Novel Hardware for Alternative Computing

Description

Fig. 1. a) Material stack structure of a magnetic tunnel junction. CoFeB and MgO are common materials used fo r the ferromagnetic and tunneling barriers, respectively. b) Transmission electron microscope image of a single magnetic tunnel junction device. c) Characteristic switching curve for a magnetic tunnel junction. The device switches from a high (RAP) to low (RP) resistance state after applying a voltage to the device higher than 500 millivolts. 

Credit: NIST

Magnetic tunnel junctions
Magnetic tunnel junctions (see Fig. 1) consist of two thin films of a ferromagnetic material separated by a few atomic layers of an insulating material.  The insulator is so thin that electrons can tunnel quantum mechanically through it. The rate at which the electrons tunnel is affected by the relative magnetic configuration of the two ferromagnetic layers.  If the magnetizations in the two layers are parallel, it is easier to tunnel than if they are antiparallel. The resulting difference in resistance makes it straightforward to read the state of the magnetic layers using electronic circuits. This ease of reading the magnetic state is only one important feature of these devices. The other is the ability to change the state of the device by passing a current through it, creating a spin torque. Practical reading and current-control of magnetic tunnel junctions are key features that are enabling the realization of fast, dense, non-volatile memory integrated into complementary metal-oxide-semiconductor (CMOS) circuits in commercial applications today.  

Fig. 2. a) Cartoon describing operation of a memristive device. The device begins in a high resistance state. Upon applying a sufficient voltage or current, a conductive filament forms, where the strength of the resistance controls the change in resistance. b) example switching curve showing the process described above. The forming process is an initial priming process that is the first time the filament is formed. c) Micrograph image of memristor devices integrated with CMOS. 

Credit: NIST

Memristors
Memristors are also two-terminal devices, consisting of a metallic top and bottom electrode sandwiching an insulating material. These devices operate by passing a current through the device which creates a conducting filament in the insulating layer that bridges the two electrodes. By changing the strength of the current, the length of the filament can be changed. This results in a continuously variable device resistance, which is determined directly by the conductive filament (see Fig. 2).