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

Abstract

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 (t_Py) and β-Ta thicknesses (t_Ta), dependent magnetization dynamics behaviour of the epitaxial Py (t_Py = 3–20 nm)/β-Ta (t_Ta = 1.5–15 nm) system. The t_Py dependence of TMS in epitaxial Py nano-layers (t_Py = 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 (H_r), line-width (ΔH), and Gilbert damping constant (α) behaviour. It is found that in Py, t_Py < 10 nm, layers TMS is dominating due to non-uniform precession of the magnetization. However in Py, t_Py ≥ 10 nm, layers the uniform magnetization precession dominates, therefore Py layers, t_Py ≥ 10 nm, are almost free from TMS. Furthermore, a nearly TMS free 12 nm epitaxial Py(200) layer is capped with β-Ta (t_Ta = 1.5–15 nm) layers to explore the t_Ta dependent magnetization precession of epitaxial Py (12 nm) in terms of change in effective Gilbert damping constant (α_eff). An anomalous decrease in α_eff from 0.0087 at t_Ta = 0 to a minimum value of 0.0077 at t_Ta = 6 nm, and its subsequent increase for t_Ta > 6 nm are observed in the epitaxial Py (12 nm)/β-Ta(t_Ta) system. Therefore the Si(400)/TiN(200) (8 nm)/Py(200) (12 nm)/β-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.