Ultrafast switching in a synthetic antiferromagnetic magnetic random-access memory device

The dynamics of a synthetic antiferromagnet (a metallic trilayer) have been explored and are shown to exhibit ultrafast switching on a time scale of tens of ps. This conclusion is based on first-principles, atomistic spin dynamics simulations. The simulations are performed at finite temperature, as well as at $T=0$ K (the macrospin limit), and we observe a marked temperature dependence of the switching phenomenon. It is shown that, to reach very high switching speeds, it is important that the two ferromagnetic components of the synthetic antiferromagnet have oppositely directed external fields to one another. Then a complex collaboration between precession switching of an internal exchange field and the damping switching of the external field occurs, which considerably accelerates the magnetization dynamics. We discuss a possible application of this fast switching as a magnetic random access memory device, which has as a key component intrinsic antiferromagnetic couplings and an applied Oersted field.

Figure 1

Schematic figure of an MRAM device utilizing antiferromagnetic exchange coupling. The bit line is the uppermost line in the figure, and generates an Oerstedt field, B. The digit line is the lowermost line. The magnetic units are represented by rectangular boxes, colored in red/dark grey (for magnetization direction into the figure) and blue/grey (for magnetization direction out of the figure). The GMR unit is represented by the blue, yellow (white) and red boxes, whereas the AF unit is represented by the red and blue boxes, separated by the bit line. An entire stack of GMR and AF unit is referred to as an MRAM unit. In order to achieve selectivity in switching of individual MRAM units a second bit line is needed as shown in the inset of the figure, where a top view of the device is shown. The second bit line is parallel to the digit line but placed in the vicinity of the AF unit. The second bit line is shown in purple (dark grey). The two bit lines give rise to magnetic fields ${\mathrm{B}}_1$ and ${\mathrm{B}}_2$, respectively, and the requirement that their sum results in a total field which is oppositely directed for the two magnetic layers of the AF unit, but with the same magnitude for these layers.

Figure 2

A simulation of the magnetization reversal of a Fe${}_3$/Cr${}_4$/Fe${}_3$ trilayer in an applied field of 0.1 T. The plot shows the time evolution of the components of the average magnetization of one of the Fe layers (top). The simulation is performed at 0 K with a damping constant of $\alpha =0.01$. The magnetization is initially directed almost along the negative $z$ direction (175${}^{ˆ}$). In the bottom panel the deviation from 180 degree antiferromagnetic coupling between the two Fe slabs is shown (in degrees).

Figure 3

The left hand figure illustrates the torques exerted by an antiparallel external fields on the two sublattices of a synthetic antiferromagnet. Note the different directions of the external field for the two sublattices. The precessional torque gives rise to a frustration between the sublattices and an effective internal exchange field. The right hand figure illustrates a top view of the torques exerted by the internal exchange field. Damping torques from the external field and precessional torques from the exchange field bring the atomic moments to alignment with the external field.

Figure 4

A switching diagram for the Fe${}_3$/Cr${}_4$/Fe${}_3$ system at 0 K. The phase diagram shows the $z$ component of the average magnetization of one Fe layer color coded. The red/blue (light grey/dark grey) coloring indicates areas where switching has occurred/has not occurred. Three regions of damping are identified, corresponding to different switching dynamics—see text.

Figure 5

Switching time as a function of the applied field for the Fe${}_3$/Cr${}_4$/Fe${}_3$ system at 0 K.The phase diagram shows the $z$-component of the average magnetization of one Fe layer color coded. The red/blue (light grey/dark grey) coloring indicates areas where switching has occurred/has not occurred. The damping parameter was fixed to 0.05 for all fields.

Figure 6

A switching diagram for the Fe${}_3$/Cr${}_4$/Fe${}_3$ system as a function of temperature. The phase diagram shows the $z$ component of the average magnetization of one Fe layer color coded. The red/blue (light grey/dark grey) coloring indicates areas where switching has occurred/has not occurred.