Utilizing Single Atom Magnets on MgO

The properties of rare earth atoms have been investigated for many decades in bulk materials through electronic transport or optical and microwave spectroscopic techniques. In this talk we will focus on experiments in Scanning Probe Microscopy, which offer the ability to study individual atoms on surfaces. We will focus on holmium on the oxygen binding site of MgO. In this environment, Ho is a single atoms magnet: it keeps its magnetization up or down along the surface normal and does not relax, resulting in a T₁ time longer than days. This long electronic spin lifetime for a single atom is even more surprising since Ho on the O binding site is a non-Kramers atom with C₄v symmetry, which enables quantum tunneling of the magnetization. In this presentation I want to focus on two recent experimental advances in this system that take advantage of this unique system.

First, we were able to measure the quantum tunneling of magnetization for Ho and found an exceedingly small tunnel gap of only 10peV, which corresponds to a tunneling timescale of 1kHz. Such fine measurements can be performed with STM by carefully ramping an external magnetic field through the avoided level crossing and using Landau-Zener tunneling. Ho has only one isotope with a nuclear spin of 7/2, which gives rise to 8 avoided level crossings at different magnetic fields. This enables us to perform single-shot projective readout of the nuclear spin state of Ho, where we find a nuclear T₁ time of 3s for Ho on a double layer of MgO and 300s on a triple layer, all grown on Ag. Hence, we have a truly unique quantum system of a single atom with electron T₁>days and nuclear T₁=5min.

Second, we can use this stable electron spin to measure the magnetic exchange force between Ho and the spin-polarized tip of an atomic-force microscope. This enables us to study the formation of a chemical bond with atomic-scale precision. We find that the overlap of wavefunctions of the 3d electrons of the tip with in-plane 5d electrons of the Ho atom gives excellent quantitative agreement with the measured interaction.