Depth-Dependent Spin Dynamics of Canonical Spin-Glass Films: A Low-Energy Muon-Spin-Rotation Study

We have performed depth dependent muon-spin-rotation and -relaxation studies of the dynamics of single layer films of AuFe and CuMn spin glasses as a function of thickness and of its behavior as a function of distance from the vacuum interface (5–70 nm). A significant reduction in the muon-spin relaxation rate as a function of temperature with respect to the bulk material is observed when the muons are stopped near (5–10 nm) the surface of the sample. A similar reduction is observed for the whole sample if the thickness is reduced to, e.g., 20 nm and less. This reflects an increased impurity spin dynamics (incomplete freezing) close to the surface although the freezing temperature is only modestly affected by the dimensional reduction.

Figure 1

Muon depolarization rate ($\lambda$) and stretch exponent ($\beta$, see inset) versus temperature for AuFe thin films of 10, 20, and 50 nm, 10 mT TF data. Muons of 1, 2, 6 keV, respectively, are stopped in the center of the layer.

Figure 2

Muon depolarization rate ($\lambda$) and stretch exponent ($\beta$, see inset) as a function of temperature ($T$) at different (mean) depths in AuFe 3 at. %. Thickness: 220 nm, ZF data.

Figure 3

Static amplitude of random fields $a_s$ in the AuFe film (thickness 220 nm) at different depths. In the inset polarization spectra (normalized to the asymmetry obtained in pure Au) at 5 K and 2 different depths are shown. The curves are fit to Eq. (1).