Investigation On The Enhancement Of Magnetoelectric Effect With The Interface Between Multiferroic Hexagonal Ferrite And Ferroelectric Thin Films

Multiferroic material is a new type of quantum functional material with ferromagnetism,ferroelasticity and ferroelectricity.These materials not only have rich physical properties,but also have broad prospects in low-power devices.In recent years,among multiferroic oxide materials,SC doped Mtype barium ferrite(BaFe12-xScxO19,x≥0.5)has attracted extensive attention because of its nonlinear conical magnetic structure and magnetic structure induced polarization.For single crystal bulk BaFe12xScxO19,its cone magnetic structure transition temperature(Tcone)can reach 370 K,which means that it has potential multiferroic properties above room temperature.However,after BaFe12-xScxO19 is prepared as epitaxial film,its tcone will be reduced below 306 K.How to further improve the tcone and magnetoelectric coupling properties of BaFe12-xScxO19 films has become an important problem in the application of BaFe12-xScxO19 films to high temperature thin film devices.The preliminary analysis shows that the interfacial strain between BaFe12-xScxO19 film and substrate may have a great influence on tcone.Therefore,in view of the above problems,firstly,the effects of different Sc doping on the magnetic structure of BaFe12-xScxO19 epitaxial films were studied.Secondly,BaFe10.2Sc1.8O19(BFSO)was selected as the research object.A ferroelectric buffer layer BaTiO3(BTO)was introduced between BFSO film and SrTiO3(STO)single crystal substrate to regulate the lattice strain in BFSO,and the change law of magnetic structure in BFSO film under different interface strain was studied.Finally,the influence of BTO buffer layer on the magnetoelectric coupling properties of BFSO thin films was further studied by electrically controlled magnetic devices and magnetic dielectric devices.The specific research results are as follows:Firstly,the effects of different Sc doping on the magnetic structure of BaFe12-xScxO19 epitaxial films were studied.The results show that.With the increase of Sc content in bascm films,the magnetocrystalline anisotropy of the films gradually changes from the uniaxial type of BaFe12O19 to the near plane type of BaFe10.2Sc1.8O19.The field coolingl zero field cooling(ZFC/FC)curves with different doping Sc content further show that the conical magnetic structure transition temperature(Tcone)of the films increases with the increase of SC content.When the doping amount reaches 1.8,its Tcone is higher than room temperature and reaches 306 K.Secondly,the effects of BFSO/BTO interface strain on BFSO hysteresis loop and tcone are studied.By changing the thickness of BFSO and BTO films and annealing process,the interfacial strain of BFSO/BTO was regulated.The results show that the magnetic properties of BFSO are greatly affected by strain.The addition of BTO buffer layer alleviates the lattice mismatch between BFSO and sto substrates.The tcone of BFSO/BTO composite film increases significantly with the increase of BTO thickness,and tends to be saturated when the BTO thickness is greater than 200 nm.Before annealing,the tcone of BFSO/BTO is up to 240 K.After annealing,the tcone of BFSO/BTO is up to 393K,which is higher than that of single crystal BFSO block(370 K).The effect of interfacial strain on tcone is further proved by fixing BTO thickness and changing BFSO thickness.With the same BTO thickness,tcone decreases with the increase of BFSO thickness.Further microstructure analysis shows that the decrease of compressive strain in BFSO leads to the obvious regulation of 4f2 lattice position in BFSO.The 4f2 position in BFSO is the position point of Sc doping,which is the main lattice position leading to the transformation of M-type ferrite collinear magnetic structure into conical magnetic structure,which is the main reason for the improvement of Tcone.At the same time,the diffusion of Ti at BFSO/BTO interface to 4f2 is also one of the factors affecting Tcone,which may be one of the reasons why Tcone of BFSO/BTO composite film is higher than that of single crystal BFSO bulk.Then,the regulation laws of electric field on the magnetic properties of BFSO/BTO composite films with different BTO buffer layer thickness are studied.The following results are obtained:(1)the regulation laws of electrostatic field(0-400 kV/cm)and square wave electric field on the hysteresis loop and magnetization m of BFSO/BTO composite films are studied.The results show that at a temperature much lower than tcone(20 K),the magnetization electric field regulation amplitude(M%)of the singlelayer BFSO film is 15%,the M%of the composite film with BTO of 80 nm is 35%,and the M%of the composite film with BTO of 300 nm is 37%.At the high temperature(390 K)close to the composite film tcone,the M%of the single-layer BFSO film is close to zero,and the M%of the composite film with BTO of 300 nm is still 6%.(2)The effect of electric field on tcone of composite film is further studied.The results show that electric field has no regulation effect on tcone of single-layer BFSO film,but the regulation of tcone of BFSO/BTO composite film is also very weak,with only 5 K change.(3)Finally,the effects of electric field on the microwave ferromagnetic resonance(FMR)characteristics of different BFSO/BTO composite films were studied.The results show that the microwave resonance field(Hr)of single-layer BFSO film and composite film with BTO of 80 nm is shifted by 3 Oe and 8 Oe respectively under the maximum electric field(±400 kV/cm).It can be seen that the transition layer BTO improves the adjustable performance of FMR electric field of BFSO.The above results show that the transition layer BTO not only improves the tcone of BFSO films,but also significantly enhances their electrically controlled magnetic properties at high and low temperatures,but has relatively little effect on the equivalent magnetic anisotropy field(Hr).Finally,the magnetic and dielectric properties of BFSO/BTO composite films with different BTO buffer layer thickness were studied.The results show that the single-layer 80 nm thick BFSO film has a maximum magnetic dielectric change amplitude of 0.25%at room temperature.For the composite films with BTO of 80 nm and 300 nm,the maximum magnetic dielectric change amplitudes at room temperature are 2.3%and 2.5%,respectively.At the same time,the composite film with BTO of 300 nm still has a magnetic dielectric change amplitude of 0.78%at 390 K.The above results show that the addition of BTO transition layer not only enhances the magnetodielectric effect of the composite film,but also increases the maximum temperature at which the magnetodielectric effect exists.In conclusion,the addition of BTO buffer layer alleviates and controllably adjusts the lattice mismatch between BFSO and sto substrate,and significantly improves the tcone of BFSO/BTO composite film,with the maximum value of 393 K,which is higher than 370 K of single crystal BFSO block.The improvement of magnetoelectric coupling performance of BFSO thin films by BTO buffer layer has been further verified in electronic magnetic devices and magnetic dielectric devices.The research of this paper is of great significance to the regulation of the magnetic structure and magnetoelectric coupling performance of the second multi iron film material,the multi field regulation of the magnetoelectric coupling performance of the film and the promotion of the improvement of the service temperature of magnetoelectric components.