Loss of spin glass behavior in ultrathin AuFe films

Abstract

We determined the temperature dependence of the magnetization of thin Au 97 Fe 3 films in the thickness range between 1 and 50 nm by performing polarized neutron reflectivity measurements in a magnetic field of 6 T in a temperature range from 200 down to 2 K. For the films in the thickness range from 50 to 2 nm, we observed a Brillouin-type behavior of noninteracting Fe atoms down to 50 K followed by a strong reduction compared to the Brillouin curve below 50 K. This reduction in the magnetization is a measure of the spin glass frustration and depends on the film thickness below 20 nm as shown in our previous study ͓M. Saoudi, H. Fritzsche, G. J. Nieuwenhuys, and M. B. S. Hesselberth, Phys. Rev. Lett. 100, 057204 ͑2008͔͒ on Au 97 Fe 3 films. The present 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 Au 97 Fe 3 layers below 1 nm do not show spin glass behavior anymore but are paramagnetic. Spin glass systems have been of interest to theorists as well as experimentalists for decades. 1 For thin films, the focus has been on finite-size effects, 2-8 the crossover from twoto three-dimensional behavior, 9 and the existence of a surface anisotropy. 8,[10][11][12][13][14] 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 15,16 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. Therefore, magnetometry experiments on thin spin glass films reported in literature were all performed on multilayers in order to increase the signal. 2,5 However, it is always questionable whether multilayer data represent the true single-film properties because of structural differences as the film gets thicker and an uncomplete decoupling of the individual spin glass layers. 17 Finite-size effects were also inferred from the vanishing of a cusp in anomalous Hall-effect measurements 3 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 smeared out in magnetic fields 15 and anomalous Hall-effect measurements showed that the cusp vanishes in large magnetic fields. 3,18 Using polarized neutron reflectometry ͑PNR͒, we were able to show 7,19 that thin AuFe films show spin glass behavior in a large magnetic field of 6 T. Therefore, one has to be cautious when dra...

Structural, Magnetic, and Superconducting Characterization of the CuNi/Nb Bilayers of the S/F Type Using Polarized Neutron Reflectometry and Complementary Techniques

“…Compare for example with Ref. [16], where magnetization 4M = 0.7 kGs was measured by PNR in the 1-nm-thick Au 97 Fe 3 film.…”

Preparation of SrCoOx thin films on LaAlO3 substrate and their reversible redox process at moderate temperatures

“…where M and M FM denote the total magnetization and the lowtemperature magnetization of ferromagnetic component, and the second term is the paramagnetic contribution from the free Co 3 þ ions, where n, μ¼4μ B , and B(S,T) are the density, magnetic moment for Co 3 þ ion and the Brillouin function with S ¼2, respectively [27]. As shown in Fig.…”

Recent Progresses in Nanometer Scale Analysis of Buried Layers and Interfaces in Thin Films by X-rays and Neutrons

“…The magnetization reversal of a CoPt/Permalloy/Ta/Permalloy heterostructure was studied with PNR in [193], and quantitative depth resolved information of the magnetization reversal processes for a multilayer of mesoscopic thickness was obtained. Polarized neutron reflectivity measurements were used in [194] to determine the temperature dependence of the magnetization of thin Au97Fe3 films in the thickness range between 1 and 50 nm and in a temperature range from 200 to 2 K. The magnetization of the 1-nm-thick film could be described with a Brillouin function below 50 K proving that ultrathin Au97Fe3 layers below 1 nm do not show spin glass behavior anymore but are paramagnetic. Design of tunable heatless neutron spin rotators, based on remnant magnetization of FeCoV film, was presented in [195].…”