Strain-Free Magnetic Tunnel Junction with Metastable bcc CoxMn100-x(001) Ferromagnetic Layers

Abstract

In spintronics, one of the long standing questions is why the MgO barrier is almost the only option to achieve a large tunnelling magnetoresistance (TMR) ratio at room temperature (RT) but not as large as the theoretical prediction [1]. This can be due to the spin fluctuation at the ferromagnet/MgO tunnel barrier interfaces and/or spin-independent hopping within the barrier in a magnetic tunnel junction (MTJ) [2]. One of the approaches to overcome these issues is to employ the lattice softening of the ferromagnetic layers in MTJ. Recently a metastable body-centred cubic (bcc) CoxMn100-x (CoMn) ferromagnetic films have been reported to exhibit the enhancement of their magnetic moments with an ideal lattice matching with MhO [3], which can be an ideal candidate for the TMR enhancement in MTJ.MTJ stacks were grown using conventional magnetron sputtering on MgO(001) substrates with the structure of Cr (40)/CoxMn100-x (10)/MgO (2.4)/CoxMn100-x (4)/Co3Fe(1.5)/IrMn (10)/Ru (5) (thickness in nm) [4]. For CoMn, four different compositions of x = 66, 75, 83 and 86 were used, allowing to control the CoMn lattice constants. To promote the crystallisation of the seed and CoMn layers, in-situ annealing was performed after the deposition of the Cr seed layer and CoxMn100-x layers at 700oC and 200oC, respectively. MTJs were then post-annealed at 325oC for their crystallisation after patterned into pillars with Ti/Au electrodes by photolithography. TMR measurements were carried out using the conventional four-terminal setup with elevating temperature, showing two distinctive TMR ratios of 229% and 142% at room temperature for x = 75 and 86, respectively. The corresponding atomic structures were imaged by cross-sectional transmission electron microscopy (TEM, JEOL JEM-2100 Plus) at 600k magnification.Figures 1(a) shows a cross-sectional high-resolution (HR) TEM image of the MTJ with x = 75 showing the larger TMR ratio. Fully epitaxial growth of the entire MTJ is confirmed with some dislocations, of which period is calculated to be (11.4 ± 0.3) nm. This indicates that the Co75Mn25 layer may induce plastic deformation rather than elastic deformation with inducing dislocations at the boundaries between crystals. The x = 86 sample, on the other hand, shows dark black regions at the CoMn/MgO interfaces indicating CoMn/MgO is not fully crystallised within the CoMn layer [see Fig. 1(b)]. These features are found to be induced by dislocations formed at the CoMn/MgO/CoMn interface with the period of (8.9 ± 0.3) nm. These dislocations may be the origin of the interfacial spin fluctuation.In the MgO layer for x = 75, two distinctive crystallographic phases are observed as shown as the regions (i) and (ii), which are crystallised along MgO[001] and [100] directions, respectively, as similarly reported in previous reports [5]. For x = 86, an additional partially crystallised MgO grains are found, which induces spin-independent hopping to reduce the corresponding TMR ratio.By measuring the lattice constants of CoMn and MgO in the HRTEM images, the top CoMn lattice constants are almost constant across MTJs apart from x = 86, confirming the lattice softening of these layers with forming almost strain-free MTJ. These results were compared with ab initio calculations on the lattice stability of the CoMn alloys. Calculations confirm the crystalline deformation stability across a broad compositional range in CoMn, proving a strain-free interface for larger TMR ratios. Further optimisation of the CoMn-based MTJs can achieve > 1,000% TMR ratio at RT. **