Study On Magnetic,Electrical And Magnetoelectric Coupling Properties Of Multiferroic Fluorides

Nowadays,the requirement of information technology is increasing,and one of the core of information technology is information storage device.Multiferroic materials have great potential applications in memory devices because they possess both ferroelectric and ferromagnetic properties and can be controlled by each other,which has a very broad application prospect in the field of information storage.The magnetoelectric coupling effect has been reported a lot,which is mainly focuses on oxides,the research on other multiferroic materials is still rare.Compared with oxides,most fluorides have the ferroelectric Curie temperature much higher than room temperature,and the compound is dominated by ionic bonds.In principle,they are not limited by the conflicting requirements of the electron configuration requirements of the outer layer（this restriction is based on oxides with covalent bonds）for the ferroelectric and ferromagnetic properties of oxides.So in theory,they are a kind of unique multiferroic systems.However,the current research on fluorides is less,and the theoretical system is not completed,so it is still important to explore more potential materials other than oxides.Compared with oxides,one of the major characteristics of fluoride is that ferromagnetism and ferroelectricity can be driven by the same structural unit.A typical example is fluoride with octahedral structure.The octahedral structure of fluorine refers to the octahedron composed of transition metal ions and six fluorine ions,and then the octahedron acts as the basic unit of the structure to form compounds.Compounds of this structure are characterized by their ferroelectric properties resulting from the polarization caused by transition metal ions shifting away from the center of the octahedron.Therefore,if magnetic ion is selected as the central ion of octahedron,ferromagnetism and ferroelectricity can be realized in principle from the same origin.Based on this consideration,Pb₅Fe₃F₁₉ and Sr Fe F₅,belonging to Pb₅M₃F₁₉ and AMF₅family,are selected as research objects in this paper.The A and M in the above series are metallic ions,in which M is usually the transition metal and A is the non-transition metal.The main experiment contents and results are as follows:1.Pb₅Fe₃F₁₉ bulk was prepared by solid state reaction method.XRD pattern showed that Pb₅Fe₃F₁₉ was pure phase and its structure met the structural requirement of ferroelectric property.Phase and amplitude curves were observed using a piezoelectric force microscope,indicating the existence of ferroelectric polarization at room temperature.The measurement of magnetic properties showed that both antiferromagnetic phase and weak ferromagnetism phase exist at low temperature,and magnetic hysteresis loops were observed.The observation of magnetodielectric effect proved that there exists obvious magnetoelectric coupling effect below Néel temperature.2.Sr Fe F₅ bulk was prepared by solid state reaction method.Sr Fe F₅ was paramagnetic at room temperature,and M-T curve showed that antiferromagnetic phase with weak ferromagnetism existed at low temperature due to the Dzyaloshinskii-Moriya interaction.In addition,spin glass state might exist.The M-H curves showed a clear well-shaped hysteresis loop with weak exchange bias at low temperatures.Clear magnetodielectric effect was obtained by the magnetic dielectric measurement.Exchange bias and magnetodielectric effect can be used as the evidence of magnetoelectric coupling in Sr Fe F₅ system with Dzyaloshinskii-Moriya interaction.