  Spin-torque memristors were proposed in 2009, which could provide fast,
low-power and infinite memristive behavior for large-density non-volatile
memory and neuromorphic computing. However, the strict requirements of
combining high magnetoresistance, stable intermediate states and spin-polarized
current switching in a single device pose difficulties in physical
implementation. Here, we experimentally demonstrate a nanoscale spin-torque
memristor based on a perpendicular-anisotropy magnetic tunnel junction with a
CoFeB/W/CoFeB composite free layer structure. Its tunneling magnetoresistance
is higher than 200%, and memristive behavior can be realized by spin-transfer
torque switching. Memristive states are maintained by robust domain wall
pinning around clusters of W atoms, where nanoscale vertical chiral spin
textures could be formed through the competition between opposing
Dzyaloshinskii-Moriya interactions and the fluctuating interlayer coupling
caused by the Ruderman-Kittel-Kasuya-Yosida interaction between the two CoFeB
free layers. Spike-timing-dependent plasticity is also demonstrated in this
device.
