Single Crystal Growth,Magnetic Properties And Magnetoelectric Effects Of Transition Metal Oxide(Fe/Co/Ni)₄Nb₂O₉

With the continuous development of information technology,the performance requirements of semiconductor devices are gradually improved along with the improvement of application requirements,which also stimulates people to explore new high-quality storage media.Multiferroic materials with two or more basic ferroics have great application potential in this field.For multiferroic materials it is possible to realize the four-state logic storage characteristics which can improve the storage capacity in the new storage devices using them as storage media.In the study of magnetoelectric materials,transition metal oxide A₄B₂O₉(A=Co,Mn,Fe,B=Nb,Ta)system has strong magnetoelectric effect at the temperature below Néel temperature due to its special spin structure,which has become potential Single-phase magnetoelectric information functional material that researchers have long paid attention to.This dissertation takes Fe₄Nb₂O₉ and Ni doped Co₄Nb₂O₉ as the research object,and deeply studies its structural transformation,magnetic phase transformation,magnetic anisotropy and magneto-electric coupling effect.The main contents are as follows:1)Fe₄Nb₂O₉ and Co_4-xNi_xNb₂O₉(x=1,2)series polycrystalline materials are synthesized by solid-state reaction method,and the corresponding high quality single crystal samples are grown by optical floating zone method.The preparation of Fe₄Nb₂O₉ polycrystalline samples was carried out by the method of step-by-step weighing grinding in a glove box,and the single crystal of Co₂Ni₂Nb₂O₉ was grown in an atmosphere of 0.6 MPa argon.The phase and crystal structure of Fe₄Nb₂O₉ and Co_4-xNi_xNb₂O₉ are analyzed by XRD.X-ray back-reflection Laue camera was used to accurately determine the direction of the crystal axis of the single crystal sample and precise cutting was done with a precision cutting machine..2)Similar to Co₄Nb₂O₉,Fe₄Nb₂O₉ is antiferromagnetic at a temperature below 90K,and the magnetic moment of Fe ions are arranged in the ab plane,showing an angle antiferromagnetic structure.Through pyroelectric test,it is found that Fe₄Nb₂O₉ has the electric polarization which can change the sign with the electric field flipping below the Néel temperature,that is,it has the ferroelectric state.In particular,the occurrence of ferroelectric states does not require the application of external magnetic field,indicating that Fe₄Nb₂O₉ is a rare multiferroic material in the A₄B₂O₉ family.When the external magnetic field is applied,the polarization intensity of Fe₄Nb₂O₉ increases with the increase of the external magnetic field,and can reach more than 300?C/m² at the external magnetic field of 70 k Oe.Fe₄Nb₂O₉ also exhibits a metagnetic transformation at 50 K,which occurs in the ab plane and is expressed by the magneto-thermal curve(M-T),isothermal magnetization curve(M-H)and pyroelectric curve(P-T).3)Co_4-xNi_xNb₂O₉(x=1,2)is antiferromagnetic at x=1,and the Néel temperature is about 32 K.The crystal structure and magnetic properties are close to Co₄Nb₂O₉,with a spin-flop phase transition below the Néel temperature,and the phase transition critical magnetic field Hc?11.9 k Oe at 20 K.When x=2,the magnetic properties of the system are different from that of the parent phase Co₄Nb₂O₉ due to the increase of Ni doping amount.The doped compound is antiferromagnetic,and the appearance temperature of macroscopic magnetic order is increased to T_N?47 K.The magneto-thermal curves along the a and[210]directions at 44 K show a spin glass-like behavior.The bifurcation of the ZFC and FC curves and the sharp peaks of the ZFC curves are observed in the experiment.Below the Néel temperature,Weak hysteresis loops can be observed in the in-plane isothermal magnetization test,which is caused by the magnetic transformation in the in-plane during magnetization.According to existing reports and test results,the magnetic moment of the doped sample is distributed in ab plane when x=2.The test below 4 K shows a magnetic transition in the A₄B₂O₉ system that is not limited to the ab plane.This may be due to the fact that the magnetic moments of the Co and Ni ions in the doped sample cannot form an anti-parallel arrangement because of the competition,and finally a new magnetic order is induced by the temperature.