Effect of the substitution of Pr for Nd on microstructure and magnetic properties of nanocomposite Nd2Fe14B/α-Fe magnets

Microstructure and magnetic properties of melt-spun nanocomposite magnets with nominal compositions of (Nd_1−xPr_x)₉Fe₈₆B₅ (x=0–1) were investigated. Substitution of Nd by Pr could significantly improve the hard magnetic properties of the nanocomposite magnets; the intrinsic coercivity (_iH_c) and the maximum magnetic energy product ((BH)_max) increase from 414 kA/m and 124 kJ/m³ for x=0 to 493 kA/m and 152 kJ/m³ for x=0.6, respectively. Further substituting Nd by Pr (x>0.6) strongly weakens exchange-coupling interaction between magnetically hard and soft phases.     

Relaxation process and ferromagnetic resonance investigation of ferrofluids with Mn–Zn and Mn–Fe mixed ferrite particles

The magnetic relaxation processes in two ferrofluids with Mn_0.4Zn_0.6Fe₂O₄ (sample F1) and Mn_0.6Fe_0.4Fe₂O₄ (sample F2) mixed ferrite particles, dispersed in n-decan and kerosene, respectively, are investigated through the determination of components χ′ and χ′′ of the complex magnetic susceptibility in the range of (2–30) MHz. The values of the saturation magnetization of the two ferrofluids are M_∞=5.28 kA/m for sample F1 and M_∞=10.99 kA/m for sample F2. A maximum of the imaginary component χ′′ was observed for both samples at frequencies of tens MHz. This maximum was assigned to relaxation processes of Néel type.The effective anisotropy constant K of the particles from the studied samples was evaluated, using both static and dynamic measurements and the values were found to be K₁=6.12×10³ J m⁻³ for the ferrofluid F1, and K₂=5.60×10³ J m⁻³ for the ferrofluid F2. From ferromagnetic resonance measurements, and based on the theoretical values computed for the Lande factor (g), the effective anisotropy constants for the mixed ferrite particles in the studied ferrofluids and the anisotropy field values were determined using a new method. The values obtained in this way for the anisotropy constants K₁ and K₂ are compared to the ones determined from magnetic relaxation measurements.     

Preparation of magnetic FeCo nanoparticles by coprecipitation route

Magnetic FeCo nanoparticles with high saturation magnetization (M_s = 148 emu/g) at 15 kOe were prepared by a coprecipitation route. The value of M_s for FeCo nanoparticles depends on the ratio of Fe to Co components. The size of the nanoparticles was confirmed by transmission electron microscopy (TEM) images, and morphology of the nanoparticles was obtained by field emission scanning electron microscopy (FE-SEM) images. The crystal structure of the nanoparticles dependent on annealing was characterized by X-ray diffraction data. The magnetic properties were characterized by saturation magnetization from a hysteresis loop by VSM.     

The lightning striking distance—Revisited

First return stroke current waveforms measured by Berger [Methods and results of lightning records at Monte San Salvatore from 1963–1971 (in German), Bull. Schweiz. Elektrotech. ver. 63 (1972) 21403—21422] and Berger and Vogelsanger [Measurement and results of lightning records at Monte San Salvatore from 1955–1963 (in German), Bull. Schweiz. Elektrotech. ver. 56 (1965) 2–22] are used to estimate the charge stored in the lightning stepped leader channel. As opposed to previous charge estimates based on the entire current waveform, only the initial portion of measured current waveforms (100 μs in duration) was used in order to avoid the inclusion of any charges not involved in the effective neutralization of charges originally stored on the leader channel. The charge brought to ground by the return stroke within the first 100 μs, Q_f,100 μs (in C) is related to the first return stroke peak current, I_pf (in kA), as Q_f,100 μs=0.61 I_pf. From this equation the charge distribution of the stepped leader as a function of the corresponding peak return stroke current is estimated. This distribution (along with the assumed average electric field of 500 kV/m in the final gap) is used to estimate the lightning striking distance S (in meters) to a flat ground as a function of the prospective return stroke peak current I (in kA): S=1.9 I_pf^0.90. For the median first stroke peak current of 30 kA one obtains S=41 m, while the traditional equation, S=10 I_pf^0.65, gives S=91 m. In our view, the new equation for striking distance provides a more physically realistic basis for the electro-geometric approach widely used in estimating lightning incidence to power lines and other structures.     

Effect of (Ni,Zn)Ru mixtures on magnetic properties of barium hexaferrites yielded by high-energy milling

The room temperature magnetic properties and the cation site preferences of NiRu and ZnRu substituted barium hexaferrite BaFe_12−x(Ni,Zn)_xO₁₉ (0⩽x⩽0.3) were investigated by vibrating sample magnetometry and Mössbauer spectroscopy. (Ni,Zn)Ru substitutions led to a great increase in the saturation magnetization (66.5 Am²/kg) at low concentrations. Mössbauer spectroscopic studies showed that both mixture ions mainly occupy the 4f₂ and 2a+4f₁ sites. NiRu and ZnRu mixtures showed differences in M_s presumably due to the magnetic nature of divalent ion. H_ci could be easily controlled from 381.1 to 37.4 kA/m with M_s enhancement for both substitutions, mainly due to selective occupation of nonmagnetic cations on sublattice sites. The tetravalent Ru⁴⁺ ion seems to enhance M_s and to decrease H_ci markedly at low substitutions.     

Pulsed-field-remanence measurements on individual magnetotactic bacteria

We measured pulsed-magnetic-field remanences of individual cells of two types of magnetotactic bacteria. Wild-type magnetic vibrios displayed square remanence curves with the reversal field, H_rev reaching values up to 825 Oe (65.7 kA/m). We attribute the generally high values of H_rev to the elongated shape of the magnetosomes (length-to-width ratio, e≈1.2) and to the comparatively short distances between the magnetosomes (8 nm). Cells of the rod-shaped Magnetobacterium bavaricum, on the other hand, which frequently contain more than 600 magnetosomes per cell, could gradually be demagnetised; the coercivity of remanence, H_cr of individual cells always ranged between 600 and 700 Oe (47.7–55.7 kA/m). The non-square remanence curves of M. bavaricum reflect a distribution of elongations and interactions between adjacent strands of magnetosomes within the braid-like chains.     

Magnetic properties of Nd–Fe–B system anisotropic HDDR powder made from segregated master ingots

In order to examine the possibility of applying the HDDR process to segregated master ingots, Nd–Fe–B system HDDR powders were made from ingots with different levels of homogeneity, and their structures and magnetic properties were evaluated in detail. HDDR powders made from segregated as-cast ingots displayed anisotropy and large coercivity. They had a nearly homogeneous Nd₂Fe₁₄B phase, although some large areas with α-Fe and Nd-rich regions of 30 μm in size were present after the HD process. With increasing in the homogeneity level of the master ingots, the anisotropy of HDDR-processed powders decreased and their coercivity increased. In addition, an intermediate Ar treatment was applied between the HD and DR processes to improve the magnetic properties. As a result, the effect of the IA treatment was clearly confirmed, and good magnetic properties of B_r=1.23 T, H_cJ=848 kA/m and (BH)_max=238 kJ/m³ were obtained.     

Magnetic properties and microstructure of Ba-ferrite powders prepared by ball milling

The magnetic properties and the phase constitution of barium ferrite powders with Fe/Ba ratios varying from 7 to 15 and heat-treated in the temperature range 1000–1200°C are reported. The results showed that heating temperature was effective in increasing the magnetization (M_s) of the powders for ratios ⩾10. For lower ratios M_s increased initially and then decreased with increasing temperature, presumably due to the formation of BaFe₂O₄. The coercivity (H_ci) presented a maximum for all the compositions and then decreased due to particle growth. In addition, the effect of the milling time on the magnetic properties and powder characteristics was investigated. It was found that 40 h of milling led to having samples with less volume fraction of second phases and that the magnetic properties were also maximized. TEM micrographs show particle sizes varying from 0.1 to 2 μm.     

Microstructure refinement and significant improvements of magnetic properties in Pr2Fe14B/α-Fe nanocomposites

Exchange coupled (Pr,Tb)₂(Fe,Nb,Zr)₁₄B/α-Fe nanocomposites have been produced by melt spinning. A trend for perpendicular and planar c-axis orientation of the 2:14:1 phase was observed in the free surface of ribbons spun at speeds below 10 m/s and at optimal speeds, respectively. Higher wheel speeds led to the formation of an amorphous phase that transformed to 2:14:1 phase around 680°C. Optimum magnetic properties were found in samples spun at 14–17 m/s and annealed at 700°C for 20 min. The loop squareness was also found to depend mainly on the microstructure that is very sensitive to the sample composition. A few percentage of Nb and Zr suppressed the grain growth, resulting in a drastic improvement of magnetic properties, with approximate 50% enhancement in the intrinsic coercivity and an increase in maximum energy product from 5.6 kOe and 14.7 MGOe for the (Nb,Zr)-free sample to 8.2 kOe and 20.3 MGOe for the (Nb,Zr)-substituted samples, respectively. The significant improvement in magnetic properties originated from a much finer and homogeneous nanocomposite microstructure with an average grain size of 20 nm, leading to a high remanence of 0.73 M_s. Henkel plots indicate the enhancement of exchange coupling between hard and soft magnetic phases.     

Microwave-induced combustion synthesis of Ni–Zn ferrite powder and its characterization

Ni–Zn ferrite powders were successfully synthesized by microwave-induced combustion process. The process takes only a few minutes to obtain calcined Ni–Zn ferrite powders. The resultant powders were investigated by XRD, SEM, VSM, TG/DTA and surface area measurements. The as-received product shows the formation of cubic ferrite with saturation magnetization (M_s)≈23 emu/g, whereas upon annealing at 850°C for 4 h, the saturation magnetization (M_s) increased to ≈52 emu/g.     

Theoretical investigation of the inversion parameter in Co3 − sAlsO4 (s = 0–3) spinel structures

The structural, energetic, and thermodynamic properties of the Co_3 ? sAl_sO₄ (s = 0, 1, 2, and 3) crystal family are studied using periodic DFT calculations. We provide a quantitative discussion of the cation distribution effect on the cell parameter, the oxygen Wyckoff position, the interatomic distances and the energies of the structures. It is demonstrated that the low cobalt containing CoAl₂O₄ spinel is the most stable structure of the Co_3 ? sAl_sO₄ (s = 0, 1, 2, and 3) crystal family.     

Giant magneto-impedance effect in CoMnSiB amorphous microwires

The giant magneto-impedance (GMI) ratio, ΔZ/Z=[(Z(H)−Z(H_max)]/Z(H_max), in a nearly zero magnetostrictive Co_68.5Mn_6.5Si₁₀B₁₅ amorphous microwire has been investigated for the frequency range 0.5–10 MHz, driving current amplitude of 0.5–2.5 mA, bias DC magnetic field up to 2400 A/m and under applied tensile stress up to 132 MPa. A maximum relative change in the GMI ratio up to around 130% is observed at a frequency of 10 MHz, magnetic DC field of about 180 A/m, driving current amplitude of 1 mA and under tension of 60 MPa. The tensile stress dependence of the magnetic field, H_m, corresponding to the maximum ΔZ/Z ratio allows to estimate the magnetostriction constant (λ_s≈−2×10⁻⁷) to be in good agreement with λ_s values estimated by different methods and in amorphous alloys with similar compositions.     

Magnetic and structural properties of the electrochemically deposited arrays of Co and CoFe nanowires

Magnetic and structural properties of the arrays of 18 nm diameter nanowires of Co and Co₉₀Fe₁₀ electrodeposited in the pores of anodic alumina are investigated. Arrays of Co and Co₉₀Fe₁₀ nanowires show perpendicular magnetic anisotropy and textured crystallographic behaviour. Coercivity H_c (⊥) and remanence M_r/M_s (⊥) values of 2275 Oe (Co₉₀Fe₁₀); 1188 Oe (Co) and 96% (Co₉₀Fe₁₀), 81% (Co) are observed. The continuous films of Co and Co₉₀Fe₁₀ on Cu substrates show in plane magnetic anisotropy and coercivity values between 109 and 288 Oe.     

Critical grain size in the γ→α martensite transformation. Thermodynamic analysis with regard to spatial scales characteristic of martensite nucleation

In the work, it is shown that taking into account the ratio of spatial scales characteristic of martensite crystal nucleation in the elastic field of an individual dislocation localized in a grain of diameter D makes it possible (i) to estimate the critical grain size D_c (～1 μm) characteristic of the γ→α transformation at a martensite starting temperature M_s ～ 100 K from the requirements that threshold strain arises in the elastic region, (ii) to consistently describe the D dependence of M_s with M_s(D_c) = 0 from analysis of the phase free energy difference, (iii) to express D_c through macroparameters and interpret the limiting case M_s∞→0, D_c→∞ (D_c ～ (M_s∞)^?1 with M_s∞ ≡ M_s (∞)), and (iv) to more exactly specify the qualitative dynamic model of the effect of dislocation grain boundaries on the onset of initial excitation.     

Giant magnetic refrigerant capacity in Ho3Al2 compound

Magnetic properties and magnetocaloric effects (MCEs) of the intermetallic Ho₃Al₂ compound are investigated by magnetization and heat capacity measurements. Two successive magnetic transitions, a spin-reorientation (SR) transition at T_SR=31 K followed by a ferromagnetic (FM) to paramagnetic (PM) transition at T_C=40 K, are observed. Both magnetic transitions contribute to the MCE and result in a large magnetic entropy change (ΔS_M) in a wide temperature range. The maximum values of ?ΔS_M and adiabatic temperature change (ΔT_ad) reach 18.7 J/kg K and 4.8 K for the field changes of 0–5 T, respectively. In particular, a giant value of refrigerant capacity (RC) is estimated to be 704 J/kg for a field change of 5 T, which is much higher than those of many potential refrigerant materials with similar transition temperatures.     

Experimental investigation of flame spreading over pure methane hydrate in a laminar boundary layer

Flame spreading over pure methane hydrate in a laminar boundary layer is investigated experimentally. The free stream velocity (U_∞) was set constant at 0.4 m/s and the surface temperature of the hydrate at the ignition (T_s) was varied between ?10 and ?80 °C. Hydrate particle sizes were smaller than 0.5 mm. Two types of flame spreading were observed; “low speed flame spreading” and “high speed flame spreading”. The low speed flame spreading was observed at low temperature conditions (T_s = ?80 to ?60 °C) and temperatures in which anomalous self-preservation took place (T_s = ?30 to ?10 °C). In this case, the heat transfer from the leading flame edge to the hydrate surface plays a key role for flame spreading. The high speed flame spreading was observed when T_s = ?50 and ?40 °C. At these temperatures, the dissociation of hydrate took place and the methane gas was released from the hydrate to form a thin mixed layer of methane and air with a high concentration gradient over the hydrate. The leading flame edge spread in this premixed gas at a spread speed much higher than laminar burning velocity, mainly due to the effect of burnt gas expansion.     

Synthesis of ammonia at atmospheric pressure with Ce0.8M0.2O2−δ (M = La,Y, Gd,Sm) and their proton conduction at intermediate temperature

Ionic conduction in fluorite-type structure oxide ceramics Ce_0.8M_0.2O_2−δ (M = La, Y, Gd, Sm) at temperature 400–800 °C was systematically studied under wet hydrogen/dry nitrogen atmosphere. On the sintered complex oxides as solid electrolyte, ammonia was synthesized from nitrogen and hydrogen at atmospheric pressure in the solid states proton conducting cell reactor by electrochemical methods, which directly evidenced the protonic conduction in those oxides at intermediate temperature. The rate of evolution of ammonia in Ce_0.8M_0.2O_2−δ (M = La, Y, Gd, Sm) is up to 7.2 × 10^− 9, 7.5 × 10^− 9, 7.7 × 10^− 9, 8.2 × 10^− 9 mol s^− 1 cm^− 2, respectively.     

Ultrasonic degradation of N-di and trihydroxy benzoyl chitosans and its effects on antioxidant activity

Modified chitosans with 3,4-dihydroxy benzoyl groups (CS-DHBA) and 3,4,5-trihydroxy benzoyl groups (CS-THBA) were synthesized and their chemical structures were determined by Fourier transform infrared (FT-IR) and ¹H nuclear magnetic resonance (¹H NMR) spectroscopy. Then, ultrasonic degradation of CS, CS-DHBA and CS-THBA in 1% acetic acid solution was investigated. The kinetics studies of degradation were followed by gel permeation chromatography (GPC). The results indicated that the weight-average molecular weight of chitosan decreased obviously after ultrasound treatment, but molecular weights of CS-DHBA and CS-THBA decreased slowly with increasing sonication time. Degradation kinetics model based on 1/M_t−1/M₀ = kt was used to estimate the degradation rate constant. It was found that the rates of degradation of CS-DHBA and CS-THBA are lower than CS, and follow the order: CS₄ > CS₈ > CS₁₂ > CS-THBA₄ > CS-THBA₈ ≈ CS-DHBA₄ > CS-THBA₁₂ > CS-DHBA₈ > CS-DHBA₁₂. The antioxidant activity of the CS, CS-DHBA and CS-THBA before and after sonication was investigated by the radical scavenging activity method using 1,1-diphenyl-2-picrylhydrazyl (DPPH). The DPPH scavenging free radical capacity of CS-THBA and CS-DHBA increased up to 89% and 74% respectively, when the concentration reached 6 μg/ml. The ultrasonic treatment of CS-DHBA and CS-THBA after 30 min decreased the DPPH free radical scavenging activity but ultrasonic treatment of CS increased the DPPH free radical scavenging activity.     

Flux pinning and superconducting properties of melt-textured NEG-123 superconductor with TiO2 addition

We studied the effect of TiO₂ doping on flux pinning and superconducting properties of a melt-grown (Nd_0.33Eu_0.33Gd_0.33) Ba₂Cu₃O_y + 35 mol% Gd₂BaCuO₅ (70 nm in size) composite (NEG-123) processed in Ar–1% O₂ atmosphere. As indicated by similar, sharp superconducting transitions, the small quantities of TiO₂ used in our experiments did not deteriorate superconducting properties of the NEG material. Transmission electron microscopy (TEM) analysis found 20–50 nm Ti-based particles in the NEG-123 matrix. However, we have not observed the clouds of <10 nm sized particles in the NEG-123 matrix, as in the case of recently reported NEG-123 composites doped by Mo and Nb nanoparticles. Nevertheless, quite a good J_c–B performance in the 0.1 mol% Ti-doped sample, namely 550 kA/cm² at the self-field and at the secondary peak field (4.5 T) was achieved at 65 K, while 320 kA/cm² was obtained at zero-field at 77 K, and 50 kA/cm² at 90.2 K. The pinning effectiveness decreased with increasing Ti content above 0.2 mol%. The analysis of the pinning force showed that higher concentration of Ti (>0.2 mol%) increased the amount of normal pins (δl pinning), indicated by the Fp(h) peak shift from h = 0.42–0.36. The maximum pinning effect in a broad field range could be achieved by optimizing Ti content and adding sub-micron Gd-211 particles.     

Crystal-field effect on the hyperfine splittings,Mössbauer spectra and specific heat of Er3+ in pyrogermanate and diglycollate hosts

Hyperfine splittings and the Mössbauer spectra (MS) of the two isomers ¹⁶⁶Er and ¹⁶⁷Er in the pyrogermanate (or ErPG) and diglycollate (or ErDG) hosts were calculated using the previously determined values of the crystal field (CF) parameters and the CF levels. MS for the 166 isomer in both the hosts show only 5 lines, whereas 27 lines are expected for the 167 isomer. In case of ErDG, the quadrupole interaction P increased considerably between 300 and 4 K, i.e., from 1.5 to 21.3 MHz in case of ¹⁶⁶Er and from −1.0 to −14.0 MHz in ¹⁶⁷Er. However for ErPG, the corresponding changes in P for both the isomers are insignificant ∼1 MHz. The hyperfine magnetic fields H_hf in ErDG and ErPG were found to be nearly the same being equal to 4.4±0.01 MG because the g_∥-values are close, i.e., 10.31 and 10.58, respectively. Specific heat C_p of ErPG was measured between 30 and 0.65 K and a sharp λ peak was detected with a transition temperature T_N=0.95±0.01 K. The calculated hyperfine and Schottky specific heat components, C_hf, and C_sch, respectively showed peaks at 32 mK and 110 K. From the experimental results of C_p, the lattice specific heat C_L/R in ErPG was found to be 3.34×10⁻⁵ T³ and the thermal characteristics of the magnetic specific heat C_M was determined. The internal energy U_M(T) was obtained from C_M by numerical integration and its change ΔE/R=[U_M(∞)/R–U_M(0)/R] was found to be 1.02 K and the corresponding value of the Weiss constant θ in the ordered state was 2.01 K. The value of the exchange interaction constant |J_ex/k| and the dipole–dipole interaction constant for nearest neighbours were found to be ∼0.5 and ∼1.03 K, respectively. The molecular field (H_mol) for ErPG was equal to 4.1 kG. From future epr and MS measurements, interesting results are expected regarding the magnetic ordering in ErPG and the phase change in ErDG.