Size Effect in the Spin Glass Magnetization of Thin AuFe Films as Studied by Polarized Neutron Reflectometry

We used polarized neutron reflectometry to determine the temperature dependence of the magnetization of thin AuFe films with 3% Fe concentration. We performed the measurements in a large magnetic field of 6 T in a temperature range from 295 to 2 K. For the films in the thickness range from 500 to 20 nm we observed a Brillouin-type behavior from 295 K down to 50 K and a constant magnetization of about $0.9{\mu }_B$ per Fe atom below 30 K. However, for the 10 nm thick film we observed a Brillouin-type behavior down to 20 K and a constant magnetization of about $1.3{\mu }_B$ per Fe atom below 20 K. These experiments are the first to show a finite-size effect in the magnetization of single spin-glass films in large magnetic fields. Furthermore, the ability to measure the deviation from the paramagnetic behavior enables us to prove the existence of the spin-glass state where other methods relying on a cusp-type behavior fail.

Figure 1

Measured neutron reflectivities $R^{+}$ (+) and $R^{-}$ (open circles) as a function of the scattering vector $q$ for a 20 nm thick ${\mathrm{Au}}_{97}{\mathrm{Fe}}_3$ film along with the fits (solid and dashed line, respectively) at a temperature $T=10\mathrm{K}$ in an external magnetic field of 6 T.

Figure 2

Asymmetry as a function of the scattering vector $q$ for a 20 nm thick ${\mathrm{Au}}_{97}{\mathrm{Fe}}_3$ film measured at 10 K (solid circles) and measured at 200 K (open circles) in a magnetic field of 6 T. The solid and dashed lines are the fits for the 10 and 200 K data, respectively.

Figure 3

Averaged magnetic moment per Fe atom versus temperature of thin ${\mathrm{Au}}_{97}{\mathrm{Fe}}_3$ films as derived from the fit to the PNR data (solid circles) compared to a Brillouin function with $J=2$ for isolated Fe atoms (solid line).