AuFe spin glass films
Spin glass systems have been of interest to theorists as well as experimentalists for decades. For thin films, the focus has been on finite-size effects, the crossover from two to three-dimensional behavior, and the existence of a surface anisotropy. The challenge from an experimental point of view is to provide reliable experimental data on the magnetization of single films in the nanometer range containing only a few atomic percent of magnetic atoms in a nonmagnetic matrix. Superconducting quantum interference device magnetometry has been successfully employed to study bulk properties but is not adequate to measure the magnetization of spin glass films in the nanometer range because of the small magnetic signal compared to the huge diamagnetic background of the substrate. PNR has the advantage to measure the magnetization directly and the contribution of the substrate to the measured signal does not obscure the tiny signal of the thin film as has been shown, e.g., in ultrathin Fe films. We have successfully employed the technique of PNR to study the properties of thin AuFe spin glass films (see references below).
Size effect in thin AuFe films
Finite-size effects were also inferred from the vanishing of a cusp in anomalous Hall-effect measurements of AuFe thin films at about 12 nm. However, the vanishing of a cusp does not necessarily mean a vanishing of the spin glass frustration. Susceptibility measurements showed that the cusp in AuFe samples is meared out in magnetic fields and anomalous Hall-effect measurements showed that the cusp vanishes in large magnetic fields. Using polarized neutron reflectometry (PNR), we were able to show that thin AuFe films show spin glass behavior in a large magnetic field of 6 T. Therefore, one has to be cautious when drawing conclusions from the vanishing of a cusp in either susceptibility or Hall-effect measurements.
Recently we observed a finite-size effect in the temperature dependence of the magnetization of single Au0.97Fe0.03 films.7 The films in the range from 500 to 20 nm showed a Brillouin-type behavior from 295 K down to 50 K with a constant magnetization of 0.9 mu_B per Fe atom below 30 K whereas the magnetization of the 10-nm-thick film could be fitted with a Brillouin function down to 20 K followed by a constant magnetization of 1.3 mu_B. So, the temperature dependence of the magnetization in AuFe films deviates from bulk behavior below 20 nm but it still shows a typical spin glass behavior at 10 nm.
The latest study is a continuation of this work extending the thickness range down to 1 nm. It shows that below 10 nm, the reduction in the spin glass magnetization compared to a Brillouin-type behavior decreases with decreasing film thickness. Finally, the magnetization of the 1-nm-thick film could be described with a Brillouin function also below 50 K proving that ultrathin Au97Fe3 layers below 1 nm do not show spin glass behavior anymore but are paramagnetic.
H. Fritzsche, J. M. van der Knaap, M. B. S. Hesselberth, and G. J. Nieuwenhuys
Loss of spin glass behavior in ultrathin AuFe films
M. Saoudi, K. Temst, C. Van Haesendonck, M. R. Fitzsimmons, and H. Fritzsche
Magnetic field dependence of the magnetization of a 29 nm thick AuFe spin glass film
M. Saoudi, H. Fritzsche, G. J. Nieuwenhuys, and M. B. S. Hesselberth
Size effect in the spin glass magnetization of thin AuFe films as studied by polarized neutron reflectometry
H. Fritzsche, M. Saoudi, K. Temst, and C. Van Haesendonck
A polarized neutron reflectometry study of the spin glass freezing in a 29 nm thick AuFe film
H. Fritzsche, J. Root, K. Temst, and C. Van Haesendonck
Magnetization cusp in a thin AuFe Spin glass film in high magnetic fields
J. Swerts, K. Temst, C. Van Haesendonck, H. Fritzsche, V. N. Gladilin, V. M. Fomin, and J. T. Devreese
Polarized neutron reflectivity on dilute magnetic alloys