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 0.9μ_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 1.3μ_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.