Spin cross-over molecules show the unique ability to switch between two spin states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensional spin cross-over layers.

Magnetism in recently discovered van der Waals materials has opened several avenues in the study of fundamental spin interactions in truly two-dimensions. A paramount question is what effect higher-order interactions beyond bilinear Heisenberg exchange have on the magnetic properties of few-atom thick compounds. Here we demonstrate that biquadratic exchange interactions, which is the simplest and most natural form of non-Heisenberg coupling, assume a key role in the magnetic properties of layered magnets. Using a combination of nonperturbative analytical techniques, non-collinear first-principles methods and classical Monte Carlo calculations that incorporate higher-order exchange, we show that several quantities including magnetic anisotropies, spin-wave gaps and topological spin-excitations are intrinsically renormalized leading to further thermal stability of the layers. We develop a spin Hamiltonian that also contains antisymmetric exchanges (e.g., Dzyaloshinskii–Moriya interactions) to successfully rationalize numerous observations, such as the non-Ising character of several compounds despite a strong magnetic anisotropy, peculiarities of the magnon spectrum of 2D magnets, and the discrepancy between measured and calculated Curie temperatures. Our results provide a theoretical framework for the exploration of different physical phenomena in 2D magnets where biquadratic exchange interactions have an important contribution.

Search for room-temperature superconductivity is inspired by the unique properties of the electron-phonon interaction in metal superhydrides. Encouraged by the recently found highest-TC superconductor fcc-LaH10, here we discover several polyhydrides of another lanthanideneodymium. We identified three novel metallic Nd-H phases at pressures of 90 to 130 GPa: I4/mmm-NdH4, C2/с-NdH7, and P63/mmc-NdH9+x (x = 0-0.5), synthesized by laser-heating metal samples in NH3BH3 media for in situ generation of hydrogen. A lower trihydride Fm3 ̅ m-NdH3 was found at pressures from 2 to 52 GPa. I4/mmm-NdH4 and C2/с-NdH7 were stable from 130 down to 85 GPa, and P63/mmc-NdH9+x -at 110 to 130 GPa. Theoretical calculations predict that all the neodymium hydrides have a strong magnetism at pressures below 150 GPa (> 2.2 μB per Nd atom): C2/c-NdH7 and hcp-NdH9 possess collinear anti-ferromagnetic [110] and [100]2 orders respectively, while NdH4 is a ferromagnetic with the (110) easy-axis. The critical Curie or Neel temperatures for new neodymium hydrides were estimated using the mean-field approximation as 100 K (NdH7), 95 K (NdH9) and 167 K (NdH4).