- 1601 Elings Hall
- Physics Seminar
Electron spin resonance (ESR) reveals magnetic properties of unpaired electron spins in materials, and recently in individual atoms and atomic-scale defects such as NV centers in diamond. We use a low-temperature scanning tunneling microscope (STM) to perform ESR of individual iron and titanium atoms on a surface, and employ these atoms as atomic-scale magnetic sensors. This technique combines the high energy resolution of spin resonance with the strengths of STM, which include atomic-resolution imaging and atom manipulation for assembly of unique nanostructures. We drive spin resonance by using the large electric field available in the tunnel junction, and sense the spin by means of magnetoresistance, using a spin-polarized STM tip. The atoms are isolated from the conducting substrate by an insulating layer of magnesium oxide two atoms thick, which reduces undesired spin relaxation while allowing electrical access to the atoms. Magnetic coupling between Fe atoms placed 1−4 nm apart shows inverse-cube dependence on distance, which indicates dipole-dipole interaction. This yields a precise measure of the magnetic moment of Fe, which is then used to probe other atoms such as Co. Placing several Fe sensor atoms at known locations allows “nano-GPS”, the use of trilateration at the atomic scale to detect the location of unknown spins. The STM can also drive spin resonance of Ti atoms, which show nearly free spin-1/2 behavior, in contrast to Fe’s large moment and easy-axis anisotropy. The combination of STM with ESR provides a flexible new tool for exploring nano-scale magnetism.