Enhancement of exchange bias with crystal orientation in NiFe/CoO and CoO/NiFe bilayers grown on MgO(100) and MgO(111)

We present the comparative study of the structural and magnetic properties of the NiFe/CoO and CoO/NiFe bilayers grown on both MgO(111) and MgO(100) substrates by using the ion beam sputtering technique. We observed that crystallographic orientation and crystal quality strongly affected the exchange bias properties of the bilayers. The NiFe layers showed fourfold magnetocrystalline anisotropy when we used MgO(100) substrate and uniaxial anisotropy when we used MgO(111) substrate. When the CoO layer is grown on MgO(111) and MgO(100) substrates as a first layer, instead of the NiFe, the increase of the crystalline quality and stoichiometric CoO phase has been achieved so that the exchange bias field increases by five and three times for NiFe/CoO/MgO(111) and NiFe/CoO/MgO(100) systems, respectively. The blocking temperature significantly increases for NiFe/CoO on MgO(111) and (100) substrates, indicating the increase of the stoichiometric stable Co1O1 phase in the exchange-biased system. The magnitude of the exchange bias field is 800 Oe at 10 K for NiFe/CoO/MgO(111) and more than two times as compared to that of NiFe/CoO/MgO(100). This high exchange bias value observed for NiFe/CoO/MgO(111) was explained with the uncompensated spins in CoO(111) surface. We also carried out training effect measurements to observe the durability of the exchange bias for technological applications.