Electric-Field Control Of Magnetization Reversal And Tunnel Magnetoresistance In Multiferroic Heterostructures

Electric-field control of magnetism has recently attracted much attention because of low-power consumption,which has potential application in low-power multifunction devices.Ferromagnetim/ferroelectric multiferroic heterostructure is a useful wa y to realize electric-field control of magnetism.And strain-mediated magnetoelectric coupling with large magnetoelectric coupling coefficient at room temperature is one of the lively research hotspot.Combination of strain-mediated magnetoelectric coupling and exchange bias is a promising method to reverse magnetization by electric field,which is still a challenge.However,electric-field control of magnetization reversal still remains elusive after some attempts and requires more investigations.On the other hand,magnetic tunnel junction(MTJ)is an important part of spintroics and electrical manipulation of tunnel magnetoresistance(TMR)is crucial for low-power spintroic device while present work have some problems such as low opertation temperature,assistance of a magnetic field and volatile.Therefore,room temperature electrical nonvolatile manipulation of tunnel magnetoresistance in the absence of bias magnetic field is highly desired while still lacking.In this thesis,PMN-PT(011)with large piezoelectric coefficients was selected as ferroelectric substrate,where magnetic multilayers deposited,to investigate electric-field control of magnetization reversal and TMR.We study the angular dependence of electric-field-controlled exchange bias and magnetization reversal in Co Fe B/Ir Mn /PMN-PT.It is demonstrated that the ratio of the exchange-coupled unidirectional anisotropy and the uniaxial anisotropy of the ferromagnetic layer,as well as their relative orientation can be dramatically and continuously tuned via electric fields.Moreover,reversible electric-field control of exchange bias and magnetization reversal w ere realized,which depend strongly on the angle between external magnetic field and pinning direction.The simulations based on a modified Stoner-Wohlfarth model confirm that the electric-field-controlled exchange bias originates from the competition between the uniaxial anisotropy induced by the piezostrain and the exchange-coupled unidirectional anisotropy.Through optimizing anisotropy configuration,electric-field-controlled magnetization reversal was realized at zero magnetic field.Combination of ferroelectric material and exchange bias pave a new way to study exchange bias and electric-field-controlled magnetism.A nonvolatile piezostrain of the PMN-PT(011)substrate was generated through a novel asymmetric poling operation(-1.6 kV/cm ~ 8 kV/cm)induces in-plane and out-plane ferroelectric polarization,and it can rotate magnetization of single Co Fe B layer by 90? through strain-mediated magnetoelectric coupling.Then we fabricate MTJ with core structure Ir Mn/Co Fe B/Mg O/Co Fe B on PMN-PT.The nonvolatile electric-field-controlled TMR was realized at zero magnetic field by regulating the spin-polarization configuration of MTJ owing to electric-field-induced magnetization rotation of the free layer.This nonvolatile electrical manipulation of TMR is significant for future ultralow energy consumption spintronic devices and information storage.