Nanocrystalline FeCo Research Articles

Eddy current and magnetic evaluation of nanostructured iron–cobalt produced by ball-milling

The nanocrystalline iron (Fe)–cobalt (Co) alloy exhibits significant properties compared with a conventional material of the same concentration due to the impact of the crystal size. Nanocrystalline FeCo was synthesized from elementary iron and cobalt powdered materials using a planetary ball mill. The complete formation of the iron–cobalt solid solution was observed after 10 h of grinding, and the crystal size reached 15 nm with a grinding time of 40 h. This research aims to investigate the electromagnetic properties of the iron–cobalt nanocrystalline system. Structural and morphological evaluations were carried out using X-ray diffraction and scanning electron microscopy, respectively. The change in electromagnetic behavior was analyzed by using the eddy current technique, which confirms that the nanostructural state can be verified by using the impedance spectrum. The magnetic results found by the vibrating-sample magnetometry measurements confirm those found by the eddy current tests.

Investigating the magnetocrystalline anisotropy and the exchange bias through interface effects of nanocrystalline FeCo

Investigating the magnetocrystalline anisotropy and the exchange bias through interface effects of nanocrystalline FeCo

Improved ductility by coupled motion of grain boundaries in nanocrystalline B2-FeCo alloys

Embedded-atom method (EAM) potential for the FeCo binary systems has been improved to reproduce the antiphase boundary (APB) energies, crucial to the mechanical properties for ordered alloys. Using the improved potential, we further carried out tensile simulations on nanocrystalline FeCo models to investigate the effect of disordered grain boundaries (GBs) on the intergranular fracture. We found that the large concentrated stress at GBs ascribed to the poor dislocation behavior and plastic-deformation compatibility caused the occurrence of microcracks. As the degree of order at GBs was reduced, the GB motion can be enhanced contributed to the smaller GB migration energy barrier. The GB-dominant plasticity can improve the plastic-deformation compatibility and release the concentrated stress at GBs to prevent the generation of microcracks and intergranular fracture. Collectively, the enhancement of coupled GB motion by disordered GBs opens more pathways to improve the mechanical properties for both nanostructured and conventional polycrystalline alloys with ordered superlattice structures.

On the origin of magnetic anisotropy of FeCo(Nb)B alloy thin films: A thermal annealing study

Abstract Influence of thermal annealing on the ion beam sputtered FeCo(Nb)B quaternary alloy thin films having thickness ∼20 nm and 80 nm was investigated. Regardless of amorphous microstructure, the as-deposited films possess uniaxial magnetic anisotropy (UMA) in the film plane. Annealing at temperature ∼503 K results in amorphous to nanocrystalline phase transformation with precipitation of bcc-FeCo phase. With increasing annealing temperature, magnetic anisotropy gradually decreases and thermal annealing at 773 K results in complete disappearance of UMA. In-plane strain distribution in such nanocrystalline FeCo(Nb)B thin films was obtained by using laboratory and synchrotron X-ray diffraction (XRD) measurements. In-plane and out-of-plane X-ray diffraction measurements suggest, growth induced long range non-uniform tensile stress, as the possible origin of in-plane uniaxial magnetic anisotropy in such films. It is demonstrated that the stress distribution becomes uniform after annealing at 773 K and results in complete disappearance of magnetic anisotropy. The study further suggests that the presence of Nb in FeCoB film improves the thermal stability, inhibits grain growth and stabilizes bcc-FeCo phase up to elevated temperatures. This information is useful for magnetic tunnel junction application, where annealing of FeCoB magnetic electrode is essential.

Atomic ordering and coercive force of nanocrystalline Fe-Co alloy films

In single crystal films of the Fe0,5Co0,5 alloys grown by the method of vacuum condensation, a metastable condition may be received that is similar to the completely disordering condition. Approximately during ten days after the condensation in the film at the room temperature the short range and then long range atomic ordering is developed. Simultaneously the changes of magnetic anisotropy, electroresistance and coercive force of the films were investigated. This investigations show that the appearance of the shot range atomic ordering increases the electroresistance of films and the long range atomic ordering reduces it. The dependence of coercive force from time to time at room temperature atomic ordering films Fe0,5Co0,5 alloys, and Fe0,75Co0,25. Found that the biggest change Hc (double) is observed in single-crystal films when changing mechanism of magnetization reverse. Most is a consequence of the changes to the crystallographic anisotropy.

Effect of pH on the Structural Properties of Electrodeposited Nanocrystalline FeCo Films

Effect of pH on the Structural Properties of Electrodeposited Nanocrystalline FeCo Films

Effect of Bath Temperature on the Microstructural Properties of Electrodeposited Nanocrystalline FeCo Films

Effect of Bath Temperature on the Microstructural Properties of Electrodeposited Nanocrystalline FeCo Films

Chemical synthesis, structure and magnetic properties of nanocrystalline Fe-Co alloys

Synthesis and some properties of Fe-Co alloys have been studied. Fe100-xCox powders were prepared by the reduction reactions involving the reduction of a mixture of iron sulphate and cobalt chloride with hydrazine-hydrate. The structure and magnetic properties were investigated by using X-ray diffraction and measurements of the saturation magnetization and the coercivity, respectively. In the system Fe-Co, solid solutions were formed. The steady-state grain size is≈5–20nm. The maximum saturation magnetization of 207 and 215 A•m2/kg was achieved at 300 and 5K, respectively. Nanocrystalline Fe-Co alloys have been shown to exhibit a hard magnetic behavior.

Effect of Nitridation Magnetic Properties of Nanocrystalline Fe-Co Alloy Powders

Nanocrystalline Fe-Co alloy powders, which were prepared by high-energy mechanical milling, were nitrided under the mixing gas of NH3/H2 in the temperature range from 380°C to 510°C. X-ray diffraction (XRD) was used to analyze the grain size and reaction during the processing. The magnetic properties of the nitrided powders were measured by Vibrating Sample Magnetometer (VSM). The results show that with the appearance of Fe4N phase after nitride treatment, and the grain-size of FeCo phase decreases with the increase of nitridation temperature between 380°C to 450°C.The saturation magnetization of nitrided alloy powder treated at 480°C is about 18% higher than that of the initial Fe-Co alloy powder, accompanied by the reduction of the coercivity. Transmission electron microscope (TEM) was used, attempting to further analyze the effect of Fe4N phase on microstructure and magnetic properties of the powder mixtures.

Magnetic domain observation in nanocrystalline FeCo Base alloys by Lorentz microscopy and electron holography

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

Synthesis and Characterization of Fe-Co Nanocrystalline Alloys

AbstractNanostructured alloys have great potential as soft magnetic materials. In particular, nanocrystalline Fe-Co based alloys are believed to be good candidates for imparting improved magnetic behavior in terms of higher permeability, lower coercivity, reduced hysteresis losses and higher Curie temperatures. In the present work, Fe-50at.%Co alloy powders have been prepared using mechanical alloying (MA) in a planetary ball mill under controlled environment. The particle size and morphology of MA powders was investigated using scanning electron microscopy. The crystal size and internal strain was measured using X-ray diffraction. It has been shown that the crystal size could be reduced down to less than 15 nm in these alloys. Finally, the influence of grain size and internal strain on the magnetic properties has been discussed.

Magnetic properties of nanocrystalline FeMCuNbSiB alloys (M: Co, Ni)

The magnetic properties and microstructure of nanocrystalline FeMCuNbSiB alloys (M: Co, Ni) annealed under a transverse field have been studied. Large induced magnetic anisotropy and good properties in the high frequency range were obtained in Co-rich nanocrystalline FeCo(Cu)NbSiB alloys.

The Effects of Ternary Elemental Additions on the Structure and Magnetic Properties of Nanocrystalline Feco Powders

ABSTRACTPowders of near-equiatomic Fe and Co were mechanically milled with additions of Zr, C, Ni, Cu and/or B for 60 hr using stainless steel balls in a Svegari attritor operated at 1300 r.p.m. under argon. The milled powders were examined before and after annealing at 600 °C. The morphologies and sizes of the powders were examined using a scanning electron microscope. The grain sizes were characterized from the widths of X-ray diffraction peaks obtained using a computer-controlled x-ray diffractometer and the lattice parameters were determined. The resulting magnetic properties were measured using a vibrating sample magnetometer.

Mossbauer measurements for a nanocrystalline Fe/sub 44/Co/sub 44/Zr/sub 7/B/sub 4/Cu/sub 1/ alloy

A two phase microstructure, consisting of nanocrystallites surrounded by an amorphous matrix, was produced by a melt spinning processing route. Alloys of this type have extrinsic properties that are dependent on the relative amounts of the amorphous and nanocrystalline phases. One method for examination of the properties of the nanocrystalline and amorphous phases is by Mossbauer spectroscopy. This paper examines ribbons with the composition of Fe/sub 44/Co/sub 44/Zr/sub 7/B/sub 4/Cu/sub 1/, both as-spun and after annealing at 650/spl deg/C for 1 hour. Three Mossbauer techniques were used to examine these materials, including: transmission measurements, conversion electron Mossbauer spectroscopy, and radio frequency Mossbauer. The transmission spectrum for the annealed HITPERM alloy is composed of two superimposed sextets corresponding to the nanocrystalline FeCo and retained amorphous phases. The rf-Mossbauer results fail to show collapse of the sextet, indicating a large magnetocrystalline anisotropy of the nanocrystalline phase.

Kinetic paths for B2 order in nanocrystalline FeCo–Mo: a MÖssbauer spectroscopic study

The kinetic evolution of B2 order in nanocrystalline FeCo–Mo alloys was studied at different temperatures using Mössbauer spectroscopy. The kinetic paths in the space spanned by the two short-range order parameters α Co/Fe and α Mo/Fe were found to be independent of temperature within experimental errors. This behavior is to be compared with that observed in coarse-grained FeCo–Mo alloys, where the paths were not the same at different temperatures. This can be attributed to the presence of more grain boundary regions in the nanophase alloys, which provide short-circuited diffusion paths for atom movements.

The magnetic properties of nanocrystalline Fe-Co(Cr)–Hf–N thin films

We have investigated the effect of Co(Cr) substitution on magnetic properties of as-deposited Fe–Hf–N films. 4πMs increases with increasing Co content (3–16 at. %) and decreases with increasing Cr content. The film Fe56Co16Hf10N18(at. %) shows excellent soft magnetic properties, with 4πMs∼16.8 kG, Hc∼0.25 Oe, and effective permeability ∼4200 at 100 MHz. In Fe71Cr3Hf9N17, the permeability is about 5200 at 100 MHz, while the values of 4πMs and Hc are ∼14.6 kG and 0.5 Oe, respectively. In order to estimate the magnetic anisotropy, the angular dependency of the resonance magnetic field and linewidth were observed by ferromagnetic resonance (FMR) at 9.84 GHz. The Fe56Co16Hf10N18 film shows a single resonance peak, whereas the as-deposited Fe–Hf–N films shows weak secondary peaks.

Mechanical and magnetic properties of nanocrystalline FeCo alloys produced by mechanical alloying

Abstract Nanostructured FeCo powders were synthesized by high energy milling, starting from Fe and Co powders in various structural states. The alloy formation is studied by X-ray diffraction and Mossbauer spectroscopy. The final grain size is shown to be less than 10 nm. Results are compared to those obtained on nanocrystalline FeCo thin films prepared by electron-gun technique under ultra-high vacuum conditions. The consolidation behavior of “as received” and mechanically alloyed powders is also reported.

Damage and recovery in nanocrystalline FeCo thin films induced by ion irradiation

Abstract Magnetic thin films of Fe60Co40 at.% alloy produced under ultrahigh vacuum conditions by electron beam evaporation from two sources are irradiated at 77 and 300 K with Xe+ ions and molecular N2+ ions. The as evaporated films are polycrystalline with a mean grain size of 7–10 nm, and EXAFS experiments suggest that these films exhibit a noncrystalline structure with a crystalline component inside the grains and a second component with a random atomic arrangement in the grain boundaries. The electrical resistivity of the as evaporated films is much higher than the bulk value, and can be related to the large density of disordered interfaces in the nanocrystalline structure. From in situ electrical resistivity measurements, performed continuously during ion irradiations at 77 K as a function of the ion dose the most important parameters, damage efficiency, spontaneous recombination volume, are deduced. The results are compared to those obtained on the annealed films. A peculiar behaviour is observed at room temperature for both Xe+ and N2+ ions with increasing dose. The electrical resistivity shows a maximum and an important decrease. An interpretation is given based upon the crystallization of the intergranular phase by defect enhanced diffusion mechanism.

Ion irradiation damage in nanocrystalline FeCo thin films

Abstract Thin films of the Fe60Co40 alloy produced by electron beam evaporation under ultra high vacuum are irradiated at 77 K with different ions: He, Ne, Ar, Xe. The as-evaporated films are polycrystalline with a very small grain size of about 8 nm and exhibit the structure of nanocrystalline materials formed of a crystalline component inside the grains and a second component in the grain boundaries with a random atomic arrangement. We have continuously measured the radiation damage as a function of the ion dose at 77 K using in situ electrical resistivity measurements. An important resistivity decrease is observed at high doses but only with heavy ions. This particular behaviour is discussed in relation with both the displacement cascade effects and the microstructural state of the films. It is suggested that nanocrystalline materials could be useful to study thermal spike behaviour and specific effects about displacement cascades.

Electrical resistivity of nanocrystalline FeCo thin films

Determination de la conductivite electrique des couches minces desordonnees Fe 60 Co 40 preparees par evaporation. Correlation de la conductivite avec la grosseur des grains de l'alliage. Explication des resultats par le modele de Vancea