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Spin-transfer torque

Spin-transfer torque is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Spin-transfer torque rather than just read about it. In short: Spin-transfer torque (STT) is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current. Charge carriers (such as electrons) have a property known as spin which is a small quantity of angular momentum intrinsic to the carrier.

Spin-transfer torque — main illustration
Spin-transfer torque — illustration

Key takeaways

  • Spin-transfer torque belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Spin-transfer torque to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Spin-transfer torque from memory before moving on to harder problems.

Reference excerpt

Spin-transfer torque (STT) is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current. Charge carriers (such as electrons) have a property known as spin which is a small quantity of angular momentum intrinsic to the carrier. An electric current is generally unpolarized (consisting of 50% spin-up and 50% spin-down electrons); a spin polarized current is one with more electrons of either spin. By passing a current through a thick magnetic layer (usually called the “fixed layer”), one can produce a spin-polarized current. If this spin-polarized current is directed into a second, thinner magnetic layer (the “free layer”), the angular momentum can be transferred to this layer, changing its orientation. This can be used to excite oscillations or even flip the orientation of the magnet. The effects are usually seen only in nanometer scale devices.

Spin-transfer torque memory Spin-transfer torque can be used to flip the active elements in magnetic random-access memory. Spin-transfer torque magnetic random-access memory (STT-RAM or STT-MRAM) is a non-volatile memory with near-zero leakage power consumption which is a major advantage over charge-based memories such as SRAM and DRAM. STT-RAM also has the advantages of lower power consumption and better scalability than conventional magnetoresistive random-access memory (MRAM) which uses magnetic fields to flip the active elements. Spin-transfer torque technology has the potential to make possible MRAM devices combining low current requirements and reduced cost; however, the amount of current needed to reorient the magnetization is presently too high for most commercial applications, and the reduction of this current density alone is the basis for present academic research in spin electronics.

Industrial development Sony Research Center published the first Japan Patent application for S.P.I.N.O.R. (Spin Polarized Injection Non-Volatile Orthogonal Read/Write RAM), a forerunner of STT RAM, in 1997. Subsequently, at IEDM 2005, Sony researchers reported the first working 4kb STT memory, dubbed Spin-RAM, with replacement of the paramagnetic spacer layer of SPINOR memory with MgO dielectric. Hynix Semiconductor and Grandis formed a partnership in April 2008 to explore commercial development of STT-RAM technology. Hitachi and Tohoku University demonstrated a 32-Mbit STT-RAM in June 2009. In 2010, Seagate presented the first 16Kb STT-RAM fabricated in the United States using commercially available semiconductor foundry technologies. On August 1, 2011, Grandis announced that it had been purchased by Samsung Electronics for an undisclosed sum. In 2011, Qualcomm presented a 1 Mbit Embedded STT-MRAM, manufactured in TSMC's 45 nm LP technology at the Symposium on VLSI Circuits. In May 2011, Russian Nanotechnology Corp. announced an investment of $300 million in Crocus Nano Electronics (a joint venture with Crocus Technology) which will build an MRAM factory in Moscow, Russia. In 2012 Everspin Technologies released the first commercially available DDR3 dual in-line memory module ST-MRAM which has a capacity of 64 Mb. In June 2019 Everspin Technologies started pilot production for 28 nm 1 Gb STT-MRAM chips. In December 2019 Intel demonstrated STT-MRAM for L4-cache In 2022 TechInsights finds 16Mb embedded STT-MRAM memory in the FitBit Luxe fitness tracker's MCU and that of several other commercially available wearable products. Other companies working on STT-RAM include Avalanche Technology, Crocus Technology and Spin Transfer Technologies.

See also Magnetoresistive RAM Spin (physics) Memristor Spintronics

References

External links Spin torque applet J.C. Slonczewski:"Current-driven excitation of magnetic multilayers(1996)", Journal of magnetism and magnetic materials volume 159, issues 1–2, June 1996, pages L1-L7 [2]

Illustrations

Spin-transfer torque: A simple model of spin-transfer torque for two anti-aligned layers. Current flowing out of the fixed layer is spin-polarized. When it reaches the free layer the majority spins relax into lower-energy states of opposite spin, applying a torque to the free layer in the process.
A simple model of spin-transfer torque for two anti-aligned layers. Current flowing out of the fixed layer is spin-polarized. When it reaches the free layer the majority spins relax into lower-energy states of opposite spin, applying a torque to the free layer in the process.
Spin-transfer torque: A schematic diagram of a spin valve/magnetic tunnel junction. In a spin valve the spacer layer (purple) is metallic; in a magnetic tunnel junction it is insulating.
A schematic diagram of a spin valve/magnetic tunnel junction. In a spin valve the spacer layer (purple) is metallic; in a magnetic tunnel junction it is insulating.

Worked examples

Example 1 — a first encounter with Spin-transfer torque

Start with the simplest possible case. Write down what Spin-transfer torque claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Spin-transfer torque before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Spin-transfer torque ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Spin-transfer torque

In research
Spin-transfer torque appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Spin-transfer torque in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Spin-transfer torque is common in secondary-school and first-year university syllabi. It links to neighbouring topics Spintronics, so understanding it makes those chapters shorter.
In everyday life
Look for Spin-transfer torque outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Spin-transfer torque in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Spin-transfer torque means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Spin-transfer torque out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Spin-transfer torque in simple terms?

Spin-transfer torque (STT) is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current. Charge carriers (such as electrons) have a property known as spin which is a small quantity of angular momentum intrinsic…

Why does Spin-transfer torque matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Spin-transfer torque?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Spin-transfer torque.

Tags

  • Spintronics

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