Introduction
Resistive memories, which can vary their resistance depending on applied voltage, were intensively studied from the 1960s to early 1980s for device applications (1). For example, chalcogenide materials, semiconductors, various kinds of oxides and nitrides, and even organic materials were found to have resistive memory properties. However, high operation voltage and current, poor endurance, and low film handling technique made the resistive memory unfavorable in comparison with capacitive memories.