text: Embodiments of the inventive concept relate generally to electronic memory technologies. More particularly, embodiments of the inventive concept relate to nonvolatile memory devices and related methods of operation.
  Semiconductor memory devices can be roughly divided into two categories according to whether they retain stored data when disconnected from power. These categories include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power. Examples of volatile memory devices include static random access memory (SRAM) and dynamic random access memory (DRAM), and examples of nonvolatile memory devices include ferroelectric random access memory (FRAM), magnetoresistive random access memory (MRAM), phase-change random access memory (PRAM), and resistive random access memory (RRAM).
  Resistive random access memory (RRAM) has the potential for high storage capacity, high performance, and low power consumption. Accordingly, extensive research is being conducted in the field of RRAM technology to improve the characteristics of RRAM devices. An RRAM device stores data using a variable resistance material layer that changes resistance according to the polarity and size of an applied electrical pulse. One type of variable resistance material layer is a colossal magnetoresistive (CMR) material layer having a perovskite structure.
  RRAM and other memories using a variable resistance material layer are referred to as variable resistance memories. Variable resistance memory devices can be classified as unipolar devices and bipolar devices according to polarity of a writing pulse. In a unipolar variable resistance device, a set pulse and a reset pulse have the same polarity. As a result, unipolar variable resistance memory devices may exhibit unstable performance in the presence of a unipolar pulse.