Magnetotransport studies of strongly disordered annealed amorphous Fe/Si multilayers.

Abstract

We present low-temperature, T\ensuremath{\le}20 K, magnetotransport measurements on amorphous Fe/Si multilayers that have been annealed at high temperatures 80 \ifmmode^\circ\else\textdegree\fi{}C\ensuremath{\le}${\mathit{T}}_{\mathrm{ann}}$\ensuremath{\le}400 \ifmmode^\circ\else\textdegree\fi{}C. Thermal annealing leads to interdiffusion of the Fe and Si layers, which increases the disorder and can lead to electrical coupling between the conducting Fe-rich layers. The Hall resistance (${\mathit{R}}_{\mathit{xy}}$) and the sheet resistance (${\mathit{R}}_{\mathit{sq}}$) of three multilayers with Fe thicknesses/Si thicknesses/numbers of layers, 15 \AA{} / 45 \AA{} / 10; 10 \AA{} / 32 \AA{} / 20; 10 \AA{} / 40 \AA{} / 20, were measured. From these quantities, we obtained the sheet conductance (${\mathit{G}}_{\mathit{sq}}$), carrier density per unit area (${\mathrm{n}}_{\mathit{A}}$), mobility (\ensuremath{\mu}), saturation magnetic field (${\mathit{B}}_{\mathrm{sat}}$), and anomalous Hall coefficient (${\mathit{R}}_{\mathit{A}}^{\mathrm{eff}}$). The unannealed multilayers exhibit an anomalous contribution to their Hall resistance that increases and a sheet conductance that decreases logarithmically with decreasing temperature. The logarithmic behavior can be attributed to the effects of disorder-enhanced electron-electron interaction effects in two dimensions. Unlike earlier work and theoretical predictions, however, the anomalous Hall conductance exhibits a clear temperature dependence. In the two samples with ${\mathit{d}}_{\mathrm{Si}}$\ensuremath{\ge}40 \AA{}, ${\mathrm{G}}_{\mathit{sq}}$, ${\mathit{n}}_{\mathit{A}}$, and ${\mathit{B}}_{\mathrm{sat}}$ all decrease and ${\mathrm{R}}_{\mathit{A}}$ and \ensuremath{\mu} increase with annealing, in the same qualitative manner as for bulk ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ as x decreases.We use these data to infer a picture of the evolution of the structure of the multilayers with annealing. Over the whole annealing range, the conductance maintains a 2D temperature dependence. The other sample with thinner Si layers, in which ${\mathit{d}}_{\mathrm{SI}}$=32 \AA{}, shows similar behavior for ${\mathit{T}}_{\mathrm{ann}}$\ensuremath{\le} 250 \ifmmode^\circ\else\textdegree\fi{}C. Above 250 \ifmmode^\circ\else\textdegree\fi{}C, however, the coupling between its Fe-rich layers becomes strong enough that its conductance shows three-dimensional behavior. Simultaneous with the onset of this interlayer coupling, ${\mathit{n}}_{\mathit{A}}$ increases dramatically and the anomalous contribution to the Hall effect, which is proportional to the susceptibility, drops significantly. We argue that the interlayer coupling increases the effective dimensionality of the multilayers, which reduces the influence of disorder effects. Consequently, ${\mathit{n}}_{\mathit{A}}$ increases and the magnetic susceptibility decreases. \textcopyright{} 1996 The American Physical Society.