Influence of alloying additions and annealing time on magnetic properties in amorphous alloys based on iron

The present paper focuses on the influence of alloying additions on magnetostriction coefficient, concentration of free volume and magnetization in Fe–X–Si–B, and Fe–X–Cu–Si–B amorphous alloys. It was shown that in one group of alloys, the enhancement of soft magnetic properties effect can be attributed to formation of nanocrystalline phase αFe(Si), a decrease of the magnetostriction coefficient and annealing out of free volume. In the second group of alloys, this effect is due to a decrease of the magnetostriction coefficient and annealing out of free volume i.e. takes place in the relaxed amorphous phase.     

Structural relaxation,crystallization and improvement of magnetic properties in FeXSiB (X=Cr,Nb)-type amorphous alloys

In the present paper, the influence of Nb and Cr on intensity of structural relaxation, crystallization processes, electric and magnetic properties in the Fe₇₆Nb₂Si₁₃B₉, Fe₇₆Cr₂Si₁₃B₉ and Fe₇₆Nb₁Cr₁Si₁₃B₉ alloys were investigated. It was shown that the improvement of magnetic permeability caused by a suitable annealing is a thermally activated process. Activation energy of this process is found to be of the order of 1 eV. Cr as an alloying addition to the Fe–Si–B alloy does not change the 1 h optimization annealing temperature and causes an increase of its efficiency. Nb as an alloying addition causes an increase of the 1 h optimization annealing temperature, and also the temperature of the first step of crystallization.     

Grain growth process of two-phase nanocrystalline soft magnetic materials

In order to clarify the origin of the high thermal stability of the microstructure in bcc-Fe/amorphous two-phase nanocrystalline soft magnetic materials, we have investigated the changes in the magnetic and microstructural properties upon isothermal annealing at 898 K for an Fe₈₉Zr₇B₃Cu₁ alloy by means of transmission electron microscopy, Mössbauer spectroscopy and DC magnetometry. The mean grain size was found to remain almost unchanged at the early stage of annealing. However, rapid grain coarsening was evident at an annealing time of 7.2 ks where the intergranular amorphous phase begins to crystallize into Fe₂₃Zr₆. The grain growth process with a kinetic exponent of 1.6 is observed for the growth process beyond this annealing time, reflecting the disappearance of the intergranular amorphous phase. Our results confirm that the thermal stability of the bcc-Fe/amorphous two-phase nanocrystalline soft magnetic alloys is governed by the residual amorphous phase.     

The influence of composition and field annealing on magnetic properties of FeCo-based amorphous and nanocrystalline alloys

We report the influence of composition and very high transverse field annealing on the magnetic properties and structure of four FeCo-based amorphous and nanocrystalline alloys. The compositions (Fe₅₀Co₅₀)₈₉Zr₇B₄ and (Fe₆₅Co₃₅)₈₉Zr₇B₄ were investigated changing the Fe:Co ratio from 50:50 to 65:35. (Fe₅₀Co₅₀)₈₅Zr₂Nb₄B_8.5 was chosen to investigate Nb substitution for Zr in an FeCo-based alloy. This substitution is shown to increase the magnetostrictive constant, _λS${\lambda }_{\mathrm{S}}$, of the nanocrystalline alloy from 36×10⁻⁶ to 54×10⁻⁶. The composition (Fe₆₅Co₃₅)₈₄Cr₅Zr₇B₄ was studied to investigate the influence of Cr on intergranular coupling across the amorphous matrix. Samples of each composition were annealed in the amorphous state at 300 °C and in the nanocrystalline state at 600 °C. Field annealing was performed in 17 T transverse field in an inert atmosphere. Frequency-dependent magnetic properties were measured with an automatic recording hysteresisgraph. Static magnetic properties were measured with a vibrating sample magnetometer. The mass-specific power loss of the alloys decreased with field annealing in both the nanocrystalline and amorphous states for some frequency and induction combinations. Furthermore, the hysteresis loops are sheared after field annealing, indicating a transverse magnetic anisotropy. The nanocrystalline (Fe₅₀Co₅₀)₈₅Zr₂Nb₄B_8.5 composition has a lower relative permeability than the other compositions.     

Advances in amorphous and nanocrystalline magnetic materials

Recent advances made in the area of amorphous and nanocrystalline alloys exhibiting high saturation inductions are reviewed. A new chemical composition was identified that achieves a saturation induction of 1.64 T in an iron-based amorphous alloy. This alloy, when used in electrical transformers, shows a much improved performance over the existing amorphous alloy. Nanocrystalline FeCoCuNbSiB alloys are found to have saturation induction levels reaching 1.7 T. These materials are suited for use in sensors and inductors carrying large currents. Some of these nanocrystalline alloys show a BH squareness ratio exceeding 90%, which can be utilized in pulse power devices. Recent developments in the applications of these materials are also pointed out.     

Studies of the relaxed amorphous phase in the Fe80Nb6B14 alloy

The paper presents measurements of magnetic permeability, magnetic after-effects, magnetostriction, DSC and XPS for the Fe₈₀Nb₆B₁₄ amorphous alloys preliminary annealed for 1 h at temperatures ranging from 300 to 770 K. It was shown that annealing out of free volume and internal stresses causes a decrease of magnetostriction coefficient and leads to the formation of the energetically stable relaxed amorphous state. The XPS spectra show local fluctuation of boron density. This effect was attributed to the formation of small iron clusters—the characteristic feature for the relaxed amorphous phase.     

Structural and magnetic correlation of Finemet alloys with Ge addition

The correlation between saturation magnetization and the magnetic moment per Fe atom in the nanocrystalline state is studied for Finemet-type alloys. These studies were performed on nanocrystalline ribbons whose compositions were Fe_73.5Si_13.5−xGe_xNb₃B₉Cu₁ (x=8, 10 and 13.5 at%). We used a simple lineal model, X-ray diffraction and Mössbauer spectroscopy data to calculate the magnetic contribution of the nanocrystals and the results were contrasted with the measured saturation magnetization of the different alloys. The technique presented here provides a very simple and powerful tool to compute the magnetic contribution of the nanocrystalline phase to the alloy. This calculus could be used to determine the volume fraction of nanocrystalline and amorphous phases in the nanocrystallized alloy, without using a very sophisticated microscopy method.     

External influence on magnetic properties of Fe-based nanocrystalline alloys

Amorphous and nanocrystalline ribbons of NANOPERM, FINEMET and HITPERM were studied by Mössbauer spectroscopy (MS) after the influence of external factors: different annealing atmospheres, tensile stress and several kinds of corrosion. MS is a suitable tool for such studies because the spectral parameters are very sensitive to changes in the vicinity of the probe — ⁵⁷Fe nuclei. The most sensitive parameters were hyperfine magnetic field in crystalline component, average hyperfine field in amorphous component and direction of net magnetic moments. Influence of external factors modified also the structure of the alloys, i.e. new or modified phases were identified by MS phase analysis.     

Preparation of large Co nanosheets with enhanced coercivity by a magnetic-field-assisted solvothermal approach free of surfactants, complexants or templates

We presented a magnetic-field-assisted solvothermal route to prepare hexagonal close-packed (hcp) large Co nanosheets free of any surfactants, complexants or templates. Scanning electron microscope (SEM) images showed that the typical size and thickness of the Co nanosheets were several microns and ca. 50 nm, respectively. Magnetic measurement indicated the coercivity of the sample reached 317.3 Oe, which was much higher than those of bulk Co metal and previously reported Co nanoplatelets. It was found that the solvents played a crucial role in determining the morphology and the crystal structures of the Co products, and the introduction of ethanol conduced to the formation of pure hcp phase Co microcrystal. The external magnetic field was also found to be indispensable for the formation of sheet-shaped morphology.     

Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles

CoFe₂O₄ ferrite nanoparticles were prepared by a modified chemical coprecipitation route. Structural and magnetic properties were systematically investigated. X-ray diffraction results showed that the sample was in single phase with the space group . The results of field-emission scanning electronic microscopy showed that the grains appeared spherical with diameters ranging from 20 to 30 nm. The composition determined by energy-dispersive spectroscopy was stoichiometry of CoFe₂O₄. The Curie temperature in the process of increasing temperature was slightly higher than that in the process of decreasing temperature. This can be understood by the fact that heating changed Co²⁺ ion redistribution in tetrahedral and in octahedral sites. The coercivity of the synthesized CoFe₂O₄ samples was lower than the theoretical values, which could be explained by the mono-domain structure and a transformation from ferrimagnetic to superparamagnetic state.     

Magnetic and magnetization properties of electrodeposited fcc CoPt nanowire arrays

Magnetic and magnetization properties of fcc Co_1−xPt_x (x?0.3) alloy nanowires fabricated by electrodeposition into self-synthesized anodic alumina templates are investigated. Magnetization curves, measured for varying wire geometries, show a crossover of easy axis of magnetization from parallel to perpendicular to the nanowire axis as a function of the diameter and length. The measured values of coercivity (H_c) and remanent squareness (SQ) of CoPt nanowire arrays, as a function of angle (θ) between the field and wire axis, support the crossover of easy axis of magnetization. The curling mode of the magnetization reversal process is observed for CoPt nanowire arrays. At low temperatures, the easy axis for magnetization of the nanowires is observed to deviate from the room-temperature orientation.     

High-temperature soft magnetic properties of Co-doped nanocrystalline alloys

In situ hysteresis measurements were used to follow the evolution of soft magnetic properties of various Co containing Finemet- and Hitperm-type nanocrystalline materials. This work is aimed at obtaining the proper chemical composition of nanocrystalline alloys, which can be applied for highest temperatures and is less affected by the inevitable decoupling phenomena, which is common to all two-phase nanocrystalline systems. Experimental results pointed out that in the Co-added Finemet-type alloys the decoupling temperature can be increased by about 50–70 K as compared to Co-free Finemet alloy. Hitperm alloys, close to equiatomic Fe/Co ratio, have much higher decoupling temperature, but their coercivity is an order of magnitude larger at low temperatures compared to the Co-free alloys so their advantage is evident above 400 °C only.     

Micromagnetism and high-frequency properties of soft magnetic films

The wavelength and amplitude of the longitudinal oscillations of the transversal and longitudinal components of magnetization was obtained from the contrast ripple observed by the Lorentz TEM. The micromagnetic ripple is connected with the internal stray field in the film. The theory, based on Landau–Lifshitz equation, is developed, where the internal stray field is taken into account. The theory predicts that the micromagnetic ripple can broaden the ferromagnetic resonance (FMR) to lower range of the frequencies. For certain range of the internal stray field, a second bump at lower frequencies arrives, which shows up in some previously reported spectra.     

Fabrication and magnetic properties of Fe3O4 nanowire arrays in different diameters

Fe₃O₄ nanowire arrays with different diameters of D=50, 100, 150 and 200 nm were prepared in anodic aluminum oxide (AAO) templates by an electrodeposition method followed by heat-treating processes. A vibrating sample magnetometer (VSM) and a Quantum Design SQUID MPMS magnetometer were used to investigate the magnetic properties. At room temperature the nanowire arrays change from superparamagnetism to ferromagnetism as the diameter increases from 50 to 200 nm. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements show that the blocking temperature T_B increases with the diameter of nanowire. The ZFC curves of D=50 nm nanowire arrays under different applied fields (H) were measured and a power relationship between T_B and H were found. The temperature dependence of coercivity below T_B was also investigated. Mössbauer spectra and micromagnetic simulation were used to study the micro-magnetic structure of nanowire arrays and the static distribution of magnetic moments of D=200 nm nanowire arrays was investigated. The unique magnetic behaviors were interpreted by the competition of the demagnetization energy of quasi-one-dimensional nanostructures and the magnetocrystalline anisotropy energy of particles in nanowires.     

Morphology and magnetic properties of Co0.8Fe2.2O4 films prepared by the chemical solution deposition

Co_0.8Fe_2.2O₄ ferrite thin films have been prepared on Si(0 0 1) substrates by the chemical solution deposition. Structural characteristics indicate all films are single phase with spinel structure and the space group and the mean grain size increases from 8 to 30 nm with the increase of annealing temperature. The magnetic properties of Co_0.8Fe_2.2O₄ thin films are highly dependent on annealing temperature. The sample annealed at 800 °C possesses high saturation magnetization, moderate coercivity and squareness ratio, making it a promising application candidate in high-density record and magneto-optical materials.     

Structure, magnetic and magnetostrictive properties of as-deposited Fe-Ga thin films

Polycrystalline Fe_100−xGa_x (19?x?23) films were grown on Si(1 0 0) substrates at different partial pressures of sputtering gas ranging from 3 to 7 μbar. Microstructural, magnetic and magnetostrictive properties were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS) and magneto-optic Kerr effect (MOKE) magnetometry respectively. X-ray diffraction showed that all films have the body-centered cubic (bcc) Fe-Ga phase with the 〈1 1 0〉 direction out of the film plane. Magnetic characterization of the films showed that the films prepared at 3 μbar had weak uniaxial anisotropy whereas films grown at Ar pressures in the range 4-7 μbar were magnetically isotropic. The effective saturation magnetostriction constants (λ_eff) of the films were measured using the Villari effect. It was found that effective saturation magnetostriction constants were almost constant over the Ga composition range achieved by varying the sputtering pressure. The measured effective magnetostriction constants fit closely to the calculated saturation magnetostriction constants of 〈1 1 0〉 textured polycrystalline films with the 〈1 1 0〉 directions slightly canted with respect to the normal to the sample surface. It was found that a high pressure of the sputtering gas effected the magnetic softness of the films. The saturation field increased and remanence ratio decreased with increase in pressure.     

Study of the magnetic properties of the permanent magnets by Mössbauer spectroscopy

This paper presents methods that can be used in order to determine the relative remanent magnetization and uniaxial magnetic anisotropy constant of particles in powder-based permanent magnets using Mössbauer spectroscopy. The methods were verified on the permanent magnet barium ferrite.