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.

We show that the magnetic moment of a nanoparticle embedded in the surface of a solid can be switched by surface acoustic waves (SAW) in the GHz frequency range via a universal mechanism that does not depend on the structure of the particle and the structure of the substrate. It is based upon generation of the effective ac magnetic field in the coordinate frame of the nanoparticle by the shear deformation of the surface due to SAW. The magnetization reversal occurs via a consecutive absorption of surface phonons of the controlled variable frequency. We derive analytical equations governing this process and solve them numerically for the practical range of parameters. PACS numbers: 75.60.Jk; 72.55.+s; Switching of the magnetic moment by means other than applying the magnetic field has been one of the paradigms of modern magnetism. Studies of the spin transfer torque have led to the commercialization of random access memory (STT-RAM) devices [1]. Recent years have been also marked by intensive research on manipulating magnetic moments by electric fields in multiferroic and composite materials [2]. As far as the speed is concerned, a direct 180-degree switching of the magnetization by the electric field would be the most desirable for applications. This approach, however, requires significant ingenuity because linear coupling of the magnetic moment to the electric field is prohibited by symmetry.In this letter, we propose switching of the magnetic moment of a nanoparticle, embedded in a solid surface, by mechanical oscillations due to surface acoustic waves (SAW). Magnetization dynamics due to magnetostriction induced by SAW in a ferromagnetic layer has been studied before [3][4][5][6][7]. The spin-rotation mechanism proposed here is based upon observation that a nanoparticle subjected to SAW undergoes rotational oscillations with the angular velocity Ω = 1 2 ∇ ×u, where u is the local displacement field, see Fig. 1. In the rotating coordinate frame of the nanoparticle its magnetic moment M feels the effective ac magnetic field, h ac = Ω/γ, with γ being the gyromagnetic ratio. This provides the linear coupling between M and Ω. We believe that this effect has been been partially responsible for the spin dynamics generated by SAW in manganites [8] and molecular magnets [9].The spin-rotation coupling responsible for the above mechanism is a manifestation of the Einstein -de Haas effect at the nanoscale [10,11]. It corresponds to the transfer of the angular momentum between spin and mechanical degrees freedom. In that sense it relies on the spin-orbit interaction that manifests itself in the crystal field acting on the magnetic moment. Interaction of SAW with the magnetic moment is provided by the rotation of  S FIG. 1: Color online: Schematic representation of the spinrotation coupling provided by SAW. A magnetic nanoparticle with a total spin S is adhered to a solid surface. Elastic displacements due to SAW (shown by small black arrows of decreasing length as one goes down away from the surface) generate fast rotatio...

We study spin-rotation effects in a magnetic molecule bridged between two conducting leads. Dynamics of the total angular momentum couples spin tunneling to the mechanical rotations. Landau-Zener spin transition produced by the time-dependent magnetic field generates a unique pattern of mechanical oscillations that can be detected by measuring the electronic tunneling current through the molecule.

We study the quantum dynamics of a system consisting of a magnetic molecule placed on a microcantilever. The amplitude and frequencies of the coupled magneto-mechanical oscillations are computed. Parameter-free theory shows that the existing experimental techniques permit observation of the driven coupled oscillations of the spin and the cantilever, as well as of the splitting of the mechanical modes of the cantilever caused by spin tunneling. PACS numbers: 85.85.+j, 75.50.Xx, 75.45.+j, 75.80.+q Magnetic molecules exhibit quantum tunneling between different orientations of the spin in macroscopic magnetization measurements [1,2]. Detection of coherent quantum spin oscillations, similar to those observed in a SQUID [3], would be of great interest. In a crystal of magnetic molecules this effect is difficult to observe because of the inhomogeneous broadening of spin levels and decoherence arising from various interactions. Magnetic measurements of individual molecules would have been more promising but insufficient sensitivity of existing magnetometers has prohibited such studies so far. The effort has been made to observe spin tunneling effects in the electron transport through a single magnetic molecule bridged between metallic electrodes [4].In this Letter we propose a different approach to the detection of quantum oscillations of the spin of a single magnetic molecule. It is based upon the resonant coupling of the spin oscillations to the mechanical modes of a microcantilever. The geometry of the proposed experiment is shown in Fig. 1. A magnetic molecule of spin S is deposited on a microcantilever of length L, Fig. 1. The y = 0 end of the cantilever is fixed while the y = L end is free. The molecule is assumed to be imbedded in or firmly attached to the cantilever at y = y 0 , with the magnetic anisotropy axis being parallel the X-direction. Let ∆ be the tunnel splitting of the two lowest energy states of the molecule,where | ± S denote two opposite spin orientations along the X-axis. A weak ac magnetic field of frequency ω = ∆/ , applied along the X-axis, will force the spin of the molecule to oscillate between the two orientations. Conservation of the total angular momentum requires that the oscillations of the spin are accompanied by the mechanical oscillations of the cantilever (Einstein -de Haas effect [5,6]). Consequently, if ω = ∆/ coincides with a resonant mode of the cantilever, one should expect the effect of the ac field on the cantilever. If the cantilever rotates by a small angle δφ at the location of the spin, the Hamiltonian of the moleculeĤ S becomes [7,8] Due to strong magnetic anisotropy the molecule can be considered as a two-state system. The Hamiltonian of such a system, H 2 , is a projection of the Hamiltonian (2) onto the two states given by Eq. (1). These states can be viewed as the eigenstates of the Pauli matrix σ z . For the geometry shown in Fig. 1 the projection can be performed by writingwithThis givesThe states with a definite X-projection of the spin are eigenstates of σ x : ...

Combination of the thermal effet in magnetic deflagration with resonance spin tunneling controlled by the dipole-dipole interaction in molecular magnets leads to the increase of the deflagration speed in the dipolar window near tunneling resonances.