| Multiferroics are defined as materials that exhibit more than one of the primary ferroic properties:ferroelectricity,ferromagnetism,and ferroelasticity.We focused on the materials with both ferromagnetism and ferroelectricity,in which magnetic fields(electric fields)can modulate the electric polarization(magnetization).However,the small magnetoelectric coupling coefficient and low magnetoelectric coupling temperature hamper the development of single-phase multiferroic materials.In recent years,the room-temperature magnetoelectric(ME)effect is realized in hexaferrites,but there are still some problems for putting it into the practical application.In this thesis we try to enhance the ME effect by searching for new materials,doping and growing thin films.The first chapter discussed the contradiction between ferroelectricity and ferromagnetism in transition metal oxides and also their coexistence mechanism.According to the inverse Dzyaloshinskii-Moriya(DM)interaction mechanism,several typical types of spiral magnetic ordering associated with the ME effects are briefly introduced.The second chapter gives the ME effects development history of three kinds of hexaferrites.In the third chapter,we successfully built a ME effect test system according to the theory of direct ME measurement,and described two methods to processing the experimental raw data.In the fourth chapter,we systematically studied the enhanced ME effect in Cr doped Ba0.5Sr1.5Zn2Fe12O22(BSZFO)hexaferrite.The polycrystalline samples show direct ME effect persistent up to 250 K with a maximum ME coefficient 1309 ps/m at 75 K.We also found non-zero polarizations at zero magnetic field,which means that Cr ions effectively modulate the magnetic ground state of BSZFO.At low temperatures,the magnetic field can reverse the electric polarization,while there is no such a phenomenon in high temperature regions.A possible field-dependent magnetic structures were proposed to qualitatively explain this transition.Based on the data of magnetization,dielectric constant and electric polarization as a function of temperatures and magnetic fields,we established a T-H phase diagram for the polycrystalline samples.Moreover,a four-state converse ME effect can be induced by different initial poling fields in the polycrystalline samples.The single crystal sample gives a maximum direct ME coupling coefficient of 2950 ps/m at 10 K.The magnetic field can also reverse the electric polarization at low temperatures,and the electric field almost linearly modulate the magnetization in this single crystal sample.In one word,the Cr ions replacement effectively modulates the feature of magnetic field induced electric polarization since a possible transverse conical magnetic ground state is created.In the fifth chapter,we investigated the ME effect of MnCr2O4 and Sr2CeIrO6 single crystals.Under the commensurate spiral spin order transition temperature of MnCr2O4,magnetic fields can reverse the electric polarization continuously and stably.For Sr2CeIr06,the electric polarization can be detected in different directions,but the magnetic field cannot modulate the electric polarization,which means there is no obvious ME effect.The origin of ferroelectricity in Sr2CelrO6 is not yet very clear and there are a lot of experimental and theoretical verifications demanded.In the sixth chapter,we grew the BaFei0.35Sc1.6Mg0.05O19 M-type ferrite epitaxial film by Pulsed Laser Deposition systems(PLD).When growing the Z-type ferrite film,an M-type ferrite film with hexagonal island-shaped nanoparticles on the surface was unexpectedly obtained.The seventh chapter is a summary and prospect of the work. |
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