Static And Dynamic Exchange Magnetic Coupling In Fe/RFeO₃ Heterostructures

The RFeO₃?R denotes Y,La to Lu?orthoferrites have been largely investigated for their particular magnetic structures and abundant phase transitions.The complex magnetic structure and abundant phase transitions enable RFeO₃ an ideal candidate to investigate the ultrafast magnet-optical recording,multiferroic,anisotropic magnetocaloric effect and unusual intrinsic exchange bias effect.However,the interfacial exchange coupling effects in ferromagnetic?FM?/RFeO₃ heterostructures are still less demonstrated.The FM/AFM heterostructure has been widely used in advanced magnetic recording and magnetic sensor device.Therefore,the understanding of interfacial exchange coupling in FM/AFM is important for both practical application and fundamental magnetism.Due to the particular magnetic structure and abundant phase transitions of RFeO₃,the particular interfacial exchange coupling effects in exchange-coupled FM films are also expected.The main results of this thesis are as follows:1.The interfacial exchange coupling effects in Fe/Er FeO₃?EFO?heterostructures are systemically demonstrated by both the MOKE and anisotropic magnetoresistance.The weak net moment in G-type AFM of EFO orthoferrite along c axis can be used as a pinning layer to fix the Fe films.The AFM interfacial exchange coupling between Fe film and adjacent net magnetic moment of EFO orthoferrite leads to a positive exchange bias in Fe/ EFO heterostructure at room temperature without any field cooling process.On the other hand,a strong UMA,originating from the interfacial exchange coupling between Fe film and compensated G-type AFM spins of EFO,is also determined by the methods of both fitting hysteresis loops and magnetic torque curves.We further demonstrate the angular dependent asymmetry of magnetization reversal from the views of both bias field and asymmetric switching field.The huge asymmetric switching field near magnetization hard axis makes it an effective method to probe weakly unidirectional magnetic anisotropy for film with nearly 180o domain wall displacement.2.Fe films were normally or obliquely deposited on single crystal EFO?010?and EFO?001?substrates and investigated by using ex-situ magnetoptical Kerr effect?MOKE?and anisotropic magnetoresistance?AMR?measurements at room temperature.We obtained two-fold,four-fold and even six-fold magnetic anisotropies in exchange-coupled Fe/EFO heterostructures from fitting magnetic torque curves.Using micromagnetic simulation approach,we found that the interfacial AFM spins are not completely rigid but rotatable for minimizing the total free energy of FM/AFM heterostructure.The spin-canted state of AFM spins is dependent on magnetization direction of FM,leading to the anisotropic total free energy in FM/AFM.The wellknown spin-flop-coupling-induced UMA and high-order magnetic anisotropies in FM obtained here are understood as the first-order and high-order effects of anisotropic total free energy in FM/AFM,respectively.The experimental and simulation results suggest that the high-order magnetic anisotropies are not only just accessible in epitaxial FM film grown on AFM with four-fold symmetry of spin structure,but more likely common effects in exchange-coupled FM/AFM heterostructures.3.The temperature driven spin-reorentation transitions in exchange-coupled Fe/EFO heterostructures are investigated by both hysteresis loops and magnetic torque measurements.Experimental and simulation results both demonstrate that,due to the interfacial exchange coupling between Fe film and compensated spins of AFM,a 90o in-plane spin reorientation transition?SRT?of Fe film grown on b-cut substrate from c axis to a axis,can be induced by the phase transition of EFO orthoferrites from 4?38?phase to 2?38?phase in the temperature range of 100-80 K.The temperature dependent UMA and four-fold magnetic anisotropy constants obtained from fitting magnetic torque in Fe/EFO?010?heterostructure are both well consistent with the values obtained from micromagnetic simulation approach,suggesting the first-order and high-order effects of total free energy density in FM/AFM can well describe the origin of complex magnetic anisotropies in Fe/EFO exchange-coupled heterostructures.In the cases of Fe/EFO?100?for temperature larger than 100 K and Fe/EFO?001?for temperature smaller than 80 K,since the compensated AFM spins stay out-of-plane,the exchange-coupling-induced in-plane magnetic anisotropies are ignorable.When cooling Fe/ EFO?100?down to 70 K and warming Fe/EFO?001?up to 100 K,the compensated AFM spins in these two heterostructures both switch from out-of-plane to in-plane,and consequently sharp increase trends of in-plane magnetic anisotropies as a function of temperature are both obtained.4.A significant difference in ultrafast spin dynamics between Fe/RFe O₃ heterostructures and RFeO₃ orthoferrite single crystals was observed.The spin precession of pure RFeO₃ orthoferrite single crystals cannot be triggered by ultrafast laser heating around room temperature.On the contrary,with coupling by Fe ferromagnetic layer,not only the quasi-ferromagnetic resonance?Q-FM?mode and the impurity mode,but also the coherent phonon mode,were successfully triggered by femtosecond laser at room temperature.We propose the improved efficiency of multimode spin dynamics excited by ultrafast laser is attributed to the optical modification of interfacial exchange coupling between Fe/RFeO₃?100?.Our results reveal that the optical modification of interfacial exchange coupling between Fe/RFe O₃?100?can open a new avenue to engineer the ultrafast spin dynamics of rareearth orthoferrites and expand the application of antiferromagnetically spintronic devices.5.Ultrafast laser control of a ferromagnetic via exchange coupling with an antiferromagnetic material by ultrafast laser is demonstrated in Fe/EFO heterostrcutures.The static and dynamic magnetic properties in Fe?3.5 nm?/EFO?100?,Fe?3.5 nm?/EFO?010?,Fe?3.5 nm?/ EFO?001?and Fe?7.0 nm?/EFO?001?heterostrcutures were studied by magneto-optical Kerr effect,anisotropic magnetoresistance and time-resolved magneto-optical Kerr effect.When applied field out of plane,the optical modification dynamic frequency is proportional to the interfacial exchange coupling-induced uniaxial magnetic anisotropy field,predicting the photoinduced reduction of interfacial exchange interaction and confirmed by using micromagnetic simulation approach.The numerical calculation shows the lattice temperature increase is only ¹7 K during the spin precession process for peak laser power,which seems impossible cause a huge modification of interfacial exchange coupling observed here.The optical modification of exchange interactions in iron oxides reported previously may explain the optical modification spin dynamics behavior in Fe/EFO heterostuctures found here,and further insight into this issue is still on the way.Furthermore,the surface morphology and surface magnetic anisotropy can also play a significant role in determing the magnetic anisotropies of ultrathin film.The tunning of in-plane spin orentation of epitaxial Fe film growning on vicinal Si?111?substrate is further developed.Due to the step-induced competing magnetic anisotropies in Fe/Si?557?film,i.e.the surface and volume strain –induced magnetic anisotropies,the in-plane spin orientation of Fe film can be modified by the thickness of Fe film,temperature and capping layer.We also investigated the magnetic anisotropy of obliquely deposited Fe films on vicinal Si?111?substrate by using in-situ and ex-situ surface magneto-optical Kerr effect?MOKE?.Thickness-induced inplane spin-reorientation transition,i.e.magnetization easy axis gradually rotate away from step direction,was observed.MOKE measurements and micromagnetic simulation demonstrate this spin-reorientation transition process largely originated from the competition between step-induced magnetic shape anisotropy and obliquedeposition-induced magnetic shape anisotropy.