Modulation Effect Of Electric-field On Magnetic Properties In "Ferromagnetic/Piezoelectric" Magnetoelectric Composite Thin Films

In this thesis,electric field modulation effect on magnetic properties of ferromagnetic phases in "ferromagnetic/piezoelectric" composite magnetoelectric thin films were investigated.The main contents and results were summarized as follows:Firstly,we investigated the effects of different deposition conditions on the structure,martensite transition temperature and magnetic entropy in NMCS/SiO2 thin film prepared by pulsed laser deposition technology.The experimental results indicate that the substrate temperature and the annealing temperature have great influence on the transition temperature of the thin film.And the peak value of the magnetic entropy change and its corresponding temperature are very sensitive to deposition conditions.Secondly,we investigated the structure and magnetic properties of NMCS/PMN-PT and NMCS/Cu/PMN-PT thin film prepared by pulsed laser deposition technology.The effects of electric field on the magnetic properties were studied.Furthermore,the modulation effect of in-situ electric field on magnetocaloric effect in NMCS phase was well investigated.The results present that,the change of martensite transition temperature Ms and magnetic entropy change ?SM have been well achieved by applying an electric field on to ferroelectric phase in the magnetoelectric heterostructures.The change of martensite transition temperature(?Ms)up to 6K,as well as the change rate of ?SM up to 18.5%,was obtained with H=2T and E=+6KV/cm in NMCS/PMN-PT heterostructure.The two values were up to 4K and 62.1%respectively for the NMCS/Cu/PMN-PT heterostructure with the same external conditions.Lastly,we studied the tuning effects of ex-situ electric field on magnetism its tuning mechanism in "soft-magnetic alloy/PMN-PT" heterostructures.In more detail,Fe3CO7/PMN-PT(011)and Ni/PMN-PT(001)magnetoelectric thin films were prepared by pulsed laser deposition technology,respectively.The results indicated that,the two samples presented obvious nonvolatile modulation effects of electric field on magnetism.The change rate of magnetization up to 2.1%was obtained when applying an electric field(3KV/cm,equal to coercive electric field(Ec))to the PMN-PT(011)single crystal.Besides,if the magnetization reaches 2.845memu,the needed external magnetic field shift(8H)up to 21840e is obtained when an electric field of+3KV/cm is applied to the substrate in the Fe3Co7/PMN-PT(011)thin film.Moreover,this tuning effect is still obvious even under zero bias magnetic field.As for the Ni/PMN-PT(001)magnetoelectric thin film,the change rate of magnetization up to 8.6%can be obtained when applying an electric field of 1KV/cm to the PMN-PT(001)single crystal.And an electric field of 3 KV/cm makes the coercive force change rate of Ni thin film reaches up to 32%.The tuning mechanisms of electric field in the two thin films were investigated by analyzing magnetoelastic effect and additional magnetic anisotropy in magnetic materials.