New insight into an intriguing state of magnetism

Dec 18, 2012 by Paul Preuss
An impressionistic rendition of helimagnetism, which can arise if a material’s crystal structure forces the spins of itinerant electrons to precess collectively.

(Phys.org)—Magnonics is an exciting extension of spintronics, promising novel ways of computing and storing magnetic data. What determines a material's magnetic state is how electron spins are arranged (not everyday spin, but quantized angular momentum). If most of the spins point in the same direction, the material is ferromagnetic, like a refrigerator magnet. If half the spins point one way and half the opposite, the material is antiferromagnetic, with no everyday magnetism.

There are other kinds of magnetism. In materials where the electrons are "itinerant" – moving rapidly through the like a gas, so that their spins become strongly coupled to their motions – certain can cause the spins to precess collectively to the right or left in a helix, producing a state called helimagnetism.

Helimagnetism most often occurs at low temperature; increasing the heat collectively excites the spin structure and eventually destroys the order, relaxing the magnetism. In quantum calculations, such collective excitations are treated like particles ("quasiparticles"); excitations that disrupt magnetism are called magnons, or . There is a well developed theory of helimagnons, yet little is known experimentally about how helimagnetism forms or relaxes on time scales of less than a trillionth of a second, the scale on which actually occur.

A team of scientists from Berkeley Lab's Materials Sciences Division, UC Berkeley's Department of Physics, and the Technical University of Munich, led by Jake Koralek and Dennis Meier, has studied magnons in a material that becomes helimagnetic below about 30 kelvin: iron silicide doped with cobalt. They investigated how helimagnons evolve as the temperature increases, destroying the magnetic order, as well as how the magnetic phases are affected by an external magnetic field.

Working in Joseph Orenstein's laboratory, the team used an to excite the cobalt-doped iron silicide crystal, then followed up with another laser pulse within quadrillonths of a second. This allowed them to measure how much the helical oscillations had relaxed and how the spin states evolved. The pump-probe experiments were performed over a range of temperatures at various strengths.

The pump pulses created spin waves, or helimagnons, which weakened the magnetic order. Because they were able to resolve the helimagnons at ultrashort time resolution, the researchers discovered striking versatility in spin relaxation according to the electrons' collective organization. They were able to reveal the underlying spin dynamics of a model itinerant magnet system, opening the way to fruitful research programs in the burgeoning field of magnonics.

Explore further: Experiment with speeding ions verifies relativistic time dilation to new level of precision

More information: Paper: prl.aps.org/abstract/PRL/v109/i24/e247204

For a related study of electron spin transport and spin helices, visit prl.aps.org/abstract/PRL/v109/i24/e246603

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rubberman
4.4 / 5 (7) Dec 18, 2012
I wonder why I constantly find myself attracted to articles on Magnetism......:P
vegitax
5 / 5 (7) Dec 18, 2012
Rubberman: Maybe you have too much iron in your diet?
Noumenon
4 / 5 (20) Dec 18, 2012
Or, maybe its only his relativistic frame of reference, and that he's really attracted to electrical effects.
Macksb
1 / 5 (1) Dec 18, 2012
In my opinion, helimagnetism is simply another example of Art Winfree's law of coupled oscillators, which he described circa 1967. His law applies to all "limit-cycle oscillators," to use his term, which according to his law have a tendency to synchronize in certain specific patterns.

Per Winfree, two oscillator systems synchronize at 0 and 180; three oscillator systems synchronize at 0 (or 360, if you prefer), 120 and 240. The precession that forms helimagnetism follows the same pattern--evenly spaced in intervals that are 360 degrees divided by the number of relevant oscillators. Quantized angular momentum is a "limit-cycle oscillator" in Winfree's terms, so one should expect such precise organization. The 0 and 180 spacing of antiferromagnetism follows the same Winfree principle.

A MacArthur prize winner, Winfree was a bio-mathematician. His coupled oscillator theory deserves more attention in physics. All quantum behavior is a periodic oscillation.
Macksb
1 / 5 (1) Dec 18, 2012
Spin waves...same thing.