Electrical Manipulation of Nanomagnets

We demonstrate that it is possible to manipulate the magnetic coupling between two nanomagnets by means of an ac electric field. In the scheme suggested, the magnetic coupling is mediated by a magnetic particle that is in contact with both nanomagnets via tunnel barriers. The time-dependent electric field is applied so that the height of first one barrier then the other is suppressed in an alternating fashion. We show that the result is a pumping of magnetization from one nanomagnet to the other through the mediating particle. The dynamics of the magnetization of the mediating particle allows the coupling to be switched between being ferromagnetic and being antiferromagnetic.

Figure 1

Schematic diagram of the system discussed in the text. Single-domain magnetic grains with magnetic moments ${\mathbf{M}}_L$ and ${\mathbf{M}}_R$ are coupled by a magnetic cluster—a mediator—with magnetic moment $\mathbf{m}$. Gate electrodes induce ac electric fields that are concentrated to the areas between the grains and the cluster. These fields control the heights of the tunnel barriers and therefore affect the exchange coupling between the different components of the system.

Figure 2

Illustration of the periodic regime of the molecular spin dynamics in which magnetization flows in one direction between the two nanomagnets of Fig. 1. The bottom parts represent the periodic dynamics of the projection of the molecular spin on the plane perpendicular to the vector ${\mathbf{M}}_L+{\mathbf{M}}_R$. Points $L$ and $R$ indicate the axes aligned with the vectors ${\mathbf{M}}_L$ and ${\mathbf{M}}_R$, respectively. If the molecular cluster is coupled to a nanomagnet, its spin performs counterclockwise rotation around axis $L$ or $R$ (depending on which nanomagnet it interacts with). The circles schematically represent the trajectories which are traced out by the end of the vector $\mathbf{m}$. The angle of rotation $\varphi$ depends on the length of time the cluster is coupled to the nanomagnets and on the magnitude of the exchange coupling, which is controlled by the amplitude of the alternating field applied to the gate electrodes. In the periodic regime of the molecular spin evolution the spin periodically oscillates between points 1 and 2. During one oscillation period a magnetic moment $\Delta \mathbf{m}$ is transferred from one nanomagnet to the other, making the nanomagnet magnetizations rotate around the axis ${\mathbf{M}}_L+{\mathbf{M}}_R$ (upper part).