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C’mon Baby Light My (Magnetic) Fire


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Living in Los Angeles for the last six years, I’ve become quite familiar with the spread of wildfires, with a corresponding deepening respect for Nature’s power. Given the devastation an out-of-control wildfire can cause, it’s not surprising that there’s been quite a bit of research into modeling the specifics of how forest fires spread over the last few decades, with an eye towards developing ever-more efficient methods for stopping the flames in their tracks. And now it seems as though the way that fires spread has something in common with the propagation of so-called “magnetic avalanches” that occur in magnetic crystals, according to a new paper published in Physical Review Letters this week.

The very word “avalanche” calls to mind snowy avalanches and sand piles, but materials get their properties, like magnetism, from atomic structure, putting this is the realm of quantum mechanics. We’re really talking about a cascade of “spin flips” that culminate in a reversal of the sample’s magnetization — a not-well-understood phenomenon called magnetic deflagration. “Spin” is the quantum mechanical version of angular momentum — electrons have spin, and hence angular momentum — except it’s never so simple at the atomic scale. I’ll let Chad Orzel of Uncertain Principles give you the “toddler” version (there’s a lot more detail and a fun toddler-centric video at that link):

“Electrons, and all other fundamental particles, have a property known as “spin.” This is an intrinsic angular momentum associated with the particles, as if they were little spinning balls of charge. … The spin angular momentum of an electron does have some strange properties, though, that are very unlike those of ordinary rotating objects. For one thing, it has only two possible states, “spin up” and “spin down.” … Electron spin is like a turntable with only forward and backward settings at a single speed, with the power cord wired directly into the mains so it can’t be shut off. It’s always spinning in one direction or the other.”

As Chad’s post makes clear, spin is really complicated (and important) but all you need to know for purposes of this post is that there are only two possible states (spin-up and spin-down) and it’s possible to flip those spins via, for example, a magnetic pulse. So back to that new paper: the scientists were interested in modeling these magnetic avalanches (uncontrolled cascades of spin-flips) because this can be a significant source of energy loss in things like electrical generators. They can also damage disk drives.

Credit: Pradeep Subedi et al. Source: Physical Review Letters.

The researchers — hailing from New York University, the University of Barcelona, City College of New York, and the University of Florida — used a molecular magnet (basically a collection of magnetic molecules) to test the hypothesis. They zapped one side of the sample with a magnetic pulses, and traced how that initial pulse spread throughout the sample material thanks to carefully placed magnetic sensors.

This enabled them to pinpoint the precise conditions that lead to magnetic avalanches. And they found that it’s actually more like a wildfire — a tiny, magnetic, spin-flipping wildfire spreading through a quantum mechanical forest.

Fire isn’t a substance, contrary to what the ancient Greeks believed; it’s a chemical reaction — oxidation, to be specific — and the three basic components needed are fuel, heat, and air (oxygen). Under the right circumstances, these basic ingredients ignite a sustained chemical chain reaction, and if that isn’t nipped in the bud, the fire spreads rapidly via conduction, convection and radiation. Take out one of those three ingredients, and you stop the spread. That’s the basis for firefighting strategies.

Photo: Federal Bureau of Land Management. Via How Stuff Works.

Exactly how wildfires spread is a complicated thing to model, but the seminal work on this was done back in 1972 by an aeronautical engineer named Richard Rothermel. His model still widely used, even though — like many mathematical models — it’s an idealized case study. It’s well suited if you’re talking a fire spreading through a uniform field of wheat, but less accurate for fires spreading through a landscape dotted with clumps of trees and shrubs, for example. But it’s quick and simple, and reasonably reliable, and hence useful in the field, where time is of the essence.

Using Rothermel’s model, it’s possible to determine just how much energy would be needed to transfer sufficient heat to ignite the fuel. (Every kind of fuel has an “ignition point” or “flash point”; the flash point for wood is 572 degrees Fahrenheit, or 300 C.) Once you know that, you can calculate the rate of ignition needed for the fire to spread rapidly, accounting for critical variables like wind speed and the slope of the ground.

What’s this got to do with flipping spins in a magnetic crystal? The magnetic pulse the scientists applied to their sample is the equivalent to a “spark” igniting a bit of fuel, kickstarting the chemical chain reaction that leads to a rampant wildfire. Or rather, when the spins flipped, they released energy and transmitted it to other nearby atoms in the crystal, which then flipped their spins, and so on, producing a runaway reaction — magnetic deflagration.

So magnetic avalanches are more like wildfires (or wildfires behave like magnetic avalanches), even though they seem like very different systems. Who knows what we could learn by mimicking combustion processes at the atomic level, both for materials and for battling forest fires?

References:

Bortolozzo, U. et al. (2007) “Light intensity distributions for spatiotemporal pulses generated in a ring cavity with a liquid crystal gain medium,” Physical Review Letters 99(2): 023901.

Rothermel, Richard C. (1972). “A mathematical model for predicting fire spread in wildland and fuels,” USDA Forest Service INT-115.

Scott, Joe H. and Burgan, Robert E. (2005). “Standard Fire Behavior Fuel Models: A Comprehensive Set for Use with Rothermel’s Surface Fire Spread Model,” USDA Forest Service General Technical Report RMRS-GTR-153.

Subedi, Pradeep et al. (2013) “Onset of a propagating self-sustained spin-reversal front in a magnetic system,” Physical Review Letters 110(20).

Jennifer Ouellette About the Author: Jennifer Ouellette is a science writer who loves to indulge her inner geek by finding quirky connections between physics, popular culture, and the world at large. Follow on Twitter @JenLucPiquant.

The views expressed are those of the author and are not necessarily those of Scientific American.





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