Research On The Magnetoelectric Effects In Several Single-phase And Composite Multiferroic Materials

In recent years,multiferroic magnetoelectric materials with coexisting magnetic and electric orders have drawn ever increasing interests for their potential applications in sensors,information storage,energy harvest,spintronics,and so on.Multiferroic magnetoelectric materials can be classified as single-phase and composite systems.For single-phase multiferroics,magnetic order and electric order coexist in single phase.While systems consisting of single ferromagnetic and ferroelectric phase through different methods are called composite multiferroics.The coexistence of magnetic and electric orders in multiferroics provides the possibility of magnetoelectric effect(ME).Accordingly,the ME effects can be classified as direct ME(DME)and converse ME(CME),which are characterized as magnetic-field-induced polarization and electric-field-induced magnetization,respectively.Due to their huge potential applications in multifunction and low-energy consuming novel devices,magnetoelectric effects in single-phase and composite magnetoelectric materials have become a hot research topic.Based on this background,we have investigated the ME effects in some single-phase and composite magnetoelectric materials.These results presented in this dissertation are mainly divided into four parts:1.ME coupling in M-type hexaferrite PbCoTiFe10O19Polycrystalline sample of PbCoTiFe10O19 with co-doped Co2+-Ti4+ ions has been synthesized by solid state reaction,in which the magnetic,ferroelectric and magnetoelectric coupling properties are investigated.Doping Co2+ ions would weaken uniaxial anisotropy and stabilize easy-plane anisotropy.Substitution of nonmagnetic Ti4+ ions would interrupt magnetic interaction and make magnetically decoupled between RS and R*S*blocks.As a result,the co-doping of Co2+-Ti4+ ions makes the magnetic moments apart from c axis and stabilizes a spiral conical spin structure below 240 K.The electric polarization of this multiferroics decreases sharply around 240 K,which is consistent with the onset temperature of conical spin structure,suggesting the intrinsic coupling between magnetic and electric orders.Further experiment results demonstrate that the electric polarization decreases with increasing magnetic fields,showing a distinct direct magnetoelectric effect.Besides,the magnetization can be controlled by in-situ electric field in this multiferroic hexaferrite,exhibiting an obvious converse magnetoelectric effect.2.Linear ME coupling in Pyroxene CoGeO3The polycrystalline sample of orthorhombic pyroxene CoGeO3 has been prepared by solid state reaction route.The magnetic,dielectric and magnetoelectric coupling properties are investigated.The magnetic structure of CoGeO3 is commensurate with the lattice,and the magnetic symmetry of this material is non-centrosymmetry under magnetic field,which allows the linear ME effect.At zero magnetic field,neither dielectric anomaly or electric polarization is observed in this compound.However,under the application of magnetic field,dielectric peak and pyroelectric current peak present simultaneously around its antiferromagnetic phase temperature.It is noted that the dielectric peak temperature and the pyroelectric current peak temperature shift to lower temperature region with increasing magnetic field.The magnitude of electric polarization monotonously increases with increasing magnetic field,demonstrating an obvious linear magnetoelectric effect in this compound.CoGeO3 has different antiferromagnetic domains with equal volume fraction and opposite ME coefficients.When CoGeO3 is cooled through the Neel temperature under magnetic and electric fields,one type of antiferromagnetic domains would favor over the other,giving rise to the existence of magnetic-field-induced polarization.3.Electric field control of magnetism in SrRuO3/PMNPT heterostructureThe composite multiferroic heterostructure of SrRuO3/PMNPT has been fabricated by pulsed laser deposition method.The in-situ electric field control of magnetic and transport properties of SrRuO3 film are investigated.As demonstrated by the experiment results,the intrinsic magnetic properties of SrRuO3 film can be controlled by electric field,with the Curie temperature of SrRuO3 film decreasing about 2 K and the saturation magnetization of SrRuO3 film decreasing about 8.6%,which is different with those previous reported electric field controls of magnetic anisotropy.Simultaneously,we also demonstrate the electric field control of resistivity of SrRuO3 film.Moreover,the magnetization and resistivity of this heterostructure can be controlled dynamically,reversibly and reproducibly by in-situ electric field.Compared with the earlier report about depositing SrRuO3 film on different substrates,this method of controlling magnetism in SrRuO3 film is adjustable and scalable,which is helpful to gain more insight on the intrinsic strain impact on the magnetic properties of this film.The electric field control of magnetic and transport properties in SrRuO3/PMNPT heterostructure can be ascribed to the modification of the RuO6 octahedral rotation angle in SrRuO3 film caused by electric-field-induced strain,which gives rise to the enhancement of the overlap and hybridization between the Ru 4d orbitals and O 2p orbitals.Therefore,the magnetic interaction and the electron hopping matrix elements would reduce,leading to the decrease of magnetization,Tc and resistivity.4.Anisotropic effect of electric-field-controlled of magnetism in(011)Co/PMN-PT heterostructureThe composite multiferroic film of(011)Co/PMN-PT has been prepared by DC magnetron sputtering.By measuring hysteresis loop,magnetic domain and anisotropic magnetoresistance under in-situ electric field,this heterostructure exhibits an obvious anisotropic effect of electric field control of magnetism.After the PMN-PT substrate is poled,the Co film shows an in-plane isotropic magnetization state.When an electric field is applied along the thickness direction of the PMN-PT substrate,the magnetic state of Co film transforms into an in-plane anisotropic state,in which the[01-1]direction is the easy axis and the[100]direction is the hard axis.In-situ magnetic domain measurement demonstrates that the electric field can impact on the magnetic domain configuration dramatically.The ME coupling coefficient,which is deduced from the on-off magnetization switch under on-off electric field can reach up to 1.4×10-6 s/m in this heterostructure.The electric field control of anisotropic magnetoresistance in this heterostructure provides potential applications in multifunctional and low-energy consuming spintronic devices.The anisotropic effect of electric field control of magnetism in(011)Co/PMN-PT system stems from the anisotropic piezoelectric effect of the PMN-PT substrate.When an electric field is applied on the substrate,the generated anisotropic piezo-strain effectively transfers to the Co film then alters the magnetic anisotropy through magnetoelastic coupling.