Study On Optimal Design And Manipulation Of Magnetoelectric Coupling Coefficients For Multiferroic Thin Film Heterostructures

Multiferroics are materials with two or more of the primary ferroic orderings(ferromagnetic,ferroelectric and ferroelastic)simultaneously in the same phase or in a composite.There are many interesting physical mechanisms for the coupling between ferromagnetic and ferroelectric orderings.The application of magnetoelectric multiferroics is promising in terms of fields of sensors,transducers,memories,and spintronics.It's of great significance for the theories of multi-field coupling and application of multifunctional devices by means of exploring the strength of magnetoelectric coupling coefficients.The objectives of this thesis on multiferroic thin film heterostructures were to investigate the effect of materials parameters on magnetoelectric coupling,including thin film magnetostriction,thickness,orientation,strain states,and substrate thickness.Two thin film heterostructure systems were chosen for the studies,i.e.,Pb(Zr_0.52Ti_0.48)O₃/La_0.67Sr_0.33Mn O₃(PZT/LSMO)epitaxial thin film heterostructure for direct magnetoelectric effect and ferrite film-PZT(polycrystalline)composite for converse magnetoelectric effect.Five sample structures were designed to study the correlation between direct magnetoelectric couling coefficient and interface coupling,phase ratio,and magnetostriction,and the correlation between converse magnetoelectric couling coefficient and anisotropic magnetostriction and geometric parameters of samples.The main results are as follows:Firstly,PZT/LSMO composite thin film heterostructures,the focus was on influence of interface coupling between LSMO and PZT,phase ratio in PZT layer,and magnetostriction of LSMO on magnetoelectric coupling coefficient(?_E31).The results show that(1)PZT and LSMO layers are under compressive and tensile strain,respectively,for PZT/LSMO heterostructure grown on(001)Sr Ti O₃(STO)substrate.The maximum?_E31 was 122 m V/cm Oe for a bias field of 4.5 k Oe.Based on the theory of Laudau-Ginzburg-Devonshire,the interface coupling coefficient k value of PZT/LSMO heterostructure increased with increasing of DC magnetic field.The experimental and theoritical?_E31 values at DC magnetic field of4.5 k Oe for(001)PZT/LSMO/STO were consistent with each other.(2)The monoclinic r-phase in PZT layer significantly enhanced the value of?_E31 and the amount of r-phase decreased with increasing thickness of PZT layer for PZT/LSMO/STO heterostructure.The maximum?_E31=1153 m V/cm Oe for a sample with PZT thickness of 32 nm and LSMO thickness of 40 nm and relative r-phase ratio of 0.31.(3)With the assistance of PMN-PT and Metglas foil to the heterostructure,a maximum?_E31 of 17000 m V/cm·Oe was achieved at DC bias field of 14 Oe for the(001)-oriented PZT/LSMO/PMN-PT/Metglas heterostructure,which was attributed to the increase of magnetostriction of LSMO layer.Theoritical and experimental studies show the direct?_E31 of multiferroic thin film heterostructures can be enchanced significantly via(1)ferromagnetic and ferroelectric layers are under different strain states,(2)the amount of asymmetric ferroelectric phase is increased,(3)the assistance of materials with high piezoelectric coefficient and high magnetostriction.Secondly,ferrite film-polycrystalline PZT composite was studied to explore the effect of anisotropic magnetostriction and geometric parameters of samples on converse magnetoelectric coefficient(A).The main results are:(1)in composites of PZT and Ni Fe₂O₄(NFO)film,different thicknesses of NFO films with anisotropy field of 1–10.4 k Oe were grown on(100)and(110)oriented Mg Ga₂O₄(MGO)and Co Ga₂O₄(CGO)substrates,respectively.The A was stronger for films on CGO than for films on MGO.In samples with(110)NFO films the A was the highest for bias magnetic field H//[11?0],and a negative A was inferred for H//[001].For composites with NFO on(100)substrates the strongest A was measured for H along[001].Based on Landau-Lifshits-Kittel equation,the calculated A was in consistent with experimental values by considering compressive and bending deformation.(2)NZFO thick films with different thicknesses were grown on(100)and(111)Mg O substrates.(100)oriented NZFO films have higher in plane anisotropy field.The A for(100)NZFO/PZT was 2–4 times higher than the value of(111)NZFO/PZT.A increase of A from2.67 Oe cm/k V to 10.3 Oe cm/k V was observed when the thickness of Mg O decreased from0.5 mm to 0.2 mm.The calculated A was found to be in agreement with experimental values based on Landau-Lifshits-Kittel equation by considering the effect of Mg O thickness.Theoretical and experimental studies show that A for multiferroic heterostructures can be enchanced via(1)tuning bending and compressive strain of asymmetric multiferroic heterostructures,(2)decreasing the thickness of ferromagnetic layer and substrate.Results of our studies have led to an overall understanding of the effects of interface coupling coefficient,asymmetric ferroelectric phase,magnetostriction of ferromagnetic layer on?_E31,and the relationship between A and anisotropic magnetostriction and geometric parameters of samples.