Effect of Hydrogen Absorption on the Magnetic Moment and Anisotropy of Fen and Con Clusters*

Theoretical investigations have been carried out to examine the effect of H absorption on the magnetic moment and anisotropy of small Fe$_{n}$ and Co$_{n}$ clusters. Our studies use a gradient corrected density functional approach and cover clusters containing up to 4 transition metal and 2 H atoms. We show that the successive addition of H atoms can lead to monotonic or oscillatory change from the free cluster magnetic moment. An analysis of the density of electronic states shows that the variations in the magnetic moment can be related to the location of the lowest unoccupied molecular orbital in the parent cluster. It is shown that the addition of hydrogen can substantially change the magnetic anisotropy, yielding a particularly high magnetic anisotropy for Co$_{3}$H$_{2}$ that is higher than the known molecular nano-magnets.