The longitudinal magnetic susceptibility of single crystals of the molecular magnet Mn 12 -acetate obeys a Curie-Weiss law, indicating a transition to a ferromagnetic phase at ϳ0.9 K. With increasing magnetic Þeld applied transverse to the easy axis, a marked change is observed in the temperature dependence of the susceptibility, and the suppression of ferromagnetism is considerably more rapid than predicted by mean-Þeld theory for an ordered single crystal. Our results can instead be Þt by a Hamiltonian for a random-Þeld Ising ferromagnet in a transverse magnetic Þeld, where the randomness derives from the intrinsic distribution of locally tilted magnetic easy axes known to exist in Mn 12 -acetate crystals, suggesting that Mn 12 -acetate is a realization of the random-Þeld Ising model in which the random Þeld may be tuned by a Þeld applied transverse to the easy axis.
The energy released in a magnetic material by reversing spins as they relax toward equilibrium can lead to a dynamical instability that ignites self-sustained rapid relaxation along a deflagration front that propagates at a constant subsonic speed. Using a trigger heat pulse and transverse and longitudinal magnetic fields, we investigate and control the crossover between thermally driven magnetic relaxation and magnetic deflagration in single crystals of Mn(12)-acetate.
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.Comment: 8 pages, 5 figure
We discuss the electrolysis mechanism of colloidal ZnO NPs (10 nm diam.) in CH 3 CN. Stripping the preconcentrated Zn(Hg) allows quantification of the ZnO electrolyzed during stochastic interactions with the Hg surface. We model the mass transport taking the charged agglomerates of ZnO NPs as ionic species to calculate their migration and diffusional contributions. In unsupported suspensions, the mobility and positive zeta potential enhance transport towards the Hg UME. The NP electrolysis generates ionic species, increasing the migration rate and allowing lower detection limits compared to weakly supported suspensions, where the electrolyte modifies agglomerate charge and colloidal properties. We determine the kinetic constant (k f, in cm/s) for the ZnO reduction from the electrolysis transient model for destructive collisions of single entities, corrected for the potentiostat time constant. While most reduction events happen within 100 ms, the single entity model is consistent with mass transport studies over longer experimental times (1800 s).
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