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In contrast to silicon-based transistors, single molecule junctions can be gated by simple mechanical means. Specifically, charge can be transferred between the junction's electrodes and its molecular bridge when the interelectrode distance is modified, leading to variations in the electronic transport properties of the junction. While this effect has been studied extensively, the influence of the molecule orientation on mechanical gating has not been addressed, despite its potential influence on the gating effectiveness. Here, we show that the same molecular junction can experience either clear mechanical gating or none, depending on the molecule orientation in the junctions. The effect is found in silver-ferrocene-silver break junctions, and analyzed in view of ab-initio and transport calculations, where the influence of molecular orbitals geometry on charge transfer to or from the molecule is revealed. The molecule orientation is thus a new degree of freedom that can be used to optimize mechanically-gated molecular junctions., Comment: 10 pages, 3 figures

Knowledge of the magnetic ordering temperature is one of the fundamental physical properties characterizing material behavior. Magnetism is not only related to the magnitude of magnetic moments within specific magnetic ordering, but it also influences other physical properties. In the present work, we determine magnetic ordering temperatures for high-entropy alloys (HEAs), with examples of the Al- and Mo-based Cantor alloy derivatives. Nowadays, HEAs represent a promising class of materials in the sense of mechanical engineering. Regarding the paramagnetic state of the well-known and well-explored Cantor alloy, we deal with its Mo and Al derivatives, which offer higher possible ordering temperatures. We discuss the differences between $p$-type and $d$-type substitution as well as their influence on the magnetic behavior. We determine the ordering temperatures based on the ab initio calculated magnetic exchange interactions in terms of the mean-field approximation, random-phase approximation, and Monte Carlo simulations. Based on the thorough description of magnetic pair exchange interactions, a particular composition's influence on the achieved critical temperatures is described.

Within the RCo2 family of compounds, a structural distortion linked with the onset of magnetic ordering around the critical temperature can be observed. One of the less explored RCo2 compounds is TmCo2 probably due to its low Curie temperature. Exceptionally this compound, given its position at the end of the ferrimagnetic series, shows discrepancies in the ordering of the Co sub-lattice because of a weak Weiss molecular field. In this paper we focus on the structural distortion in TmCo2, which appears together with the magnetic ordering around the critical temperature of T C ∼ 3.6 K. Poly-crystals as well as single-crystals of TmCo2 were used in our experiments. For both kinds of samples we observed the same type of the rhombohedral distortion along the [111] direction from the cubic Fd 3 ¯ m to R 3 ¯ m space group. The relation between observed magnetic and structural properties in this compound is discussed.

Doping ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ by magnetic ions represents an interesting problem since it may break the time-reversal symmetry needed to maintain the topological insulator character. Mn dopants in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ represent one of the most studied examples here. However, there is a lot of open questions regarding their magnetic ordering. In the experimental literature different Curie temperatures or no ferromagnetic order at all are reported for comparable Mn concentrations. This suggests that magnetic ordering phenomena are complex and highly susceptible to different growth parameters, which are known to affect material defect concentrations. So far theory has focused on Mn dopants in one possible position, and neglected relaxation effects as well as native defects. We have used ab initio methods to calculate the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ electronic structure influenced by magnetic Mn dopants, and exchange interactions between them. We have considered two possible Mn positions, the substitutional and interstitial one, and also native defects. We have found a sizable relaxation of atoms around Mn, which affects significantly magnetic interactions. Surprisingly, very strong interactions correspond to a specific position of Mn atoms separated by a van der Waals gap. Based on the calculated data we performed spin dynamics simulations to examine systematically the resulting magnetic order for various defect contents. We have found under which conditions the experimentally measured Curie temperatures ${T}_{\mathrm{C}}$ can be reproduced, noticing that interstitial Mn atoms appear to be important here. Our theory predicts the change of ${T}_{\mathrm{C}}$ with a shift of Fermi level, which opens the way to tune the system magnetic properties by selective doping.

Multiprincipal element alloys, called high-entropy alloys represent a promising group of materials. They possess unique mechanical or electrical properties, which provide a wide range of potential applications. In general, mechanical or electrical properties of a material are influenced by the magnetic behavior. Therefore, in our work, we are exploring magnetic behavior of so-called ``Cantor alloy'' CoCrFeMnNi and its molybdenum based derivatives. Mo alloys were studied not only to carefully describe their properties but also to probe the magnetic behavior of the parent alloy by adding a nonmagnetic element. Based on ab initio calculations using the TB-LMTO-ASA (tight-binding linear-muffin-tin-orbital atomic-sphere approximation) method within CPA (coherent-potential approximation), we have found the ground-state magnetic structures of a particular alloy. We deal with various magnetic structures including complex structures beyond the simple FM or DLM phases. We show the influence of the presence of a particular element on the magnetic properties. It includes, e.g., magnitudes of magnetic moments or preferred magnetic phases. The calculations were extended by studying the magnetic exchange interaction employing the Liechtenstein formula. We clearly show the contribution of each element to the magnetism as a function of the composition or crystal structure. We provide a thorough description of magnetic behavior in the mentioned compounds.