New Multiferroic Ceramics With Perovskite-Like Structure

Recently, magnetoelectric multiferroic materials have attracted unprecedented interest because of their great theoretical significances and prospects of practical applications. The perovskite-like type multiferroic materials are one of the hottest research topics among these materials. In the present thesis, the crystal structures, dielectric, magnetic and magnetodielectric properties of Ca2FeAO5+δ (A= Al, Ga) ceramics were investigated, and the origin of their magnetodielectric (MD) effects were deeply analyzed. Also the crystal structures, ferroelectric properties and phase transition of Ca3(Ti1-xMnx)2O7 (x= 0,0.05,0.1,0.15) ceramics were investigated, and the effects of Mn4+ on crystal structures and ferroelectric properties were analyzed.Partial of Fe3+ ions would be oxidized to Fe4+ ions, since the hyperstoichiometric oxygens were presented in Ca2FeAO5+δ (A= Al, Ga) ceramics. Dielectric relaxations were attributed to polaronic hole hopping between Fe4+ and Fe3+ ions. Antiferromagnetic ordering was found to be dominant in both ceramics and the weak ferromagnetism was likely to be originated from non-cancelled spins of antiferromagnetic-ordered Fe3+ and Fe4+ ions. The dielectric constants at room temperature were very sensitive to weak magnetic fields, and the largest room-temperature MD coefficient (-31.7%) was obtained in a magnetic field strength of 1.4 T at a frequency of 10 MHz in the Ca2FeGaO5+δ ceramic. The MD effects at relatively low frequencies could be attributed to extrinsic effects, whereas the MD effect at relatively high frequencies wre likely to be originated from intrinsic effects. The intrinsic giant MD effect could be attributed to the suppression of charge fluctuation between Fe3+ and Fe4+ ions in the magnetic field. The antiferromagnetic order-disorder transition around room temperature also contributed to the large MD effect.The space groups of Ca3(Ti1-xMnx)2O7 (x= 0,0.05,0.1,0.15) ceramics were A21am, which belong to polar point groups. And their polarizations should be aroused from the a-a-c+ octahedral rotation patterns. All the ceramics showed the typical ferroelectric hystersis loops, and the current density peaks near coercive fields indicated all ceramics were ferroelectrics. The largest polarization was 3.09 μC/cm2, which was found in Ca3Ti2O7 ceramic. The Pmax decreased monotonously with the increasing amount of Mn4+ ions. The significant thermal hysteresis, which was confirmed by the difference between the endothermic and exothermic peaks in DSC spectrum, indicated the present paraelectric-ferroelectric transition should belong to the first order phase transition. With the increasing amount of Mn4+ ions, the temperatures of the endothermic and exothermic peaks dropped linearly.