Granular multilayers [Fe(tnm)∕MgO(3nm)]N with 0.4nm⩽t⩽1.5nm were prepared by sequential pulsed laser deposition. Transmission electron microscopy (TEM) images show that increasing t causes the growth of the sizes of Fe nanoparticles and broadening of the particle size distribution. For t>0.81nm, continuous Fe layers are formed. The evolution of the shapes and sizes of the particles is reflected in the magnetic properties of the investigated films. A crossover from superparamagnetic to ferromagnetic behavior upon formation of a continuous Fe layer is observed. The fit of zero field cooled and field cooled susceptibility measurements and magnetization curves using Curie–Weiss law and a weighted sum of Langevin functions, respectively, allows the estimation of the average granule size for the films with t<0.61nm. The results of the estimations correlate with the data obtained from TEM images. Reduction of saturation magnetization for Fe nanoparticles and an increase of the coercivity up to 1200Oe at low temperatures were found. It is attributed to the formation of Fe-core∕FeOx-shell structured nanocrystals. The oxide shell gives rise to a strong contribution of surface anisotropy. Isotropic tunneling magnetoresistance up to ∼3% at room temperature and in magnetic field up to 18kOe was found for the film with t=0.61nm. For higher t, an anisotropic magnetoresistance typical for continuous ferromagnetic films was observed.

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We have investigated the transport and magnetotransport properties in Fe/MgO multilayers around the Fe percolation threshold as a function of the temperature and the nominal thickness of iron layer ͑t͒. Electrical resistivity measurements allowed us to disclose the charge transport mechanisms involved, which are closely related to the degree of discontinuity in the Fe layers. The samples with Fe thickness below percolation threshold ͑t ϳ 0.8 nm͒ exhibit isotropic magnetoresistance ͑MR͒, which can be understood considering spin-polarized electron tunneling between nanometer-sized, superparamagnetic Fe grains. The MR ratio increases with decreasing temperature from ϳ3% at room temperature to ϳ10% at 30 K. The temperature dependence of MR can be explained satisfactorily in terms of a modified Mitani's model. © 2010 American Institute of Physics. ͓doi:10.1063/1.3298504͔A new generation of devices combining the advantages of semiconductor electronics with spin-dependent transport is emerging as a consequence of the strong potential for magnetoelectronic applications, such as storage media, sensors, and potentiometers. 1 The interest in the development of these spintronic devices inspired the search for new materials in the nanometric length scale, where the magnetic and transport properties differ substantially from those of the bulk. Among them we can mention granular cermets-in which nanometer-sized magnetic metallic particles are embedded in an insulating matrix. [2][3][4][5] Discontinuous metal-insulator multilayers ͑DMIMs͒ are a special kind of granular cermets where the metallic particles are located not randomly but in layers, and this arrangement causes the enhancement of the low-field dependence and relatively high magnetoresistance ͑MR͒ ratios at room temperature. 6 DMIMs are easier to prepare than planar, epitaxial magnetic tunnel junctions ͑MTJs͒ showing huge MR values, 7,8 but strongly reduced by structural defects at interfaces. 9,10 The physical principle that underlies the MR in both cases is a spin-dependent electric charge transfer between two magnetized electrodes or grains via tunneling effect. 11,12 This effect is called tunneling MR ͑TMR͒, and the tunneling probability depends on the relative orientation of magnetization of the electrodes or grains. 13 Experimentally TMR ratios up to 180% in fully epitaxial Fe͑001͒/ MgO͑001͒/Fe͑001͒ junction were found at room temperature, 7 and first-principles calculations predict even higher values. 14 Despite extensive research on planar Fe/ MgO/Fe MTJs, little attention has been paid to Fe/MgO granular cermets. 15,16 In this paper we present our results on the magnetotransport properties of Fe/MgO DMIMs prepared by pulsed laser deposition ͑PLD͒.Polycrystalline ͓Fe͑t͒ / MgO͑3 nm͔͒ N multilayers with 0.4 nmՅ t Յ 1.5 nm were deposited by PLD on glass substrates. The preparation procedure was reported in detail elsewhere. 16 Electrical resistance and MR measurements were carried out using the four-point method in the temperature range 30-295 K and in-plane app...