The ruthenium based molecular magnet [Ru2(D(3,5-Cl2Ph)F)4Cl(0.5H2O)·C6H14] (hereafter Ru2) behaves as a two-level system at sufficiently low temperatures. The authors performed spin detection by means of single-crystal measurements and obtained magnetic hysteresis loops around zero bias as a function of field sweeping rate. Compared to other molecular systems, Ru2 presents an enhanced irreversibility as shown by "valleys" of negative differential susceptibility in the hysteresis curves. Simulations based on phonon bottleneck model are in good qualitative agreement and suggest an abrupt spin reversal combined with insufficient thermal coupling between sample and cryostat phonon bath.Molecular magnets have been explored intensively in the recent years for their potential application in information technology and as test beds for quantum mechanics at macroscopic scale. These systems are tunable, identical, two-or multilevel systems that can be produced in large numbers and therefore are potential candidates for qubit implementation in quantum computing algorithms [1]. In crystalline form, the molecules are relatively well isolated from each other and thus the quantum physics of their spin is not strongly affected by the collective nature of the sample. Large spin molecular magnets have demonstrated key quantum effects, such as quantum tunneling of the magnetization through an anisotropy barrier [2,3] and Berry phase interference [4]. Low spin molecules have shown interesting effects when tuning phonon dissipation [5] that provide increased relaxation and decoherence times for the molecular qubits.In the case of low-spin molecular systems, the anisotropy barrier against spin reversal is often very small. However, magnetic hysteresis has been recently measured for a series of low-spin molecules (V 15 S=1/2, Fe 10 S=1, V 6 S=1/2) [5,6,7,8] and explained in the frame of the phonon-bottleneck (PB) model [6,9]. In this model, the phonons' low heat capacity keeps the lattice and the spins at the same temperature T s , slightly different from cryostat temperature T , as a result of inherent limited sample thermalization in the experimental setup. Cycling of an external magnetic field induces a delay (hysteresis) in the dynamics of the spin temperature which is observed by measuring the magnetic moment of a single crystal. Other theoretical interpretations treat * Electronic email: ichiorescu@fsu.edu the lattice at equilibrium with the cryostat at a temperature T = T s [10]. Though magnetic hysteresis can be simulated for a given bath temperature, these models cannot explain the observed dependence on sample thermalization [5] and the picture of phonons having very low heat capacity at low temperatures [9]. On the other hand, the PB model presented in Ref. [6] gives a good quantitative agreement with the experiments. The present study introduces a particular aspect of the phonon-bottleneck phenomena as observed in the Ru 2 paddle wheel molecular system (structure in Fig. 1). Single crystal measurements reveal enhanced magnet...

We present a magnetic study of the Gd 3 N@C 80 molecule, consisting of a Gd-trimer via a Nitrogen atom, encapsulated in a C 80 cage. This molecular system can be an efficient contrast agent for Magnetic Resonance Imaging (MRI) applications. We used a low-temperature technique able to detect small magnetic signals by placing the sample in the vicinity of an on-chip SQUID. The technique implemented at NHMFL has the particularity to operate in high magnetic fields of up to 7 T. The Gd 3 N@C 80 shows a paramagnetic behavior and we find a spin transition of the Gd 3 N structure at 1.2 K. We perform quantum mechanical simulations, which indicate that one of the Gd ions changes from a 8 S 7/2 state (L = 0, S = 7/2) to a 7 F 6 state (L = S = 3, J = 6), likely due to a charge transfer between the C 80 cage and the ion.

We report on multi-photon Rabi oscillations and controlled tuning of a multi-level system at room temperature (S = 5/2 for Mn 2+ :MgO) in and out of a quasi-harmonic level configuration. The anisotropy is much smaller than the Zeeman splittings, such as the six level scheme shows only a small deviation from an equidistant diagram. This allows us to tune the spin dynamics by either compensating the cubic anisotropy with a precise static field orientation, or by microwave field intensity. Using the rotating frame approximation, the experiments are very well explained by both an analytical model and a generalized numerical model. The calculated multi-photon Rabi frequencies are in excellent agreement with the experimental data.

PACS 75.45.+j -Macroscopic quantum phenomena in magnetic systems PACS 75.50.Xx -Molecular magnets PACS 75.10.Jm -Quantized spin modelsAbstract. -A characteristic of spin reversal in the presence of phonon-bottleneck is the deviation of the magnetization cycle from a reversible function into an opened hysterezis cycle. In recent experiments on molecular magnets (e.g. V15 and Ru2), the zero-field level repulsion was sufficiently large to ensure an otherwise adiabatic passage through zero-field and the magnetization curves can be described by using only a phonon-bottleneck model. Here, we generalize the phonon-bottleneck model into a model able to blend the non-adiabatic dynamics of spins with the presence of a nonequilibrium phonon bath. In this simple phenomenological model, Bloch equations are written in the eigenbasis of the effective spin Hamiltonian, considered to be a two-level system at low temperatures. The relaxation term is given by the phonon-bottleneck mechanism. To the expense of calculus time, the method can be generalized to multi-level systems, where the notion of Bloch sphere does not apply but the density matrix formalism is still applicable. Introduction. -Molecular magnets [1] or less complex systems containing diluted spins [2] have been explored intensively in the recent years for their potential application in information technology. In addition, despite their macroscopic dimensions, crystals containing magnetic molecules show measurable manifestations of fundamental quantum mechanical phenomena at large scale. This is due only to the fact that the magnetic molecules are relatively well separated from each other and their quantum properties are amplified by the large number contained in a tiny monocrystal. Consequently, they show remarkable phenomena, like Quantum Tunneling of the Magnetization (QTM) measured first [3] in spin S = 10 systems (Mn 12 , Fe 8 ) and Berry Phase quantum interference [4] (Fe 8 ). This large spin molecular magnets show significant anisotropy barriers protruded by tunneling channels which are generated by a transverse magnetic field or anisotropy terms. A sweeping longitudinal field (that is, parallel with the main spin quantification axis) will force the spin to sequentially visit the tunneling channels and obey the Landau-Zener tunneling mechanism each time. Consequently, distinct jumps are generated in the hys-