Observation of magnetic hysteresis at the nanometer scale by spin-polarized scanning tunneling spectroscopy

Using spin-polarized scanning tunneling microscopy in an external magnetic field, we have observed magnetic hysteresis on a nanometer scale in an ultr athin ferromagnetic film. An array of iron nanowires, being two atomic laye rs thick, was grown on a stepped tungsten (110) substrate, The microscopic sources of hysteresis in this system-domain wall motion, domain creation, a nd annihilation-were observed with nanometer spatial resolution. A residual domain 6.5 nanometers by 5 nanometers in size has been found which is inhe rently stable in saturation fields. Its stability is the consequence of a 3 60 degrees spin rotation. With magnetic memory bit sizes approaching the su perparamagnetic limit with sub-10 nanometer characteristic lengths, the und erstanding of the basic physical phenomena at this scale is of fundamental importance.