The Magnetic Tunnel Junction cell is made of two ferromagnetic layers separated by a thin insulating layer that acts as tunnel barrier (Fig. 4). In contrast to giant magneto-resistive (GMR) structures in which the sense current usually flows parallel to the layers of the structure, the current is passed perpendicular to the layers of the MTJ sandwich. Similar to GMR, the resistance of the MTJ sandwich depends on the magnetic arrangement of the magnetic moments of the two ferromagnetic layers with respect to each other. Typically, the resistance of the MTJ is lowest when these moments are aligned parallel to one another, and it is highest when anti-parallel, thereby giving rise to magneto-resistance. The read operation is performed by measuring spin-dependent tunneling current between the magnetic layers, thus the impedance is high and the difference between the 0 and 1 level can achieve 50% of cell resistance. The biggest problem of MRAM is scalability because undesired cross-talk effects by magnetic fields are difficult to suppress.
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Figure 4: Magnetic tunnel junctions |
Phase change memory: PC-RAM, also called ovonic unified memory (OUM), is based on rapid reversible phase changes in some materials under the influence of external pulses [1, 5]. In particular, OUM cells use the reversible structural phase change in thin chalcogenide film material as data storage mechanism. The small volume of active media acts as a programmable resistor between a high and low resistance with high dynamic range. “1” and “0” are thus presented by crystalline versus amorphous phase states of the active material. Phase states are often programmed by the application of a current pulse through a MOSFET which drives the memory cell into a high or low resistance state, depending on current magnitude. Measuring resistance changes in the cell performs the function of reading data. OUM cells can be programmed to intermediate resistance values; e.g., for multi-state data storage.
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Figure 5: Phase change memory, write/erase operation |
The potential advantage of OUM is its relatively simple materials science approach which can be furthermore aggressively scaled. Since the energy required for phase transformation decreases with cell size, the write current can scale with cell size, thus facilitating high-density memory scaling.