  We demonstrated current-induced four-state magnetization switching in a
trilayer system using spin-orbit torques. The memory device contains two Co
layers with different perpendicular magnetic anisotropy, separated by a space
layer of Pt. Making use of the opposite spin current at the top and bottom
surface of the middle Pt layer, magnetization of both Co layers can be switched
oppositely by the spin-orbit torques with different critical switching
currents. By changing the current pulse forms through the device, the four
magnetic states memory was demonstrated. Our device provides a new idea for the
design of low power and high density spin-orbit torque devices.
