Spinodal decomposition of Fe-Cu nanocrystals: Control of atomic-magnetic-moment and magnetic properties

Experimental results corresponding to the saturation magnetization and coercive field during the decomposition, upon annealing, of bcc and fcc Fe_xCu_(1-x), obtained by mechanical alloying are reported. The overall behavior points out that the decomposition takes place in two steps: (i) at low temperatures a decrease of the saturation magnetic moment as well as an anomalous thermal dependence of coercive field are observed, however, no phase transformation is detected, and (ii) for further annealing temperatures a new phase appears; the magnetization tends to increase and the coercive field abruptly increases. The analysis of the results leads us to conclude that the first step corresponds to a spinodal decomposition. Fluctuations in the local composition give rise to coexistence of adjacent regions with Curie temperature varying continuously in a range of 1000 K across distances of a few nanometers, thus allowing the tailoring of the magnetic nanostructures

Mechanical-alloying and lattice distortions in ball-milled CuFe

A least-square fitting analysis of EXAFS data collected from partially-crystallized Fe_80B_20 thin films (t=15 nm), using data collected from pure phase standards of the crystallization products, was found effective in determining the relative atomic fraction of each crystalline phase present. This fitting scheme provides a means for the quantitative treatment of crystallization and precipitation kinetics in thin films and multilayered structures

Mechanically driven alloying of immiscible elements (Comment)

In conclusion we have proven that the fact that both fcc FeCu and bcc Fe magnetization agree at 300 K is simply an accident and our data at low temperature show clearly that the Fe contribution after precipitation from the metastable phase has a deficiency in magnetization of at least 20% with respect to the Fe state in fcc FeCu metastable solid solution

Extended x-ray-absorption fine-structure studies of heat-treated fcc-Fe_50Cu_50 powders processed via high-energy ball milling

The local structure and chemistry of a ferromagnetic fcc-Fe_50Cu_50 solid solution obtained through high-energy ball milling were measured before and after heat-treatment-induced decomposition using extended x-ray-absorption fine-structure measurements. The decomposition is first evident with the phase separation of a-Fe after a heat treatment at 523 K. Analysis of the residual fee component revealed that the Fe atoms were predominantly surrounded by other Fe atoms, suggesting that the Fe has coalesced within the fee structure. The Fe atoms within the fee phase likely exist in low-spin clusters which provide an explanation for the reduced values of low-temperature magnetization previously measured in annealed samples [P. Crespo et aZ., Phys. Rev. B 48, 7134 (1993)]

Anomalous low temperature stair like coercivity decrease due to magnetostatic coupling between superconducting and ferromagnetic particles in mixed powders

Magnetization curves of mixed Nb and FeSi based micrometric particles have been analyzed. The influence of the dispersion of Nb particles on the mixture remanence and coercivity has been studied above and below the Nb superconducting critical temperature. The hysteresis loop shows, at 5K and low applied fields, a decrease of both remanence and coercivity with respect to the one of pure ferromagnetic powders as well as a stair like profile. These features are explained as a consequence of the diamagnetic hysteresis loop of Nb giving rise to local stray fields acting on the ferromagnetic particles at its nearest neighboring

Magnetic behavior of metastable fcc Fe-Cu after thermal treatments

A ferromagnetic and supersaturated fcc Fe_51Cu_49 solid solution has been obtained by mechanical alloying. After subsequent thermal treatments the fcc phase undergoes a spinodal decomposition which finally, at 780 K, yields a mixture of fcc and bcc phases. In this work, a systematic magnetic study is carried out on samples at diferent decomposition states in order to determine the process of transformation into the stable phases. We observe a 20% maximum diminution on the magnetic moment with increasing temperatures of the thermal treatment. The Mössbauer spectrum taken at 8 K shows that 20% of the Fe atoms are in a nonferromagnetic state. On the other hand, upon heating up to 723 K the roomtemperature coercive field increases dramatically to 640 Oe, and after cooling down to 10 K it decreases to 270 Oe. Deviations from the T law in the temperature dependence of the magnetization have been observed. This behavior is explained by fluctuations in composition due to the spinodal decomposition, which lead to fluctuations of the magnetic order parameters, i.e., magnetic moment and Curie temperature

Magnetic and structural-properties of electrodeposited Co1-xPx amorphous ribbons

The specific magnetic moment, coercive force, anisotropy field, and saturation magnetostriction constant have been measured in Co(1-x)P(x) amorphous ribbons with 0.04 less-than-or-equal-to x less-than-or-equal-to 0.27. Differential scanning calorimetry, x-ray diffraction, and transmission electron microscopy analysis have been made in order to study the transition from the amorphous state to the crystalline one. Results suggest that transition takes place when x decreases from 0.19

Magnetic phase diagram of nanostructured zinc ferrite as a function of inversion degree delta

Magnetic properties of spinel zinc ferrites are strongly linked to the synthesis method and the processing route since they control the microstructure of the resulting material. In this work, ZnFe_2O_4 nanoparticles were synthesized by the mechanochemical reaction of stoichiometric ZnO and alpha-Fe2O3, and single-phase ZnFe_2O_4 was obtained after 150 h of milling. The as-milled samples, with a high inversion degree, were subjected to different thermal annealings up to 600 ºC to control the inversion degree and, consequently, the magnetic properties. The as-milled samples, with a crystallite size of 11 nm and inversion degree delta = 0.57, showed ferrimagnetic behavior even above room temperature, as shown by Rietveld refinements of the X-ray diffraction pattern and superconducting quantum interference device magnetometry. The successive thermal treatments at 300, 400, 500, and 600 degrees C decrease delta from 0.15 to 0.18, affecting the magnetic properties. A magnetic phase diagram as a function of delta can be inferred from the results: for delta 0.5, a new antiferromagnetic order appeared due to the overpopulation of nonmagnetic Zn on octahedral sites that leads to equally distributed magnetic cations in octahedral and tetrahedral sites