Interface-induced Magnetic Anisotropy And Anisotropy Of Magnetic Damping In Ferromagnetic/Non-ferromagnetic Films

Magnetic anisotropy has been applied in spintronic devices including magnetic random access memories,magnetic logic devices and magnetic field sensors,while magnetic damping is one of the key factors governing the performance of these spintronic devices.It not only determines the speed of magnetization reversal but also influences the energy consumption of these spintronic devices.Therefore,investigation on magnetic anisotropy,magnetic damping and the correlation between them favors to design spintronic devices with lower energy consumption,faster processing speed and higher integration.Ferromagnetic/non-ferromagnetic(FM/NM)film has been used as the core constituent in plenty of spintronic devices.In this thesis,we have performed studies on the interface-induced in-plane magnetic anisotropy and anisotropy of magnetic damping in a couple of FM/NM films and tried to disclose the correlation between them.The main contents are listed in the following.1.Magnetic anisotropy and anisotropy of magnetic damping in Ga As/Co Fe B filmsSo far more and more theoretical prediction and experimental findings have demonstrated that magnetic damping is anisotropic,which favors us to manipulate the magnetic damping conveniently by changing the azimuthal angle within the same device.Recently,damping anisotropy has been mainly found in single-crystalline or high-textured polycrystalline films and it is closely related to the magneto-crystalline anisotropy(MCA)of these films.However,whether or not magnetic damping is anisotropic in amorphous films,in which MCA has been ruled out,remains unknown.The amorphous Co Fe B films with different thicknesses were deposited on Ga As(001)single-crystalline substrate by magnetron-sputtering.Large in-plane uniaxial magnetic anisotropy(UMA)has been found in Ga As/Co Fe B films with easy axis(EA)along Ga As[110]and hard axis(HA)along Ga As[1-10].Moreover,the strength of UMA decreases with increasing the Co Fe B film thickness,implying a kind of typical interface-induced magnetic anisotropy.The ferromagnetic resonance(FMR)results show that resonance field(H_r),linewidth(?H)and damping factor all behave in-plane UMA.The fitting results of in-plane H_r-frequency(f)curves show that only pure in-plane UMA exists in Ga As/Co Fe B films while the fourfold symmetry corresponding to MCA is absent,agreeing well with the amorphous structure of Co Fe B.Similar to UMA,the damping anisotropy in Ga As/Co Fe B films also decreases with increasing the Co Fe B film thickness,indicating the interfacial effect.Moreover,Ga As(001)/Co Fe B films with fixed B content and varying the relative composition between Co and Fe were deposited.The maximum UMA and the maximum damping anisotropy appear in the Ga As/Co Fe B films when Co and Fe have equal compositions(Co₄₀Fe₄₀B₂₀),which is consistent with the prediction of Néel-Taniguchi model.By analyzing the characterization result of the films'microstructure,we find that the Ga As-Co Fe B interface is relatively flat along the EA while rough along the HA.This special interfacial morphology induces not only the in-plane UMA but also the anisotropic two-magnon scattering(TMS),which finally results in the anisotropy of magnetic damping.2.Exchange bias in Ni Fe2O4/Bi Fe O3 bilayersBesides UMA,unidirectional anisotropy is also a common kind of magnetic anisotropy induced by interfacial coupling,which often accompanies with the appearance of exchange bias(EB)effect.By using the multiferroic material Bi Fe O₃(BFO)whose antiferromagnetic and ferroelectric phase transition temperatures both well above room temperature,people can design low-energy-consumption magnetoelectric random access memories(MERAM)device with the crucial part of BFO/FM bilayer.Usually,metal material is employed as the FM layer,but it is easily destroyed by the influence of interfacial diffusion,interfacial chemical reaction and so on.Therefore,an alternative to use all-oxide BFO/FM bilayer is a smart choice.However,significant exchange bias has not been realized at room temperature(RT)yet in this kind of bilayers.In this present work,we fabricated Ni Fe₂O₄(NFO)/BFO bilayers by using pulsed laser deposition(PLD).X-ray diffraction(XRD)and high-resolution transmission electron microscopy(HRTEM)characterizations demonstrate that both the NFO and the BFO layers have good crystallinity.Element mappings of TEM and depth profile X-ray photoemission spectroscopy(XPS)spectra confirm the well layered structure of NFO/BFO bilayers.Upon field cooling from RT,obvious EB effect appears at low temperature region in the NFO/BFO bilayers,which is attributed to spin-glass(SG)phase formation in the BFO layer which provides the pinning effect to the NFO layer.Upon field cooling from temperatures higher than RT,obvious EB effect appears at RT in the NFO/BFO bilayers.When the field cooling temperature(T_FC)is 600 K and the cooling field(H_FC)is 0.5 T,the EB field(H_E)and the ratio of H_E/coercivity(H_C)can be achieved to be 48 Oe and 0.6,respectively.We consider that the EB effect at RT is resulted from the pinned and uncompensated spins near the NFO-BFO interface.3.Anisotropy of magnetic damping in Fe Ni/Ir Mn exchange bias structuresBesides UMA,the unidirectional anisotropy induced by FM-AFM interfacial coupling in FM/AFM bilayer can also generate damping anisotropy.In this present work,we choose Fe Ni/Ir Mn bilayer system,which should have significant EB effect,to investigate the anisotropy of magnetic damping.The Fe Ni/Ir Mn and Ir Mn/Fe Ni bilayer films with opposite deposition sequences were fabricated by magnetron-sputtering under the field of 300 Oe.The measurement results of M-H loops in these two kind of films demonstrate that H_E is inversely proportional to the FM thickness and exhibit the in-plane unidirectional anisotropy as well.The FMR results show that H_r,?H and magnetic damping all have unidirectional anisotropies in these two kind of films.The in-plane angular dependences of magnetic damping and H_E can be fitted by trigonometric functions.Moreover,the change of magnetic damping(difference of magnetic damping between HA and EA)is linear with H_E.Continuous measurements of M-H loop in the Fe Ni/Ir Mn films show that both H_E and H_C decrease with increasing the cycle number n,indicating obvious EB training effect.Meanwhile,the magnetic damping along EA and HA also decrease with increasing n,especially a sharp decrease between n=2 and n=1 can be observed at the HA direction.In addition,the stronger EB training effect is,the more obvious decrease of magnetic damping with increasing n can be observed.Therefore,the magnetic damping also has significant training effect.