Young scientist discovers magnetic material unnecessary to create spin current

July 24, 2015 by Carla Reiter
Typically when referring to electrical current, an image of electrons moving through a metallic wire is conjured. Using the spin Seebeck effect (SSE), it is possible to create a current of pure spin (a quantum property of electrons related to its magnetic moment) in magnetic insulators. However, this work demonstrates that the SSE is not limited to magnetic insulators but also occurs in a class of materials known as paramagnets. Since magnetic moments within paramagnets do not interact with each other like in conventional ferromagnets, and thus do not hold their magnetization when an external magnetic field is removed, this discovery is unexpected and challenges current theories for the SSE. New ways of generating spin currents may be important for low-power high-speed spin based computing (spintronics), and is also an area of great fundamental interest. The paramagnetic SSE changes the way we think about thermally driven spintronics, allowing for the creation of new devices and architectures where spin currents are generated without ferromagnetic materials, which have been the centerpiece of all spin-based electronic devices up until this point.

It doesn't happen often that a young scientist makes a significant and unexpected discovery, but postdoctoral researcher Stephen Wu of the U.S. Department of Energy's Argonne National Laboratory just did exactly that. What he found—that you don't need a magnetic material to create spin current from insulators—has important implications for the field of spintronics and the development of high-speed, low-power electronics that use electron spin rather than charge to carry information.

Wu's work upends prevailing ideas of how to generate a current of spins. "This is a discovery in the true sense," said Anand Bhattacharya, a physicist in Argonne's Materials Science Division and the Center for Nanoscale Materials (a DOE Office of Science user facility), who is the project's principal investigator. "There's no prediction of anything like it."

Spin is a quantum property of electrons that scientists often compare to a tiny bar magnet that points either "up" or "down." Until now scientists and engineers have relied on shrinking electronics to make them faster, but now increasingly clever methods must be used to sustain the continued progression of electronics technology, as we reach the limit of how small we can create a transistor. One such method is to separate the flow of from the flow of electron current, upending the idea that information needs to be carried on wires and instead flowing it through insulators.

To create a current of spins in insulators, scientists have typically kept electrons stationary in a lattice made of an insulating ferromagnetic material, such as yttrium iron garnet (YIG). When they apply a heat gradient across the material, the spins begin to "move"—that is, information about the orientation of a spin is communicated from one point to another along the lattice, much in the way a wave moves through water without actually transporting the water molecules anywhere. Spin excitations known as magnons are thought to carry the current.

Wu set out to build on previous work with spin currents, expanding it to different materials using a new technique he'd developed. He worked on making devices a thousand times smaller than the typical systems used, giving him more control over the heat and allowing him to create larger thermal gradients in a smaller area. "That was the key to why we were able to do this experiment," he says.

Wu looked at a layer of ferromagnetic YIG on a substrate of paramagnetic gadolinium gallium garnet (GGG). He expected to see no action from the GGG: in a paramagnet the spins aren't aligned as they are in a ferromagnet. They generate no magnetic field, produce no magnons, and there appears to be no way for the spins to communicate with one another. But to everyone's surprise, the spin current was stronger in the GGG than it was in the YIG. "The spins in the system were not talking to each other. But we still found measurable ," says Wu. "This effect shouldn't happen at all."

The next step is to figure out why it does.

"We don't know the way this works," said Bhattacharya. "There's an opportunity here for somebody to come up with a theory for this."

The scientists also want to look for other materials that display this effect. "We think that there may be other new physics working here," said Bhattacharya. "Because, since the material is not a ferromagnet, the objects that are moving the spin are not what we typically understand."

In the meantime, said Wu, "We've just taken ferromagnetism off its pedestal. In a spintronic device you don't have to use a ferromagnet. You can use either a paramagnetic metal or a paramagnetic insulator to do it now."

For more information, see the paper, "Paramagnetic Seebeck effect," published in the journal Physical Review Letters.

Explore further: Ultrafast heat conduction can manipulate nanoscale magnets

More information: "Paramagnetic Spin Seebeck Effect." Phys. Rev. Lett. 114, 186602 – Published 5 May 2015.

Related Stories

Ultrafast heat conduction can manipulate nanoscale magnets

June 8, 2015

Researchers at the University of Illinois at Urbana-Champaign have uncovered physical mechanisms allowing the manipulation of magnetic information with heat. These new phenomena rely on the transport of thermal energy, in ...

Spintronics just got faster

July 20, 2015

In a tremendous boost for spintronic technologies, EPFL scientists have shown that electrons can jump through spins much faster than previously thought.

Recommended for you

Changing semiconductor properties at room temperature

October 28, 2016

It's a small change that makes a big difference. Researchers have developed a method that uses a one-degree change in temperature to alter the color of light that a semiconductor emits. The method, which uses a thin-film ...

Novel light sources made of 2-D materials

October 28, 2016

Physicists from the University of Würzburg have designed a light source that emits photon pairs, which are particularly well suited for tap-proof data encryption. The experiment's key ingredients: a semiconductor crystal ...

Shocks in the early universe could be detectable today

October 27, 2016

(—Physicists have discovered a surprising consequence of a widely supported model of the early universe: according to the model, tiny cosmological perturbations produced shocks in the radiation fluid just a fraction ...

Bubble nucleus discovered

October 27, 2016

Research conducted at the National Superconducting Cyclotron Laboratory at Michigan State University has shed new light on the structure of the nucleus, that tiny congregation of protons and neutrons found at the core of ...


Adjust slider to filter visible comments by rank

Display comments: newest first

Whydening Gyre
not rated yet Jul 24, 2015
Great work...
not rated yet Jul 24, 2015
Some of the most important scientific discoveries started with "That shouldn't have happened."
not rated yet Jul 24, 2015
This is a very beautiful experiment. I hope you will not take offense if I note a truth. The theory of such already exist. See "A Theory of the Relativistic Fermionic Spinrevorbital" ( http://www.academ...AB049585 ) Click "Full Text PDF" for free copy. In such manuscript, insulator states in particular involving 2p orbitals and hybrids and associated gaps form hidden 'continuum' states which can be excited by heat, pressure, electric field, gravityational field and/or magnetic field of the surroundings. See pages 16-19.
not rated yet Jul 24, 2015
In particular on page 19 where I use the example of superconductivity in MgB2 and explain it by the gap in such insulator and phonon scatter Cooper pair (boson) into continuum to form fermionic pair (magnon) and the conversion of activating heat, pressure, electric to gravitomagnetic field for binding such magnons at higher temperatures. Such continuum states bind the paramagnetic spins more strongly by a type superexchange involving gravitational and magnetic fields formed by the heat, pressure and/or electric activation!
not rated yet Jul 24, 2015
The stronger binding of the paramagnetic pair in the hidden state allows the spin wave to propagate! I have used such to explain high temperature superconductivity, possible coexistence of magnetism and antiferromagnetic states involved with high temperature superconductivity
Jul 24, 2015
This comment has been removed by a moderator.
1 / 5 (1) Jul 24, 2015
The next step is to figure out why it does.

"We don't know the way this works," said Bhattacharya. "There's an opportunity here for somebody to come up with a theory for this."

The scientists also want to look for other materials that display this effect. "We think that there may be other new physics working here,"….

Maybe the problem is because we do not know what an electron is, understanding its nature could help ….
5 / 5 (1) Jul 25, 2015
Electrons begin to spin as they approach the speed of light. Planck called it "...the action of the electron upon itself". Just a little over a century in beginning to understand the electromagnetic properties of atoms.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.