Here is some Christmas holiday reading. HP's claim that RRAM, PCM, and MRAM are all memristor technologies is bullshit, the memristor is not a fourth fundamental circuit element, HP didn't find it, and its developing memristor product is a not a memristor - really.

The flame from Blaise

So says ex-USPTO patent examiner and inventor Blaise Mouttet1 of Arlington, Virginia. He commented to me on a story about HP and Hynix producing the memristor goods by 2013, saying: "You are either an idiot or trying to get in good with HP for contributing to their bullshit claiming that RRAM, PCM, and MRAM are all memristor technologies. Aren't you capable of distinguishing between corporate propaganda and legitimate technological development? What a disgrace."

Upon asking why he thought that he replied:

The original 1971 definition of a memristor given by Chua was a 2-terminal non-linear passive circuit linking electric charge and magnetic flux linkage. None of the memory types you listed in your article have been shown to conform to this definition and it is irresponsible to claim them as memristors unless this is shown. The drift of oxygen vacancies in titanium dioxide is non-linearly dependent on current rather than linearly dependent as required by the memristor model proposed in Strukov and Williams 2008 paper2. The "memristor" is a propaganda campaign by HP to take credit for emerging memory devices invented by other companies. Chua's memristor theory does nothing to explain the actual physics in any of these devices. I gave a presentation at ISCAS 2010 which expands on some of these points ... as well as an online article attempting to explain why the memristor model is wrong and providing some background historical context. It is very disappointing to me that so many in the scientific press so easily buy into this memristor crap without attempting to look more closely into the motivations behind why HP is promoting it so much or whether their claims stand up to scrutiny.

To understand his points we have to delve into a little electronics and a little history.

First we have to note that a classic resistor in electronics is a linear circuit element whose resistance to electric current depends upon a proportional relationship between current and voltage as represented by Ohm's law; i=v/r.

Memsistor and memresistor history

Some special electronic devices have a resistance that depends upon the history of a previously applied current or voltage. Bernard Widrow of Stanford University devised the term 'memistor' in 1960 for this, while working with electro-chemical resistors in an attempt to build a artificial neuron-like circuit called ADALINE (ADAptive LInear NEuron). Various scientific papers discussed solid-state, thin-film memistors after that3.

'It appears that Chua has recently broadened the definition to save face for HP'

Mouttet says that Leon Chua, a a UC Berkeley professor of non-linear circuit theory, coined the term "memristor" in 1971 to describe what he considered to be a missing circuit and the “4th fundamental circuit element” after the resistor, capacitor, and inductor. This 4th circuit element claim is fallacious in Mouttet's view.

HP Lab's Stanley Williams, a senior HP fellow and founding Director of the HP Quantum Science Research (QSR) group, has a different view:

[Chua's original memristor definition did not] postulate any mechanism at all. Moreover, the memristor definition did not even require causality. In other words, the mathematical relationship between flux and charge could be the result of some other cause – any mechanism that led to the constraint embodied by the equation dφ = M dq would lead to a device with the properties of a memristor. He published these initial findings4 essentially as a curiosity – it was not obvious at that time that such a circuit element existed. However, some people (as I did at first) have taken this paper too literally and thought that a memristor must involve a direct interaction of a charge with a magnetic flux – in fact, there was no such requirement or restriction in the memristor definition.

A documented brief history of memristor development by Williams can be downloaded here (pdf). It is a very good read.