Synchrotron Radiation Study On Magnetoelectric Properties And Interface Exchange Bias Effect Of RFeO₃/FM System

Perovskite based orthoferrite compounds have been an active area of research due to possibility of exploring new materials with multifunctional applications.Their unique electric and magnetic phenomena are consequences of strongly correlated electronic systems.The quest for understanding these effects represents one of the most promising fields in materials science.Rare earth orthoferrites(RFeO₃)are non-collinear antiferromagnet and play an important role due to possessing a number of interesting magnetic,optical and magneto-optical,ultrafast laser-induced spin-reorientation phase transitions switching,multiferroic properties.RFeO₃possessing two inequivalent magnetic subsystems,namely,4f electron based rare earth R sublattice and 3d electron-based Fe sublattice that are coupled in an antiparallel fashion.Noncollinear antiferromagnetism in the Fe sublattice gives rise to weak ferromagnetism in the orthoferrites at relatively high temperatures,while the R sublattice typically orders antiferromagnetically at much lower temperatures.The energy of R³⁺-Fe³⁺(4f-3d electrons)interaction which plays crucial roles in Spin-reorientation(SR)is rather small compared with that of Fe³⁺-Fe³⁺interaction,but larger than or of the same order of the anisotropy energy of R³⁺-R³⁺interaction.This leads to a complex magnetic phase diagram and related phenomena in these materials.In RFeO₃/FM system,the good performance of RFeO₃ is well reflected.In the aspect of magnetism,RFeO₃ mainly show inclined antiferromagnetic structure,which can achieve pinning effect on the ferromagnetic layer FM,resulting in exchange bias effect at the interface.The exchange bias system of Bi FeO₃/FM has been widely studied.In this paper,exchange bias effect and orbital hybridization in RFeO₃/FM system are studied.The details are listed as following:(1)SmFeO₃ thin films are grown on Sr Ti O₃(100)substrates by pulsed laser deposition.SmFeO₃ powders are prepared by standard solid state reaction method.The electronic structure and magnetic properties of SmFeO₃ thin films and powders are studied.SmFeO₃ films and powders show different magnetic behaviors and different exchange bias effect.Combined with the X-ray absorption spectrum analysis of Fe and O element,the ionic bond length between Fe-O in SmFeO₃ thin films becomes longer and the hybridization between Fe 3d-O 2p weakens,which affects the magnetic behavior of the whole sample.(2)The effect of different orbital hybridization at the interface of SmFeO₃/FM bilayers on the exchange bias effect are studied.SmFeO₃/Ni Fe(3 nm)and SmFeO₃/Co Fe B(3 nm)bilayers on STO(100)substrates are grown by pulsed laser deposition and magnetron sputtering.More antiferromagnetic Co O in SmFeO₃/Co Fe B can provide more pinning sources for the ferromagnetic layer,resulting in higher cutoff temperature of exchange bias in SmFeO₃/Co Fe B bilayers.At 5 K,the values of H_EB and H_C in SmFeO₃/Ni Fe bilayers are higher than those in SmFeO₃/Co Fe B bilayers.The hybridization strength between Ni 3d-O 2p in SmFeO₃/Ni Fe bilayers is higher than that of Co 3d-O 2p in SmFeO₃/Co Fe B bilayers.This may be one of the reasons why the two bilayers have different values of H_EB and H_C at 5 K.(3)The effects of oxygen partial pressure on the magnetoelectric properties of SmFeO₃/Ni bilayers are studied.SmFeO₃/Ni(P_O2-21 Pa)and SmFeO₃/Ni(P_O2-4 Pa)bilayers on Nb doped Sr Ti O₃(100)substrates are grown by pulsed laser deposition and magnetron sputtering.The reason is that the roughness of the interface in SmFeO₃/Ni(P_O2-4 Pa)bilayers is lower than that in SmFeO₃/Ni(P_O2-21 Pa)bilayers.The higher the roughness of the interface,the smaller the exchange bias field.In addition,the appearance of mixed valence states of Fe ions in SmFeO₃/Ni(P_O2-4 Pa)bilayers leads to the formation of different hybrid orbitals,which may be one of the reasons for the different exchange bias fields and coercivity in the two bilayers.