Propagation of Avalanches inMn12-Acetate: Magnetic Deflagration

Abstract

Local time-resolved measurements of fast reversal of the magnetization of single crystals of Mn12-acetate indicate that the magnetization avalanche spreads as a narrow interface that propagates through the crystal at a constant velocity that is roughly two 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. 1 . Arranged in a centered tetragonal lattice, the spin of the Mn 12 clusters is subject to strong magnetic anisotropy along the symmetry axis (the c-axis of the crystal). Below the blocking temperature of ≈ 3.5 K, the crystal exhibits remarkable staircase magnetic hysteresis due to resonant quantum spin tunneling between energy levels on opposite sides of the anisotropy barrier corresponding to different spin projections, as illustrated in Fig. 1(a).2 This and other interesting properties of Mn 12 -ac have been intensively studied in the last decade (see Refs. 3,4,5,6 for reviews).It has been known for some time 7 that Mn 12 -ac crystals exhibit an abrupt reversal of their magnetic moment under certain conditions. This phenomenon, also observed in other molecular magnets, has been attributed to a thermal runaway (avalanche) in which the initial relaxation of the magnetization toward the direction of the field results in the release of heat that further accelerates the magnetic relaxation. Direct measurements of the heat emitted by Mn 12 -ac crystals, 8 as well as measurements of the magnetization reversal in pulsed magnetic fields, 9 have confirmed the thermal nature of the avalanches. More recently, the electromagnetic signal associated with avalanches was detected 10,11 and it was argued that if the radiation is of thermal origin it would indicate a significant increase in the temperature of the crystal. This has not been confirmed by direct bulk measurements of the temperature using a thermometer. Evidence has been obtained 12 that the avalanche may not be a uniform process throughout the sample. No clear understanding of the avalanche process has emerged to date.In this Letter we report local time-resolved measurements of fast magnetization reversal (avalanches) in mmsize single crystals of Mn 12 -ac. We show that a magnetic avalanche takes the form of a thin interface between regions of opposite magnetization which propHall Sensors (a) (b)Spin-up

Quantum Magnetic Deflagration inMn12Acetate

“…In the experiment of Suzuki et al [5] avalanches were ignited in a stochastic way on sweeping magnetic field between ÿ5 T and 5 T. In such an experiment one cannot control the ignition process. Consequently, the probability that the avalanche occurs at a tunneling resonance is low, which may explain why no oscillation of v on H due to resonant spin tunneling has been observed.…”

“…In the avalanche, the initial relaxation of the magnetization towards the direction of the field results in the release of heat that further accelerates the magnetic relaxation. Recent local magnetic measurements of Mn 12 crystals [5] have demonstrated that during an avalanche the magnetization reversal occurs inside a narrow interface that propagates through a crystal at a constant speed of a few meters per second. It has been argued that this process is analogous to the propagation of a flame front (deflagration) through a flammable chemical substance.…”

“…It has been argued that this process is analogous to the propagation of a flame front (deflagration) through a flammable chemical substance. The conventional theory of deflagration, in the first approximation, yields the following expression for the velocity of the flame front [5][6][7]:…”

Enhanced Spin Tunneling in a Molecular Magnet Mixed with a Superconductor

“…It represents an unusually large jump from the positive to negative magnetization in a narrow field interval from 20 Oe to −20 Oe. Such jumps have only been seen in molecular magnets under the conditions of magnetic deflagration -propagation of the front of combustion of the Zeeman energy which is similar to the combustion of a chemical substance 8,9 . However, in a zero field the Zeeman energy is zero and deflagration is not possible.…”

“…They provide an ultimate limit of the miniaturization of magnetic memory and are promising candidates for qubits -elements of quantum computers 2 . Quantum effects observed in molecular magnets include quantum tunneling of the magnetic moment [3][4][5][6] , topological Berry phase 7 , quantum magnetic deflagration 8,9 , and Rabi oscillations 10,11 . Most recently it was demonstrated that measurement of the electric current through a magnetic molecule permits readout of quantum states of an individual atomic nucleus 12,13 .…”