediate compositions on the Y–S line. The effective easy direction of magnetization in composites will lie in between the easy direction in cubic nickel ferrite and the easy plane (0 0 1) in $Y-Ni_2$. The increase in coercivity for intermediate compositions is due to the increase in anisotropy field for these composites.