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).

A universal curve for the change in the magnetic entropy has been recently proposed for materials with second-order phase transitions. In this work we have studied the universal behavior of the magnetocaloric effect in the family of cobalt Laves phases, RCo 2 , and mixed manganites, La 2/3 ͑Ca x Sr ͑1−x͒ ͒ 1/3 MnO 3 , which exhibit first-and second-order phase transitions. The rescaled magnetic entropy change curves for different applied fields collapse onto a single curve for materials with second-order phase transition as opposed to the first-order phase transition compounds, for which this collapse does not hold. This result suggests that the universal curve may be used as a further criterion to distinguish the order of the phase transition.

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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

The magnetic behavior of Fe 3Àx O 4 nanoparticles synthesized by either high-temperature decomposition of an organic iron precursor or low-temperature coprecipitation in aqueous conditions is compared. Transmission electron microscopy, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and magnetization measurements show that nanoparticles synthesized by thermal decomposition display high crystal quality and bulklike magnetic and electronic properties, while nanoparticles synthesized by coprecipitation show much poorer crystallinity and particlelike phenomenology, including reduced magnetization, high closure fields, and shifted hysteresis loops. The key role of the crystal quality is thus suggested, because particlelike behavior for particles larger than about 5 nm is observed only when the particles are structurally defective. These conclusions are supported by Monte Carlo simulations. It is also shown that thermal decomposition is capable of producing nanoparticles that, after further stabilization in physiological conditions, are suitable for biomedical applications such as magnetic resonance imaging or biodistribution studies. V

The magnetic anisotropy of Co clusters with diameters ranging from 1.1 nm to 4.5 nm turns out to be significantly larger than in bulk and strongly increasing with decreasing cluster size. The dominating role of the surface can be used to modify the anisotropy by changing the electronic properties of the matrix surrounding the clusters. We find that capping the clusters by a metallic (Cu and Au) layer significantly enhances the anisotropy, thus also stabilizing the magnetization against thermal fluctuations. The observed anisotropy enhancement is attributed to the bonding of the Co 3d electrons to the conduction band of the capping layer, which depends on the electronic band structures of both metals.

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