Electric Field Control Of Magnetis Min Epitaxial Ferro Magnetic/Ferroelectric Heterostructures

Non-volatile,high read/write speed,high storage density,low power consumption and device miniaturization are the main developing trends for magnetic memory in the future.Magnetic random access memory is the most promising candidate to achieve this goal.In terms of magnetic information reading,the magnetic tunnel junctions based on the tunneling magnetoresistance effect have been successfully applied to the storage devices and exhibit obvious advantages.In the field of magnetic information writing,the magnetic field generated by conduction wires,the spin transfer torque and the spin-orbit torque can be used to realize the controllable magnetization reversal through current induced relevant effect.However,the applied current density must be larger than the critical current density to achieve the goal.These approaches will cause a significant increase in energy consumption.In contrast,through electric field control of magnetic properties to realize the magnetization switching can not only significantly decrease the effect of the joule heat,but also realize the high density data storage with low power consumption.The research on the electric field control of magnetism mainly focuses on the composite multiferroic heterostructures with combined ferroelectricity,ferromagnetism and ferroelasticity and magnetoelectric coupling effects.In an artificial ferromagnetic/ferroelectric?FM/FE?heterostructures with the coupling of ferromagnetic layer having high magnetostrictive to ferroelectric layer having large piezoelectric effect,the magnetism of FM/FE heterostructures can be adjusted by inverse magnetoelectric coupling effect.The coupling mechanisms in such FM/FE heterostructures include charge mediated effect through charge accumulation/dissipation at the interface,exchange bias-mediated effect resulting from ferromagnetic-antiferromagnetic coupling,and strain mediated effect by means of transferring strain from the piezoelectric FE layer to the FM layer.Among them,the strain-mediated mechanism has received a lot of attention recent years,since the strain-mediated mechanism is particularly convenient and effective at room temperature.In the current study,the isotropic CoFeB or the FeNi,FeCo,and Co with uniaxial magnetic anisotropy was usually selected as FM layer to fabricate FE/FM heterostructures.Among them it has realized the volatile and non-volatile magnetization switching of FM layer by modifying the ferroelectric polarization direction of FE layer under appropriate electric fields.However,few works have been performed regarding the magnetic properties and electric field control magnetism of FM with cubic magnetocrystalline anisotropy.The main problem is that the strain induced uniaxial magnetic anisotropy is too weak to overcome the original hard axis barrier of cubic magnetocrystalline anisotropy energy.In this thesis,we first study the structure,static magnetic and dynamic magnetic properties of the epitaxial ferromagnetic thin films,and the magnetization switchingmechanismundertheexternalmagneticfield.Thenthesinglecrystal[Pb(Mg_1/3Nb_2/3)O₃]_x-[PbTiO₃]_1-x?PMN-PT?with piezoelectric memory effect are used as the ferroelectric layer of the heterostructures,and epitaxial Fe Si film with appropriate magnetocrystalline anisotropy are used as ferromagnetic layer.We study the reversible and non-volatile magnetization switching,as well as the magnetic anisotropy variation under the applied electric field.The research contents are as follows:?1?The Co₆₁Fe₂₆Si₁₃?CoFeSi?,Fe₈₆Si₁₄ and Fe₈₀Si₂₀?FeSi?thin films were deposited on?001?MgO substrates by radio frequency magnetron sputtering.The influences of post annealing temperature on crystallization,magnetic anisotropy and dynamic high frequency magnetic properties of CoFeSi films were investigated.The epitaxial relationship of?001?[100]MgO//?001?[110]FeSi is characterized by x-ray diffraction and transmission electron microscopy.The static magnetic properties measurements show the four-fold symmetric cubic magnetic anisotropy of FeSi thin films,demonstrating the?001?epitaxial relation of FeSi films through the magnetic viewpoint.The magnetization reversal process is also investigated by measuring mico-focused magneto optical Kerr hysteresis loops.Moreover,in the Fe₈₆Si₁₄/MgO?001?thin films,the large cubic anisotropy field significantly increases the natural resonance frequency up to 8.0 GHz,and the natural resonance phenomena can be observed in any in-plane directions.It exhibits the advantage comparing with the soft magnetic thin films with in-plane uniaxial anisotropy,in which the natural resonance phenomenon disappears in some specific directions.?2?The epitaxial Fe₈₆Si₁₄ films were fabricated on?001?PMN-0.32PT single crystal substrate by radio frequency magnetron sputtering.The magnetic anisotropy transition from four-fold cubic anisotropy to two-fold uniaxial magnetic anisotropy occurs after applying negative electric fields in Fe₈₆Si₁₄/?001?PMN-0.32PT heterostructures.Unfortunately,the magnetic anisotropy transition is non-volatile and irreversible.Based on theoretical analysis and simulation results,we modified the composition of the ferromagnetic layer and the ferroelectric substrate.In the fabricated epitaxial Fe₈₀Si₂₀/?001?PMN-0.3PT heterostructures,we study the non-volatile and reversible 90^o magnetization switching with the uniaxial magnetic anisotropy dominated under±6 kV/cm by measuring the angular dependent remanent magnetization.By measuring mico-focused magneto optical Kerr hysteresis loops under electric fields,the non-volatile magnetization switching and internal competitive relation between cubic magnetocrystalline anisotropy energy and in-plane uniaxial anisotropy energy are further confirmed.?3?The feasibility of reversible and non-volatile magnetic transition between four-fold and two-fold magnetic anisotropy by electric field is demonstrated through free energy analysis and simulation in epitaxial FeSi/?011?PMN-PT heterostructures.In the epitaxial Fe₈₀Si₂₀/?011?PMN-0.3PT heterostructures with 300 ^oC post annealing process,the reversible and nonvolatile transition between the four-fold cubic magnetocrystalline anisotropy and uniaxial magnetic anisotropy under saturated electric field and converse coercive electric field is demonstrated by means of static magnetic properties measurements.Meanwhile,we also achieved electric-field mediated reversible and non-volatile 90^o and 180°magnetization switching with the assistance of the magnetic field pulse.Finally,the importance of the selection of the ferroelectric substrate and the crystallization of the ferromagnetic film on the electric field modulation of the magnetic properties in Fe₈₀Si₂₀/?011?PMN-PT heterostructures were also verified experimentally.