Superparamagnetic limit to spin-polarized vacuum tunneling from Fe-coated W(001) tips.

Superparamagnetic fluctuations of magnetization in thin Fe films that coat a sharp W(001) tip are evident in the spin polarization of field emitted electrons. Fluctuations of polarization between four transverse directions that align with the tip <100> directions are consistent with the in-plane biaxial magnetization of Fe films on macroscopic W(001) surfaces. This superparamagnetic behavior imposes a limit to spin polarized vacuum tunneling applications that proves to be more stringent than the suppression of the Curie temperature in thin films due to finite size effects. [ABSTRACT FROM AUTHOR]