Using a wire heater to ignite magnetic avalanches in fixed magnetic field applied along the easy axis of single crystals of the molecular magnet Mn 12 acetate, we report fast local measurements of the temperature and time-resolved measurements of the local magnetization as a function of magnetic field. In addition to confirming maxima in the velocity of propagation, we find that avalanches trigger at a threshold temperature which exhibits pronounced minima at resonant magnetic fields, demonstrating that thermally assisted quantum tunneling plays an important role in the ignition as well as the propagation of magnetic avalanches in molecular magnets.

The local time-dependent theory of Einstein-de Haas effect is developed. We begin with microscopic interactions and derive dynamical equations that couple elastic deformations with internal twists due to spins. The theory is applied to the description of the motion of a magnetic cantilever caused by the oscillation of the domain wall. Theoretical results are compared with a recent experiment on the Einstein-de Haas effect in a microcantilever.

Crystals of the molecular magnet Mn12-acetate are known to contain a small fraction of lowsymmetry (minor) species with a small anisotropy barrier against spin reversal. The lower barrier leads to faster magnetic relaxation and lower coercive field. We exploit the low coercive fields of the minor species to make a direct determination of the dipole field in Mn12-ac. We find that the dipolar field of a fully magnetized crystal is 51.5 ± 8.5 mT, consistent with theoretical expectations.

Using micron-sized thermometers and Hall bars, we report time resolved studies of the local temperature and local magnetization for two types of magnetic avalanches ͑abrupt spin reversals͒ in the molecular magnet Mn 12 acetate, corresponding to avalanches of the main slow-relaxing crystalline form and avalanches of the fastrelaxing minor species that exists in all as-grown crystals of this material. An experimental protocol is used that allows the study of each type of avalanche without triggering avalanches in the other, and of both types of avalanches simultaneously. In samples prepared magnetically to enable both types of avalanches, minor species avalanches are found to act as a catalyst for the major species avalanches.