Anomalous Hall Effect In Cofeb-based Perpendicular Films

It is widely recognized that the magnetic tunnel junction with a perpendicular magnetic anisotropy (PMA) is a candidate for current-induced spin transfer torque devices because the PMA enables a small critical current density for current induced magnetization switching. For example, perpendicular tunnel junctions have been prepared due to the observation of PMA in a CoFeB layer sandwiched by the MgO and Ta layers. As a powerful tool, anomalous Hall effect (AHE) has been intensively used to study the magnetic materials, in particular the thin films with PMA. The mechanisms of AHE is still elusive. Most recently, we have found that the Ta/CoFeB/MgO and MgO/CoFeB/Ta films show a strong PMA when the CoFeB thickness is1.04nm and1.2nm, respectively. Compared with the conventional perpendicular Pt/Co/Al2O3, the perpendicular Ta/CoFeB/MgO films can well meet the requirements of perpendicular magnetic tunnel junction. Meanwhile, we have found that the [CoFeB/Pt]N multilayers have a clear PMA within CoFeB thickness from0.3nm to0.65nm, which is quite similar to the case of CoFeB/Pd. Compared with the conventional perpendicular Co/Pt multilayers, the CoFeB/Pt multilayers have much small coercivity and weak perpendicular anisotropy, which can meet the requirements of low operation field and high sensitivity for the applications of magnetic sensors. By investigating the transport properties and the magnetic properties, we analyze the PMA and mechanisms of the anomalous Hall effect in Ta/CoFeB/MgO films,[CoFeB/Pt]N multilayers and amorphous CoFeB films and get following conclusions.Firstly, the PMA in as-deposited [CoFeB/Pt]N multilayers has been studied by anomalous Hall effect. A clear PMA has been observed in the ultrathin (0.3～0.6nm) amorphous CoFeB layer sandwiched by Pt. Moreover, the PMA in as-deposited [CoFeB/Pt]n multilayers is strongly dependent on the thickness of CoFeB,Pt,and the number of CoFeB/Pt bilayers. With the increasing the number of CoFeB/Pt bilayers, the hysteresis loops change from rectangle to bow-tie shaped, and then the net moment approaches zero at the remnant state.Moreover, we also found that the annealed Ta/CoFeB/MgO and MgO/CoFeB/Ta films show a strong PMA when the CoFeB thickness is1.04nm and1.2nm, respectively. The PMA arises from the CoFeB/MgO interface and has a strong dependence on the CoFeB, Ta and MgO layer thickness.The anomalous Hall effect in the perpendicular CoFeB/Pt multilayers obey the common scaling relation of Rs=apxx+bp2xx. Different from the conventional perpendicular Co/Pt multilayers, it is found that a small side jump contribution combines with a large skew scattering component to generate AHE in CoFeB/Pt multilayers. The experiments also show that the skew scattering component is enhanced by the number of the CoFeB/Pt bilayer. Using the sensitivity of X-Ray to the interface, We found that the interface roughness increased with the increasing repetition number of CoFeB-Pt bilayers. Therefore, we strongly suggest that the enhancement of the AHE in CoFeB/Pt multilayers is due to the increasing interface scattering caused by the CoFeB-Pt interface roughness. The anomalous Hall effect in the perpendicular Ta/CoFeB/MgO has been investigated. The contribution of side jump is near3times larger than that of skew scattering, indicating that the side jump mechanism dominates the AHE in perpendicular Ta/CoFeB/MgO films. Furthermore, the anomalous Hall conductivity exhibits a linear dependence of the magnetization, σA∝M(T). We assume that the anomalous Hall conductivity from the SJ contribution also shows a linear dependence on magnetization, although the reason is not clearly now.The anomalous Hall effect (AHE) in the amorphous CoFeB films as a function of temperature has been investigated. It is found that both the anomalous Hall resistance RAH and longitudinal resistance Rxx shows a logarithmic temperature dependence in low temperature in a wide range of CoFeB thickness, which is due to a weak-localization correction in two-dimensional (2D). Based on the inelastic cutoff scaling of the localization effect, we present a dimensional crossover from2D to3D, which indicates that the inelastic scattering length is about13nm in weak disorder amorphous CoFeB films. The experiments also show that the weak-localization correction to the anomalous Hall conductance is observable in strong disorder, but is completely quenched in the weak disordered ((kFl)-1<<1) ferromagnetic systems. Moreover, the weak localization will modulate the scaling exponent of the anomalous Hall effect. We have found an unconventional low temperature AHE scaling (σAH=σxxγ,γ≥2) in amorphous CoFeB films in the dirty regime, which is accompanied by a quantum correction to σAH in weak disordered region. The scaling relation shows that the γ=.57after subtracting the contribution of the weak-localization.