Layer thickness dependence of the current-induced effective field vector in Ta|CoFeB|MgO

Abstract

However, the size and direction of the current induced effective field seems to vary depending on the system and the underlying mechanism of such field generation is not well understood. For example, the effective field in Pt|Co|AlOx is reported to be ~3000 to ~10000 Oe for a current density of 10 8 A/cm 2 , pointing perpendicular to both the film normal and the current flow direction (defined as a transverse field hereafter) 4,13 . More recently, in the same system, signs of current induced effective field directed along the current flow, i.e. Here we show a systematic study of the current induced effective field in Ta|CoFeB|MgO.We use a low current excitation technique to quantitatively evaluate the size and direction of the effective field. We find that the size and even the sign of the transverse and longitudinal effective fields vary as the Ta layer thickness is changed, suggesting competing contributions from two distinct sources. We find that the transverse effective field is larger than the longitudinal field, by nearly a factor of three, when the Ta layer thickness is large. In contrast, the relative size of the two components shows somewhat an oscillatory dependence on the Ta thickness for films with thin Ta.Films are sputtered on a highly resistive silicon substrate coated with 100 nm thick thermally oxidized SiO 2 . We use a linear shutter during the sputtering to vary the thickness of one layer in each substrate. Two film stacks are prepared here: Ta wedge: Si-sub|d Ta Ta|1 Co 20 We set the thickness of each layer, d Ta and t CoFeB , to vary from ~0 to ~2 nm. Throughout this paper, the nominal thickness is used for the Ta wedge. For the CoFeB wedge film, correction of the thickness was required due to technical reasons, and thus we use our resistivity results to calibrate the thickness. All films are annealed at 300 °C for one hour ex-situ after the film deposition. Photo-lithography and Ar ion etching are used to pattern Hall bars from the film and a lift off process is used to form the contact electrodes (10 Ta|100 Au). Prior to the deposition of the contact electrodes, we etch the Ta capping layer and nearly half of the MgO layer to avoid large contact resistance. Although etching of the MgO layer significantly influences the magnetic anisotropy of the CoFeB layer under the etched region 24 , here we assume that this has little effect on the evaluation of the current induced effective fields since we limit the applied field smaller than the magnetization switching field. 4Schematic illustration of the experimental set up and definition of the coordinate system are shown in Fig 1(a). The width and length of typical wires measured are 10 m and 60 m, respectively. We measure wires with different width, ranging from 5 m to 30 m, and find little dependence on the width for most of the parameters shown here. Positive current is defined as current flowing along the +y direction in Fig 1(a). Current is fed into the wire and the Hall voltage is measured in all experiments. Using the Extraordinary Hal...

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