Electric-Field Control Of Magnetization In Ferromagnetic/Ferroelectric Heterostructures

Recently,with the development of new information technology,e.g.the cloud computing and big data,it is more urgent to develop low-power,high-density,fast reading and writing information storage devices.One feasible access to meet this need is to use electric-writing and magnetic-reading.Therefore,the studies of multiferroic with the purpose of realizing the electric-writing and magnetic-reading have achieved rapid developments.The strain-transferred composite multiferroics are most likely to be used in devices due to the rich selection of materials,simple structures,and large coupling coefficients,and therefore become the research focus in multiferroics.Even though plenty of results on the strain-transferred composite multiferroics have been achieved,many problems leave to be solved,e.g.the electric field control of magnetization(M-E)based on different ferroelectric phases,what is the difference between the micrometer size magnetic islands and continuous films on the aspect of the electric field control of magnetization.Theses problems are vital for understanding the M-E behaviors in multiferroic heterostructures as well as the applications.To solve the two problems above,the thesis chose the strain-transferred Co40Fe40B20/ Pb(Mg1/3Nb2/3)(1-x)TixO3(CoFeB/PMN-xPT)multiferroic heterostructures to explore the electric-field-controlled-magnetization.Different ferroelectric phases can be achieved by changing composition and temperature.The electric field control of magnetization in CoFeB/PMN-xPT multiferroic heterostructures with different ferroelectric phases demonstrated that the non-volatile loop-like bipolar-electric-field-controlled magnetization occurs for PMN-xPTs with Rhombohedral(R)phase,monoclinic(M)phase,and R phase far away from the MPB,while volatile bipolar-electric-field-controlled magnetization with a butterfly-like behavior occurs when ferroelectric phase changes to the tetragonal phase(T).Further analysis indicates that the nonvolatile M-E behavior is related to the change of in-plane strain component due to the 109° domain switching or the similar one.Moreover,for the R-phase sample far away from the MPB,an abrupt and giant increase of magnetization is observed at a characteristic temperature in the temperature dependence of magnetization.These results are discussed in terms of macro-microdomain transformation of relaxors.This is the first time to demonstrate the effect of ferroelectric domains’ size changes on magnetism for multiferroic heterostructures.In addition,we fabricated ferromagnetic islands on PMN-0.30 PT single crystals with micrometer sizes using the microfabricating methods.By observing the magnetic behavior with an in-situ-electric-field MOKE,we can realize the space resolution of magnetic anisotropy and electric-field-controlled magnetization.Results show that different ferromagnetic islands exhibit diversed M-E behaviors,but three types are concluded,and can be explained qualitatively by different types of ferroelectric domain switching.The results can promote the understanding of the electric field control of magnetization in FM/FE heterostructures,and is valuable to applications.