| In 2007,Science magazine reported that the multiferroic materials would be one of the top seven research hotspots in 2008,then set off a wave of multiferroics research.The ferromagnetic/ ferroelectric(FM/FE)heterostructures are more popular than those of single-phase multi-ferrous materials with weak magnetic coupling strength and low Curie temperature.In this field,the study of E-Field control of magnetism mainly focused on two major points:First,for the application of the demand,how to achieve stable nonvolatile electronic control of the magnetism? Second,the mechanism for the control of magnetic field is not clear,how to explore the new physical effects of magnetoelectric coupling? In this thesis,we investigated the behavior of E-field control of magnetic properties in Co/PMN-PT heterostructure.Co(CoZr)thin films were deposited on a PMN-PT(011)substrate by radio frequency magnetron sputtering.The static magnetism and high frequency magnetic properties of the samples under electric field were characterized by hysteresis loop,ferromagnetic resonance absorption spectrum and vector network analyzer.The main results were as follows:1.The four-state magnetoelectric memory caused by strain-mechanism magnetoelectric coupling was found.By changing the in-plane easy axis of the Co film in different directions along the ferroelectric substrate,it is possiable to gain that in-plane uniaxial static magnetic anisotropy field of electric field regulation and even to achieve the easy axis of the reversal.Natural resonance frequency and magnetic permeability shown a Butterfly dependence on the electric field,indicating that due to stress mechanism of magnetoelectric effect.By changing the temperature,the strain curve of PMN-PT substrate can be transformed from Butterfly to Loop,and then at different temperatures by adjusting the excitation pulse electric field and magnetic field,the stable nonvolatile four remnant magnetization were obtained,which is crucial for the four-state magnetoelectric memory.2.The new effect of magnetoelectric coupling caused by interfacial spin polarization charges was found.The phenomenon of electric field induced ferroresonance(FMR)spectral deformation was found in Co(15 nm)/PMN-PT heterostructures.This new experimental phenomenon originates in the interfacial spin polarization charge-mechanism magnetoelectric coupling effect,which was proved by changing film thicknesses and inserting non-magnetic layers Ta between the ferromagnetic layer and the ferroelectric substrate.According P dependence on temperature,it was found that P was smaller or even disappeared at low temperature,and the FMR spectrum was not changed with the electric field under 200 K,which further confirmed that the phenomenon came from the P & M coupling effect.In theory,considering the electrostatic shielding effect of the metal charge,combined with the spin diffusion and magnetic exchange in the ferromagnetic,a model of interfacial spin polarization charge induced magnetoelectric coupling effect is constructed.3.The anisotropy caused by interfacial spin polarization charge-mechanism magnetoelectric coupling was found.Combined strain-mediated and interface spin polarization charge-mediated magnetoelectric coupling is expected in Co/PMN-PT heterostructures.Because of two mechanisms for the different symmetries of the electric field response,the anisotropic fields were obtained,which are induced by the strain-and interface spin polarization charge-mechanism respectively.It was found that the interface spin polarization charge-mechanism plays a key role in the controlling of anisotropic magnetic effective fields,in the case of small electric field(E ≤ 5 kV/cm).In addition,by adjusting magnetic field and electric field,the effective damping behavior of Butterfly and Loop can be obtained,which anisotropy was different from the traditional ferromagnetic films.4.The variable dynamic permeability caused by the interfacial spin polarization chargemechanism magnetoelectric coupling was found.In Co/PMN-PT heterostructures,spectral deformation was obtained by changing magnetic field in remnant polarization situation.In addition to the increase in the resonant frequency with increasing magnetic field,the permeability values can be varied not only in the positive range but also from positive to negative.It was proved that the occurrence of negative dynamic permeability due to interfacial spin polarization charge-mechanism magnetoelectric coupling at the FM/FE interface.This new experimental phenomenon will provide a new way and important reference for the design and application of new green information function devices. |
