Zero-field-cooled magnetization, magnetic relaxation, and a.c. susceptibility measurements have been performed on a Mn12-Ac oriented powdered sample as a function of temperature, field, and orientation. Magnetization jumps and susceptibility peaks have been observed about magnetic-field values Bn = nB1, where B1 ≈ 5 kG. These anomalies are due to the existence of relaxation rate maxima near Bn. From these experimental results we infer the existence of resonant spin tunnelling between degenerate excited levels of opposite spin orientation.

X-ray magnetic circular dichroism ͑XMCD͒ measured at T = 6 K and 0 H = 5 T on the ␣-phase Fephthalocyanine ͑FePc͒ textured thin films shows that the Fe 2+ ions present an unusually large, highly unquenched m L = 0.53Ϯ 0.04 B orbital component, with planar anisotropy. The spin m S = 0.64Ϯ 0.05 B and the intra-atomic magnetic dipolar m T components were also obtained. The m L / m S = 0.83 ratio is the largest measured in 3d complexes and compounds. The origin of this unusually high orbital moment is the incompletely filled e g level lying close to the Fermi energy. This explains the unusually large and positive hyperfine field detected by Mössbauer spectroscopy in FePc. The FePc film strong planar anisotropy inferred from XMCD experiments is fully confirmed by magnetization measurements.

We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.4 nm (25 to 7500 atoms) prepared by sequential sputtering of Co and Al2O3. The particle size distribution has been determined from the equilibrium susceptibility and magnetization data and it is compared to previous structural characterizations. The distribution of activation energies was independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant K ef f . We find that K ef f is enhanced with respect to the bulk value and that it is dominated by a strong anisotropy induced at the surface of the clusters. Interactions between the magnetic moments of adjacent layers are shown to increase the effective activation energy barrier for the reversal of the magnetic moments. Finally, this reversal is shown to proceed classically down to the lowest temperature investigated (1.8 K).

We present a theory of resonant quantum tunneling of large spins through thermally activated states. It gives numerical results that are in good agreement with data from recent magnetic quantum tunneling experiments in Mn acetate. We show that neither dipolar fields nor crystal-field perturbations, acting separately, can account for the resonances observed. However, we find that these two perturbations acting jointly produce a highly nonlinear effect that enhances tunneling rates up to their observed values. Resonant tunneling through lowlying energy-state pairs is blocked. We show that the tunneling frequency T and the lifetime 0 of the thermally populated pairs of states through which tunneling proceeds fulfils T 0 ӷ1. A superposition of these two states becomes incoherent approximately in time 1/ T . Using the master equation that follows, and spin-phonon-induced transition rates that we calculate, we obtain relaxation rates and magnetization hysteresis curves that agree reasonably well with experiment.

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The magnetic ac susceptibility of oriented Mn 12 Ac crystallites has been measured as a function of temperature, field, and frequency. The field has been applied at different values of the angle with respect to the sample easy axis. For Tϭ5 K, the isothermal and adiabatic limits have been determined as a function of field. For ϭ0°and intermediate frequencies, Lorentzian-shaped peaks have been observed at magnetic field values H n ϭnH 1 with nϭ0, 1, and 2 where H 1 ϭ4.1 kOe. As increases these maxima shift to higher fields, that satisfy H n cosϭconst, and decrease in amplitude. The relaxation time 1 follows Arrhenius' law with respect to temperature and decreases sharply at HϭH n . The observed phenomenology unambiguously proves the existence of field-tuned tunneling between excited magnetic states which are thermally populated. At 5 K, the effective activation energy and the spin states involved in the tunneling process have been obtained.

Luis et al. Reply: Hansen and Mørup (HM) argue [1] that our interpretation of magnetic relaxation of interacting Co clusters [2] was ''based on unrealistic assumptions'' and suggest an alternative interpretation in terms of collective dynamics. We shall next (a) discuss the validity of our assumptions and (b) argue that the interpretation proposed by HM is not supported by the experiments reported in [2].In our calculations [2], we assume that the magnetic moments of the neighboring particles have time to reach equilibrium between two flips of the central spin. We

We report on the magnetic properties of the supra-molecular compound iron(II) phthalocyanine in its α-form. dc-and ac-susceptometry measurements and Mössbauer experiments show that the iron atoms are strongly magnetically coupled into ferromagnetic Ising chains with very weak antiferromagnetic interchain coupling. The transition to 3D magnetic ordering below 10 K is hindered by the presence of impurities or other defects, by which the domain-wall arrangements along individual chains become gradually blocked/frozen, leading to a disordered 3D distribution of ferromagnetic chain segments. Below 5 K, field-cooled and zero-field-cooled magnetization measurements show strong irreversible behavior, attributed to pinning of the domain-walls by the randomly distributed defects in combination with the interchain coupling. High-field magnetization experiments reveal a canted arrangement of the moments in adjacent ferromagnetic chains.