We have prepared nonoriented and magnetically oriented specimens of Co2Z-type Ba ferrite Ba3Co2Fe24O41 (Ba3Z) and those with Sr2+ substitution for Ba2+, i.e., Ba1.5Sr1.5Co2Fe24O41 (Ba1.5Sr1.5Z) and Sr3Co2Fe24O41 (Sr3Z) with the conventional solid-state reaction method. Permeability measurements of nonoriented specimens have shown that this substitution improves the frequency characteristic of permeability, though the permeability in Sr3Z significantly decreases. X-ray diffraction (XRD) and magnetization measurements of magnetically oriented specimens have shown that the magnetic moments of iron and cobalt ions in Ba3Z and Ba1.5Sr1.5Z lie in the c plane, but that those in Sr3Z deviate from the c plane. We have studied the substitution effect of Sr2+ for Ba2+ on the crystal structures and the effective sizes and directions of magnetic moments and their temperature dependences with high-temperature neutron diffraction technique. This substitution induces the change in the distribution of cobalt ions and moment directions of magnetic ions. Magnetic moments in Ba3Z and Ba1.5Sr1.5Z turn to the c axis when temperature rises from 523to573K. However, the moments in Sr3Z turn to the c axis at the lower temperature of 50K. These results were consistent with the present measurements on permeability, XRD, and magnetization. The change in moment direction caused by temperature rise must be attributed to the disappearance of effective moments of cobalt ions in these temperature regions.

We investigated the correlation between the thermomagnetic curve of Co 2 Z-type hexagonal barium ferrite and its magnetic moment direction. We measured the thermomagnetic curve of Ba 3 Co 1:8 Fe 24:2 O 41 , prepared using the conventional solid-state reaction method, in the temperature range from 294 to 773 K with a vibrating sample magnetometer under 70 Oe. The curve shows two significant magnetization slumps at 540 K and 680 K. High-temperature XRD patterns show that no crystal transformation occurs in the temperature region from 294 to 773 K. High-temperature neutron diffraction experiments were performed to investigate the magnetic moment orientation at elevated temperatures. The Rietveld analyses of the neutron diffraction patterns indicate that the temperature rise from 523 to 573 K makes the magnetic moments turn to the c-axis from a direction parallel to the c-plane most significantly. The slump in magnetization at 540 K may be attributed to the change in easy magnetization direction from the c-plane to the c-axis. The change in average orientation of the magnetic moments must be induced by the disappearance of the contribution of cobalt to magnetism in this temperature range.