  The processional switching mechanism governs magnetic switching in magnetic
tunnel junctions (MTJs) in the sub-nanosecond range, which limits the
application of spin transfer torque magnetic random access memory (STT-MRAM) in
the ultrafast region. In this paper, we propose a new picosecond magnetic
switching mechanism in a synthetic antiferromagnetic (SAF) structure using the
adjustable Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction controlled by an
external electric field (E-field). It is shown that along with the sign change
of the RKKY interaction in the SAF structure with an external E-field, the
critical switching current density can be significantly reduced by one order of
magnitude compared to that of a normal MTJ design at 100 ps; thus, this novel
STT-MRAM can be written with a very low switching current density to avoid the
MTJ breakdown problem and reduce the writing energy. To understand the physical
origin of this abnormal phenomenon, a toy model is proposed in which the
external-E-field-controlled sign change of the RKKY interaction in the SAF
structure provides an extra contribution to the total energy that helps
thespins overcome the energy barrier and break the processional switching
mechanism.
