Impact of MgO Thickness on the Performance of Spin-Transfer Torque Nano-Oscillators
A magnetic tunnel junction (MTJ) stack incorporating an MgO wedge was deposited over a 200 mm wafer and nanofabricated into circular nanopillars with a diameter of 200 nm. Due to the variable MgO thickness, we could obtain MTJ nanopillars with resistance $\times $ area (RA) ranging from less than $1~\Omega \mu \text{m}^{2}$ up to $15~\Omega \mu \text{m}^{2}$ and tunnel magnetoresistance ratios up to $\sim 100$ %. It was observed that the ferromagnetic coupling ( $H_{F})$ displayed by the transfer curves increases for smaller values of RA. This variation is attributed to the orange-peel coupling, which is caused by the MgO roughness, with a larger impact for thinner barriers. The RF output of the nanopillars caused by the spin-transfer torque-induced magnetic precession of the free layer was measured as a function of bias current and external magnetic fields for nanopillars with different RA values. Results concerning typical devices in the thin MgO region ( $1.3~\Omega \mu \text{m}^{2})$ and in a thicker MgO region ( $2.7~\Omega \mu \text{m}^{2})$ are shown to illustrate the dependence found. It was observed that the higher output power ( $\sim 300$ nW) and the smaller linewidth ( $\sim 90$ MHz) could be obtained with the higher RA region. These results suggest that there must be an optimum RA value for the production of nano-oscillator devices, which depends on the tradeoffs between the need for large breakdown voltages and strong spin polarization of the bias current (favored for larger RA values) and between the need of endurance under large current densities (favored for smaller RA values). Provided the breakdown voltage of the MgO barrrier can be made large enough, nanopillars based on MTJ stacks with an intermediate RA can provide a solution for the production of large output power spin-transfer nano-oscillators.