The anhysteretic magnetization of the granular metallic alloy Cu 90 Co 10 is experimentally studied over a wide temperature range ͑2-700 K͒. The measurements definitely exclude that this alloy is a simple superparamagnet, even in the high-temperature limit, although some features of granular systems ͓such as the typical Langevin-like form of the anhysteretic magnetization curves M (H)͔ are often taken as evidence of superparamagnetism. A phenomenological theory is proposed, explicitly considering that particle moments interact through long-ranged dipolar random forces, whose effect is pictured in terms of a temperature T*, adding to the actual temperature T in the denominator of the Langevin function argument. This simple formula explains all features of the experimental M (H) curves. The theory indicates that the actual magnetic moments on interacting Co particles are systematically larger than those obtained fitting the magnetic data to a conventional Langevin function. The Cu 90 Co 10 granular alloy is therefore identified as an ''interacting superparamagnet'' ISP. The ISP regime appears as separating the high-temperature, conventional superparamagnetic phase from the low-temperature, blocked-particle regime. In this way, a magnetic-regime diagram can be drawn for each granular system. The competition between single-particle and collective blocking mechanisms is briefly analyzed. The proposed interpretation is thought to be applicable to other fine particle systems; its main features and intrinsic limits are discussed.

Room-temperature measurements of magnetization and giant magnetoresistance were performed on rapidly solidified granular Cutoo, Co, systems (X=5,10,15). The magnetoresistance of melt-spun Cu, oo "Co, ribbons was enhanced either by suitable furnace annealings or by exploiting the dc Joule-heating technique in the attempt of precipitating smaller magnetic particles. The particle-size distribution, the particle density, and mean distance are obtained for all compositions and heat treatments through a suitable analysis of the magnetic behavior of samples. The magnetoresistance is plotted as a function of the reduced magnetization, and a significant deviation from the quadratic behavior predicted by the independent-moment approach is observed at low fields. A simple theory taking explicitly into account the correlation existing among the magnetic particles is proposed. A general expression for the magnetoresistance in granular magnetic systems is obtained, and shown to accurately fit all the experimental curves, indicating that this effect is basically determined by the ratios between two distinct correlation ranges for the magnetic-moment fluctuations and the electronic mean free path.

A fluidflowtemperature model for the casting of amorphous metal ribbon by melt extraction A dc technique of Joule heating has been developed with the purpose of developing unconventional crystallization products in amorphous metallic ribbons. The thermal effects of a continuous current flow on Fe,ONi,B,O and FesOSilOB1O amorphous ribbon strips are examined. The evolution of the temperature in samples submitted to different values of electrical current is followed by measuring the variations in the electrical resistance of the ribbon strips. A model of sample heating is developed and shown to predict both the steady-state temperature of the ribbon strips and the law of approach to the steady state. A number of experimental data, obtained from multiple-thermocouple measurements, x-ray diffractometry, and differential scanning calorimetry, support the model's assumptions. The effect of fluctuations in the room-temperature resistance is discussed. The temperature behavior of samples submitted to free cooling after a Joule-heating treatment is considered.

Nanocrystalline Fe3O4 and a composite system constituted by nanocrystalline Fe and Fe3O4 have been synthesized by ball-milling commercial magnetite and an equimolar mixture of iron and magnetite powders. The physical parameters governing the milling process have been strictly controlled so as to achieve the nanocrystalline state of the precursor material and to avoid chemical reactions. X-ray diffraction and Mössbauer spectroscopy measurements have been carried out both on as-milled powders and on samples previously subjected to annealing treatments in the 100–600 °C temperature range. The results, providing information on the structural and compositional features of the produced samples, are discussed in terms of structural disorder which is healed by subsequent annealing. In the case of the composite system, this analysis indicates that a high mixing degree between the constituent phases has been reached. In particular, the presence of a sextet with anomalous hyperfine parameters in the Mössbauer spectrum of as-milled Fe+Fe3O4 has been associated with an alteration of the magnetite structure at the interface with bcc Fe. For both sets of samples, the influence of the structural features on the macroscopic magnetic behavior has been investigated by performing magnetic hysteresis loop measurements at room temperature.

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Magnetic, structural, and transport properties of as-quenched and annealed ColOCugO samples have been investigated using x-ray diffraction and a SQUID magnetometer. The largest value of MR change was observed for the as-quenched sample annealed at 450 "C for 30 min. The magnetic and transport properties closely correlate with the microstructure, mainly with Co magnetic particle size and its distribution. For thermal annealing the as-quenched samples below 600 "C, the Co particle diameters increase from 4.0 to 6.0 nm with a magnetoresistance (MR) drop from 33.0% to 5.0% at 10 K. Comparison with the theory indicates that the interfacial electron spin-dependent scattering mechanism correlates with GMR for Co particle diameters up to about 6.0 nm. 0 1995 American Institute of Physics.

In recent years, magnetic-based technologies, like nanomagnet logic (NML), are gaining increasing interest as possible substitutes of CMOS transistors. The possibility to mix logic and memory in the same device, coupled with a potential low power consumption, opens up completely new ways of developing circuits. The major issue of this technology is the necessity to use an external magnetic field as clock signal to drive the information through the circuit. The power losses due to the magnetic field generation potentially wipe out any advantages of NML. To solve this problem, new clock mechanisms were developed, based on spin transfer torque current and on voltage-controlled multiferroic structures that use magnetoelastic properties of magnetic materials, i.e., exploiting the possibility of influencing magnetization dynamics by means of the elastic tensor. In particular, the latter shows an extremely low power consumption. In this paper, we propose an innovative voltage-controlled magnetoelastic clock system aware of the technological constraints risen by modern fabrication processes. We show how circuits can be fabricated taking into account technological limitations, and we evaluate the performance of the proposed system. Results show that the proposed solution promises remarkable improvements over other NML approaches, even though state-of-the-art ideal multiferroic logic has in theory better performance. Moreover, since the proposed approach is technology-friendly, it gives a substantial contribution toward the fabrication of a full magnetic circuit and represents an optimal tradeoff between performance and feasibility

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