A. Rettori

Magnetic Relaxation in Big Magnetic Molecules

The relaxation time of the magnetization of a big magnetic molecule is evaluated. Isotropic exchange is assumed to be dominant, resulting in a fixed spin modulus s (equal to 20 in the case of interest). A perturbation of the form ASz2 results from spin-orbit coupling and produces an energy barrier As2 which separates positive and negative values of Sz. Spin-phonon interactions are treated by second-order perturbation theory. An upper limit of the pre-exponential factor in the Arrhenius law is obtained, which is consistent with the very large experimental value.

Finite-size effects in single chain magnets: an experimental and theoretical study.

The problem of finite-size effects in s=1/2 Ising systems showing slow dynamics of the magnetization is investigated introducing diamagnetic impurities in a Co2+-radical chain. The static magnetic properties have been measured and analyzed considering the peculiarities induced by the ferrimagnetic character of the compound. The dynamic susceptibility shows that an Arrhenius law is observed with the same energy barrier for the pure and the doped compounds while the prefactor decreases, as theoretically predicted. Multiple spin reversal has also been investigated.

Glauber slow dynamics of the magnetization in a molecular Ising chain

The slow dynamics (10−6 s–104 s) of the magnetization in the paramagnetic phase, predicted by Glauber for the 1d Ising model, has been observed with ac susceptibility and SQUID magnetometry measurements in a molecular chain comprising alternating Co2+ spins and organic radical spins strongly antiferromagnetically coupled to give a 1d ferrimagnet. An Arrhenius behavior with activation energy Δ = 152 K has been observed for ten decades of relaxation time and found to be consistent with the Glauber model. We have extended this model to take into account the ferrimagnetic nature of the chain as well as its helicoidal crystal structure.

The classical and quantum dynamics of molecular spins on graphene

Controlling the dynamics of spins on surfaces is pivotal to the design of spintronic1 and quantum computing2 devices. Proposed schemes involve the interaction of spins with graphene to enable surface-state spintronics3,4, and electrical spin-manipulation4-11. However, the influence of the graphene environment on the spin systems has yet to be unraveled12. Here we explore the spin-graphene interaction by studying the classical and quantum dynamics of molecular magnets13 on graphene. While the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated. The couplings to graphene phonons, to other spins, and to Dirac fermions are quantified using a newly-developed model. Coupling to Dirac electrons introduces a dominant quantum-relaxation channel that, by driving the spins over Villain’s threshold, gives rise to fully-coherent, resonant spin tunneling. Our findings provide fundamental insight into the interaction between spins and graphene, establishing the basis for electrical spin-manipulation in graphene nanodevices.

Fast switching of bistable magnetic nanowires through collective spin reversal

The use of magnetic nanowires as memory units is made possible by the exponential divergence of the characteristic time for magnetization reversal at low temperature, but the slow relaxation makes the manipulation of the frozen magnetic states difficult. We suggest that finite-size segments can show a fast switching if collective reversal of the spins is taken into account. This mechanism gives rise at low temperatures to a scaling law for the dynamic susceptibility that has been experimentally observed for the dilute molecular chain Co(hfac)2NitPhOMe. These results suggest a possible way of engineering nanowires for fast switching of the magnetization.

Finite-size effects on the dynamic susceptibility of CoPhOMe single-chain molecular magnets in presence of a static magnetic field

The static and dynamic properties of the single-chain molecular magnet [Co(hfac)2NITPhOMe] are investigated in the framework of the Ising model with Glauber dynamics, in order to take into account both the effect of an applied magnetic field and a finite size of the chains. For static fields of moderate intensity and short chain lengths, the approximation of a mono-exponential decay of the magnetization fluctuations is found to be valid at low temperatures; for strong fields and long chains, a multi-exponential decay should rather be assumed. The effect of an oscillating magnetic field, with intensity much smaller than that of the static one, is included in the theory in order to obtain the dynamic susceptibility �(!). We find that, for an open chain with N spins, �(!) can be written as a weighted sum of N frequency contributions, with a sum rule relating the frequency weights to the static susceptibility of the chain. Very good agreement is found between the theoretical dynamic susceptibility and the ac susceptibility measured in moderate static fields (Hdc ≤ 2 kOe), where the approximation of a single dominating frequency turns out to be valid. For static fields in this range, new data for the relaxation time, � versus Hdc, of the magnetization of CoPhOMe at low temperature are also well reproduced by theory, provided that finite-size effects are included.

Dynamic control of magnetic nanowires by light-induced domain-wall kickoffs

Controlling the speed at which systems evolve is a challenge shared by all disciplines, and otherwise unrelated areas use common theoretical frameworks towards this goal. A particularly widespread model is Glauber dynamics, which describes the time evolution of the Ising model and can be applied to any binary system. Here we show, using molecular nanowires under irradiation, that Glauber dynamics can be controlled by a novel domain-wall kickoff mechanism. In contrast to known processes, the kickoff has unambiguous fingerprints, slowing down the spin-flip attempt rate by several orders of magnitude, and following a scaling law. The required irradiance is very low, a substantial improvement over present methods of magneto-optical switching. These results provide a new way to control and study stochastic dynamic processes. Being general for Glauber dynamics, they can be extended to different kinds of magnetic nanowires and to numerous fields, ranging from social evolution to neural networks and chemical reactivity.

Dipolar interaction and incoherent quantum tunneling: a Monte Carlo study of magnetic relaxation

Abstract:We study the magnetic relaxation of a system of localized spins interacting through weak dipole interactions, at a temperature large with respect to the ordering temperature but low with respect to the crystal field level splitting. The relaxation results from quantum spin tunneling but is only allowed on sites where the dipole field is very small. At low times, the magnetization decrease is proportional to

Gd(hfac)3NITEt molecular magnetic chain

\sqrt t