Two magnon scattering and anti-damping behavior in a two-dimensional epitaxial TiN/Py(tPy)/β-Ta(tTa) system

The prime requirements for the spin transfer torque based ferromagnetic (FM)/nonmagnetic (NM) bilayer spin devices are (i) the absence of two-magnon scattering (TMS) noise, (ii) minimum energy dissipation and (iii) fast switching. To realize these objectives we have studied the thickness, Py (permalloy) thicknesses (tPy) and b-Ta thicknesses (tTa), dependent magnetization dynamics behaviour of the epitaxial Py (tPy 1⁄4 3–20 nm)/b-Ta (tTa 1⁄4 1.5–15 nm) system. The tPy dependence of TMS in epitaxial Py nanolayers (tPy 1⁄4 3–20 nm) grown on a Si(400)/TiN(200) (8 nm) substrate is explored in terms of uniform and non-uniform magnetization precession regimes by employing ferromagnetic resonance field (Hr), linewidth (DH), and Gilbert damping constant (a) behaviour. It is found that in Py, tPy < 10 nm, layers TMS is dominating due to non-uniform precession of the magnetization. However in Py, tPy $ 10 nm, layers the uniform magnetization precession dominates, therefore Py layers, tPy $ 10 nm, are almost free from TMS. Furthermore, a nearly TMS free 12 nm epitaxial Py(200) layer is capped with b-Ta (tTa 1⁄4 1.5–15 nm) layers to explore the tTa dependent magnetization precession of epitaxial Py (12 nm) in terms of change in effective Gilbert damping constant (aeff). An anomalous decrease in aeff from 0.0087 at tTa 1⁄4 0 to a minimum value of 0.0077 at tTa 1⁄4 6 nm, and its subsequent increase for tTa > 6 nm are observed in the epitaxial Py (12 nm)/b-Ta(tTa) system. Therefore the Si(400)/TiN(200) (8 nm)/Py(200) (12 nm)/b-Ta(200) (6 nm) epitaxial system with nearly uniform magnetic precession and minimum effective Gilbert damping is suitable for low energy loss and ultrafast switching applications in spin transfer torque devices.