Observation of the Conduction Electron Spin Polarization in the Ag Spacer of a $\mathrm{F}\mathrm{e}/\mathrm{A}\mathrm{g}/\mathrm{F}\mathrm{e}$ Trilayer

The spatially oscillating electron spin polarization in the Ag spacer of a $4\mathrm{n}\mathrm{m}\mathrm{F}\mathrm{e}/20\mathrm{n}\mathrm{m}\mathrm{A}\mathrm{g}/4\mathrm{n}\mathrm{m}\mathrm{F}\mathrm{e}(001)$ epitaxial trilayer has been determined by means of low energy muon spin rotation. It oscillates with the same period as the interlayer exchange coupling, but shows a much weaker attenuation at large distances $x$ from the interface. The measured magnetization profile from the inner 14 nm of the spacer is described by an oscillating polarization decaying as $x^{-0.8(1)}$. This unusual behavior may arise from a full confinement of electron states within the spacer.

Figure 1

LE-$\mu \mathrm{S}\mathrm{R}$ decay asymmetry as a function of time obtained at a temperature of 20 K in an external field of $B_{\mathrm{e}\mathrm{x}\mathrm{t}}=8.8\mathrm{m}\mathrm{T}$. Only muons stopping in the inner 14 nm of the 20 nm Ag spacer contribute to $A(t)$. Muons thermalized in Fe or in interface regions of $\approx 3\mathrm{n}\mathrm{m}$ sense too high magnetic fields to be detected with the LE-$\mu \mathrm{S}\mathrm{R}$ experiment. The solid line is a maximum entropy fit to the data using Eq. (1). Inset: Muon stopping profile for a muon implantation energy $E=3\mathrm{k}\mathrm{e}\mathrm{V}$, calculated with the Monte Carlo code TRIM.SP.

Figure 2

Measured field distribution (points) inside the inner 14 nm of the Ag spacer at 20 K in an external field of $B_{\mathrm{e}\mathrm{x}\mathrm{t}}=8.8\mathrm{m}\mathrm{T}$. The local field at the muon includes positive and negative hyperfine field contributions, which arise from the spatially oscillating polarization in the spacer. The measured fields can be directly converted into the local induced magnetization $M_{\mathrm{l}\mathrm{o}\mathrm{c}}$ (upper scale) at the muon sites at various depths within the layer. By comparison with a model distribution (shaded areas) it is possible to determine the spatial magnetic profile within the Ag spacer (inset). The model is described in the text and has been convoluted with a Gaussian of width 0.2 mT to represent the finite frequency resolution of the $\mu \mathrm{S}\mathrm{R}$ technique arising from the muon lifetime.