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A magnetic tunnel junction (MTJ) is a nanoscale device with two magnetic layers separated by an ultrathin insulator. Electrons can cross that barrier by quantum tunneling, and the junction’s resistance changes with the layers’ relative magnetization. That resistance difference lets a circuit read the magnetic state.
What is a magnetic tunnel junction?
An MTJ is a spintronic device built from two ferromagnetic layers, also called electrodes, with an ultrathin insulating barrier between them. The barrier is only a few atomic layers thick in the structures described by NIST. Although it is an insulator, it is thin enough for electrons to cross by quantum tunneling rather than through an ordinary metallic contact.
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The basic stack is ferromagnet / insulator / ferromagnet. Common examples use cobalt–iron–boron (CoFeB) magnetic layers and a magnesium oxide (MgO) barrier; aluminum oxide is another barrier material. These are examples, not fixed requirements: compositions, thicknesses and geometries vary among MTJ designs. IEEE’s overview of spin-transfer-torque MRAM describes thin barriers including aluminum oxide and crystalline MgO.
How does an MTJ produce a resistance signal?
The key is the relative direction of the magnetization in the two ferromagnetic layers. When the directions are parallel, tunneling is easier and resistance is lower. When they are antiparallel, tunneling is less probable and resistance is higher. This orientation-dependent resistance change is called tunneling magnetoresistance (TMR); the relative difference between the antiparallel and parallel resistance is commonly reported as a TMR ratio. IEEE explains this TMR effect in the context of MTJs.
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To read an MTJ, a circuit measures the resistance through the stack. It can then infer which magnetic configuration is present from the resulting electrical signal. In memory, that provides a way to read a stored magnetic state; in a sensor, the resistance response can indicate a change in magnetic field. NIST describes MTJs as straightforward to read electronically.
How is an MTJ switched?
Switching is distinct from reading. In suitable designs, a current can exert spin-transfer torque and change the magnetization of a free layer; the change in resistance can then be read through TMR. IEEE describes spin-polarized current switching a free-layer magnetization without an external magnetic field, while NIST describes current-induced spin torque as a way to change an MTJ state. Not every design uses the same writing scheme.
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In a figure caption, NIST reports that its pictured junction switches from a high-resistance to a low-resistance state after a voltage higher than 500 millivolts is applied. That value belongs to the specific device shown; it is not a universal MTJ switching threshold or a general operating-voltage recommendation. NIST figure caption.
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Magnetic random-access memory
Magnetic random-access memory (MRAM) uses magnetic states to store information, with MTJs providing an electrical resistance contrast for reading those states. IEEE’s MRAM overview discusses MTJs in this memory context.
Magnetic sensors and read heads
TMR sensors use an MTJ’s resistance response to sense magnetism. IEEE’s review of magnetic-field vector sensors includes TMR alongside approaches such as anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), Hall, fluxgate, search-coil and SQUID sensors. These sensing approaches are not interchangeable by default: their operating principles, error sources and calibration needs differ. IEEE Sensors Journal review.
IEEE also identifies MTJs as sensing elements in hard-disk read heads. IEEE overview.
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MTJ essentials at a glance
- Structure: Two ferromagnetic layers separated by an ultrathin insulating tunnel barrier.
- Parallel state: Magnetizations point in the same direction; resistance is lower.
- Antiparallel state: Magnetizations point in opposite directions; resistance is higher.
- Read principle: A circuit measures the resistance contrast, known as TMR.
- Write principle: Some designs use current-induced spin torque to switch a magnetic layer; writing and reading are separate operations.
- Applications: MRAM, TMR magnetic sensors and hard-disk read heads.
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