Preparation And Magnetoelectric Properties Of Multiferroic BaFe₁₂O₁₉/Sr₃Al₂O₆/SrTiO₃ Heterojunction And Polycrystal Mn₃O₄

Multiferroic materials,with magnetic and ferroelectric orders,have a wide range of uses in magnetic sensing,magnetic memory devices,and other fields.BaFe₁₂O₁₉（Ba M）hexagonal ferrite has wide applications in microwave and vertical magnetic recording due to the large magnetocrystalline anisotropy,chemical stability and high coercivity.In particular,Ba M and Mn₃O₄ are good carriers for studying the magnetoelectric coupling mechanism of multiferroic materials due to their special magnetic structure,which can induce ferroelectric polarization.Therefore,the preparation conditions and magnetoelectric properties of Ba M hexagonal ferrite films and polycrystal Mn₃O₄ are investigated in detail.Frist,Ba M films were deposited on SrTiO₃<111>（STO）substrates using pulsed laser technology,and the effects of different substrate temperatures on the orientation,microstructure of the Ba M films were explored.Subsequently,a buffer layer of Sr₃Al₂O₆（SAO）was added between the STO substrate and the Ba M film,and its water-soluble property was used to investigate the peeling process of flexible film,and the magnetic properties of the Ba M film with and without the buffer layer were compared.The results show that:（1）the films have a single orientation along the c-axis,obvious hexagonal morphology and low roughness when the substrate temperature is 720℃.（2）Ba M films with the buffer layer show stronger uniaxial magnetocrystalline anisotropy along the c-axis,large coercivity（up to about 16 k Oe）and a remanence ratio of 0.99.Second,based on the above optimal preparation conditions,the relationship between thicknesses of the films and their magnetic properties was investigated by changing the deposition time of Ba M films.The result demonstrates that as the deposition time increased,the c-axis orientation of the films become worse,leading to the decrease of saturation magnetization.Then,Au/Ba M/SAO/Nb:STO film heterojunctions were deposited with Nb:STO（111）as the substrate to investigate the magnetodielectric properties of the films.The results showed that the dielectric anomaly peak did not vary with the frequency near 80K,which may be caused by the ferroelectric polarization induced by the conical magnetic structure.Third,(Mn_3-xCr_x)₃O₄（x=0-0.5）single phase polycrystalline samples were successfully prepared by the conventional solid-phase method.The crystal structure,morphology and magnetoelectric properties of x=0,0.4 samples were investigated.The results are as follows:（1）The space group of the doped sample changes from I4₁/amd to Fd3^—m,and the morphology changes from a tight mosaic structure to discrete pentahedra and octahedra.（2）The Yafet-Kittel type ferrimagnetic magnetic phase transition point increased from 43 K to 60 K after doping,which is due to the enhancement of the magnetic interaction from the decrease of the Mn³⁺(Cr³⁺)/Mn²⁺-O bond length and the increase of the Mn³⁺(Cr³⁺)-O-Mn³⁺(Cr³⁺)bond angle.（3）The dielectric curves show that there are dielectric anomalous peaks near the magnetic phase transition point,which do not move with frequency in the samples before and after doping,indicating the existence of magnetic-order-related ferroelectric polarization in the system.