A. Caneschi

Nonadiabatic Landau-Zener tunneling in Fe8 molecular nanomagnets

The Landau-Zener method allows to measure very small tunnel splittings Δ in molecular clusters Fe8. The observed oscillations of Δ as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topological quantum interference of two tunnel paths of opposite windings. Studies of the temperature dependence of the Landau-Zener transition rate P gives access to the topological quantum interference between excited spin levels. The influence of nuclear spins is demonstrated by comparing P of the standard Fe8 sample with two isotopically substituted samples. The need of a generalised Landau-Zener transition rate theory is shown.

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.

Landau–Zener method to study quantum phase interference of Fe8 molecular nanomagnets (invited)

We present details about an experimental method based on the Landau–Zener model which allows one to measure very small tunnel splittings Δ in molecular clusters Fe8. The measurements are performed with an array of microsuperconducting quantum interference devices. The observed oscillations of Δ as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topological quantum interference of two tunnel paths of opposite windings. Transitions between M=−S and (S−n), with n even or odd, revealed a parity (symmetry) effect which is analogous to the suppression of tunneling predicted for half integer spins. This observation is the first direct evidence of the topological part of the quantum spin phase (Berry phase) in a magnetic system. The influence of intermolecular dipole interactions on the measured tunnel splittings Δ is shown.

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.

Intercalation of a nitronyl nitroxide radical into layered inorganic hosts.: Preparation and physico-chemical characterization

Abstract The synthesis of new intercalation compounds obtained introducing the organic radical 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H -imidazol-1-oxyl-3-oxide (NITpBAH) in layered inorganic hosts via topotactic anion exchange reactions or exchange of anionic ligands is presented. Three different inorganic hosts were employed, two LDH-phases (MgAlCO 3 and ZnAlCl) and the butanoxy derivative of λ-zirconium phosphates. In all cases the intercalation procedure was successfully accomplished. The percentage of exchanged anions obtained was 27, 45, 40% for MgAlLDH, ZnAlLDH and λ-ZrPO 4 [O(CH 2 ) 3 CH 3 ]DMSO, respectively. The magnetic properties of the intercalated compounds were measured with a SQUID magnetometer and compared to those of a polycrystalline powder of the pure radical. The intercalation process does not change the nature of magnetic intermolecular interactions which remain antiferromagnetic but reduces their magnitude.

Quantum resonance of the magnetisation in a single-crystal of Mn12-ac

Abstract In this study we raise a few questions regarding the hypothesis of conservation of the collective spin in molecular crystals, some of them of interest for a deeper understanding of resonant tunnelling in Mn 12 -ac. We also show experimentally that in this system, resonant tunnelling is not solely thermally activated, but takes place from the ground state m = 10 below 2 K. The effects of a quartic anisotropy term on magnetization curves and resonance shifts are discussed.

Specific heat and μ+SR measurements in Gd(hfac)3NITiPr molecular magnetic chains: Indications for a chiral phase without long-range helical order

Low-temperature specific heat C(T) and zero-field muon spin resonance (μ + SR) measurements were performed in Gd(hfac) 3 NITiPr, a quasi-one-dimensional molecular magnet with competing nearest-neighbor and next-nearest-neighbor intrachain exchange interactions. The specific heat data exhibit a λ peak at T 0 =2.08 ′0.01 K that disappears upon the application of a 5 T magnetic field. Conversely, the μ + SR data do not present any anomaly at T2 K, proving the lack of divergence of the two-spin correlation function as required for usual three-dimensional long-range helical order. Moreover, no muon spin precession can be evinced from the μ + SR asimmetry curves, thus excluding the presence of a long-range-ordered magnetic lattice. These results provide indications for a low-T phase where chiral order is established in absence of long-range helical order.

1H nuclear magnetic resonance and spin dynamics in the tetranuclear iron(III) cluster {Fe4}

The spin dynamics of a cluster of four iron(III) ions characterized by a total spin ground state ST=5 and Ising anisotropy have been investigated by the 1H nuclear magnetic resonance (NMR) linewidth and 1H NMR spin-lattice relaxation rate 1/T1 as a function of the temperature (0.5–295 K) and external magnetic field (0.3–7.2 T). At very low T (0.5 K) the spectrum becomes very broad indicating freezing of the Fe3+ moments in a superparamagnetic state. The temperature dependence of T1−1 and T2−1 shows strong field dependent enhancement below 50 K which must be related to the spin dynamics of the Fe3+ moment freezing process, although no quantitative theory of this is currently available.

Evidence for a helical and a chiral phase transition in the Gd(hfac)3NITiPr magnetic specific heat

New specific heat data taken at very low temperatures (0.03<T<0.18K) in quasi one-dimensional helimagnet Gd(hfac)3NITiPrGd(hfac)3NITiPr show a clear λλ anomaly at TN=0.039K signaling the onset of the 3D helimagnetic phase. They match fairly well with previously reported data which showed the onset of the chiral phase transition at T0=2.08K. Also new magnetic susceptibility data taken in the neighborhood at T0T0 are repeated.

Evidence for a helical and a chiral phase transition in the Gd(hfac)3NITiPrGd(hfac)3NITiPr magnetic specific heat

New specific heat data taken at very low temperatures (0.03<T<0.18K) in quasi one-dimensional helimagnet Gd(hfac)3NITiPrGd(hfac)3NITiPr show a clear λλ anomaly at TN=0.039K signaling the onset of the 3D helimagnetic phase. They match fairly well with previously reported data which showed the onset of the chiral phase transition at T0=2.08K. Also new magnetic susceptibility data taken in the neighborhood at T0T0 are repeated.