Co 72 Fe 20 B 8 : Structure, magnetism, and tunneling spin polarization

We present measurements of tunneling spin polarization of ${\mathrm{Co}}_{72}{\mathrm{Fe}}_{20}{\mathrm{B}}_8$ in $\mathrm{Al}∕\mathrm{Al}{\mathrm{O}}_x∕\mathrm{Co}\mathrm{Fe}\mathrm{B}$ tunnel junctions using superconducting tunneling spectroscopy. We consistently observe high spin polarization (above 53%), higher than that measured for comparable CoFe alloys, indicating a crucial role played by the 8% atomic boron. X-ray diffraction measurements on $700\mathrm{\AA }$ thick films reveal an amorphous structure for as-deposited films, while anneals above $250°\mathrm{C}$ initiate a gradual crystallization. Superconducting quantum interference device magnetometery shows that the magnetic moment exhibits minor changes after anneals, but there is a dramatic change in the coercivity after anneals above $400°\mathrm{C}$ . However, these structural and magnetic changes induce only a slight change in the tunneling spin polarization of CoFeB.