Spin resonance amplitude and frequency of a single atom on a surface in a vector magnetic field

To characterize the local magnetic environment at the tunnel junction in a scanning tunneling microscope, the authors utilize as a rotatable spin sensor a spin-1/2 hydrogenated Ti atom on a surface. They measure the electron spin resonance at different Ti spin directions as controlled by two-axis magnets and clarify the crystal-field and magnetic-tip effects at the STM junction, identifying the origin of the giant (15%) anisotropy of $g$ factors for Ti on the surface and the variable driving and readout of spin resonance.