Stacking faults were found to play a critical role in the evolution of the structural and magnetic properties of Co subjected to ball milling. This was revealed by using techniques such as magnetometry and torque measurements or nuclear magnetic resonance and X-ray diffraction. After short milling times, a stacking-fault driven transformation from face-centered cubic to hexagonal close-packed Co was observed. This was accompanied by an increase in the effective magnetic anisotropy, the nuclear magnetic resonance restoring field and the coercivity. The results suggested that small numbers of stacking faults could be beneficial in enhancing the coercivity of hexagonal Co. At longer milling times, both X-ray diffraction and nuclear magnetic resonance results showed that the hexagonal close-packed  phase became heavily distorted because of the large numbers of stacking faults which accumulated. This induced a decrease in the magnetic anisotropy, which led to overall softening of the material.

Role of Stacking Faults in the Structural and Magnetic Properties of Ball-Milled Cobalt. J.Sort, S.Suriñach, J.S.Muñoz, M.D.Baró, M.Wojcik, E.Jedryka, S.Nadolski, N.Sheludko, J.Nogués: Physical Review B, 2003, 68[1], 014421 (7pp)