Research On Voltage-controlled Magnetization Rotation Mechanism And Applications Based On Multiferroic Heterostructures

Non-volatile magnetic random access memory（MRAM）based on spin current controlled magnetization rotation,is a powerful competitor in next generation of integrated memories.With the development of the adaptability in embedded storage and single architecture memory structure,how to further reduce energy consumption of driving magnetization rotation in memory cell,has become the primary problem in the development of MRAM.The coexistence of ferroelectricity and ferromagnetism in multiferroic materials can realize voltage control of magnetism with the converse magnetoelectric（CME）coupling effect as the bridge.It opens up a new way for achieving ultra-low power consumption memory by using voltage replace the magnetic field and spin current to control magnetic moment.Among them,the strain-mediated ferromagnetic（FM）/ferroelectric（FE）multiferroic heterostructure has great potential in practical application due to its advantages of simple structure,large ME effect and room temperature operation.Therefore,based on FM/FE multiferroic heterostructure,this dissertation utilizes voltage-controlled CME effect to investigate CME effect’s microscopic mechanism,voltage assisted and full voltage controlled magnetism manipulation and magnetization rotation of magnetic storage cell.This dissertation achieves ultra-low power controlled magnetization rotation in anisotropic magnetoresistance（AMR）,giant magnetoresistance（GMR）,and tunneling magnetoresistance（TMR）magnetic storage structures,which offers the experimental and theoretical foundation for the development of low power consumption MRAM.Meanwhile,by expanding the investigation of this dissertation,a switching-field-adjustable GMR magnetic sensor is proposed.It provides a new idea for the development of voltage-controlled multifunctional magnetic devices.The main research contents of this dissertation are as follows:1.Based on the CME theory combined with micromagnetic simulation,the model of strain assisting a low magnetic field to rotate magnetic moment is proposed.It can effectively break the limit of CME effect controlled 90°magnetic moment rotation,and achieve completely 180°magnetic moment rotation.The conventional critical magnetic field can relatively be reduced 88%.2.Based on the theoretical research,this dissertation selects conventional Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃（PMN-PT）with linear strain characteristic and special PMN-PT with defect dipoles as FE substrates.This dissertation focuses on the special FE single crystal PMN-PT’s strain、FE characteristics,and the microscopic mechanism of voltage controlled FE domain polarization and lattice strain.And it demonstrated that defect dipoles in PMN-PT can effectively offer nonvolatile strain.This offers the foundation for nonvolatile magnetism manipulation and magnetic moment rotation of magnetic storage cell.3.To realize CME effect controlled magnetization rotation in magnetic film and magnetic storage cell,this dissertation firstly using Ni₈₀Co₂₀with a large magnetostriction coefficient to fabricate NiCo/PMN-PT heterostructure.On the conventional PMN-PT substrate,the easy axis of NiCo film is produced along 45°direction via magnetic field deposition.The magnetic moment can be turned to the[001]（0°direction）and[1-10]（90°direction）direction of PMN-PT by applying appropriate voltages.Thus,voltage-controlled three reversible and repeated magnetization states of NiCo film can be achieved.This dissertation proposes a new full voltage controlled low power consumption three-state magnetic storage cell based on AMR effect;Secondly,this dissertation fabricates NiCo/PMN-PT heterostructure on the special PMN-PT substrate with defect dipoles.The easy axis of NiCo film is produced along the[1-10]direction.The magnetic moment can be rotated to the[001]direction after applying a positive voltage pulse with a wide optional range.And the magnetic moment can return to the[1-10]direction after applying a negative voltage pulse.Therefore,a reversible,repeated and non-volatile AMR modulation can be obtained at zero magnetic field,realizing the voltage-controlled non-volatile magnetic storage.4.Based on the strain control of magnetic moment in magnetic thin film,this dissertation fabricates spin valve（SV）and magnetic tunnel junction（MTJ）on PMN-PT substrates.The CME effect controlled magnetism of storage cell is investigated:Based on SV（MTJ）multiferroic heterostructures,this dissertation investigates the voltage-controlled non-volatile 90°magnetic moment rotation of free layer in SV（MTJ）.The voltage-controlled nonvolatile magnetic storage without magnetic field is achieved;Then by using a sequential voltage pulse assisting a small magnetic field,the 180°magnetization rotation of free layer in SV（MTJ）storage cell is realized,which can effectively,repeatedly write information of“0”,“1”.This dissertation successfully achieves low power and large GMR,TMR modulation,which offers the foundation of the development of low power consumption and nonvolatile ME memory,offers the specific evidence of the feasibility of developing magnetic storage devices with low writing current.5.Based on the voltage controlled magnetization rotation in magnetic storage cell,this dissertation extends CME effect to GMR switch sensor.By applying positive and negative voltage pulses,the magnetic moment of free layer can be rotated to[1-10]and[001]directions,respectively,and the saturation magnetic field of free layer can be modulated.It can achieve adjustable switching fields of a sensor according to the application requirements;By introducing positive and negative voltage pulses at appropriate moments in the detection process,four voltage-pulse-adjustable switching field ranges of±100 Oe,±350 Oe,-350～+100 Oe and-100～+350 Oe,can be obtained in a GMR switch sensor.This offers a new way to develop voltage-controlled multifunctional magnetic devices.