Magnetoresistive RAM (MRAM)

MRAM is a memory family, not a promise that any part can replace DRAM or flash unchanged. Start with its interface and completion rules, then check retention and cycling limits for the required temperature range.

A magnetic tunnel junction (MTJ) places a thin insulating barrier between a reference magnetic layer and a switchable free layer. Electrons tunnel across the barrier. In the conventional junctions described here, parallel magnetizations produce lower resistance and antiparallel magnetizations higher resistance. The circuit interprets these states as bits. [1]

The read mechanism does not determine the write mechanism. Toggle MRAM uses magnetic fields from write lines. STT-MRAM instead passes spin-polarized current through the junction to switch the free layer. MRAM, MTJ and spin-transfer torque therefore name a family, a device structure and a switching mechanism, respectively. [1] [2]

Retention and endurance answer different questions: how long stored information remains recoverable under specified conditions, and how much cycling the device supports. In laboratory MTJs with MgAl2O4 barriers, Choi and colleagues found that voltage-pulse polarity and interface conditions affected breakdown under 50 ns pulses. That experiment demonstrates a stress mechanism; its cycle counts are not specifications for commercial MgO-based MRAM. [4]

For a concrete specification, EM128LX v1.7 lists 10^14 write cycles per architecture-defined 32-byte unit and 10-year retention at 85 degrees C for industrial-grade parts. Those conditions and counting units belong with the numbers. The same device supports writes without prior erasure, while also providing erase commands for interface compatibility. [3]

Completion also needs a device-level check: EM128LX routes writes through an internal cache and can still report Write in Progress (WIP) after chip select (CS#) rises. Finishing transmission is not sufficient evidence that the array is idle. The 32-byte endurance unit is not an atomic-write guarantee. [3]

Nonvolatility preserves stored state, not every interrupted operation. The EM128LX reset section warns that resetting an ongoing WRITE, PROGRAM or ERASE can abort it and corrupt data. It does not promise an all-or-nothing application record. [3]

For memory-mapped deployments, caches, ordering and the persistence boundary must also be considered. SNIA distinguishes failure atomicity from visibility to concurrent threads. A multi-store update needs a protocol that leaves a recoverable state after interruption; the presence of MRAM alone does not supply one. [5]

When evaluating a design, document the exact part and revision, power/reset sequence, acknowledged-write boundary and recovery procedure. Treat product ratings, junction experiments and system-level guarantees as different evidence.