Clusters of metal ions are a class of compounds actively investigated for their magnetic properties, which should gradually change from those of simple paramagnets to those of bulk magnets. However, their interest lies in a number of different disciplines: chemistry, which seeks new synthetic strategies to make larger and larger clusters in a controlled manner; physics, which can test the validity of quantum mechanical approaches at the nanometer scale; and biology, which can use them as models of biomineralization of magnetic particles.
A particularly sensitive heat capacity measuring device has allowed us to measure the tunneling process of Mn 12 O 12 -acetate single crystals (mass: 1 and 20 mg) from the irreversible tunneling process below the blocking temperature T B to the reversible resonant tunneling process above T B . Above the T B (typically 3.5 K) we find specific heat anomalies at the magnetic field values that correspond to the crossing of spin up and spin down levels of different magnetic quantum numbers. Below T B , heat relaxation pulses at the crossing of crystal field levels are observed for fields applied antiparallel to the initial magnetization. These measurements give a new scope to Mn 12 O 12 -acetate investigations and show the great interest of nanocalorimetry for studies of big magnetic molecules. [S0031-9007(97)03794-0] PACS numbers: 75.45. + j, 61.46. + w, 07.20.Fw Crystals of Mn 12 O 12 -acetate clusters [1] are molecular spin systems that exhibit spectacular effects [2]. Twelve manganese ions (4 Mn 31 and 8 Mn 41 ) are coupled by ferromagnetic exchange to a S 10 macrospin. The Mn 12 O 12 clusters are embedded in an organic matrix and show no exchange coupling from one cluster to another.The crystals are regular parallelepipeds with a strong magnetocrystalline anisotropy (ϳ60 K) along the longitudinal axis. Once oriented, they present at low temperatures and zero magnetic field a very long relaxation time of the magnetization (two months at 2 K [3]). Recently, quantum tunneling of molecular spins through the anisotropy, at magnetic field values that correspond to the crossing of spin up and spin down levels of different magnetic quantum numbers, has been demonstrated [4][5][6].The anisotropy lifts the 2S 1 1 degeneracy of the magnetic levels in zero field, creating a double well configuration [5] as sketched in Fig. 1. The manganese system is superparamagnetic and can be described by a Hamiltonian of the form [7]where D 0.6 K is the anisotropy energy per cluster, H the magnetic field applied parallel to the easy axis of magnetization, g Ӎ 2 is the gyromagnetic factor, and S is the spin per cluster. H l is a term that does not commute with S z and is due to the demagnetizing field, dipole coupling, higher anisotropy terms, and/or hyperfine splitting. A great effort is underway to understand how it comes about [7,8]. Thermal measurements as a function of a magnetic field enlighten the interplay between the spin system and the lattice of the crystal, e.g., clarifying how the phonons influence the tunneling of the macrospins, therefore, giving a great deal of new information.We have performed two kinds of measurements on Mn 12 O 12 monocrystals, the heat capacity as a function of an applied static magnetic field (C[H]) and the temperature as a function of a slowly scanned magnetic field ͑T ͓H͔͒. The measure of very small single crystals promises a high quality of the sample and avoids the broadening found with powder samples due to the slightly different characteristics of every crystal. On the other side, it implies a gr...
The solid-state molecular structure of I consists of a triangular array of iron(III) ions connected by a triply bridging methoxide and three /^-methoxide ligands. The oxygen donors of a monodentate methoxide and of a chelating dbm ligand complete the coordination sphere of each metal ion. The resulting mononegative Fe3(/i3-OCH3)(,M2-OCH3)3(OCH3)3(
The energy splitting of the low-lying levels has been investigated on two magnetic molecular clusters Fe 6 and Fe 10 by means of low-temperature zero-field specific-heat measurements. Significant deviations from the usual CϳT Ϫ2 law were observed above the maximum of the main Schottky anomalies as a result of nonnegligible contributions from the excited spin states with SϾ1 and the estimated lattice contributions follow a phenomenological power law C/RϳT ␣ with ␣ϳ2.7 for both these compounds. The singlet-triplet energy gaps evaluated by the Schottky anomaly, T 0 ϭ19.2 K for Fe 6 and 4.56 K for Fe 10 , are smaller than what we can estimate by a simplified spin-Hamiltonian approach in the strong exchange approximation and using the energy levels obtained by the high-field magnetization and susceptibility measurements. This discrepancy asks for a more complex description of the low-lying states of these molecular clusters, beyond the strong exchange approximation. At very low temperatures TӶ1 K, two low-energy Schottky anomalies were also observed in Fe 10 , probably due to a small fraction of defected rings or to hyperfine contributions.
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