Electric-Field Control Of Magnetization In Ferromagnetic/Ferroelectric Composite Multiferroic Materials

In recent years,the multiferroic composites consisting of ferromagnetic and ferroelectric materials have rapidly developed in the investigation of magnetoelectric effect.The electric-field-controlled magnetization effect based on the strain coupling mechanism has the advantages of wide selection of materials,simple structure,and large room-temperature magnetoelectric effect,which has become a hot research topic due to its important applications in low-energy-consuming and new multi-functional magnetoelectric devices.In this dissertation,we investigate the electric-field control of magnetization in ferromagnetic/ferroelectric multiferroic composites.The results in this dissertation are mainly divided into three parts:1?Electric Field Manipulation of 180° Magnetization Reversal in FePt/(011)PMN-PT HeterostructureFePt/(011)PMN-PT heterostructure was synthesized by co-sputtering FePt film on a(011)-cut PMN-PT substrate.We demonstrate that electric-field control of magnetic anisotropy can be achieved by the strain-mediated converse magnetoelectric effect at room temperature.When the electric field increase,the magnetization process of the FePt film along the[100]direction become harder.On the contrary,the magnetization process along the[01-1]direction become easier with increasing electric field.Taking advantage of the change of coercivity and magnetization,180°magnetization reversal can be realized in FePt film by adjusting the electric field.These results suggest a promising application for realizing storage devices with low-power consumption.2?Electric Field Manipulated Multilevel Magnetic States Storage in FePt/(011)PMN-PT HeterostructureIn the previous chapter,manipulation of magnetization was achieved by applying electric field in the[100]and[01-1]directions of PMN-PT substrate,respectively.However,this effect is volatile.After the removal of electric field,the strain returns to zero and then the magnetization of FePt thin film go back to its original state,which is disadvantageous to the application of information storage.The realization of multilevel states in electric-writing magnetic-reading storage technology will remarkably increase the storage density,which has important application value in the field of information storage.In this chapter,we investigate the electric-field manipulated multilevel magnetic states in FePt/PMN-PT heterostructure.We demonstrate that multilevel states of remanent strain can be obtained by applying unipolar electric field across the PMN-PT substrate.Further,we use this non-volatile strain to control the magnetic anisotropy of FePt film.Without bias magnetic field,different values of remanent magnetization can be obtained in FePt film by these remanent strains.These multilevel magnetic signals can serve as writing data bits in a storage device,which remarkably increases the storage density.Using a commercial giant magnetoresistance magnetic sensor,these multilevel remanent magnetization values can be read nondestructively and distinguishably by converting magnetic signal to voltage signal.Moreover,the writing of multistate remanent magnetization signals and reading of corresponding voltage signals have good retention and switching properties.These results enable the promising applications in electric-writing magnetic-reading memory devices with low-power consumption and high storage density.3?Nonvolatile Manipulation of Magnetocaloric Effect in Ni-Co-Mn-Sn/(011)PMN-PT Composite by Electric Field Magnetic refrigeration based on the magnetocaloric effect is a new type of refrigeration technology with environment friendliness and high-efficiency.Heusler-type Ni-Mn-based ferromagnetic shape memory alloys have a first-order martensitic transformation.Around the phase transition,these alloys exhibit large changes of magnetization,leading to large magnetic entropy changes.However,due to the first-order nature of this transition,these alloys have narrow operating temperature region,which largely hinder their application as refrigerants.In order to extend the operating temperature region,the electric control of magnetocaloric effect is investigated in the Ni-Co-Mn-Sn/(011)PMN-PT composite.The nonvolatile strains can be obtained by controlling the unipolar electric fields in(011)PMN-PT substrate.Because the phase transition of Ni-Co-Mn-Sn ribbons is very sensitive to the stress,the nonvolatile control of magnetocaloric effect are realized in this composite by the electric-field-induced nonvolatile strains.With the increase of the electric field,phase transition temperature can be significantly increased,which remarkably extends the refrigeration temperature region.These results show that nonvolatile control of magnetocaloric effect by the electric field have important applications in magnetic refrigeration.