Propagation of Avalanches in ${\mathrm{Mn}}_{12}$-Acetate: Magnetic Deflagration

Local time-resolved measurements of fast reversal of the magnetization of single crystals of ${\mathrm{Mn}}_{12}$-acetate indicate that the magnetization avalanche spreads as a narrow interface that propagates through the crystal at a constant velocity that is roughly 2 orders of magnitude smaller than the speed of sound. We argue that this phenomenon is closely analogous to the propagation of a flame front (deflagration) through a flammable chemical substance.

Figure 1

(a) The potential energy function in a longitudinal magnetic field of 1.5 T. (b) Schematic diagram of a sample mounted on an array of Hall sensors used to detect $B_x$. Pronounced nonuniformity in magnetization during an avalanche generates large values of $B_x$.

Figure 2

Hysteresis loops for a single crystal of ${\mathrm{Mn}}_{12}$-ac (sample 1). $B_x$ was recorded by a Hall sensor situated about $80\mu \mathrm{m}$ above the middle of the sample. Steps due to quantum tunneling of the magnetization are observed, as well as a sharp spike in $B_x$ associated with an avalanche, shown in the inset.

Figure 3

Signals recorded by seven equally spaced Hall sensors situated near the center of sample 1 during an avalanche triggered at 4 T. The left (right) trace corresponds to the top (bottom) sensor (see Fig. 1). Sensor positions are measured relative to the top of the sample, with the center at $\approx 320\mu \mathrm{m}$. The inset shows sensor position versus the time at which the sensor recorded peak amplitude. A straight line fit yields a constant velocity of propagation of $12\mathrm{m}/\mathrm{s}$.

Figure 4

Velocity of propagation of avalanches versus the magnetic field at which the avalanches occurred (samples are identified by number). ZFC denotes data obtained for the sample cooled in the absence of a magnetic field. From top to bottom, the curves are fits obtained for $\Delta M/2M_{\mathrm{sat}}=1$, 0.75, and 0.5 (see text).

Figure 5

Velocity of propagation of avalanches versus energy released per molecule. The line is drawn to guide the eye.