Magnetic Burning

A new experiment suggests that the fast flipping of the magnetic orientation of some molecules in a solid sample resembles the propagation of a flame front through a material being burned, and that the "magnetic burning" process can be used to study flammable substances without actually having flames present. In a chemical fire---say, the burning of the pages of a book---the flame front marks a dividing point: ahead of the front is intact unburned material, while behind the front is ash, the state of material that has been oxidized in the combustion process. Now, consider the magnetic equivalent as studied by a collaboration of scientists from CUNY-City College (Myriam Sarachik, sarachik@sci.ccny.cuny.edu, Yoko Suzuki, yoko@sci.ccny.cuny.edu), CUNY-Lehman College (Eugene Chudnovsky (eugene.chudnovsky@lehman.cuny.edu), the Weizmann Institute, and the University of Florida. A crystal of manganese12-acetate (Mn12-ac) molecules, each with a net spin of 10 units, is quite susceptible to magnetic influence. Turning on a strong external magnetic field opposed to the prevailing magnetic orientation of the crystal can cause a sudden reversal of spins of the molecules. The reversal propagates along a front through the crystal (which can be thought of as a stack of nanomagnets) just as a flame moves through a solid in the case of a conventional combustion. In the magnetic case, much heat will be generated as the spins get flipped (the heat energy being equal to the difference in energy of the before and after spin states), but there will be no destructive burning (see movie at http://www.sci.ccny.cuny.edu/~sarachik/MagBurn.mpeg ). The "ash" consists of the molecules in their new spin state. In summary, magnetic burning in molecular magnets has several of the qualities of regular burning (a flame front and combustion) but not the destructiveness. Myriam Sarachik says that magnetic burning might offer a more controlled way of learning how to control and channel flame propagation. (Suzuki et al., Physical Review Letters, upcoming article; http://www.sci.ccny.cuny.edu/~sarachik/)