Spontaneous exchange-bias in Fe/Mn thin multilayers

Fe/Mn multilayers were grown by means of a molecular beam epitaxy system onto quartz substrates changing the thickness of the elemental layers. A spontaneous unidirectional anisotropy develops for thickness of Fe or Mn layer of about 35 Å. Since the samples were no subjected to field cooling treatments during or after the growth, this kind of anisotropy can be explained considering besides the exchange coupling at the Fe/Mn interface, the structural disorder due to dislocations and defects. In effect, the appearance and strength of the exchange-bias field are depending on the surface roughness of the samples and are significantly enhanced by the formation of a structure constituted by islands showing a snake-like morphology. The fitting of the angular dependence of the exchange-bias field indicates that the associated anisotropy is due to the superposition of two contributions, the principal one with unidirectional symmetry and the other showing uniaxial characteristics.