The spin-reorientation transition of ultrathin Ni films is investigated by
means of in situ magneto-optical Kerr imaging as a function of the temperat
ure. A critical thickness d(c) of about 11.4 monolayers (ML) at T = 170 K a
nd 10.3 ML at 370 K has been found. Adsorbates strongly reduce this critica
l thickness. In particular the adsorption of about 2 langmuir H-2 reduces d
(c) to 7 ML at 170 K. The magnetocrystalline anisotropy energy of the clean
Ni surface K-2s = -153 mu eV/atom is strongly reduced by hydrogen adsorpti
on. [S0163-1829(99)13229-6].