Texture formation in W-type hexaferrite by cold compaction of non-magnetic interacting anisotropic shaped precursor crystallites

Abstract

Crystallites of the W-type hexaferrites, Sr(Ni_1−xZn_x)₂Fe₁₆O₂₇ (x = 0, 0.5, 1) have been aligned without applying magnetic field nor hot compaction, but through a simple synthesis process taking advantage of easy alignment of non-magnetic interacting, anisotropic-shaped precursor crystallites of goethite. The goethite precursor was prepared through a simple hydrothermal synthesis route, forming lathlike crystallites with apparent dimensions of 23.3 × 40.1 × 11.0 nm³ as extracted from powder X-ray diffraction along the a-, b- and c-axis, respectively. The calcined pellets consisted of almost phase pure W-type hexaferrites with relative small impurities of spinel ferrite (≤9.02(3) wt%). The high synthesis temperature resulted in large crystallites, which in turn caused low coercivities (H_c ≤ 5.4(1) kA m⁻¹) and a squareness ration (M_r/M_s, remanence (M_r) over saturation magnetisation (M_s)) close to zero for all samples. The vanishing coercivity makes M_r/M_s an unsatisfying measure of preferred orientation. Quantitative texture analysis of the samples was carried out based on 2D transmission synchrotron diffraction data collected at different orientations of the samples. The texture investigations revealed alignment of the crystallites with the c-axis normal to the pressing surface of the pellets. The SrNi₂Fe₁₆O₂₇ sample showed the highest texture index of 7.5 m.r.d.².