Experimental upper bound on superradiance emission fromMn12acetate

Abstract

We used a Josephson junction as a radiation detector to look for evidence of the emission of electromagnetic radiation during magnetization avalanches in a crystal assembly of Mn12-Acetate. The crystal assembly exhibits avalanches at several magnetic fields in the temperature range from 1.8 to 2.6 K with durations of the order of 1 ms. Although a recent study shows evidence of electromagnetic radiation bursts during these avalanches [J. Tejada, et al., Appl. Phys. Lett. 84, 2373], we were unable to detect any significant radiation at welldefined frequencies. A control experiment with external radiation pulses allows us to determine that the energy released as radiation during an avalanche is less than 1 part in 10 4 of the total energy released. In addition, our avalanche data indicates that the magnetization reversal process does not occur uniformly throughout the sample. PACS numbers: 75.50.Xx, 07.57.Hm, 75.45.+j, 61.46.+w Single-molecule magnets have been subject to intense experimental and theoretical investigations during the past decade. At low temperatures, they are bistable and exhibit hysteresis similar to classical magnets. 1,2 In addition, they exhibit fascinating quantum mechanical properties such as tunneling between "up" and "down" orientations, 3,4,5 and interference between tunneling paths. 6 Furthermore, their potential use in quantum computation has been proposed, 7 and experiments with millimeter-wave radiation have been carried out showing that the relaxation rate for magnetization reversal, 8,9 and the energy-level populations can be controlled. 10,11,12 Chudnovsky and Garanin, 13 have proposed theoretically that single-molecule magnets could be used to generate superradiance. In a recent experiment, Tejada et al. reported that during magnetization avalanches of the molecular magnet Mn 12 acetate millimeter-wave radiation was released. 14 They interpret the results as evidence of superradiance in the system, in which coherent radiation is produced at frequencies corresponding to transitions between spin states of the molecule. The bolometer used in that study, however, was not sensitive to radiation frequency and so no information about the spectrum of the radiation was available. Here we report on a study of the fast dynamics of magnetization reversal during avalanches in a crystal assembly of Mn 12 acetate. We used a Josephson junction as a radiation detector, exploiting the AC Josephson effect to create a frequency-sensitive detector. We were unable to detect any radiation at the predicted superradiance frequencies during magnetization avalanches. Using external radiation, we were able to set limits of the power and duration of radiation that might be emitted during an avalanche.Magnetization avalanches in Mn 12 were found at low temperatures 15 even before resonant magnetization tunneling was discovered in this material. 3 In brief, the avalanches are thermal runaway events in which the heat released by the reversal of a metastable spin in an external field induces other spins to flip...

Magnetic deflagration inGd5Ge4

“…This avalanche-like process, that occurs when the field reaches a certain value, has striking similarity with avalanches observed in the magnetization curves of molecular magnets [15][16][17][18] and of some manganites. [19][20][21] The latter avalanches have been investigated and successfully described [22][23][24][25][26][27][28][29] within theory of magnetic deflagration.…”

Intrinsic avalanches and collective phenomena in a Mn(II)-free radical ferrimagnetic chain

“…Another important question concerns the propagation of the avalanche and thus the associated time scale required for a macroscopic magnetization reversal to develop. For example, recent avalanches studies in Mn 12 ac have shown that the magnetization reversal is not uniform inside the sample 9 and that the avalanches propagate at a constant velocity, requiring a threshold energy 10 . In most cases, the propagation of the avalanche can be attributed to thermally assisted phenomena.…”

Resonant Tunneling in Truly Axial SymmetryMn12Single-Molecule Magnets: Sharp Crossover between Thermally Assisted and Pure Quantum Tunneling

“…It was also the first system to show thermally assisted tunneling of magnetization [2,3,4]. During the last several years, many more SMMs have been discovered and they are now among the most promising candidates for observing the limits between classical and quantum physics since they have a well defined structure, spin ground state and magnetic anisotropy [5,6,7,9].Nevertheless, Mn 12 -Ac is still the most widely studied SMM [10,11,12,13,14,15,16,17,18,19,20,21,22]. While a rough understanding of the quantum phenomena in Mn 12 -Ac was clear from the early studies, a detailed understanding has not yet emerged.…”

“…Nevertheless, Mn 12 -Ac is still the most widely studied SMM [10,11,12,13,14,15,16,17,18,19,20,21,22]. While a rough understanding of the quantum phenomena in Mn 12 -Ac was clear from the early studies, a detailed understanding has not yet emerged.…”