In 2021 the group proved the use of XMCD to detect the spins of rare-earth atoms resolving their electron orbital structure. Since the 4f electrons are hidden close to the nucleus, they are very well protected from external perturbations. However, when one tries to measure their spin using electric transport (such as in an STM), these 4f electrons provide a very little signal to the current, making their measurement rather difficult. However, they can be coupled to more external shells, such as 5d, 6s, or 6p, which can operate as an intermediary between the internal 4f spins and the flow of electric current across the atom. Using this newly developed orbital-resolved XMCD technique (Figure 1), we were able to map the presence or absence of spins in the different orbitals of gadolinium and holmium atoms deposited on MgO. This approach allows us to understand the mutual coupling between electrons in different orbitals and determine if an atom can provide a large signal in transport experiments. Accepted for publication at ACS Nano (2021).

Also in 2021 we identified a new single-atom rare earth magnet, dysprosium on MgO films on silver. Using STM, XMCD, and theoretical calculations, we could determine that its anisotropy barrier (one of the key properties determining the stability of magnetic orientation of an atom) is the largest ever measured for a spin on a surface, about twice as much as the previous record held by a holmium atom (Dy’s neighbor in the periodic table). The magnetic orientation of these atoms is stable for many hours at low temperatures. These atoms also show an interesting change in the charge state and magnetic stability when increasing the thickness of the oxide film onto which they are deposited. The change in charge state is due to a process called charge transfer, which occurs when the oxide film is just a few atomic layers thick. These results have been published in Nature Communications (2021) and another paper is submitted for publication.

In 2020, we determined the relaxation mechanism that limits the stability of holmium single-atom magnets. Holmium atoms have been the first atoms to show magnetic stability and are still the surface spins with the highest blocking temperature (45 K) up to date. The magnetic orientation of these atoms becomes less and less stable with increasing temperature. In this XMCD study, done in collaboration with the EPFL and ETH Zurich in Switzerland, we found that the reason for this instability is a thermally activated vibration that mostly involves the local oscillation of the Ho around its equilibrium position (Figure 2). This study is published in Physical Review Letters.

In 2019, in collaboration with the scientists at the Swiss Light Source, we unraveled the main magnetic relaxation processes that limit the magnetization stability of two very popular rare-earth-based molecules, TbPc2 and DyPc2, adsorbed on MgO films. In this study, we combined XMCD and theoretical calculations to understand the role of specific mechanisms that are well known to occur in solid-state materials, but much less understood for the cases of molecules on surfaces. The study was published in Advanced Science (2019).

*Fig.1 In orbital resolved XMCD, we can select specific X-ray transitions that can bring an electron from orbitals very close to the nucleus (3s, 3p and 3d) to outer orbitals (left). The signal coming from these transitions (right) tells how many electrons are present in each of the outer orbitals.

*Fig.2 the lifetime (τ) of Ho atoms depends on the magnetic field and temperature (top), due to oscillations of the Ho atom around its equilibrium position (bottom), which are activated when we increase the temperature.