Electric-Field Control Of Magnetization And Electronic Transport In Ferromagnetic/Ferroelectric Heterostructures

With the fast development of information storage, exploiting new concepts fordense, fast, and non-volatile random access memory with reduced energyconsumption is a significant and challenging task. To realize this goal, electric-fieldcontrol of magnetism is crucial. A promising way to control magnetism via electricfields is using the converse magnetoelectric effect, which permits control ofmagnetism with electric fields rather than with electric currents or with magneticfields. In that context, multiferroic materials which exhibit simultaneous magneticand ferroelectric orders with coupling between them are among the top candidates forrealizing electric-field control of magnetism. However, single-phase multiferroicmaterials are rare at room temperature and the converse magnetoelectric effects aretypically also too small to be useful. The use of artificial two-phase systemsconsisting of ferromagnetic and ferroelectric materials, especially various roomtemperature ferromagnetic （FM） and ferroelectric （FE） materials, serves as analternative approach to achieve electric-field control of magnetism and has beenwidely studied in recent years. In this thesis, we have investigated the electric-fieldcontrol of magnetization as well as the spin dependent transportation based on aCo₄₀Fe₄₀B₂₀（CoFeB）/Pb(Mg_1/3Nb2/30.7Ti_0.3O₃（PMN-PT） FM/FE heterostructure. Themain work can be divided to two parts as follows:Multiferroic samples were fabricated by depositing amorphous CoFeB films ontop of PMN-PT substrates with a magnetron sputtering system and the electric-fieldcontrol of magnetization was performed by magnetic property measurement systemwith in situ electric fields. We have found that the magnetization of CoFeB film onthe （001） oriented PMN-PT exhibits a giant loop-like response to electric field atroom temperature instead of the butterfly-like behavior commonly observed in thestrain-mediated FM-FE structures. Through systematic experimental investigationand theoretical analyse, it was demonstrated that the loop-like magnetization responseto electric field originates from the combined action of109°ferroelastic domainswitching in PMN-PT and absence of magnetocrystalline anisotropy in CoFeB, which is a new story for the strain-mediated FM-FE two phase systerm. What’s more, thislarge electric-field control of magnetization is tunable and non-volatile, which issignificant for applications.In another aspect, we have found even larger electric-field control ofmagnetization of CoFeB on the （110） oriented PMN-PT due to the in-plane strainanisotropy. Further experiments have demonstrated that the easy axis of CoFeB canrotate90degree at electric fields. Based on this large converse magnetoelectriceffect, spin-valves were fabricated on top of （110） oriented PMN-PT. It was shownthat both of the magnetization and the giant magnetoresistance of the structure can betuned by electric field at room temperature, which is very important for applications,especially in terms of the electric-writing and magnetic-reading random accessmemories.