I. Introduction
Memristors have many different facets. A memristor can be considered as the theoretical missing fundamental element originally proposed by Leon Chua in 1971 [1]. This theoretical device is a resistor with varying resistance, where the resistance changes according to the charge passed through the memristor over its entire history. Chua extended the theory of memristors to ‘memristive devices’ in 1976 with his student, Steve (Sung Mo) Kang [2]. A memristive device is basically any resistor with a resistance that only changes due to the voltage across the device or, alternatively, the current flowing through the device. Since the resistance does not change when there is no voltage applied across the device, memristive devices are nonvolatile. It is acceptable to use the term ‘memristor’ to describe a ‘memristive device.’