Texture and magnetic properties of aligned hexaferrite obtained from spark plasma sintering of nanosized sixline-ferrihydrite platelets resulting in (BH)max = 33 kJ m−3.

No abstract

Combining multiple characterization probes (neutrons, X-rays, electrons) illustrated the hierarchical structure of exchange-spring nanocomposites & revealed unreported degradation of parent phase.

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.

Nanocrystalline platelets of Sr hexaferrite, SrFe 12 O 19 , were prepared by four techniques (two hydrothermal and two sol-gel techniques) and calcined at 1000 °C for 1 h, 2 h, 4 h, 8 h and 16 h. The microstructure of these samples was analyzed using Rietveld refinements of high-resolution synchrotron powder X-ray diffraction (PXRD) data, and the obtained results were correlated with the magnetic properties obtained from vibrating sample magnetometer (VSM) measurements. The calcination treatment causes the crystallites to preferentially grow along the c-axis, leading to more isotropic crystallites. Moreover, the microstructural changes induced by calcination alter the magnetic properties, yielding higher saturation magnetizations in all samples. The attained coercivity is correlated with the crystallite size along the width of the platelets. Despite the pronounced changes of the microstructure and the magnetic properties after calcination, the calcination duration has a minor effect on the properties, i.e. in most cases steady state is obtained after 1 hour. The starting material has a profound impact on the microstructural change during calcination, despite the high calcination temperature.