Types of Memristors
There are quite a few vectors of inquiry researching various types of memristors. The material implementation of a memristor is important to how they behave in a memristive system. its important to understand the difference between a memristor, and a memristive system, because the specific type of memristor can highlight different strengths and weaknesses, and they can be used in a memristive system for different applications of scale or purpose. There are currently no memristor datasheets available, as much of the material implementations are experimental and in development. In general, though, for any material, Hysterisis, an accelerating rate of change of a property as objects move from one state to another, is an indicator of memristive properties.
Currently Hewlett Packard’s version of the Titanium Dioxide susbtrate memristor is the most generally pursued type of memristor, but the list of different memristor types below shows there are a wide variety of systems that exhibit memristive behavior, and more are being discovered as industries begin to build out their research, prototyping, and manufacturing infrastructures.
1. Molecular and Ionic Thin Film Memristive Systems
These type of memristors primarily rely on different material properties of thin film atomic lattices that exhibit hysterisis under the application of charge.
a). Titanium dioxide memristors
The Titanium Dioxide memristor first developed at HP Labs is based on a two-layer thin “sandwich” of titanium dioxide films, composed of symmetrical lattices of titanium and oxygen atoms. (Titanium dioxide changes its resistance in the presence of oxygen, which is why its used in oxygen sensors.) The motion of atoms in the films are tied to the movement of electrons in the material, which allows a state change in the atomic structure of the memristor. The bottom layer acts as an insulator, and the top film layer acts as a conductor via oxygen vacancies in the titanium dioxide. The oxygen vacancies in the top layer are moved to the bottom layer, changing the resistance, and maintaining the state. To access the memristive properties, crossbars of nanowires are placed above and below the top and bottom layers, so that a charge can be passed through.
Its interesting to note that Stan Williams at HP came to the material property of titanium dioxide memristive effects in part through his interests in the miniaturization of sensor technology for distributed sensing.
b. Polymeric (ionic) memristors
Utilizing the properties of various solid-state ionics, one component of the material structure, the cationic or anionic, is free to move throughout the structure as a charge carrier. Polymeric memristors explore dynamic doping of polymer and inorganic dielectric-type material to attempt and provoke hysterisis type behaviors. Usually, a single passive layer between an electrode and an active thin film attempt to exaggerate the extraction of ions from the electrode. The terms polymeric and ionic are often used somewhat loosely and generically.
c. Manganite memristive systems
A substrate of bilayer oxide films based on manganite, as opposed to titanium dioxide, were exhibited as describing memristive properties at the University of Houston in 2001.
d. Resonant-tunneling diode memristors
Certain types of quantum-well diodes with special doping designs of the spacer layers between the source and drain regions have been shown to exhibit memristive properties.
2. Spin Based and Magnetic memristive systems
Spin-based memristive systems, as opposed to molecular and ionic nanostructure based systems, rely on the property of degree of freedom in electron spin. In these types of system, electron spin polarization is altered, usually through the movement of a magnetic “domain” wall separating polarities, allowing for hysteresis like behaviors to occur.
a). Spintronic Memristors
A type of magnetic memristor under development by several labs, notably Seagate, is called a spintronic memristor. In same way that the titanium dixoide memristor changes state by altering oxygen vacanccies between two seperate layers, changing a spintronic memristors resistance state uses magnetization to alter the spin direction of electrons in two different sections of a device. Two sections of different electron spin directions are kept separate based on a moving “wall”, controlled by magnetization, and the relation of the wall dividing the electron spins is what controls the devices overall resistance state.
b). Spin Torque Transfer (STT) MRAM
Since the 1990s, the development of MRAM has shown, in certain cases, memristive properties. The configuration known as a spin valve, the simplest structure for a MRAM bit, allows for state change. The resistance in a memristive effective spin-torque transfer is controlled by a spin torque induced by a current flowing through a magnetic junction, and is dependent on the difference in spin orientation between the two sides of the junction. Depending on the material used to construct some MRAM bits, these spin torque constructions can exhibit both ionic and magnetic properties, and are sometimes referred to as “second-order memristive systems”.
3. 3-terminal memistors
As an early outlier from the 1960s, the advanced technology of Electroplating ;), was used to demonstrate the viability of a non solid state, three terminal memristor by Bernard Widrow at Stanford. The conductance was described as being controlled by the time integral of current. Interesting to note here is the research was part of a larger research project into the mathematics of early neural network modeling. The Adaptive Linear Element of Widrow (and his then-student Ted Hoff, of Intel fame) is a single layer neural network based on the McCulloch-Pitts neuron, and shows that even in the early days, the modeling of memristive systems was closely related to neuronal learning algorithms.