The Researches On Growth And Properties Of Multiferroic Fluorides

Multiferroic materials now are being extensively studied,due to the coexistence of ferromagnetism?or antiferromagnetism?and ferroelectricity in one material.The two ferroic properties can be mutually controlled through the magnetoelectric coupling,making multiferroic materials applicable in various fields,especially spin electronics.Because of the higher electronegativity of F than O,the multiferroic fluorides containing 3d transition metal atoms are more possible to break the limitation of the coexistence of ferromagnetism and ferroelectricity in perovskite oxides and realize the single-phase multiferroicity.Fluorides BaMF₄?M=Mn,Ni,Co,Fe,etc.?are typical multiferroic materials,which have the advantages of ferroelectricity at room temperature with high Curie temperature.However,BaMF₄ is generally antiferromagnetic with a very low Néel temperature,making it difficult to get practical application.We have prepared several pure phase Ba MF₄ fluorides.BaNiF₄ was prepared in epitaxial thin films and its magnetic and ferroelectric properties were studied.Pure phase solid solution of BaCoF₄ and BaNiF₄ with molar ratio of 1:1 was prepared and its magnetic properties were studied.The main work and conclusions are summarized as following:?1?BaNiF₄ films were epitaxially deposited on Al₂O₃?0001?sigle-crystal substrates by pulsed laser deposition?PLD?.The epitaxial growth of BaNiF₄ films is comfirmed by the observation of only?040?,?080?and?0120?diffration peaks in X ray diffration patterns.Using piesoresponse force microscopy?PFM?,the ferroelectricity at room temperature has been inferred from the observation of PFM hystersis loops.The wasp-wasted hysteresis loops are clearly observed below 10 K,which is due to the weak antiferromagnetic nature of the canted spins.Clear weak ferromagnetism has been observed up to 300 K,which is considered to be due to the epitaxial strains.?2?Pure phase solid solution of BaCoF₄ and BaNiF₄ with molar ratio of 1:1(BaCo_0.5Ni_0.5F₄)is prepared by solid state reaction,which has been confirmed by X ray diffraction patterns.Field dependent magnetization measurements only show the linear curve with temperature down to 5 K,indicating the antiferromagnetic nature.Compared with BaCoF₄ and BaNiF₄,no significant enhancement of magnetization is observed,indicating the absence of ferrimagnetism and random distribution of Co and Ni ions.The low temperature magnetic anomalies are studied by zero field cooled?ZFC?and field cooled?FC?temperature dependent magnetization?M-T?measurements.A bifurcation between FC and ZFC M-T curves happens at 118 K,indicating the onset of 2-dimensional antiferromagnetism.The magnetization maximum at 87 K is attributed to the 2-dimensional antiferromagnetic clusters,followed by the drastic decrease of magnetization,which is due to the onset of 3-dimensional antiferromagnetism.A dip is observed in FC M-T curve at 40 K,which is attributed to the3-dimensional antiferromagnetic clusters.A drastic increase of magnetization is observed at 9K,which is due to the uncompensated isolated spins.Exchange bias is clearly observed,with blocking temperature of 90 K.The contribution from surface spin glass has been excluded by the AC magnetization measurements,and the mechanism has been explained by the exchange coupling between the two antiferromagnetic phases.