Investigation On The Electric Field Control Of Magnetism In Magnetoelectric Heterostructures

More and more attentions have paid on the investigation of magnetoelectric(ME)composites,since they can exhibit large ME effect at room temperature and have potential applications in multifunctional novel devices,such as the spintronics,data storage,magnetic filter and novel sensors.Recently,with the development of thin film growth technique,it is possible to obtain high-quality thin-films,which creates favorable conditions to fabricate the magnetoelectric film heterostructure by combining the ferromagnetic and ferroelectric materials.Moreover,from the application viewpoint,there is a surge in the investigation of the electric field control of magnetism in magnetoelectric film heterostructure,since it would meets in device design requirement of miniaturization,multifunctionality and low energy consumption.In this dissertation,based on the analysis and investigation of the previous works,we perform the work on electric field control of magnetism and transport properties in magnetoelectric film heterostructure.The main contents in this dissertation are divided into three parts:1.The electric field manipulation of magnetization and transport properties in La1-xSrxCoO3/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructures.La1-xSrxCoO3(LSCO)films with different Sr content doping(x=0.18,0.33 and 0.5)were grown epitaxially on piezoelectric Pb(Mg1/3Nb2/3)O3-PbTiO3Substrates by pulsed laserdeposition.The magnetization of these films varies with the external electric field,showing the magnetoelectric effect.With different doping content of Sr2+ions,the change of magnetization for these films caused by the applied electric field show different behavior with increasing temperature,which can be attributed to the competition between electric-field-induced changes of spin state and double exchange interaction.It is well known that LSCO is a kind of strongly correlated material,in which the magnetic and transport properties couple intimately.In order to further characterize the effect of electric field on magnetism,we measure the temperature dependence of resistance(R-T)for LSCO film(x=0.18)under the electric field.Interesting,temperature dependence of the relative change in resistivity ?R-T shows a different behavior from that of AM-T,and a positive AR is observed in the measured temperature region and decreases with increasing temperature.This is due to the fact that the SST induced by electric field plays a crucial role in determining the resistivity of LSCO(x=0.18).In addition,it is worth noting that,compared with magnetization,a remarkable change of resistance is observed under the same electric field,suggesting that the measurement of transport property is not only an alternative but also a sensitive method to investigate the electric field control of magnetism in the strongly correlated LSCO system.This work presents an alternative mechanism to investigate the electric field control of magnetism in magnetoelectric heterostructure by tuning the spin state.2.The electric field manipulation of magnetization and transport properties in La0.7Sr0.3MnO3/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructures.The La0.7Sr0.3MnO3(LSMO)/Pb(Mg1/3Nb2/3)O3-PbTiO3 magnetoelectric film heterostructure was fabricated by using the method similar to LSCO/PMN-PT heterostructure for measuring the magnetization under different polarity electric fields.We find that the magnetization of LSMO film increases under the positive electric fields while decreases under the negative electric fields.Moreover,the manipulation of magnetization under the negative electric fields is more dramatic than the same positive electric field,which can be explained according to electric field induced DE interaction.Moreover,we measure the time dependence of the magnetization of LSMO film under application of electric fields,which exhibits very stable on-off or ladder response,which is favorable for the device application.As we know,LSMO is a typical strong correlated material,in which the electron and lattice exhibit intimate coupling.So we also investigate the change in resistance and magnetoresistance of LSMO film under the application of different electric fields.We find that the manipulation of resistance of LSMO film under the negative electric fields is in agreement with the magnetization.However,the electric field manipulation of magnetoresistance of LSMO film is not obvious,which can be understood according to the phase competetion model.3.Electrically controlled magnetization and switching in CoFe2O4/Pb(Mg1/3Nb2/3)O3-PbTiO3 magnetoelectric film heterostructure.The CFO films with different thickness were grown on one-side polished single-crystalline(001)PMN-PT substrate by pulsed laser deposition.Here the CFO target was prepared by sol-gel method with citric acid as a precipitating agent and sintered at higher temperature.The manipulation of the out-of-plane and in-plane magnetization under the application of electric fields was in situ investigated by a magnetic force microscopy(MFM)and a superconducting quantum interference devices magnetometer(SQUID),respectively.The dramatic changes in M-H loops and MFM images are observed in the CFO/PMN-PT heterostructure by applying different electric fields,indicating a large magnetoelectric effect.Meanwhile,we note that the values of saturated magnetization under application of electric fields are almost the same,which suggests that applying electric field on the substrate can only change the magnetic anisotropy of CFO film and has no effect on the intrinsic magnetization.In addition,we find that the manipulation of magnetization in CFO film with 200 nm-thick under the same electric fields is more obvious than that in 100 nm-thick CFO film,since there exists different strain state in CFO film with different thickness.The mechanism and potential applications for electric control of magnetism in CFO/PMN-PT layered films are discussed.The larger residulal strain is built in 100 nm CFO film during the film growth progress due to the difference in latticeparameters and the thermal expansion coefficients between CFO bulk and PMN-PT substrate.Most importantly,since the electric field can control He of the CFO film,the magnetization reversal in CFO/PMN-PT heterostructure can be realized at room temperature,which has a promising application in energy conversion information memory devices.