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Register a new userMultilevel model for magnetic deflagration in nanomagnet crystals
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We extend the existing theoretical model for determining the characteristic features of magnetic deflagration in nanomagnet crystals. For the first time, all energy levels are accounted for calculation of the the Zeeman energy, the deflagration velocity, and other parameters. It reduces the final temperature and significantly changes the propagation velocity of the spin-flipping front. We also consider the effect of a strong transverse magnetic field, and show that the latter significantly modifies the spin-state structure, leading to an uncertainty concerning the activation energy of the spin flipping. Our front velocity prediction for a crystal of Mn$_{12}$-acetate in a longitudinal magnetic field is in much better agreement with experimental data than the previous reduced-model results.
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Experimental evidence of the anisotropy of the magnetic deflagration associated with the low-temperature first order antiferromagnetic (AFM) --> ferromagnetic (FM) phase-transition in single crystals of Gd5Ge4 is reported. The deflagrations have been induced by controlled pulses of surface acoustic waves (SAW) allowing us to explore both the magnetic field and temperature dependencies on the characteristic times of the phenomenon. The study was done using samples with different geometries and configurations between the SAW pulses and the direction of the applied magnetic field with respect to the three main crystallographic directions of the samples. The effect of temperature is nearly negligible, whereas observed strong magnetic field dependence correlates with the magnetic anisotropy of the sample. Finally, the role of the SAW pulses in both the ignition and formation of the deflagration front was also studied, and we show that the thermal diffusivity of Gd5Ge4 must be anisotropic, following kappaa>kappab>kappac.
The reversal of spins in a magnetic material as they relax toward equilibrium is accompanied by the release of Zeeman energy which can lead to accelerated spin relaxation and the formation of a well-defined self-sustained propagating spin-reversal front known as magnetic deflagration. To date, studies of Mn$_{12}$-acetate single crystals have focused mainly on deflagration in large longitudinal magnetic fields and found a fully spin-reversed final state. We report a systematic study of the effect of transverse magnetic field on magnetic deflagration and demonstrate that in small longitudinal fields the final state consists of only partially reversed spins. Further, we measured the front speed as a function of applied magnetic field. The theory of magnetic deflagration, together with a modification that takes into account the partial spin reversal, fits the transverse field dependence of the front speed but not its dependence on longitudinal field. The most significant result of this study is the finding of a partially spin-reversed final state, which is evidence that the spins at the deflagration front are also only partially reversed.
For the first time, the morphology and dynamics of spin avalanches in Mn12-Acetate crystals using magneto-optical imaging has been explored. We observe an inhomogeneous relaxation of the magnetization, the spins reversing first at one edge of the crystal and a few milliseconds later at the other end. Our data fit well with the theory of magnetic deflagration, demonstrating that very slow deflagration rates can be obtained, which makes new types of experiments possible.
The discovery of magnetic bistability in Mn$_{12}$ more than 20 years ago marked the birth of molecular magnetism, an extremely fertile interdisciplinary field and a powerful route to create tailored magnetic nanostructures. However, the difficulty to determine interactions in complex polycentric molecules often prevents their understanding. Mn$_{12}$ is an outstanding example of this difficulty: although it is the forefather and most studied of all molecular nanomagnets, an unambiguous determination of even the leading magnetic exchange interactions is still lacking. Here we exploit four-dimensional inelastic neutron scattering to portray how individual spins fluctuate around the magnetic ground state, thus fixing the exchange couplings of Mn$_{12}$ for the first time. Our results demonstrate the power of four-dimensional inelastic neutron scattering as an unrivaled tool to characterize magnetic clusters.
The magnetic instability at the front of the spin avalanche in a crystal of molecular magnets is considered. This phenomenon reveals similar features with the Darrieus-Landau instability, inherent to classical combustion flame fronts. The instability growth rate and the cut-off wavelength are investigated with respect to the strength of the external magnetic field, both analytically in the limit of an infinitely thin front and numerically for finite-width fronts. The presence of quantum tunneling resonances is shown to increase the growth rate significantly, which may lead to a possible transition from deflagration to detonation regimes. Different orientations of the crystal easy axis are shown to exhibit opposite stability properties. In addition, we suggest experimental conditions that could evidence the instability and its influence on the magnetic deflagration velocity.
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