Different dielectric, magnetic, and magnetodielectric mechanisms in M-type BaFe12O19 hexaferrite regulated by doping Ga3+ and In3+ cations
We systematically investigated the magnetic, dielectric, and MD properties of BaFe12-xMexO19 ceramics prepared by a solid-state reaction method. The Ga3+ cations with a smaller radius preferentially substitute the Fe3+ ions in FeO6 octahedra while the In3+ cations with a larger radius tend to replace the Fe3+ ions in FeO5 bipyramids of R blocks, inducing different physical characteristics. The pure BaFe12O19 and Ga-doped samples show ferrimagnetism in the temperature range from 10 K to 300 K. The In-doped samples exhibit a transition from non-collinear magnetism to collinear ferrimagnetism. The dielectric decrease of pure BaFe12O19 at around 10-175 K is attributed to the quantum paraelectric state, and the shoulder peaks of loss at about 140-200 K are from electron hopping. The dipole glass state is responsible for the dielectric peak of Ga-doped samples at around 20-40 K. The dielectric increase and plateau of In-doped samples are mainly ascribed to the electron hopping at low temperatures. Their dielectric properties at high temperatures are all attributed to the interfacial polarization caused by the Maxwell-Wagner effect. The MD effect also has different origins for the various samples at low temperatures. For the pure BaFe12O19, the negative MD effect at extremely low temperatures and the positive MD effect after warming are ascribed to spin-phonon coupling and field-dependent electron hopping, respectively. The positive MD effect in Ga-doped hexaferrites results from the field-dependent electric dipoles inside FeO5 bipyramids. For the In-doped samples, the negative MD effect and subsequent transformation to the positive MD effect originate from the field-dependent non-collinear spin ordering and electron hopping, respectively. The MD effect at high temperatures is attributed to the combination of magnetoresistance and Maxwell-Wagner effects.