Magnetic And Ferroelectric Properties Of Multiferroic Fluorides

With the increasing demand for device miniaturization,new materials with two or more functions need to be developed urgently.In the field of industrial application and laboratory research,ferroelectric materials and magnetic materials have been widely and deeply studied.If ferroelectricity and ferromagnetism exist in a material at the same time,and the two can be regulated by each other,then this material has a very high application value in the field of information storage.At present,a lot of work has been done to report the magnetoelectric coupling effect of multiferroic materials.However,till now,the single-phase multiferroic materials are very rare.There are many magnetic ferroelectrics in the field of fluoride,which can be intensively studied to realize the single-phase multiferroic materials fulfilling the requirements of practical applications.Fluorine is highly electronegative,forms ionic bonds with transition metal ions,and has strong coulomb interaction,which may break the empirical rule of the contradictory requirements of outer shell electrons in transition metal for ferroelectricity and ferromagnetism in oxide perovskites.However,till now,the magnetic ordering temperature of fluorides are mostly much lower than room temperature,which severely limits their practical application.Therefore,it is of great significance to study the ferromagnetism and magnetoelectric coupling effect of multiferroic fluorides at room temperature.BaMnF₄ thin films and BaCoF₄ thin films were prepared based on BaMF₄（M=Mn,Co,Ni,etc.）family.The main contents and results are as follows:1.BaMnF₄ films have been successfully prepared on（0001）Al₂O₃ substrates by pulsed laser deposition.The X-ray diffraction pattern confirms the（010）orientation of the films.The clear observation of amplitude and phase hysteresis loops by piezoresponse force microscopy confirms the ferroelectricity at room temperature.Magnetization measurements show the hysteresis loop clearly,together with the bifurcation at 300 K between zero field cooling and field cooling temperature dependent magnetization cures,indicating the Curie temperature higher than room temperature.Furthermore,exchange bias effect is observed below 200 K,due to the magnetization pinned by the spontaneous polarization.The improved ferromagnetism at room temperature in BaMnF₄ films might be due to the epitaxial strain from the lattice mismatch between the film and substrate.2.The（010）oriented BaCoF₄ thin films have been successfully prepared on the（0001）Al₂O₃ substrates by pulsed laser deposition.The clear observation of amplitude and phase hysteresis loops by piezoresponse force microscopy confirms the ferroelectricity at room temperature.In contrast to the antiferromagnetism in bulk BaCoF₄ with much lower Néel temperature of 69.6 K,clear magnetic hysteresis loops are observed in BaCoF₄ films,indicating the weak room temperature ferromagnetism.The nearly unchanged saturated ferromagnetic magnetization of about 30 emu/cm³ between 5 K and 300 K suggests that the Curie temperature of BaCoF₄ films is much higher than room temperature.Exchange bias is clearly observed,with blocking temperature of 250 K.Negative magnetocapacitance effect is observed in the frequency range up to 800 k Hz,confirming the magnetoelectric coupling at room temperature.