Magnetism of Single Surface Adsorbed Atoms Studied with Radio-Frequency STM

This thesis investigates the magnetic properties of single atoms and molecules adsorbed on thin magnesium oxide decoupling layers, grown on a silver single crystal. To address these systems experimentally, we use a low temperature scanning tunneling microscope capable of applying radio-frequency bias modulations in the tunneling junction. The main motivation is to investigate and control the spin dynamics of single surface adsorbed atoms and molecules using spin-polarized scanning tunneling microscopy within and beyond the I/V converter bandwidth.

We first present the technical upgrades implemented on the system that enable a high radio-frequency bias transmission up to 40 GHz in the tunneling junction. The presentation of the corresponding transfer function is followed by electron spin resonance results on adsorbed hydrogenated titanium molecules at 0.4 K and 4.2 K. We determine the magnetic moment and g -factor of the molecules and achieve measuring narrow signal linewidths, below 40 MHz at 0.4 K, further motivating the use of this technique with our scanning tunneling microscope to address new systems.

The magnetism of single dysprosium and terbium atoms adsorbed on the top-Oxygen site of magnesium oxide thin films is investigated. The magnetic bi-stability of these systems, with an out-of-plane easy axis of the magnetization, allows us to record two-state noise at low temperatures and in an externally applied magnetic field. An analysis of the magnetization switching rate is presented and shows consistent results for dysprosium with the existing literature. We report a first energy threshold of the system at 140 meV. A similar analysis is done for terbium, from which magnetization reversal pathways are identified from quantum chemical calculations and estimated experimentally for the first time. We find three energy onsets at 72 meV, 123 meV and 164 meV.

Terbium and dysprosium atoms adsorbed on the bridge-Oxygen site, however, do not show two-state noise within the real-time limit of our instrument. On this site, terbium atoms show spectroscopic spin excitation features at ±23 meV. A comparison with a spectroscopy database in the gas phase allows to speculate on their complete electronic configuration, including the valence electrons. We find two possible configurations based on this comparison: 4f 85d16s2 and 4f 85d16s1.

Finally, quantum stochastic resonance applied to the magnetism of single iron atoms on thin magnesium oxide films is presented. This technique also benefits from the high radio-frequency transmission, as a bias modulation is employed to drive the spin dynamics of the adatom, at frequencies beyond the I/V converter bandwidth and at 0.4 K. We introduce the time t* that corresponds to the radio-frequency period at which the ground and first excited state populations are identical. We show that this time scales linearly with the relaxation time of the system, in current- and magnetic field-dependent measurements of the r