Coexisting antiferromagnetism and ferromagnetism in mechanically alloyed Fe-rich Fe–Ni alloys: implications regarding the Fe–Ni phase diagram below 400°C

Fe–Ni alloys below the Invar region with compositions Fe_100−xNi_x (x=21, 24, and 27 at%) were prepared by high-energy ball milling technique (mechanical alloying). The as-milled samples, characterized by X-ray diffraction and Mössbauer spectroscopy, contain a mixture of (BCC) and γ (FCC) phases, whereas the samples annealed at 650°C for 0.5 h show a single γ (FCC) phase displaying a single line Mössbauer spectrum at room temperature (RT). At low temperature, the Mössbauer spectra of annealed Fe₇₆Ni₂₄ and Fe₇₃Ni₂₇ alloys show the existence of a magnetically split pattern together with a broad singlet, which are ascribed to a high-moment ferromagnetic Ni-rich phase and a low-moment Fe-rich phase, respectively. The Fe-rich phase in annealed Fe₇₆Ni₂₄ alloy, which is paramagnetic at RT, undergoes antiferromagnetic ordering at 40 K, estimated from the dramatic line broadening of its spectrum, giving rise to a small hyperfine field (e.g. 2 T at 6 K). The coexistence of these phases is attributed to phase segregation occurring in these alloys as a result of enhanced atomic diffusion. The stability of these alloys towards martensitic (FCC→BCC) transformation at low temperatures is discussed in connection with the Fe–Ni phase diagram below 400°C.     

Ultra short pulse laser annealing of FeB metallic glasses

We present the results of picosecond laser annealing of as-quenched Fe₈₅B₁₅ and Fe₈₂B₁₈ metallic glasses. The influence of the laser radiation on the surface and bulk properties are studied using CrK X-ray diffraction and transmission Mössbauer spectroscopy. The X-ray data show that the amorphous nature of the surface of the samples can be improved with laser treatment. The mat (cooling) surfaces of the ribbons appear to be more affected by the laser treatment, and show a higher stability. The Mössbauer data reveal that laser annealing of ribbon surfaces also affects the bulk properties of these materials due to induced stresses from the surface layer. The magnetic properties of these materials can be modified by laser annealing.     

Magnetic properties of Sm–Co–B-based nanocomposite magnets

The magnetic properties of nanocomposite melt-spun magnets with composition Sm_16−xCo_68+xB₁₆ (x=0–10, 2 at% interval) and Sm₈Co_92−yB_y (y=10–18, 2 at% interval) have been studied systematically. Several ribbons were fabricated with a wheel speed of 50 m/s, followed by annealing in the temperature range of 700–800°C for 2.5–40 min. XRD results and magnetization versus temperature curves showed that almost all of the samples were composed of the tetragonal Sm₂Co₁₄B and rhombohedral SmCo₁₂B₆ phases which are not magnetically hard at room temperature. However, a relatively high coercivity in the range of 3.5–5.5 kOe has been obtained in these samples. The highest coercivity of 5.5 kOe and a very promising β value of −0.28%/°C were obtained in Sm₈Co₇₄B₁₈ ribbons annealed at 750°C for 5 min. The high coercivities are attributed to the small grain size of the 2 : 14 : 1 phase, in which the large surface areas enhance its effective anisotropy, and make it uniaxial type.     

Observation of hydrogen induced intermediate borides in PrFeB based alloys by Mössbauer effect spectroscopy

In the present work, a quantitative analysis of the phase compositions by Mössbauer effect spectroscopy of solid and conventional hydrogen disproportionated Pr_13.7Fe_80.3B_6.0 and Pr_13.7Fe_63.5Co_16.7Zr_0.1B_6.0 alloys was carried out. Significant amounts of intermediate borides t-Fe₃B and Pr(Fe, Co)₁₂B₆ were detected after solid hydrogen disproportionation treatment in Pr_13.7Fe_80.3B_6.0 and Pr_13.7Fe_63.5Co_16.7Zr_0.1B_6.0 alloys, respectively. After conventional hydrogenation–disproportionation–desorption–recombination treatment these phases were not detected and in no case residual Pr₂Fe₁₄B-phase was found. It was observed that the amount of intermediate borides after disproportionation can be correlated with the degree of texture after recombination at various temperatures.     

Effect of Ti and C additions on structural and magnetic properties of (Pr,Nd)–Fe–B nanocrystalline magnetic materials

Effects of titanium carbide (TiC) addition on structural and magnetic properties of isotropic (Pr,Nd)–Fe-B nanocrystalline magnetic materials have been investigated. In this work, we investigate the effect of TiC addition on a (Pr,Nd)-poor and B-rich composition, as well as on a B-poor and (Nd,Pr)-rich composition. Rapidly solidified (Pr,Nd)–Fe–B alloys were prepared by melt-spinning. The compositions studied were (Pr_1−xNd_x)₄Fe₇₈B₁₈ (x=0, 0.5, and 1) with addition of 3 at% TiC. Unlike the (Pr_xNd_1−x)_9.5Fe_84.5B₆ materials that present excellent values for coercive field and energy product, the (Pr,Nd)-poor and B-rich composition alloys with TiC addition present lower values. Rietveld analysis of X-ray data and Mössbauer spectroscopy revealed that samples are predominantly composed of Fe₃B and -Fe. For the RE-rich compositions (Pr_xNd_1−x)_9.5Fe_84.5B₆ (x=0.1, 0.25, 0.5, 0.75, and 1) with the addition of 3 at% TiC, the highest coercive field and energy product (8.4 kOe and 14.4 MGOe, respectively) were obtained for the composition Pr_9.5Fe_84.5B₆.     

Effect of Cu doping in MgB2 superconductor at various processing temperatures

The effects of Cu doping in MgB₂ superconductor has been studied at different processing temperatures. The polycrystalline samples of Mg_1−xCu_xB₂ with x = 0.05 were synthesized through the in-situ solid sate reaction method in argon atmosphere at different temperature range between 800–900 °C. The samples were characterized through X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and low temperature R–T measurement techniques for the phase verification, microstructure and superconducting transition temperature, respectively. The XRD patterns of Mg_1−xCu_xB₂ (x = 0.05) do not exhibit any impurity traces of MgB₄ or MgB₆ and they show the sharp transition in the samples prepared at 850 °C. The onset transition temperature of the prepared samples is around 39 K, which is almost the same as that for the pure MgB₂. This indicates that Cu doping in MgB₂ does not affect the transition temperature. The SEM micrograph of Mg_0.95Cu_0.05B₂ has shown that the sample is dense with grain size smaller than 1 μm.     

Annealing effects on structure and laser-induced damage threshold of Ta2O5/SiO2 dielectric mirrors

The effects of annealing on structure and laser-induced damage threshold (LIDT) of Ta₂O₅/SiO₂ dielectric mirrors were investigated. Ta₂O₅/SiO₂ multilayer was prepared by ion beam sputtering (IBS), then annealed in air under the temperature from 100 to 400 °C. Microstructure of the samples was characterized by X-ray diffraction (XRD). Absorption of the multilayer was measured by surface thermal lensing (STL) technique. The laser-induced damage threshold was assessed using 1064 nm free pulsed laser at a pulse length of 220 μs.

It was found that the center wavelength shifted to long wavelength gradually as the annealing temperature increased, and kept its non-crystalline structure even after annealing. The absorbance of the reflectors decreased after annealing. A remarkable increase of the laser-induced damage threshold was found when the annealing temperature was above 250 °C.     

Superconductivity in non-stoichiometric and tin-substituted La2CuO4: preparation, characterization, Mössbauer and microwave-absorption studies

Two samples of non-stoichiometric La₂CuO₄ were synthesized, one with La/Cu<2, and the other with 10% Sn substituting Cu. They were investigated by X-ray diffraction, Mössbauer spectroscopy, and microwave-absorption techniques. The microwave-absorption data indicated that they were both superconducting, with the transition temperatures T_c of 40.5 and 41.5 K, the one doped with Sn possessing the higher T_c. The Mössbauer spectra revealed that there exist two kinds of Sn(IV) atoms disordered with Cu. Their isomer shift, δ=−0.244(4) mm/s, is in agreement with Sn(IV) coordinated by oxygen. One site was characterized by a single Mössbauer line, being associated with a weakly distorted environment, wherein the Sn, coordinated more symmetrically, is surrounded by four Cu²⁺ ions. On the other hand, the other site, characterized by a Mössbauer doublet exhibited a quadrupole splitting Δ=1.07(2) mm/s, being associated with a highly distorted coordination, explained to be due to Sn occupying two adjacent cationic sites. To our knowledge, such a substitution for copper ions not resulting in a decrease of T_c has not been reported previously.     

Kinetic interface roughening and magneto resistance of sputter-deposited Fe/Ni75B25 multilayers

The roughening of interfaces as a function of layer thickness and magneto transport properties have been investigated on sputter-deposited Fe/Ni₇₅B₂₅ multilayer films. X-ray reflectivity data were recorded for Ni₇₅B₂₅(72 nm) film and for [Fe(2 nm)/Ni₇₅B₂₅(2 nm)]₁₆ and [Fe(4 nm)/Ni₇₅B₂₅(4 nm)]₈ multilayer films. A power law dependence of the interfacial width of growing Fe/Ni₇₅B₂₅ interfaces was observed. The resulting growth exponents β were found to be in the range of 0.55–0.58 in the initial growth stage of the multilayer with lower Fe/Ni₇₅B₂₅ repetition thickness and at approximately 0.34 for multilayer with higher repetition thickness. The growth exponents were compared with theoretical calculations. High resolution electron microscopy revealed the columnar growth of the Fe/Ni₇₅B₂₅ multilayer. Additionally, an increase of magnetoresistance was observed by the multilayering of Ni₇₅B₂₅ films with Fe interlayers.     

非晶态合金Fe81B13.5Si3.5C2在退火过程中磁各向异性和晶化的穆斯堡尔研究

用穆斯堡尔谱和X射线衍射方法,研究了急冷非晶态合金薄带Fe₈₁B_13.5Si_3.5C₂在空气中退火时磁各向异性的变化,及其与表面结晶和整体结晶的相关性。实验表明,在空气中退火时,磁各向异性的反常变化亦与表面结晶向体结晶的发展密切相关。 关键词：     

Investigation of the static and dynamic magnetic properties of CoFe2O4 nanoparticles

We have investigated the magnetic behavior of cobalt ferrite nanoparticles with a mean diameter of 7.2 nm. AC susceptibility of colloidal cobalt ferrite nanoparticles was measured as a function of temperature T from 2 to 300 K under zero external DC field for frequencies ranging from f=10 to 10,000 Hz. A prominent peak appears in both χ′ and χ″ as a function of T. The peak temperature T₂ of χ″ depends on f following the Vogel–Fulcher law. The particles show superparamagnetic behavior at room temperature, with transition to a blocked state at T_B^m94 K in ZFC and 119 K in AC susceptibility measurements, respectively, which depends on the applied field. The saturation magnetization and the coercivity measured at 4.2 K are 27.3 emu/g and 14.7 kOe, respectively. The particle size distribution was determined by fitting a magnetization curve obtained at 295 K assuming a log-normal size distribution. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy, K=6×10⁶ erg/cm³. Mössbauer spectra obtained at higher temperatures show a gradual collapse of the magnetic hyperfine splitting typical for superparamagnetic relaxation. At 4.2 K, the Mössbauer spectrum was fitted with two magnetic subspectra with internal fields H_int of 490, 470 and 515 kOe, corresponding to Fe³⁺ ions in A and B sites.     

Magnetic behavior of 3d dopants in superconducting REBa2Cu3−xFexO7+δ (RE=Dy, Er)

Mössbauer studies on ⁵⁷Fe-doped superconducting REBa₂Cu₃O_7+δ (RE=Er, Dy) were made as a function of temperature for x=0.15 and 0.30. The magnetic behavior of the 3d dopants, which mainly occupy Cu(1) sites, undergoes antiferromagnetic ordering which is coexistent with superconductivity at low temperature. The dimensionality of the magnetic interaction changes from 2D to 3D when the rare earth changes from Er to Dy. the line-widths of the Mössbauer subspectra are characteristic of magnetic fluctuation behavior in the vicinity of a phase transition. Combining these results with those of Fe-doped Y-123 (pseudo 1D) and Gd (3D), the magnitude of the rare earth moments appears to be strongly correlated with the dimensionality of the magnetic interaction of Fe dopants in these compounds. However, the Mössbauer spectrum for ¹⁵⁵Gd in GdBa₂Cu_2.85Fe_0.15O_7+δ (T_{N(Fe) 14 K}) shows no magnetic order at 4.9 K.     

Magnetic properties of the ternary carbides Tm2Fe17Cx

We have investigated the crystallographic and magnetic properties of the ternary carbides Tm₂Fe₁₇C_x by means of X-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy, ¹⁶⁹Tm Mössbauer spectroscopy and magnetic measurements. It is shown that small amounts of carbon raise the Curie temperature in Tm₂Fe₁₇C_x from below room temperature to about 500 K, at the same time increasing the average Fe moment. Important conclusions regarding the rare-earth sublattice anisotropy were derived from the quadrupole splitting of the ¹⁶⁹Tm Mössbauer spectra and from the strong concentration dependence of the spin reorientation temperature in Tm₂Fe₁₇C_x.     

Structure and optical properties of sol–gel derived Gd2O3 waveguide films

Pure and rare earth doped gadolinium oxide (Gd₂O₃) waveguide films were prepared by a simple sol–gel process and dip-coating method. Gd₂O₃ was successfully synthesized by hydrolysis of gadolinium acetate. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were used to study the thermal chemistry properties of dried gel. Structure of Gd₂O₃ films annealed at different temperature ranging from 400 to 750 °C were investigated by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that Gd₂O₃ starts crystallizing at about 400 °C and the crystallite size increases with annealing temperature. Oriented growth of (4 0 0) face of Gd₂O₃ has been observed when the films were deposited on (1 0 0) Si substrate and annealed at 750 °C. The laser beam (λ=632.8 nm) was coupled into the film by a prism coupler and propagation loss of the film measured by scattering-detection method is about 2 dB/cm. Luminescence properties of europium ions doped films were measured and are discussed.     

RF-sputtered CrB2 diffusion barrier for Ni/Au Ohmic contacts on p-CuCrO2

Ohmic contacts to p-type CuCrO₂ using Ni/Au/CrB₂/Ti/Au contact metallurgy are reported. The samples were annealed in the 200–700 °C range for 60 s in flowing oxygen ambient. A minimum specific contact resistance of 2 × 10⁻⁵ Ω cm² was obtained after annealing at 400 °C. Further increase in the annealing temperature (>400 °C) resulted in the degradation of contact resistance. Auger Electron Spectroscopy (AES) depth profiling showed that out-diffusion of Ti to the surface of the contact stacks was evident by 400 °C, followed by Cr at higher temperature. The CrB₂ diffusion barrier decreases the specific contact resistance by almost two orders of magnitude relative to Ni/Au alone.     

Field-induced local magnetic moments in γ (FCC) Fe–Ni anti-Invar alloys

Mössbauer spectroscopy in longitudinal external fields (up to 7 T) and SQUID magnetometry (up to 5 T) measurements have been carried out on mechanically alloyed (MA) γ (FCC) Fe_100−xNi_x (x=21, 24, and 27 at%) alloys at room temperature. The zero-field Mössbauer spectra of these alloys show only singlets. The high field Mössbauer results indicate that large amounts of the material is in the paramagnetic state, giving rise to two spectral components with their effective fields almost linearly depend on the external field, but with slopes that are smaller than unity. The in-field Mössbauer spectra of the x=27 at% alloy show an additional component with a hyperfine field of ≈21 T, which is attributed to Ni-rich (>30 at% Ni) clusters (domains) of ferromagnetically ordered HM phase that behaves superparamagnetically at room temperature and shows a non-linear character in the magnetization (M–H) curves at low fields. This HM phase is also present in the x=21 and 24 at% samples but with smaller amounts. The results suggest induced hyperfine fields and hence induced moments in the paramagnetic components, which increases with increasing Ni contents. Taenite-enriched samples from the metal particles of two stony meteorites, Al Kidirate (H6) and New Halfa (L4), are also studied by high field Mössbauer spectroscopy and the results are compared to that of MA samples.     

The influence of ingot annealing on the corrosion resistance of a PrFeCoBNbP alloy

The influence of the annealing time on the corrosion resistance of a Pr–Fe–Co–B–Nb alloy with the addition of 0.1 wt% P was investigated here using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The cast ingot alloys were annealed at 1100 °C for 10, 15 and 20 h. The specimens were immersed for 30 days in naturally aerated 0.02 M Na₂HPO₄ solution at room temperature, during which period the evolution of the electrochemical behavior was assessed using EIS. The results indicated that the corrosion resistance of the Pr₁₄Fe_balCo₁₆B₆Nb_0.1P_0.25 alloy was related to the annealing time and, hence, to its microstructure. Annealing at 1100 °C for 10 h was insufficient to eliminate the Fe- phase from the alloy microstructure, whereas annealing for 15 and 20 h removed an increasing amount of Fe- phase, thereby increasing the alloy's corrosion resistance.     

Annealing dependence of magnetic properties in nanostructured Sm0.5Y0.5Co5

Nanocrystalline Sm_0.5Y_0.5Co₅ powders with high coercivity H_C and enhanced remanence M_r were prepared by mechanical milling and subsequent annealing. Annealing temperatures T ranging from 973 to 1173 K, and times t ranging from 1 to 5 min were used. X-ray diffraction (XRD) and DC-magnetization measurements were carried out to study the microstructure and magnetic properties of these samples. XRD patterns demonstrate that the average grain size D of the nanocrystalline powders depends on the annealing temperature T and time t: D ranges from 11 nm (for T=973 K and t=1 min) to 93 nm (for T=1173 K and t=5 min). Magnetic measurements performed at room temperature indicate high coercivity values (H_C>955 kA/m), and enhanced remanence (M_r/M_max>0.5) for all samples. A strong annealing-induced grain size dependence of these magnetic properties was found.     

Magnetic properties in BaFe12O19 nanoparticles prepared under a magnetic field

It was observed that the nanocrystallites of BaFe₁₂O₁₉ formed at 140°C under a 0.25 T magnetic field exhibited a higher saturation magnetization (6.1 emu/g at room temperature) than that of the sample (1.1 emu/g) obtained under zero magnetic field. Both of the two approaches yielded plain-like particles with an average particle size of 12 nm. However, the Curie temperature (T_c), a direct measuring of the strength of superexchange interaction of Fe³⁺–O²⁻–Fe³⁺, increased from 410°C for the nanoparticles prepared without an external field applied to 452°C for the particles formed under a 0.25 T magnetic field, which indicates that external magnetic fields can improve the occupancy of magnetic ions and then increase the superexchange interaction. This was confirmed by electron paramagnetic resonance and Mössbauer spectrum analysis. The results present in this paper suggest that in addition to oxygen defects, surface non-magnetic layer and a fraction of finer particles in the superparamagnetic range, cation vacancies should be responsible for the decreasing of saturation magnetization in magnetic nanoparticles.     

Magnetic properties of UFe5Sn single crystals

Millimetre-size UFe₅Sn single crystals were grown by the top seed solution growth method and characterized by magnetization, ⁵⁷Fe Mössbauer spectroscopy and specific heat measurements in order to study the magnetic transitions detected in powder samples at 248 and 178 K. The magnetization measurements show different behaviour along the three crystallographic directions but with similar values of spontaneous magnetization along a and c. The transition at 248 K is associated with ferromagnetic ordering of iron moments along the c-axis, while the transition at lower temperature is associated with a reorientation towards b. Mössbauer data show that this reorientation is concomitant to the ordering of the Fe2 sites, which in a large proportion remain paramagnetic between the two transition temperatures. Specific heat measurements are consistent with the establishment of magnetic ordering at 248 K, followed by a spin reorientation at 178 K, yielding γ(0 K)140 mJ/(mol K²) and θ290 K for UFe₅Sn.