Research On Room Temperature Controllable Exchange Bias Effect Based On MFe₂O₄/BiFeO₃(M=Ni?Co)Heterojunction

Magnetoelectric memories based on multi-iron materials are non-volatile,high speed,low power and high density.Disk storage and cache memory functions are important research directions in the field of next-generation ultra-low-power storage technology.The research on this type of magnetoelectric memory has always focused on the magnetoelectric coupling effect and the exchange bias effect at the ferromagnetic/anti-ferromagnetic interface.Direction.In the field of magnetoelectric coupling,the strength of all types of materials is difficult to reach the electrically controlled magnetic conditions,and development is slow.The exchange bias effect at the ferromagnetic/antiferromagnetic interface,on the other hand,is developing faster,and the spin valve structure,magnetic random memory(MRAM),etc.are The exchange bias effect is used to the ferromagnetic/antiferromagnetic interface.And in recent years,many attempts have been made to use the electronically controlled exchange bias to create a new type of magnetoelectric memory device,but the material system itself performance,it has always been difficult to achieve the electronically controlled exchange bias at room temperature.This paper focuses on the heterojunction based on multi-iron/ferromagnetic materials to achieve an electronically controlled exchange bias at room temperature.This paper focuses on the exchange bias effect of ferromagnetic/antiferromagnetic heterojunction and the electronically controlled exchange bias based on the change of magnetoresistance.The choice of materials for the antiferromagnetic layer is mainly focused on the multi-iron material BiFeO3,which has a ferroelectric Curie temperature(TC)of0.5±0.5±0.5.(=1103K)and the antiferromagnetic Neal temperature(TN=643K)both have the advantage of being much higher than room temperature and have been since their discovery Hot materials for scientific research.Taking advantage of the fact that BiFeO3 possesses both ferroelectric and antiferromagnetic properties,we control the ferromagnetic/antiferromagnetic layer interface through electronically controlled polarization.of the exchange bias effect.In addition,we adopted the antispinel-type ferrite MFe2O4(M = Co,Ni)with high Curie temperature as the ferromagnetic materials,as a way to try to increase the cut-off temperature for the ferromagnetic layer/BiFeO3 heterojunction exchange bias effect,to achieve the exchange bias at room temperature.Both of them possess relatively high Curie temperature,and their Curie temperature is comparable to the Neal temperature of BiFeO3 film.Therefore MFe2O4/BiFeO3 heterojunctions have a great potential to increase the cut-off temperature of the exchange bias effect of these heterojunctions.The main studies are as follows:(1)In this paper,a BiFeO3 thin film was spin-coated on SiO2 substrate with Pt electrode by sol-gel method.The preparation of BiFeO3 film was optimized by changing the content of Bi elements in the sol-gel ratio and the annealing temperature of the film.process,the BiFeO3 thin film prepared with good phase structure,no other impurities,high crystallinity,and a grain size of 41 nm.Good ferroelectric properties,saturated polarization intensity Ps of28.16?C/cm(?)2,the remaining polarization intensity 2Pr up to 42.43 ?C/cm(?)2.and NiFe2O4 and CoFe2O4 thin films were prepared by magnetron sputtering.The preparation process of the two films was optimized by varying the conditions such as sputtering time,annealing temperature,and annealing time.(2)In this paper,CoFe2O4/BiFeO3 and NiFe2O4/BiFeO3 were observed at low temperature of 3K The heterojunctions all have a more obvious exchange bias.From the plots of the tested coercivity field and exchange bias field with respect to temperature,it can be seen that CoFe2O4/BiFeO3 and The NiFe2O4/BiFeO3 heterojunction exchange bias and coercivity field both decay exponentially with increasing temperature,and finally in the exchange of At the cut-off temperature of the bias,the exchange bias effect disappears.This decay is faster in the low-temperature region and the change is relatively flat in the high-temperature region.And CoFe2O4/BiFeO3 and NiFe2O4/BiFeO3 heterojunctions were also observed at room temperature.With a certain exchange bias phenomenon,the exchange bias cut-off temperature of the FM/AFM system was successfully increased.(3)Based on the heterojunction of MFe2O4/BiFeO3(M=Ni,Co),the heterojunction was studied at a certain temperature of 0.5 °C.The change of resistance with polarization voltage under magnetic field is used to observe its electronically controlled exchange bias.The electronically controlled exchange of MFe2O4/BiFeO3(M=Ni,Co)heterojunction was investigated using an electrical transport test system.Bias performance.Electronically controlled exchange bias at near room temperature was achieved via NiFe2O4/ BiFeO3 heterojunction through the The CoFe2O4/BiFeO3 heterojunction achieves an electronically controlled exchange bias at room temperature.The heterojunction relies on the electric field to control the anisotropy of the antiferromagnetic layer(KAFM)and thus the interfacial interaction energy(JAFM-).(FM)forms a high JAFM-FM state and a low JAFM-FM state to achieve controlled and reversible electronically controlled exchange at room temperature.