Fig. 3: Micromagnetic simulations of a giant spearhead magnetofossil.

Simulations were performed with an elliptical cross section (r_x = 0.55 µm; r_y = 0.4675 µm), [113] parallel to z and [111] (the magnetocrystalline easy axis) in the x-z plane. a–d Visualisations of the lowest energy remanence state. Surface spins are coloured according to the component of magnetisation in the corresponding view direction: (a) M_y, (b) M_x and (c) M_z (red is positive, blue is negative). The vortex core is coloured according to M_z. The green surface in (c) and (d) represents M_z = 0, i.e., the medial domain wall separating lateral halves of the particle with positive and negative M_z components. e Visualisation of the remanent state obtained after taking the lowest energy state in (a), applying a −26 mT back field along z, and then switching the field off. The change in colour of the vortex core from red to blue corresponds to a Bloch point (see inset to Fig. 2a). f A simulated hysteresis loop in field of ± 300 mT applied parallel to z. An expanded view of the region highlighted by the dashed box is shown in (g).