Influence of Ta insertions on the magnetic properties of MgO/CoFeB/MgO films probed by ferromagnetic resonance

Abstract

We show by vector network analyzer ferromagnetic resonance measurements that low Gilbert damping α, down to 0.006, can be achieved in perpendicularly magnetized MgO/CoFeB/MgO thin films with ultrathin insertions of Ta in the CoFeB layer. Although increasing the number of Ta insertions allows thicker CoFeB layers to remain perpendicular, the effective areal magnetic anisotropy does not improve with more insertions, which come with an increase in α.Perpendicular magnetic anisotropy (PMA) is the key to further downscaling of spin transfer torque magnetoresistive random memory devices, as it allows two key requirements to be satisfied: low critical current I c0 and high thermal stability ∆, the latter of which is proportional to the energy barrier E b between the two stable magnetic states. The spin torque switching efficiency, defined as E b /I c0 , is commonly used as a metric to account for both requirements. For a StonerWohlfarth model, it is given by [1] ( /4e) · (η/α), where α is the Gilbert damping parameter, and η is the spin polarization factor, which is related to the tunnel magnetoresistance ratio (TMR) by η = [TMR(TMR + 2)] 1/2 /[2(TMR + 1)]. It thus becomes evident that for high switching efficiency, one has to decrease α while keeping TMR high. Magnetic tunnel junctions (MTJs) based on CoFeB/MgO systems are well known to provide high TMR [2] and have recently been shown to possess PMA, which is attributed to the CoFeB/MgO interface.[3] A Ta layer is usually placed adjacent to the CoFeB to induce the proper crystallization necessary for PMA and high TMR [4]. In Ta/CoFeB/MgO systems, however, spin pumping to the Ta increases α.[5] Moreover, the CoFeB layer also needs to be ultrathin (typically less than 1.5nm) in order to exhibit PMA. [3] To improve the thermal stability as devices are scaled down to smaller diameters, increasing the effective areal anisotropy energy density K e f f t is desired.One approach to address these issues is the use of double-MgO structures, i.e., those in which both the barrier layer and capping layer straddling the free layer are made of MgO. Improved I c0 and/or ∆ have been reported in devices using double MgO free layers. [6,7,8,9] The improvement in thermal stability is attributed to the additional CoFeB/MgO interface, whereas lower I c0 is associated with low α. Indeed, α down to 0.005 has been measured in in-plane MgO/FeB/MgO films,[10] which agrees with device measurements.[11] The stacks investigated in these damping studies, however, did not have the Ta layer used in practical free layers with perpendicular anisotropy. [9,12] In addition, although the interfacial anisotropy in the out-of-plane devices measured by Tsunegi et al.[11] can be as high as 3.3 mJ/m 2 , the effective perpendicular anisotropy was rather low (K e f f t ≈ 0.04mJ/m 2 ) relative to that of a Ta/CoFeB/MgO stack [3]. In this work, we explore the influence of Ta insertions within the CoFeB layer of MgO/CoFeB/MgO films by magnetometry and vector network analyzer ferromagnetic resonance (VNA-...

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