Memristor Definition and Criticism
According to the original 1971 definition the memristor was considered the fourth fundamental circuit element forming a non-linear relationship between electric charge and magnetic flux linkage. In 2011 Leon Chua has argued for a broader definition so that all 2-terminal non-volatile memory devices based on resistance switching should be considered memristors. Stan Williams of HP Labs has also argued that MRAM, phase change memory, and RRAM should be considered memristor technologies. Some researchers have argued that biological structures such as blood and skin should also be considered to be memristors. Others have argued that the memory device under development by HP Labs and other forms of RRAM are not actually memristors or memristive systems but part of a broader class of variable resistance systems and that a broader definition of memristor is a scientifically unjustifiable land grab to favor the memristor patents of Hewlett-Packard.
Some researchers have pointed out problems with the memristor models of HP Labs. A paper by P. Meuffels and H. Schroeder published in Applied Physics A noted that one of the early memristor papers included a mistaken assumption regarding ionic conduction. A paper was posted on arXiv by P. Meuffels and R. Soni discussing possible issues and problems in the realization of memristors in terms of Landauer's principle. The physics behind the concept of memristive systems like the HP memristor seems to be in conflict with fundamentals of non-equilibrium thermodynamics. Following the dynamic state equations of such systems, one would be able to violate Landauer's principle of the minimum possible amount of energy required to change "information" states in a system.
Researchers from Mitre Corporation have noted that memristor models based on the assumption of linear ionic drift do not account for asymmetry between set time (high-to-low resistance switching) and reset time (low-to-high resistance switching) and do not provide ionic mobility values consistent with experimental data. Non-linear ionic drift models have been suggested to compensate for this deficiency.
Martin Reynolds, an electrical engineering analyst with research outfit Gartner, has commented that Stan Williams is being sloppy in calling HP's device a memristor but that the memristor critics are being pedantic and it does not matter how HP's device works as long as it provides the ability to build devices with really high density storage.
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