Synthesis And Physical Properties Of Hexaferrites

Hexaferrites have received much attention recently due to the demonstration of magnetoelectric coupling, especially its ferroelectric polarization can be induced around room temperature as well as by a small magnetic field, which suggests the potential application in room-temperature multiferroic devices. In this work, Y-type and M-type hexaferrites samples were prepared by solid-state reaction method, effects of ion-substitution in hexaferrites on the crystal structure, surface morphology, magnetic and dielectric properties was investigated subsequently, finally magnetocaloric effect and its mechanism in Y-type hexaferrites were explored. The main results and conclusions are showed as follows:(a) Polycrystalline samples of Al-doped Y-type Bao.5Sr1.5Zn2(Fe1-xAlx)12O22and (Co,Ti)-doped M-type BaFe12-2xCoxTixO19were synthesized by solid-state reaction method, and then effect of ion-substitution on its physical properties. Though the formation of the hexaferrites is an extremely complicated process, and all the different hexagonal phases overlap in different sintered temperatures, the pure Y-type hexaferrites can be obtained by modulating Al content. The results suggest the hexagonal grains are destroyed when x≥0.08; The grain size of M-type hexaferrites becomes large with increasing Co and Ti content; On the other hand, ion-substitution can affect the magnetic properties of Y-type hexaferrites such as magnetic phase transition temperatures, saturation moment and coercive field etc., which can be assigned to that the superexchange of Fe-O-Fe can be modulated by ion-substitution, and collinear (longitudinal conical) magnetic spin-order transforms into noncollinear (transverse conical) spiral magnetic spin-order.(b) Measuring and analyzing the dielectric spectra of Y-type hexaferrites at different temperature utilizing a special dielectric platform. There have three kinds of dielectric response:grain, grain boundary and electrode, however, the generation mechanism of dielectric relaxation has not very clear in Y-type hexaferrites. The impedance spectra at different temperature can be fitted utilizing a revised equivalent circuit model, and the mechanism is discussed combined with Maxwell-Wagner theory. The results indicate intrinsic dielectric response derived from grain can be observed at low temperature and high frequency, the grain boundary and electrode start to work near room temperature. Through the fitting of relaxation activation energy, the kind of carriers can be determined, which is from the hopping between Fe2+and Fe3+. On the other hand, the permittivity enhances with increasing Al content, the relaxation peaks from grain boundary and electrode shift to high frequency, which might be assigned to that the change of grain conductivity due to ion-substitution leads to the variation of accumulation of the carriers in grain boundary and elextrode, finally frequency shift occurs.(c) Measuring and analyzing the magnetocaloric effect (MCE) of pure-phase Y-type hexaferrites. Very recently, the multicaloric effect in strongly spin-polarization coupled multiferroic materials has been theoretically introduced. However, this kind of materials has not been studied widely. Furthermore, Y-type hexaferrites exhibits a sharp peak in magnetization curve, implying a drastic magnetic phase transition takes place in a small temperature range, based on these the MCE was investigated in a pure-phase Y-type hexaferrites. Above50K, the maximum values of the magnetic entropy change (-△Sm) and relative cooling power (RCP) are1.53JKg-1K-1and280JKg-1for a field change of7T, respectively, which can be attributed to the second-order magnetic transition. Furthermore,-△SMmax can be observed around room temperature (T<<Tc), in this case the magnitude of MCE is often determined primarily by the rotation of the magnetization vector. In addition, inverse MCE is also observed, which might associate with the first-order magnetic transition between two incommensurate longitudinal conical phases, where the occurrence of mixed exchange interactions leads to the increase of the orientation disordered spins in applied an external magnetic field.(d) SrTiO3thin films were epitaxially grown on (100) SrTiO3substrates using molecular beam epitaxy. The etching effects of substrates under buffer-modulation acid solution with different PH values were investigated, and the optimized growth parameters were obtained. Oxygen-vacancy was modulated by thermal annealing in different oxygen-deficient oxygen plasma partial pressure. The results show that oxygen-vacancy can induce and enhance luminescence and magnetism in SrTiO3films.