physics

Spin valve

Spin valve is a physics 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 valve rather than just read about it. In short: A spin valve is a device, consisting of two or more conducting magnetic materials, whose electrical resistance can change between two values depending on the relative alignment of the magnetization in the layers. The resistance change is a result of the giant magnetoresistive effect.

Key takeaways

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

Reference excerpt

A spin valve is a device, consisting of two or more conducting magnetic materials, whose electrical resistance can change between two values depending on the relative alignment of the magnetization in the layers. The resistance change is a result of the giant magnetoresistive effect. The magnetic layers of the device align "up" or "down" depending on an external magnetic field. In the simplest case, a spin valve consists of a non-magnetic material sandwiched between two ferromagnets, one of which is fixed (pinned) by an antiferromagnet which acts to raise its magnetic coercivity and behaves as a "hard" layer, while the other is free (unpinned) and behaves as a "soft" layer. Due to the difference in coercivity, the soft layer changes polarity at lower applied magnetic field strength than the hard one. Upon application of a magnetic field of appropriate strength, the soft layer switches polarity, producing two distinct states: a parallel, low-resistance state, and an antiparallel, high-resistance state. The invention of spin valves is credited to Dr. Stuart Parkin and his team at IBM Almaden Research Centre. Dr. Parkin is now serving as the Managing Director of the Max Planck Institute of Microstructure Physics in Halle, Germany.

How it works Spin valves work because of a quantum property of electrons (and other particles) called spin. Due to a split in the density of states of electrons at the Fermi energy in ferromagnets, there is a net spin polarisation. An electric current passing through a ferromagnet therefore carries both charge and a spin component. In comparison, a normal metal has an equal number of electrons with up and down spins so, in equilibrium situations, such materials can sustain a charge current with a zero net spin component. However, by passing a current from a ferromagnet into a normal metal it is possible for spin to be transferred. A normal metal can thus transfer spin between separate ferromagnets, subject to a long enough spin diffusion length. Spin transmission depends on the alignment of magnetic moments in the ferromagnets. If a current is passing into a ferromagnet whose majority spin is spin up, for example, then electrons with spin up will pass through relatively unhindered, while electrons with spin down will either 'reflect' or spin flip scatter to spin up upon encountering the ferromagnet to find an empty energy state in the new material. Thus if both the fixed and free layers are polarised in the same direction, the device has relatively low electrical resistance, whereas if the applied magnetic field is reversed and the free layer's polarity also reverses, then the device has a higher resistance due to the extra energy required for spin flip scattering.

Antiferromagnetic and non-magnetic layers An antiferromagnetic layer is required to pin one of the ferromagnetic layers (i.e., make it fixed or magnetically hard). This results from a large negative exchange coupling energy between ferromagnets and antiferromagnets in contact. The non-magnetic layer is required to decouple the two ferromagnetic layers so that at least one of them remains free (magnetically soft).

Pseudo spin valves The basic operating principles of a pseudo spin valve are identical to that of an ordinary spin valve, but instead of changing the magnetic coercivity of the different ferromagnetic layers by pinning one with an antiferromagnetic layer, the two layers are made of different ferromagnets with different coercivities e.g., NiFe and Co. Note that coercivities are largely an extrinsic property of materials and thus determined by processing conditions.

Applications Spin valves are used in magnetic sensors and hard disk read heads. They are also used in magnetic random access memories (MRAM).

See also Spin-transfer torque Magnetic tunnel junction RKKY interaction

External links The Spin Valve - Condensed Matter Physics Group

References

Illustrations

Worked examples

Example 1 — a first encounter with Spin valve

Start with the simplest possible case. Write down what Spin valve claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, 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 valve 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 valve 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 valve

In research: Spin valve appears in physics 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 valve 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 valve is common in secondary-school and first-year university syllabi. It links to neighbouring topics Quantum electronics, Spintronics, so understanding it makes those chapters shorter.
In everyday life: Look for Spin valve 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 valve in 20 minutes

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

Frequently asked questions

What is Spin valve in simple terms?

Why does Spin valve matter?

Because it connects several physics 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 valve?

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 valve.