Research On Magnetocrystalline Anisotropy Magnetoresistance Of Fe_xCo_100-x Epitaxial Films

The angular-dependent magnetoresistance（ADMR）not only reflects the law of magnetic scattering,but is also an effective scheme to reflect the direction of magnetization and magnetization reversal mechanism.For ADMR of polycrystalline materials,including oridinary magnetoresistance（OMR）,anisotropic magnetoresistance（AMR）,spin Hall magnetoresistance（SMR）and so on,almost all of which satisfy the twofold symmetry.However,in single crystals,in addition to the above contributions,ADMR also has high-order symmetry and phase shift.Phenomenological theory and electronic structure calculation have always considered this single crystal ADMR as AMR and failed to unify the understanding of its physical mechanism.In addition,due to the same twofold symmetry of the different magnetoresistances mentioned above,it is difficult to independently and quantitatively separate and investigate the single crystal ADMR.In this thesis,baesd on the magnetocrystalline anisotropy（MCA）symmetry-adapted model,the dominant contribution of magnetocrystalline anisotropy magnetoresistance（MMR）to single crystal ADMR is determined in Fe_xCo_100-x epitaxial films;the unified description of=₀+++in total ADMR was proved,where₀ is the fundamental resistivity contributed by impurity and phonon scattering,,,andare OMR,AMR,and MMR contributed by magnetic field,magnetization,and MCA,respectively;the dependence of OMR,AMR,and MMR on temperature and magnetic field was researched quantitatively,in order to reveal the physical laws of MMR by regulating MMR dominated ADMR.The corresponding innovative results are as follows:1.The unsaturated characteristics of magnetization direction dependent ADMR are determined in-plane and out-of-plane.In Co/Mg O（001）epitaxial films,we found that the ADMR does not satisfies the cos-square angle-dependent relationship in both in-plane xy and out-of-plane xz planes.This originates from the fact that the magnetization can never be completely saturated when the magnetic field deviates from the easy and hard axes in the coherent rotation model.Using the small-angle approximation,the approximate analytical expression of the magnetization direction-dependent ADMR is obteined.This expression can not only describe the unsaturated phenomenon well,but also obtain the anisotropic field of the material accurately.In the yz plane,where the magnetization is always perpendicular to the current direction,non-AMR-contributed ADMR is also found.2.It is proposed that the single crystal ADMR should be divided into phaseless AMR and MMR with a phase based on the MCA symmetry-adapted model.In the Fe_xCo_100-x/Mg O（001）epitaxial films,the association between single crystal ADMR and MCA is first confirmed,and the ADMR is divided into MCA-independent AMR and MCA-dependent MMR.Analyzing the ADMR in Fe₃₀Co₇₀ epitaxial films,it is found that when the current flows along the easy axis,the negative MMR will exceed the positive AMR and make the whole magnetoresistance appear abnormal negative ADMR;when the current flows along the hard axis,the ADMR will become positive under the combination of positive AMR and positive MMR;when the current flows along other axes,the ADMR will even be tuned out a phase-shift.3.The unified relationship between the three-dimensional ADMR determined by the AMR and MMR are established.The MCA symmetry-adapted model is extended to the three-dimensional space,and the results are consistent with the ADMR experimental results of yz and xz planes in Fe₃₀Co₇₀ epitaxial films.It is also shown that the non-AMR ADMR of yz plane in the single crystal originates from MMR.When the current is applied at the easy and hard axes,AMR,MMR or total ADMR in any two planes can describe the third plane;when the current is not applied to the easy or hard axis,AMR and MMR of any two planes can still describe the third plane independently,but the total ADMR does not exist the relationship because of the phase shift of the xy plane.4.The dependences of OMR,AMR and MMR on temperature and magnetic field are investigated.ADMR in Fe/Mg O（001）epitaxial film is researched at different temperatures.The temperature dependences of OMR,AMR and MMR in ADMR are determined quantitatively.With decreasing temperature,the amplitudes of negative OMR and positive AMR with twofold symmetry increase and decrease,respectively.The amplitude of MMR with twofold and fourfold symmetry hardly changes with temperature and magnetic field.Moreover,when the current applied at easy and hard axes,the OMR and AMR are identical respectively,but the MMR will produce an inverse signal.At low temperature,the contribution of OMR to ADMR is even larger than that of AMR and MMR.5.The influence of different magnetic anisotropy on MMR are established.In Fe/Si（111）epitaxial films with twofold symmetric uniaxial anisotropy（UMA）and intrinsic sixfold symmetric MCA,the temperature and magnetic field dependences of OMR,AMR,and MMR are utilized to regulatory ADMR dominating by fourfold symmetric MMR.By building sixfold symmetric MCA-adapted model,it is found that UMA only affects the shape of ADMR curves by changing the direction of magnetization;MCA is the root cause of the fourfold MMR with phase shift.Under different symmetric MCA,the twofold and fourfold MMR exhibit the scaling law of amplitude independent of current direction and phase linear consistent with MCA direction.