How MRAM Uses Electron Spin to Store Binary States

Magnetoresistive Random-Access Memory (MRAM) is an advanced non-volatile storage technology that stores binary data using electron spin rather than electrical charges. By utilizing the Tunnel Magnetoresistance (TMR) effect inside nanoscopic Magnetic Tunnel Junctions (MTJs), MRAM assigns binary values (0 and 1) to low and high electrical resistance states. This article explores the architecture of the Magnetic Tunnel Junction, the physics of electron spin orientation, the mechanisms used to switch magnetic states, and the reasons why MRAM retains data indefinitely without power.

The Magnetic Tunnel Junction (MTJ) Architecture

The foundational storage element of an MRAM cell is the Magnetic Tunnel Junction (MTJ). An MTJ consists of three primary thin-film layers:

The Reference (Pinned) Layer: A ferromagnetic layer with a fixed, unchangeable magnetic orientation.
The Tunnel Barrier: An ultra-thin insulating layer, typically made of magnesium oxide (MgO), measuring only a few atoms thick.
The Free Layer: A ferromagnetic layer whose magnetic orientation can be dynamically altered using electrical currents.

Encoding Binary States via Electron Spin Alignment

In quantum physics, electrons possess an intrinsic angular momentum known as spin, generally categorized as “spin-up” or “spin-down.” Ferromagnetic materials align their internal magnetic fields based on the majority spin orientation of their electrons.

MRAM determines binary values by comparing the relative magnetic orientations of the Free Layer and the Reference Layer:

Parallel State (Binary 0): When the magnetic moments of both the Free Layer and Reference Layer point in the same direction, majority-spin electrons from one layer find available, matching quantum energy states in the other layer. Electrons tunnel through the insulating barrier with minimal resistance. This low-resistance state represents a binary 0.
Anti-Parallel State (Binary 1): When the magnetic moment of the Free Layer points opposite to the Reference Layer, majority-spin electrons encounter a mismatch in available quantum states on the other side. This significantly restricts electron tunneling, causing high electrical resistance. This high-resistance state represents a binary 1.

Parallel Alignment (Low Resistance = 0)
[ Reference Layer:  → → → ]
[ Insulating Barrier (MgO) ]
[ Free Layer:       → → → ]

Anti-Parallel Alignment (High Resistance = 1)
[ Reference Layer:  → → → ]
[ Insulating Barrier (MgO) ]
[ Free Layer:       ← ← ← ]

Reading and Writing Data

Reading Data

To read an MRAM cell, a small sensing voltage is applied across the MTJ. The system measures the resulting current to determine the junction’s electrical resistance. Because the sensing current is minimal, it reads the parallel (low resistance) or anti-parallel (high resistance) state without altering the magnetic alignment of the Free Layer.

Writing Data (Spin-Transfer Torque)

Modern MRAM uses Spin-Transfer Torque (STT-MRAM) to write binary states: 1. A write current passes through a polarizing magnetic layer, aligning the spins of the passing electrons. 2. When this spin-polarized current enters the Free Layer, the electrons transfer their angular momentum directly to the magnetic moments of the Free Layer. 3. Depending on the direction of the electrical current, the transferred torque flips the Free Layer’s magnetization to either align with (Parallel / 0) or oppose (Anti-Parallel / 1) the Reference Layer.

Why MRAM is Non-Volatile

Traditional volatile memory, such as Dynamic RAM (DRAM) and Static RAM (SRAM), relies on electric charges trapped in capacitors or continuous current through transistor latches. When power is lost, the charge dissipates, and data is erased.

MRAM achieves non-volatility because ferromagnetic materials maintain their magnetic orientation (magnetic remanence) intrinsically. Once the electron spins in the Free Layer are aligned, they remain locked in place by the material’s magnetic anisotropy energy barrier. No continuous power supply or periodic refresh cycles are needed to preserve the binary states, combining the speed of SRAM with the persistence of Flash memory.