Converse Magnetoelectric Coupling And Its Manipulations In Multiferroic Composite Films

Nonvolatile and reversible manipulation of magnetism is one of the central issues in the last decade due to its fundamental importance and potential applications.The converse magnetoelectric(ME)effect makes it possible to control magnetism with electric fields rather than electric currents or magnetic fields,which allows faster and lower-power operations.Due to the weak ME coupling and the low operation temperature in single phase multiferroics,the most studies of electrically controlling magnetism focus on the composite multiferroics materials,and some remarkable progress has been made so far.Strain-mediated,exchange-mediated and charge-mediated magnetoelectric couplings are the major mechanisms in these manipulations.However,the strain-mediated converse ME effect usually can change extrinsical magnetism such as the magnetic anisotropies,in which the strain in the ferroelectric(FE)layer can be transferred to the ferromagnetic(FM)layer and then tune the lattice strain,coercive field,magnetization and the resistance of the films upon ferroelectric poling or polarization switching.And how to maintain the strain after removing the electric field is still a great challenge.In this work,Pb(Zr0.52Ti0.48)O3/CoFe2O4/NiO,Co:TiO2(10 at.%Co doped)/Pb(Zr0.52Ti0.48)O3 and Mn:ZnO(5 at.%Mn doped)/Pb(Zr0.52Ti0.48)O3 multiferroic composite films on Pt/Ti/SiO2/Si(100)wafer were prepared by a sol-gel process and spin coating technique,respectively.The microstructures of the above films were characterized,the electric and magnetic properties were measured.Furthermore,the converse magnetoelectric coupling and the manipulations of magnetism by electric fields were systematically studied and realized.The main research contents are summarized as follows:(1)Manipulating the exchange bias effect of Pb(Zr0.52Ti0.48)O3/CoFe2O4/NiO heterostructural films by electric fields.The Pb(Zr0.52Ti0.48)O3/CoFe2O4/NiO heterostructural films with exchange bias(EB)effect have been prepared on Pt/Ti/SiO2/Si wafers using a sol-gel process,and reversible manipulation of EB effect by electric fields has been realized.Compared with the exchange bias field(Heb=-75 Oe)at as-grown state,the modulation gain of Heb by electric fields can reach 83%(Heb=-12.5 Oe)in the case of +5.0 V and 283%(Heb=-287.5 Oe)in the case of-5.0 V,respectively.Moreover,such electrically tunable EB effect is repeatable and has good endurance and retention.Through analyzing the energy band structures in different electric treatment states,we discuss the mechanism of such electric-field-tunable EB effect.Two factors,i.e.,the filling(or releasing)of electrons into(or from)the defect levels produced by oxygen vacancies at positive(or negative)electric voltages,and the redistribution of electrons due to the ferroelectric polarization,both of which give rise to the variation of the strength of exchange interaction in the CFO layer,have been revealed to be responsible for the electric modulation of EB effect.This work provides a promising avenue for electrically manipulating the EB effect and developing high-performance memory and storage devices with low power consumption.(2)Regulating the magnetism of Co:TiO2 films by ferroelectric polarization.The Co:TiO2/Pb(Zr0.52Ti0.48)O3(PZT)heterostructrural films have been prepared on Pt/Ti/SiO2/Si wafers using a sol-gel process,and the composite films appear room temperature ferromagnetic property.The saturation magnetization(Ms)can be regulated by the ferroelectric polarization in PZT layer,and the manipulation amplitude can reach 8%at 300 K.The magnetism of Co:TiO2 originates from bound magnetic polaron.The carrier density variation in Co:TiO2 film due to the ferroelectric field effect in PZT layer leads to the change of Ms.(3)Manipulating the resistance and magnetism of Mn:ZnO/Pb(Zr0.52Ti0.48)O3 heterostructural films through interface polarization coupling.The Mn:ZnO/Pb(Zr0.52Ti0.48)O3(PZT)heterostructrural films have been prepared on Pt/Ti/SiO2/Si wafers using a sol-gel process,and reversible manipulation of the resistance and magnetism through the interface polarization coupling has been realized.When the ferroelectric polarization in PZT layer pointing from PZT layer to ZnO layer,the composite film is in low resistance state(LRS)and the Ms reduces.However,when the polarization in PZT layer switching and pointing from ZnO layer to PZT layer,the composite film is switched into high resistance state(HRS)and the Ms increases.The regulation to the resistance is repeatable and non-volatile,and the manipulation of Ms is available in the temperature range of 10～300 K.Furthermore,the modulation gain of Ms can reach 270%at 300 K.The resistance change is due to the interface potential barrier height variation and the formation of electron accumulation(depletion)layer at the Mn:ZnO/PZT interface,which can be regulated by ferroelectric polarization direction.The magnetism of Mn:ZnO originates from the bound magnetic polaron.The mobile carriers variation in Mn:ZnO due to ferroelectric field effect gives rise to the manipulation of the Ms in Mn:ZnO/PZT heterostructrural films.This work presents an effective method to modulate the magnetism of magnetic semiconductor,which is significant in designing high-performance ferroelectric-heterostructures for actual applications.