4.2 Simulational model

Figure 4.4: Schematic picture showing the system geometry and the cubic anisotropy easy axes in the simulations.

We have modeled the field induced reversal processes taking place in Fe particles having dimensions in the range $4 nm$ (thickness) $\times 30 - 250 nm$ (width) $\times 400 nm$ (length). The particle dimensions range, although not covering completely the experimentally observed particle size scale, is affordable from the point of view of the computing time. Due to the above mentioned isolated character of the Fe ribbons, we assume that the behavior of the samples can be well represented by that of an isolated simulated ribbon. We considered the occurrence in the particles of a (110) texture and, therefore, easy axes oriented along directions forming 45$^\circ$ with the applied field direction and the ribbon long axis (see Fig. 4.4). The particles were discretized in elements of size $1.29 nm$ ($1/10$ of the exchange correlation length of Fe, $l_{ex,Fe} =12.88 nm$). A particular relaxation stage was terminated when achieved a value lower than $10^{-4}$ for the error (see Section 2.2.3). The hysteresis cycles were followed by starting with the application of a $2 T$ field which was always enough to achieve the saturation. The field was varied through variable size increments which were large in the saturation region and of the order of $5\cdot10^{-4} T$ in the coercive force region. The considered Fe parameters were biaxial anisotropy value $K=5\cdot 10^4 J/m^3$, micromagnetic exchange value $A=8.3\cdot 10^{-12} J/m$ and saturation magnetization value $\mu_0 M_s=2.15 T$ and were taken from [Skomski 99, Page 158].