Magnetic And Electrical Transport Properties Of Mn-based Alloy Films And CoFeB Multilayers

In recent years,with the rapid development of spintronics and the development of emerging magnetic memory devices,many new electronic materials,novel and special magnetic transport phenomena have been discovered,such as magnetic skyrmions,topological Hall effect,anomalous magnetoresistance effect,etc.,which have gradually attracted considerable research topic in condensed matter physics.In this paper,theoretical calculation and experimental methods are combined,based on Mn-based alloys or thin films and CoFeB thin films with perpendicular anisotropy.Also the related magnetic and new transport properties are explored.The physical mechanism and theoretical basis of these magnetic structures and transport properties are further studied.The following results are obtained:（1）In Mn-based Mn₃Ga films,the samples with ultra-high coercivity（～22 k Oe）at room temperature were prepared on Si substrates by adjusting the annealing temperature.It is confirmed that in addition to the main D0₂₂phase,there is a second phase in the Mn₃Ga films by X-ray diffraction combined with transmission electron microscopy.Combined with the hysteresis loops at different temperatures,the second phase is antiferromagneticα-Mn.The large coercivity of Mn₃Ga films is also present in the electronic properties through the anomalous Hall effect.Subsequent analysis by atomic force microscopy and magnetic force microscopy revealed that the large coercivity in the films was mainly derived from finer grains and the pinning of magnetic domain walls.（2）In Mn-based hexagonal MnFeGe alloys,anomalous Hall effects and obvious magnetoresistance（MR）effect can be observed due to the spin-dependent scattering in measurements of transport properties.More importantly,it is found that the MR curves exhibit a non-monotonic change under the low-field,which first increases and then decreases with the increase of the magnetic field,giving an M-type to the whole curve.The M-type MR can be well reproduced in the sputtered MnFeGe films and even become more pronounced by changing the film thickness.Micromagnetic simulations were performed to further investigate the magnetic domain evolution in the MnFeGe films and its accordance with the M-type magnetoresistance is investigated.The transition from dense labyrinthine domains to bubble domains was found to occur with increasing external field,and the critical field for the bubble domains is also roughly consistent with the peak of the MR curves.（3）In the CoFeB thin films,the magnetic anisotropy wass regulated by continuously adjusting the thicknesses of the CoFeB and the MgO layers in the[Ta/CoFeB/MgO]₁₅ multilayer films.When studying the Hall curve that the external field rotating out-of-plane,an interesting evolution of the angular-dependence of the Hall effect with the external field rotating from the normal direction of the sample to the in-plane direction has detected.An abnormal cusp-like peak appears at some angles when the external field deviates from the vertical direction,such as 2d single-crystalline Fe₃Ge Te₂.Then the correlation between magnetic anisotropy and peak anomaly was studied.It was found that peak anomaly mainly appeared in the samples with moderate perpendicular magnetic anisotropy,but did not exist in the samples with in-plane anisotropy.（4）Based on the results of micromagnetic simulation and experiments,an effective magnetic domin evolution model is developed for the first time to explain the different behaviors of the Hall curve in all systems,including those that can be explained by the single domain Stoner–Wohlfarth model.Simulation found:When the anisotropy constant（_u）is too large,the curve will show a monotonically increasing trend with the decrease of the the positive field in the whole external magnetic field,and will always be single domain.However,when the_uis small,the curves show a smooth trend without any abnormal changes,and the in-plane vortex domain is formed due to the magnetic moment always tends to align in-plane.Based on the above discussion,a suitable_uis required to generate the abnormal peak in the curve.Considering that the formation of skyrmion magnetic domains also requires appropriate_u,magnetic skyrmions were indeed observed in thin films with spike transitions by magnetic force microscopy,thus establishing a correlation between them.This work can not only largely enrich our understanding of the evolution of the angular-dependent Hall effect,which can benefit the studies of spin–orbit torque electronics,but also provide alternative tools of probing the magnetic domain states.