Microstructure and magnetic properties of non-stoichiometric M-type hexaferrite with barium surplus

In this paper we present a modified coprecipitation method to obtain single-phase non-stoichiometric M-type hexaferrite with barium surplus Ba1+xCoTiFe10O19+x (x=0.00, 0.05, 0.10, 0.15, 0.20). X-ray analysis confirmed that the crystal structure of Ba1+xCoTiFe10O19+x did not collapse as the x increased in the range 0-0.20. The inconsonant variations of a-axis and c-axis lengths indicate that the excess barium is in the large holes along a-axis of Ba1+xCoTiFe10O19+x. Crystal growth and crystal shape transformation from plane to uniaxial associated with the x value were observed with a scanning electron microscope and transmission electron microscope. The dependence of grain size on the barium surplus (x) shows that the magnetic domain structure of Ba1+xCoTiFe10O19+x can be easily transformed from single domain to multidomain by changing the x value. Magnetic measurements show that the powders of Ba1+xCoTiFe10O19+x for x=0.05 have a maximum coercivity value of Hc=566.22Oe and a maximum remanent magnetization value of σr=19.97emu/g, both have been ascribed to the pinning effects of excess barium and more grain boundaries on the coherent magnetization rotation reversal mechanism of randomly packed single-domain particle ranging from 150 to 300 nm. Specific saturation magnetization values (σs) of Ba1+xCoTiFe10O19+x are unchanged in the x range 0-0.05 but decreased monotonously in the x range 0.05-0.20, and this may be explained in terms of the dilution effect of excess barium on the magnetic Fe3+ cations due to the increase of cell volume. The dilution effect and the magnetization reversal mechanism by the domain wall motion of randomly packed multi-domain structure particles result in the continuous reduction of coercivity and remanent magnetization values of Ba1+xCoTiFe10O19+x as the x value increases in the range 0.05-0.20.