Figure 1

(Color online) The magnetic energy as a function of magnetic field strength. The arrows next to the curves are indicative of the direction of the magnetizations in each particle in the system; i.e., there are four system orientations represented here. At a field strength equal to zero the difference in the energies of the ground state and the excited level is about $8\mathrm{eV}$, giving a value of $J_0$ of around $4\mathrm{eV}$ if $S_i=\pm 1$.Reuse & Permissions

Figure 2

(Color online) Diagrams of different states existing in a magnetic tunnel junction which arise with an increasing external magnetic field. The field is applied in the plane parallel to the main axis of anisotropy. The original antiferromagnetic orientation of the particles and their shape are associated with the parameter $K$. Large positive values of $K$ indicate that the monodomain particles are strongly elongated along the $X$ direction. These phase diagrams are obtained when we start from large positive or negative values of magnetic field. The diagrams of the states associated with the case of decreasing magnetic field may be obtained by a simple reflection.Reuse & Permissions

Figure 3

(Color online) The magnetization angles of the two particles as a function of magnetic field and anisotropy parameter $K=1$. Here the magnetic field is going from a positive value downwards into the negative range.Reuse & Permissions

Figure 4

(Color online) The total magnetization as a function of applied magnetic field strength. This hysteresis curve is illustrative of the process highlighted in Fig. 2a at level (i). The arrows below the curve are those imported from Fig. 2a at this level of anisotropy $(K=4.3)$, marking the transition between the flipped states of the ferromagnetic alignments. When following this line in the direction of the arrows, in Fig. 2a, a jump occurs at $H=0.29$ from ↓↓ to AF. The system then holds its orientation in the antiferromagnetic alignment until the magnetic field reaches the value $H=5.97$ where it becomes ferromagnetic again, where the both spins are flipped, ↑↑.Reuse & Permissions

Figure 5

(Color online) The total magnetization as a function of applied magnetic field strength. This hysteresis curve is illustrative of the process highlighted in Fig. 2 at level (ii) of the anisotropy $(K=2.3)$. The arrows below the curve are those seen in Fig. 2a at this level, marking the transition between the different alignments. Those above the hysteresis curve are those for the reversal of the magnetic field. Looking at the forward evolution of the magnetic field from a negative applied magnetic field, the transition from ↓↓ to ↙↘ happens at around $H=1.71$ before jumping to the antiferromagnetic alignment at $H=-1.03$. This state persists until $H=3.8$ where the critical point is found where the jump into ↑↑ occurs. The reveral of the field gives the inverse mirror image of this evolution.Reuse & Permissions

Figure 6

(Color online) The total magnetization as a function of applied magnetic field strength. This hysteresis curve is illustrative of the process highlighted in Fig. 2 at level (iii) of the anisotropy $(K=0.8)$. The arrows below the curve are those imported from Fig. 2a at this level, marking the transition between the different alignments. Those above the hysteresis curve are those for the reversal of the magnetic field. Looking at the forward evolution of the magnetic field from a negative applied magnetic field, the transition from ↓↓ to ↙↘ happens at around $H=-3.21$ before jumping to the antiferromagnetic alignment at $H=-1.31$. This state persists until $H=1.95$ where the critical point is found where the jump to ↖↗ occurs. When the field becomes strong enough at $H=3.21$ the transition to ↑↑ happens. The reveral of the field gives the inverse mirror image of this evolution.Reuse & Permissions

Figure 7

(Color online) The field is cycled from a negative value and back with the arrows indicating the direction of evolution throughout the system. Here $K=2.1$. The stable minima are shown as the lightest color (yellow online) and the description “Min,” the saddle points as an intermediate colour (orange online) and the description “SP,” and the maxima by the darkest shade (red online) and the description “Max.”Reuse & Permissions

Figure 8

(Color online) The hysteresis curve for $K=4.3$ is indicated by the direction of the arrows through the stable minima. The stable minima are shown as the lightest color (yellow online) and the description “Min,” the saddle points as an intermediate color (orange online) and the description “SP,” and the maxima by the darkest shade (red online) and the description “Max.”Reuse & Permissions