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We report magnetic and exchange bias studies on Ni-NiO nanoparticulate systems synthesized by a two-step process, namely, chemical reduction of a Ni salt followed by air annealing of the dried precipitate in the temperature range 400-550 °C. Size of Ni and NiO crystallites as estimated from X-ray diffraction line broadening ranges between 10.5-13.5 nm and 2.3-4 nm, respectively. The magneto-thermal plots (M-T) of these bi-magnetic samples show a well developed peak in the vicinity of 130K. This has been identified as the superparamagnetic blocking temperature "TB" of NiO. Interestingly, all samples exhibit exchange bias even above their respective NiO blocking temperatures, right up to 300K, the maximum temperature of measurement. This is in contrast to previous reports since exchange bias requires the antiferromagnetic NiO to have a stable direction of its moment in order to pin the ferromagnet (Ni) magnetization, whereas such stability is unlikely above TB since the NiO is superparamagnetic, its moment flipping under thermal activation. Our observation is elucidated by taking into account the core-shell morphology of the Ni-NiO nanoparticles whereby clustering of some of these nanoparticles connects their NiO shells to form extended continuous regions of NiO, which because of their large size remain blocked at T > TB, with thermally stable spins capable of pinning the Ni cores and giving rise to exchange bias. The investigated samples may thus be envisaged as being constituted of both isolated core-shell Ni-NiO nanoparticles as well as clustered ones, with TB denoting the blocking temperature of the NiO shell of the isolated particles. [ABSTRACT FROM AUTHOR]

A nanogranular Co–CoO/Ag system (with core-shell/matrix structure), synthesized by reactive cosputtering, has been subjected to thermal treatments at progressively higher temperatures. The exchange-bias onset temperature increases in close correlation with the average superparamagnetic blocking temperature of the magnetic particles. This behavior, together with the nonmonotonic evolution of the exchange-bias field, is explained in terms of two structural relaxation processes previously characterized in simple Co/Ag granular alloys: (i) segregation of Co solute atoms from the Ag-rich matrix, and (ii) particle coarsening. [ABSTRACT FROM AUTHOR]

The interdiffusion process between Gd and Co has been investigated in several series of multilayered films consisting of Co and different GdCo alloys (Gd[sub 1-x]Co[sub x])/Co (x=0, 0.37, and 0.60). Grazing incidence x-ray profiles provided a preliminary insight into the structure of the samples. Electrical resistivity measurements were performed in a temperature range between 8 and 300 K. For the three series, the room temperature electrical resistivity shows a clear fall once a critical Co thickness is surpassed, which we interpret as due to the growth of pure polycrystalline Co between the alloy layers. A simple model allows us to quantify the amount of Co diffused into the alloy layer. The results will be discussed considering the possible decrease of interdiffusion effects when Co is deposited onto the more stable GdCo alloys. This leads to a well defined multilayered structure, something that is essential for practical applications and for the theoretical study of its underlying physics. [ABSTRACT FROM AUTHOR]

We present magnetic characterization of two series of samples grown by reactive sputtering of Co and Ag: (i) thin films obtained by cosputtering of these metals at different oxygen pressures, and (ii) Co/Ag multilayer films deposited with fixed Co layer thickness (1.1 nm) and oxygen pressure (2×10-5 mbar), and varying Ag layer thickness tAg. For certain preparation conditions the samples obtained with both synthesis methods consist of a dispersion of core-shell Co–CoO nanoparticles embedded in a Ag matrix. Regarding series (i), it is remarkable that the interesting core-shell/matrix structure, suggested by exchange-bias and giant magnetoresistance properties, could be achieved using the simple one-step technique of reactive cosputtering. In series (ii), the exchange-bias and coercivity fields strongly depend on the spacer layer thickness for tAg<4 nm, and then become roughly thickness independent. A discontinuous-continuous transition in the silver layers with increasing tAg, similar to that observed in a previous study, is hypothesized as the origin of the behavior in both fields. [ABSTRACT FROM AUTHOR]

We have investigated the influence of the granular microstructure in the electrical resistivity of RF-sputtered Ti[SUB100-x]Co[SUBx] thin films. Their x-rays characterization has been correlated with dc resistivity measurements from 7 to 300 K. A minimum in the resistivity, ρ(T), centered around T[SUBm]∼50 K, was recorded for Ti[SUB93]Co[SUB7] (samples grown at 30 W. Contrarily, samples grown at 110 W and the same x value displayed no evidences of that low-temperature anomaly. This phenomenon correlated with the change of the majority Ti granular phase: hcp-Ti for the sample grown at 30 W and bcc-Ti for the 110 W sample. That minimum arises from the great difference in Co solubility into both Ti granular phases (0.8 at. % in hcp-Ti and 14.5 at. % in bcc-Ti). Hence, the very small quantity of Co diluted into the hcp-Ti grains allows us to consider Co particles as uncorrelated magnetic impurities which scatter the conduction electrons. This magnetic Kondo scattering would be the mechanism responsible for the observed minimum in the resistivity of Ti[SUB93]Co[SUB7] samples grown at 30 W. [ABSTRACT FROM AUTHOR]

The strong interdiffusion that takes place in the Gd/Co multilayered system greatly influences the structure, composition, and thus the magnetic properties of the layers. We have conducted grazing incidence x-ray scattering experiments using synchrotron radiation on Gd[SUB1-x]Co[SUBx]/Co multilayers with x = 0, 0.37 and x = 0.60 to study the interdiffusion process that takes place and the interfacial structure that results. Probing the scattering vector normal to the surface (specular scattering) showed excellent multilayer growth for all samples. For alloyed Gd samples, good multilayer growth was observed with Co thickness down to below 6 Å. Fits to the reflectivity data show a reduction in the thickness of the Co layer, significantly away from its nominal value. This effect was more exaggerated in the pure Gd sample (x = 0). We have successfully modeled this by assuming a significant portion of the Co has interdiffused into the pure Gd layer. Off-specular scattering experiments show that the interface roughness was conformal in all samples and was not related to the interdiffusion processes at the interfaces. [ABSTRACT FROM AUTHOR]

The combination of Mössbauer spectroscopy and magnetization measurements is demonstrated to be efficient in the determination of the phases responsible for the glassy magnetic dynamics of the highly disordered alloy Fe[sub 35]Al[sub 50]B[sub 15] produced by mechanical alloying. Its spin-glass behavior is apparent from the FC-ZFC magnetization irreversibility, the thermoremanent magnetization, and the slightly frequency-dependent peak in the temperature dependence of the ac susceptibility. Furthermore, the maximum at T[sub p] ≈ 22 K in the real component of the ac susceptibility is accompanied by a sudden onset of the imaginary component. The fit of the frequency dependence of T[sub p] to the critical slowing down law yielded a critical exponent z v = 8 ± 1. However, Mössbauer spectroscopy uncovered the existence of two different Fe environments, of which only the minor one freezes at low temperatures into a distribution of magnetic sextets. This result, combined with the superparamagnetic behavior detected at moderate fields, allowed us to ascribe the spin glass behavior to the presence of interacting, very fine, ferromagnetic clusters (estimated size ∼ 1 nm). The origin of this nanoscale compositional heterogeneity is argued to be related to regions of partially unalloyed Fe. The conclusions are contrasted with other mechanically alloyed systems with reported spin glass behavior or compositional inhomogeneity. [ABSTRACT FROM AUTHOR]

We report the observation of spin-glass-like features in a Fe[sub 61]Re[sub 30]Cr[sub 9] sample obtained after high energy milling for 460 h. X-ray diffraction showed a broad nanocrystalline peak. The zero-field-cooled and field-cooled magnetization curves exhibited a clear irreversibility, the former showing a relatively sharp maximum for low fields at T[sub max]approx. 60 K. In addition, the temperature dependence of the magnetic AC susceptibility (χ[sub ac]) also showed a peak at approximately the same temperature. The frequency sensitivity of such maximum (p=ΔT[sub max]/T[sub max]Δ log wapprox. 0.02) is small compared to that of typical superparamagnets. However, we discuss the insufficiency of this set of phenomena to assert the existence of a true spin-glass transition and propose the blocking of interacting ferromagnetic nanoparticles as the origin of the observed behavior. The effect of annealing, rendered into nanocrystallite growth, on the χ[sub ac] maximum is studied in an attempt to test this explanation. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The climatic conditions and geographical constraints in Spain have led to the development of a highly heterogeneous agricultural sector. Hence the need for a classification of farms into different farm types that allows for the development of solutions that are specific to each group. Many techniques can be used to perform such a classification, but only a few techniques use agricultural census data. This paper presents a methodology based on the combined use of Principal Component Analysis (PCA) and a Combinatorial Method to classify farms into farm groups at a national level according to their location and production system. The results presented in this paper correspond to the population of farms that produce cereals in Spain, according to agricultural census data [ABSTRACT FROM AUTHOR]