Exchange bias on rippled substrates – step induced uniaxial versus unidirectional anisotropy

Ripples with a periodicity of 35 nm and a height of 2 nm were formed on a Si(001) substrate
by means of Ar+ ion erosion. The primary energy was 500 eV and the angle of incidence 70° with respect to the surface normal. One of the main characteristics of such a surface is the large step density which is usually only achieved for vicinal miscut substrates. After ripple preparation a 0.5 nm Pt buffer layer and a 5 nm Permalloy (Ni81Fe19) ferromagnetic layer were deposited by means of molecular beam epitaxy. In order to allow for an easy comparison between ferromagnetic layer and exchange biased system on half of the sample a 10 nm Fe50Mn50 antiferromagnetic layer was deposited. Then the whole sample was covered by a 2 nm Cr protection layer. Additional AFM images confirmed that the ripple structure remains throughout the whole layer system.

The magnetization reversal behavior of the ferromagnetic layer and the exchange bias system was then investigated by means of magneto-optic Kerr effect (MOKE). The Permalloy layer itself exhibits already a uniaxial anisotropy of Ku = 3.74×104 erg/cm3 which was determined from the saturation field HS of the hard-axis magnetization reversal curve. This value is already about 20 times larger than the uniaxial anisotropy observed in Permalloy films deposited on flat substrates. Since no field annealing has been performed only a strongly enhanced coercivity with respect to the Permalloy film is found. This is due to the coupling of the ferromagnetic layer to the antiferromagnetic layer. Two additional field annealing procedures have then been performed in order to set the exchange bias direction either along or perpendicular to the ripple direction. Since the uniaxial anisotropy contribution is much larger than the unidirectional one the easy- and hard-axes remain the same. However, if the exchange bias is set along the ripple direction a shift of the easy-axis loop is found. In contrast, if the exchange bias field is set perpendicular to the ripple direction the hard-axis loop is shifted. In order to evaluate this behavior in more detail a complete angular dependency of the magnetization reversal behavior has been investigated in both cases and compared to simulations based on a Stoner-Wohlfarth coherent rotation model. A good agreement between experimental data and simulations is found.