MRAM vs ReRAM vs PCM vs FeRAM: Emerging Memories Compared

These four emerging non-volatile memories all try to combine the speed of RAM with data retention without power. They differ in what stores the bit: the magnetization of a thin film (MRAM), the resistance of a filament in an oxide (ReRAM), the crystalline or amorphous state of a material (PCM) or the polarization of a ferroelectric (FeRAM). None has replaced DRAM or flash broadly yet, but each has found a niche.

AspectMRAMReRAMPCMFeRAM
What stores the bitMagnetization direction of a free layer in a magnetic tunnel junction, read as low or high resistanceResistance of a conductive filament that forms and breaks in a metal-oxide layerCrystalline (low resistance) or amorphous (high resistance) state of a phase-change materialPolarization direction of a ferroelectric material in a capacitor
Write methodSpin-transfer torque (STT) or spin-orbit torque (SOT) currentVoltage pulses (SET and RESET)Heat pulses: melt and quench to make it amorphous, anneal to crystallizeAn electric field flips the polarization
ReadResistance sensingResistance sensingResistance sensingCapacitor type: destructive, so the data must be rewritten after reading
SpeedFast: writes in a few to tens of nsFast to moderate (tens of ns to microseconds)Fast reads; writes slower and need more energyFast, tens of ns
EnduranceVery high (over 10¹⁰ cycles)Moderate to high, varies by designModerate (roughly 10⁶ to 10⁹ cycles)Very high for capacitor type
DensityModerate: larger cells, hard to stackHigh potential: simple cells, crossbar arraysHigh potentialLower: the capacitor is hard to shrink
Main challengesWrite energy, cell size and process complexityVariability between cells, the initial forming stepHigh write current, resistance drift over timeScaling the capacitor, integrating ferroelectric materials
Typical usesEmbedded memory in microcontrollers and automotive devices, data loggingEmbedded non-volatile memory, in-memory computing for AIStorage-class memory, embedded memorySmall low-power memory in smart cards, meters and IoT devices

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

MRAM
When you need non-volatile memory with flash-like retention but DRAM-like speed and almost unlimited endurance, for example embedded memory or frequent logging.
ReRAM
When you want a simple, dense, low-voltage cell, including for in-memory computing that does multiply-accumulate in the memory array.
PCM
When you want a denser memory between DRAM and flash, and can accept higher write energy.
FeRAM
When you need very low-power, fast, high-endurance memory in small amounts, such as smart cards and sensors.

FAQ

Which emerging memory is closest to replacing DRAM or SRAM?

MRAM comes closest in speed and endurance, with writes in a few to tens of nanoseconds and over 10¹⁰ cycles. But its larger cells keep it far behind DRAM and flash in capacity and cost per bit, so it is used as embedded memory.

How does phase-change memory store data?

It heats a material to switch it between a crystalline, low-resistance state and an amorphous, high-resistance state. The resistance is then read as 0 or 1.

Why is the read in capacitor-type FeRAM destructive?

Reading flips the polarization to sense it, which erases the stored value, so the data must be written back after each read.

What is in-memory computing with ReRAM?

A ReRAM array can perform multiply-accumulate operations directly in the memory by using each cell resistance as a weight. This suits AI workloads and avoids moving data between memory and processor.

Check your understanding

Which correctly describes how MRAM stores data?

  1. AThrough differences in resistance due to the magnetization direction of a magnetic tunnel junction (MTJ)
  2. BThrough differences in resistance between the crystalline and amorphous states of a chalcogenide material
  3. CThrough the presence or absence of charge stored in a capacitor
Show answer and explanation

Answer: A. Through differences in resistance due to the magnetization direction of a magnetic tunnel junction (MTJ)

MRAM uses a magnetic tunnel junction (MTJ): two magnetic layers sandwiching an insulating layer about 1 nm thick. Parallel magnetizations give low resistance and antiparallel give high resistance (tunnel magnetoresistance), distinguishing 0 from 1, and since magnetization persists without power the memory is non-volatile. Near-unlimited endurance and high speed are its strengths. Crystalline-versus-amorphous resistance is phase-change memory, and stored charge is DRAM.

Which correctly describes how phase-change memory (PCM) stores data?

  1. AThrough differences in the magnetization direction of magnetic layers
  2. BThrough differences in resistance between the crystalline and amorphous states of a chalcogenide material
  3. CThrough the polarization direction of a ferroelectric
Show answer and explanation

Answer: B. Through differences in resistance between the crystalline and amorphous states of a chalcogenide material

PCM heats chalcogenide materials such as GeSbTe with Joule heating from current and quenches them, switching between a low-resistance crystalline state and a high-resistance amorphous state. The resistance ratio is large—two to three orders of magnitude—so intermediate levels can store multiple bits. The same materials, whose reflectivity also changes between phases, have been used in rewritable DVDs and Blu-ray discs. The large write current is a challenge.

Which property does FeRAM (ferroelectric memory) use to store information?

  1. AFormation and dissolution of conductive filaments in an oxide
  2. BThe magnetoresistance effect
  3. CThe direction of spontaneous (remanent) polarization of a ferroelectric
Show answer and explanation

Answer: C. The direction of spontaneous (remanent) polarization of a ferroelectric

FeRAM uses the remanent polarization of ferroelectrics—polarization that persists after the voltage is removed—storing 0 or 1 in its direction. Writing simply flips the polarization, so no flash-style high voltage is needed; it is fast (tens of ns), low-power and endures 10¹² cycles or more. PZT (lead zirconate titanate) has been the traditional material. Conductive filaments are ReRAM, and magnetoresistance is MRAM.

  1. Which correctly describes how ReRAM (resistive memory) stores data?
  2. Compared with flash memory, which is a general advantage of MRAM?
  3. What is the main application expected for in-memory computing using resistive memories and similar devices?

Practice Emerging memories questions

Key terms explained

MRAM
Non-volatile memory that stores data as the difference in resistance caused by the magnetization direction of a magnetic tunnel junction (MTJ).
PCM (phase-change memory)
Nonvolatile memory that stores information by the difference between the crystalline (low-resistance) and amorphous (high-resistance) states of a chalcogenide material.
ReRAM (resistive RAM)
Nonvolatile memory that applies voltage to form and break conductive paths in an oxide or similar material, storing information as differences in resistance.
FeRAM (ferroelectric RAM)
Nonvolatile memory that stores information in the polarization direction of a ferroelectric. Writes are fast and low-power, and it is used in IC cards and the like.
Magnetic tunnel junction (MTJ)
A structure with a thin insulating film between two magnetic layers. Resistance is low when the layers' magnetizations are parallel and high when antiparallel. It is the storage element of MRAM.

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