Structure and magnetic properties of W-type hexaferrites.

W-type hexaferrites (WHFs) (SrMe 2Fe16O27, Me = Mg, Co, Ni and Zn) are hard magnetic materials with high potential for permanent magnet applications owing to their large crystalline anisotropy and high cation tunability. However, little is known with regards to their complex structural and magnetic characteristics. Here, the substitution of metals (Me = Mg, Co, Ni and Zn) in WHFs is described and their crystal and magnetic structures investigated. From joined refinements of X-ray and neutron powder diffraction data, the atomic positions of the Me atoms were extracted along with the magnetic dipolar moment of the individual sites. The four types of WHFs exhibit ferrimagnetic ordering. For Mg, Ni and Zn the magnetic moments are found to be ordered colinearly and with the magnetic easy axis along the crystallographic c axis. In SrCo2Fe16O27, however, the spontaneous magnetization changes from uniaxial to planar, with the moments aligning in the crystallographic ab plane. Macromagnetic properties were measured using a vibration sample magnetometer. The measured saturation magnetization (M s) between the different samples follows the same trend as the calculated M s extracted from the refined magnetic moments of the neutron powder diffraction data. Given the correlation between the calculated M s and the refined substitution degree of the different Me in specific crystallographic sites, the agreement between the measured and calculated M s values consolidates the robustness of the structural and magnetic Rietveld model.