A study of La-substituted strontium hexaferrite by hydrothermal synthesis

The possibility of replacing Sr by La and Fe by Co and the subsequent improvement of the magnetic properties of the M-type ferrite stimulated our interest in studying other rare-earth ion substitutions. We have found that Sm and Nd substitutions can increase the coercivity of the hydrothermally synthesised Sr hexaferrite powder without causing any significant fall in the saturation magnetisation. In the present work, La-substituted Sr hexaferrite particles were prepared by hydrothermal synthesis and subsequent calcinations. The effects of the initial La/Sr ratio and the calcination temperature on the structure, particle morphology and magnetic properties of La-substituted Sr hexaferrite were investigated by X-ray diffraction, scanning electron microscopy, vibrating sample magnetometry and a pulsed field magnetometer. It was found that, under hydrothermal conditions, the La3+ additives did not substitute exclusively into the SrM structure but this could be achieved to a greater extent by subsequent calcinations at high temperatures. Compared to the effect of Sm and Nd substitutions, La substitution only increases slightly the coercivity for the samples with initial La/Sr ratios up to 1/8, and for the samples with La/Sr ratios such as 1/4 and 1/2, the coercivity is smaller than that of the sample without La. This can be attributed to grain growth during calcination. Most of the La-substituted samples exhibit a similar magnetisation to that of SrFe12O19