Luis, F., et al.: Enhancement of the magnetic anisotropy of nanometer-sized Co clusters: influence of the surface and of interparticle interactions. Phys. Rev. B 65, 094409

University of Zaragoza, Caesaraugusta, Aragon, Spain
Physical Review B (Impact Factor: 3.74). 02/2002; 65(9). DOI: 10.1103/PhysRevB.65.094409
Source: arXiv


We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.2 nm (25–7000 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 with previous structural characterizations. The distribution of activation energies has been independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant Keff. We find that Keff 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 process is shown to proceed classically down to the lowest temperature investigated (1.8 K).

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    • "In these systems, together with Fe and Co ultrathin film structures [8], the magnetic moment has been observed to increase with respect to the bulk value, the reason being the enhancement of the orbital magnetic moment due to the narrowing of the electron bands and the loss of symmetry of the crystal field at the surface. However, when the 3d-clusters are coated or embedded in a matrix, changes of the magnetic moment have been reported that point either towards an increase [9] [10] or a decrease [11] [12] [13] [14] with respect to bulk, depending on the coating or matrix material. "
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    ABSTRACT: Downsizing to the nanoscale has opened up a spectrum of new magnetic phenomena yet to be discovered. In this context, we investigate the magnetic properties of Ni clusters embedded in a metallic Ag matrix. Unlike in Ni free-standing clusters, where the magnetic moment increases towards the atomic value when decreasing the cluster size, we show, by tuning the Ni cluster size down to the sub-nanoscale, that there is a size limit below which the clusters become non-magnetic when embedded in Ag. To this end, we have fabricated by DC-sputtering a system composed of sub-nanometer sized and non interacting Ni clusters embedded into a Ag matrix. A thorough experimental characterization by means of structural techniques (x-ray diffraction, x-ray absorption spectroscopy) and DC-magnetization confirms that the cluster size is in the sub-nanometric range and shows that the magnetization of the system is dramatically reduced, reaching only 38% of the bulk value. The experimental system has been reproduced by density functional theory calculations on Nim clusters (m = 1–6, 10 and 13) embedded in Ag. The combination of the experimental and theoretical analysis points out that there is a breakdown of magnetism occurring below a cluster size of six atoms. According to our results, the loss of magnetic moment is not due to Ag–Ni hybridization but to charge transfer between the Ni sp and d orbitals, and the reduced magnetization observed experimentally is explained on the basis of the presence of a narrow cluster size-distribution where magnetic and non-magnetic clusters coexist.
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    • "In the absence of an applied magnetic field, the magnetization direction of a large single-domain particle is along an easy direction. However, since the anisotropy energy decreases as the particle size decreases (Luis et al. 2002), the thermal energy may become comparable to the anisotropy energy of a small particle. In such a case, its magnetic moment may fluctuate during the measurement process. "
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    • "In equation (1), the contributions of the core and surface to the total effective anisotropy are assumed to be solely additive excluding cross-linked effects. In spite of the simplicity of this assumption and the fact that there is no theoretical justification for it, equation (1) has been successfully applied to show that experimental values of K eff , determined from ac susceptibility measurements, scale with 1/D in some fine particle systems [12]. Moreover, magnetic resonance experiments in maghemite nanoparticles [7] have revealed an anisotropic contribution to the internal field associated with a positive uniaxial anisotropy originating at the particle surface which dominates over the cubic anisotropy contribution of the maghemite core and scales [7] with 1/D. "
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