Upon cooling, glass-forming liquids experience a dynamic decoupling in the fast β and slow α process, which has greatly influenced glass physics. By exploring an extremely wide temporal and temperature range, we find a surprising gradual change of the relaxation profile from a single-step to a two-step decay upon cooling in various metallic glasses. This behavior implies a decoupling of the relaxation in two different processes in a glass state: a faster one likely related to the anomalous stress-dominated microscopic dynamics, and a slower one associated with subdiffusive motion at larger scales with a broader distribution of relaxation times.
Vitrification from physical vapor deposition is known to be an efficient way for tuning the kinetic and thermodynamic stability of glasses and significantly improve their properties. There is a general consensus that preparing stable glasses requires the use of high substrate temperatures close to the glass transition one, Tg. Here, we challenge this empirical rule by showing the formation of Zr-based ultrastable metallic glasses (MGs) at room temperature, i.e., with a substrate temperature of only 0.43Tg. By carefully controlling the deposition rate, we can improve the stability of the obtained glasses to higher values. In contrast to conventional quenched glasses, the ultrastable MGs exhibit a large increase of Tg of ∼60 K, stronger resistance against crystallization, and more homogeneous structure with less order at longer distances. Our study circumvents the limitation of substrate temperature for developing ultrastable glasses, and provides deeper insight into glasses stability and their surface dynamics.
Well‐aligned carbon nanowalls with a thickness of a few nanometers and a lateral size in the micrometer range have been grown on various types of substrates. The nanowalls exhibit a remarkably different surface morphology as compared to fullerenes and carbon nanotubes, in particular their two‐dimensionality and high surface area. In this work, we focused on the second aspect and developed a templating method to fabricate a class of nanostructured materials based on the novel surface morphology of the carbon nanowalls. These structures may have potential applications in batteries, gas sensors, catalysts, and light‐emission/detection, field‐emission, and biomedical devices.
This paper presents magnetic properties of highly ordered ultrathin FeRh films deposited on Si/SiO wafers with MgO as a buffer layer. The antiferromagnetic to ferromagnetic (FM) transition is observed with a thickness as low as 3 nm. However, as the thickness decreases, the residual magnetization (M rs ) at low temperature increases and the amplitude of the transition decreases. In addition, the transition becomes much broader for the thinner films. This broadening is related to the grain size reduction in the thinner films. The temperature dependence of the magnetization of a highly ordered B2 FeRh film with a thickness of 10 nm was carefully measured as a function of field. The results show that the transition temperature decreases almost linearly with a rate of 0.93 K/kOe (heating) and 0.97 K/kOe (cooling) close to the value for the bulk samples, while M rs obtained at 100 K increases rapidly at low field and then linearly at a field larger than 10 kOe, which clearly demonstrates that an applied field would induce FM stabilization in ultrathin FeRh films. 3 For these applications, FeRh film is used to reduce the switching field of the storage layer through the exchange coupling between the ferromagnetic (FM) FeRh layer and the storage layer at elevated temperatures without sacrificing the thermal stability of the storage layer at ambient temperature, since the antiferromagnetic (AF) FeRh layer provides negligible exchange coupling to the storage layer. The major challenge is to prepare ultrathin FeRh films with a single AF phase at ambient temperature, which will convert to the FM phase at elevated temperatures, and vice versa. So far, a sharp AF-FM transition of FeRh film was only reported at a thickness above 14 nm. 4 For ultrathin films with a thickness of 10 nm or below, a large residual magnetization (M rs ) is generally observed. 5 The origin of this low temperature FM phase remains unclear. Fan et al.6 observed the existence of a FM phase in a region within 6-8 nm near the top and bottom interfaces of a FeRh film. Based on ab initio calculations, Lounis et al.7 found that a FM state is stable up to 9 atomic layers for Rh-terminated FeRh films. In addition, the AF structure would become unstable when the amount of the site-exchange defect density exceeds a threshold of 0.8%/f.u. 8 Furthermore, based on the phase diagram, 9 the AF to FM transition can be only achieved in the a 00 phase which is formed within a narrow Fe atomic concentration range from 45% to 51%. Due to slow diffusion rate of Rh, it is likely that a mixture of the FM Ferich a 0 phase and the paramagnetic Rh-rich c phase is formed in the film. In this paper, the magnetic stability of ultrathin (10 nm) FeRh films is examined. It is found that for such ultrathin FeRh films, the low temperature FM stabilization is sensitive to the film thickness and the applied magnetic field. An AF to FM transition is observed at a thickness as low as 3 nm, which is close to the FM stabilization thickness based on ab initio calculations for Rh-terminated fi...
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