The Manipulation Of Magnetic Moment And Spin In Multiferroic Heterostructures

In the last few years,electric manipulations in ferromagnetic heterostructures have been proposed for developing next generation spintronic devices.Two methods are used to realize electric manipulations:current and voltage.For the method via voltage,multiferroic materials play a key role due to their magnetoelectric coupling(ME)effects.Therefore multiferroic materials have attracted much attentions and the electric manipulation through them has already become one of the research hotspots in the field of spintronics.The thesis has explored the possiblity of the applications of multiferroic materials in future spintronics devices.The perpendicular magnetic anisotropy(PMA)and spin-orbit torque(SOT)in multiferroic heterostructures have been systematically studied.The main results are listed as follows.(1)The PMA has been realized in a BiFeO3(BFO)/Al2O3/Pt/Co/Pt multiferroic structure at room temperature.Interestingly,a distinct change of coercivity field(～400%)has been observed in the structure with opposite polarization directions,which can be attributed to the different oxidation degree at the Pt/Co interface.This spontaneous polarization-controlled switching not only provides us a platform to study interfacial effect in multiferroic heterostructures but also paves a way to manipulate PMA or even SOT through oxygen vacancies.(2)A method is proposed to adjust the switching current and spin Hall angle(SHA)in the SOT switching.The deterministic magnetization reversal induced by SOT has been realized in the BFO based heterostructures.With opposite spontaneous polarizations fields of BFO,the critical current for the SOT switching shows a huge change of～300%.The damping-like torques were detected and the variation of effective SHAs for the heterostructures with opposite polarizations was estimated to be 272%.These changes can be attributed to the distribution of oxygen vacancies inside the BFO film.We have also demonstrated the possibility of the applications of our structure in memory and reconfigurable logic devices.(3)A Pb(Zr0.2Ti0.8)O3(PZT)/SrRuO3(SRO)multiferroic heterostructure was prepared for realizing large ME coupling.We have investigated the PMA behaviors of the PZT/SRO heterostructure below the Curie temperature(TC).Interestingly,a relatively non-volatile ME coupling effect was observed below TC.Under the opposite polarization fields,the saturated magnetization and the coercivity field of the heterostructures can be modulated by 300%and 45%,respectively.These changes can be attributed to the distribution of oxygen vacancies at the interface of PZT/SRO.(4)The PM A is realized in BFO-based heterostructures with different phases.The anomalous Hall effect(AHE)switching shows a single step behavior in the pure rhombohedral-(R-)or tetragonal-(T-)phase BFO-based heterostructures.However,a step-like AHE switching was observed in the mixed-phase BFO-based heterostructure.We have demonstrated the distorted R-like phase and T-like phase regions in the mixed-phase BFO which are the key to generate this step-like switching.Damping-like torques have been detected in the BFO-based heterostructures and also show two steps character for the mixed-phase structure.Considering the mixed-phase BFO can be reversibly controlled via polarization field,our structures have potential to be applied in future spintronic and memory devices.