Determination of exchange anisotropy by means of ac susceptometry in Co/CoO bilayers

The strength, or stiffness $Kud$⁠, of the magnetic unidirectional anisotropy in Co (90–300 Å)/CoO bilayers has been studied with ac susceptometry. We develop a model to describe the ac susceptibility which indicates that the stiffness $Kud$ decreases linearly with temperature as $(1−T/Tcrit)$⁠. As the temperature increases and $Kud$ decreases, a rotation of the magnetization to a uniaxial easy direction is observed. The values of $Kud$ determined by this small amplitude field oscillation technique are found to be larger than the values obtained when evaluated by the usual hysteresis loop technique. We observe that the exchange field, $Hex,$ could be greatly enhanced and its deterioration after repeated magnetization reversals significantly reduced by partially capping the Co surface prior to oxygen exposure.