Analysis on three-sublattice model of magnetic properties in rare-earth iron garnets under high magnetic fields

In this paper, based on the molecular field theory, a new and improved three-sublattice model on studying the magnetic properties of ferrimagnetic rare-earth iron garnet in high magnetic fields is introduced. Here, the effective exchange field is described as H_i = λM = λχH_e, where λ is the coefficient associated with the molecular field, χ is the effective magnetic susceptibility, and H_e is external magnetic fields. As is known, the magnetic sublattices in rare-earth iron garnets can be classified three kinds labeled as a, c and d, in our calculations, whose magnetizations are defined as M_a, M_c and M_d, respectively. Then, using this model, the temperature and field dependences of the total magnetization in Dy₃Fe₅O₁₂ (DyIG) are discussed. Meanwhile, the magnetizations of the three kinds of magnetic sublattices are analyzed. Furthermore, our theory suggests that the coefficients α_i associated with λ and χ in DyIG show obvious anisotropic, temperature-dependence and field-dependence characteristics. And, the theoretical calculations exactly fit the experimental data.     

Nanocrystallization and soft magnetic properties of Fe23M6 (M: C or B) phase in Fe-based bulk metallic glass

Nanocrystallization effect of primary Fe₂₃M₆-type phase (M: C or B) on magnetic properties of Fe-based bulk metallic glass (BMG) has been investigated. Under controlled microstructural modification with Fe₂₃M₆-type phase, the soft magnetic properties of Fe_76.5C₆Si_3.3B_5.5P_8.7 (at.%) BMG were significantly improved: saturation magnetization (M_s) and initial permeability (μ_i) of the alloy were dramatically enhanced from 1.35 T and 3500 to 1.57 T and 9890, respectively. The present study about introducing medium range atomic orderings and nanocrystallization of Fe₂₃M₆-type structure in amorphous matrix suggests a strategy to enhance the soft magnetic properties of ferromagnetic BMGs having the primary crystallized phase Fe₂₃M₆. This result provides useful insight to develop soft magnetic nanocrystalline materials, stating from BMG precursor with large glass forming ability.     

Comparative study of one-dimensional NiCo alloy nanostructures assembled by in situ and ex situ applied magnetic fields

For the study of magnetic field-assisted assembly behavior, one-dimensional (1D) NiCo alloy nanostructures were solvothermally obtained at 180 °C under an in situ magnetic field (the magnetic field as applied during the chemical reduction) and ex situ field (after the chemical reduction was finished). Microscopic morphology and magnetic properties differences were investigated using scanning electronic microscope (SEM) and vibrating sample magnetometer (VSM) for these products. Magnetic measurement results show that 1D ordered microstructures under in situ magnetic field possess higher saturation magnetization M_s, remnant magnetization M_r, coercivity H_c and reduced magnetization M_r/M_s than 1D ordered microstructures under ex situ field, and the four magnetic parameters of the two ordered microstructures are much higher than those randomly distributed alloy particles prepared in the absence of external magnetic field.     

Plastic versus elastic deformation effects on magnetic Barkhausen noise in steel

A study was performed to differentiate the effects of elastic and plastic deformation on magnetic Barkhausen noise (MBN) signals. Linear and angular MBN measurements were made on a number of mild steel plate samples subjected to varying degrees of uniaxial elastic and plastic deformation up to ∼40% strain. Elastic strain effects on the MBN_energy were determined to be far more significant than plastic strain effects. MBN_energy increases in the early plastic deformation region were attributed to a slight increase in elastic strain due to work hardening. Magnetic anisotropy studies indicated that elastic strain significantly alters the magnetic anisotropy (α) in the sample, but changes the isotropic signal (β) very little. Plastic deformation has a smaller, but apparently opposite effect, in that it appears to change β but has little influence on α. As plastic deformation levels become large, however, behaviour becomes more complex and is less well understood.     

Diffusional transformation in Ti6Al4V alloy during isothermal compression

Thermodynamic calculation of the two-phase Ti alloy was completed using CompuTherm Pandat™ and Ti data base, followed by isothermal compression of Ti6Al4V (Grade 5), with an initial colony lamellar structure that was performed in the (α+β) and β-phase field. Microstructural evolution and phase transformation were investigated using X-ray diffraction, scanning and transmission electron microscopy. The presence of the Ti₃Al or α₂ (hcp), the phase stability and transition temperatures were predicted by the Gibbs free energy−temperature and phase fraction−temperature diagrams. The isothermal compression in the (α+β)-phase field is characterized by reorientation and localized kinking of α/β lamellae, and cracking at α/β interphase regions. While in the α→β-phase transformation area, deformation in β-phase and at α/β interphase boundaries, extensive transformation of α into β-phase, martensitic transformation and spheroidization of α-laths mainly characterize this isothermal compression. A complete transformation of α into β single phase occurs in the β-phase field. Ti₃Al or α₂ (hcp), β (bcc) and α (hcp)-phase, and additional hcp α' and orthorhombic α” phases in a deformed Ti6Al4V are revealed. The flow stress level, the dynamic recovery and dynamic globularization are affected by deformation temperature.     

Experimental Study of Phase Equilibria in the System Cu-Al-Sn

Phase equilibria in the Cu-rich corner of the ternary system Cu-Al-Sn have been re-investigated. Final equilibrium microstructures of 20 ternary alloy compositions near Cu₃Al were used to refine the ternary phase diagram. The microstructures were characterized using optical microscopy (OM), x-ray diffraction (XRD), electron probe microanalysis and transmission electron microscopy. Isothermal sections at 853, 845, 833, 818, 808, 803 and 773 K have been composed. Vertical sections have been drawn at 2 and 3 at% Sn, showing β₁ as a stable phase. Three-phase fields (α + β + β₁) and (β + β₁ + γ₁) result from β → α + β₁ eutectoid and β + γ₁ → β₁ peritectoid reactions forming metastable β₁ in the binary Cu-Al. With the lowering of temperature from 853 to 818 K, these three-phase fields are shifted to lower Sn concentrations, with simultaneous shrinkage and shifting of (β + β₁) two-phase field. The three-phase field (α + β + γ₁) resulting from the binary reaction β → α + γ₁ shifts to higher Sn contents, with associated shrinkage of the β field, with decreasing temperature. With further reduction of temperature, a new ternary invariant reaction β + β₁ → α + γ₁ is observed at ~813 K. The β disappears completely at 803 K, giving rise to the three-phase field (α + β₁ + γ₁). Some general guidelines on the role of ternary additions (M) on the stability of the ordered β₁ phase are obtained by comparing the results of this study with data in the literature on other systems in the systems group Cu-Al-M.     

Electrochemical investigation of chloride induced pitting of stainless steel under the influence of a magnetic field

The effect of a magnetic field on chloride induced pitting of stainless steel was studied by potentiodynamic measurements in aqueous NaCl solution in a cylindrical cell with the field perpendicular to the surface under test. Compared to identical tests without field, a significant shift of the repassivation potential E_r in the cathodic direction was observed, together with the formation of small pits at high density in the periphery of the electrode. These pits develop under the influence of a vortical flow induced by the magnetohydrodynamic effect. The shift of E_r is explained as the consequence of the occluded morphology of these pits.     

Structural, magnetic and magnetocaloric properties in Pr0.5M0.1Sr0.4MnO3 (M = Eu, Gd and Dy) polycristalline manganites

Structural, magnetic and magnetocaloric properties of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) powder samples have been investigated by X-ray diffraction (XRD) and magnetic measurements. Our samples have been synthesized using the solid state reaction method at high temperature. Rietveld refinements of the X-ray diffraction patterns show that all our samples are single phase and crystallize in the distorted orthorhombic system with Pbnm space group. Magnetization measurements versus temperature in a magnetic applied field of 50 mT show that all our samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to be 270 K, 258 K and 248 K for M = Eu, Gd and Dy, respectively. Arrott plots show that all our samples exhibit a second order magnetic phase transition. From the measured magnetization data of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) samples as a function of magnetic applied field, the associated magnetic entropy change |ΔS_M| and the relative cooling power RCP have been determined. In the vicinity of T_C, |ΔS_M| reached, in a magnetic applied field of 1 T, maximum values of 1.37 J/kg K, 1.23 J/kg K and 1.18 J/kg K for M = Eu, Gd and Dy, respectively.     

Structural, dielectric and magnetic properties of Nd-doped Co2Z-type hexaferrites

Z-type hexaferrites doped with Nd³⁺, Ba_3−xNd_xCo₂Fe₂₄O₄₁ (x = 0, 0.05, 0.10, 0.15, and 0.25), were prepared by solid-state reaction. The effect of the Nd³⁺ ions substitution for Ba²⁺ ions on the microstructure and electromagnetic properties of the samples was investigated. The results reveal that an important modification of microstructure, complex permeability, complex permittivity, and static magnetic properties can be obtained by introducing a relatively small amount of Nd³⁺ instead of Ba²⁺. SEM image shows that the grains of the ferrites doped with Nd³⁺ were smaller, more perfect and homogeneous than that of the pure ferrite. The real part (?′) of complex permittivity and imaginary part (?″) increase at first, and then decrease with increasing Nd content. At low frequency, the imaginary part μ″ of complex permeability decreases with Nd content and then increases when frequency is above 7.0 GHz. The magnetization (M_s) and the coercivity (H_c) are 79.38 emu g⁻¹ and 36.94 Oe for Ba_2.75Nd_0.25Co₂Fe₂₄O₄₁. The data of magnetism show that the ferrite doped with Nd³⁺ ions is a better soft magnetic material due to the higher magnetization and lower coercivity.     

Residual magnetic field variation induced by applied magnetic field and cyclic tensile stress

Magnetic memory testing (MMT) method is a novel non-destructive testing technique due to its unique advantages of stress concentration identification and early damage detection for ferromagnetic materials. However, a thorough understanding of the impact of exciting magnetic source and cyclic stress on the residual magnetic field variation has not been clearly addressed. The surface magnetic memory signal H_p(y) induced by applied magnetic field and cyclic tensile stress was measured throughout the fatigue process. The correlation of H_p(y) and its gradient K changes with loading cycles and applied magnetic field intensity H reported. The results show that applied magnetic field can only change the magnitude of MMT signal instead of changing the H_p(y) curve׳s profile. The H_p(y) value increases with the increase of the H, and the K value is approximately linear to the H. The maximum gradient K_max indicating the degree of stress concentration increases with the increase of either stress cycles or H. The phenomenon was also discussed from the view of the magnetic dipole in a ferromagnet.     

Phase equilibria in the Ti–Al binary system

New experimental results on the phase equilibria between the β-Ti (A2), α-Ti (A3), α₂-Ti₃Al (D0₁₉) and the γ-TiAl (L1₀) phases in the Ti–Al system using specimens with low levels of oxygen are presented. The results obtained on the α/γ and the α₂/γ equilibria are in good agreement with the previous experimental and calculated phase boundaries, while the ones obtained on the α/β equilibrium deviate significantly from the previously proposed phase diagram. It is confirmed that the β phase field extends to higher aluminum contents and that the width of the α+β two-phase region is very narrow, less than 1 at.% Al. The presence of the A2/B2 order–disorder transition in the β phase is also confirmed by a combination of differential scanning calorimetric (DSC) analysis and extrapolation of ordering phase boundaries from the Ti–Al–X (X=Cr, Fe) ternary systems. A thermodynamic analysis has been carried out taking into account the ordering configurations in the β-Ti (A2)/β₂-TiAl (B2), f.c.c.-Al (A1)/γ-TiAl (L1₀) and α-Ti (A3)/α₂-Ti₃Al (D0₁₉) equilibria. It is proposed that the anomalous α/β equilibrium is due to the A2/B2 ordering reaction.     

Magnetocaloric effect in the intermetallic compound Gd7Pd3

As a part of a current research concerning the physical characterisation of Rare Earth intermetallic compounds for magnetic refrigeration devices, the magnetocaloric properties of the Gd₇Pd₃ phase, which shows a ferromagnetic transition at 318 K, are presented. Heat capacity measurements have been performed in the 5–360 K temperature range and for applied magnetic fields up to 5 T (Tesla). Basing on the heat capacity data the isothermal entropy change ΔS_M and the adiabatic temperature rise ΔT_ad have been calculated ( ΔS_M =2.5 and 6.5 J/K kg and ΔT_ad=3.0 and 8.5 K at 320 K and respectively at 2 and 5 T). From the ΔS_M vs. T plot the refrigerant capacity in the 260–360 K temperature range has been calculated to be 100 J/kg for an applied magnetic field of 2 T and 380 J/kg for an applied magnetic field of 5 T.     

Magnetic anisotropy in Fe-25Cr-12Co-1Si alloy induced by external magnetic field

Structural and magnetic properties of Fe-25Cr-12Co-1Si alloy thermo-magnetically treated under different external magnetic field conditions were investigated. Orientation and morphology of the ferromagnetic α1 phase embedded in α2 phase matrix before and after step ageing are characterized by transmission electron microscope（TEM）. The results show that the ellipsoidal particles of ferromagnetic al phase are aligned along the direction of external magnetic field during isothermal magnetic ageing. Approximately 28% of the total coercivity can be attributed to the shape anisotropy of al phase particles induced by external magnetic field for Fe-25Cr-12Co-1Si alloy thermo-magnetically treated with a parallel magnetic field.     

Large magnetocaloric effect in a wide temperature range induced by two successive magnetic phase transitions in Ho2Cu2Cd compound

The magnetic and magnetocaloric properties of Ho₂Cu₂Cd compound have been studied. The compound reveals two successive magnetic phase transitions at 30 K and 15 K which are corresponding to the paramagnetic to ferromagnetic transition and a spin reorientation, respectively. Two successive magnetic transitions in Ho₂Cu₂Cd resulting two peaks in the temperature dependence of magnetic entropy change curves, −ΔS_M (T). Two peaks are partly overlapped and induced a large MCE in a wide temperature range, i. e., large refrigerant capacity (RC) and relative cooling power (RCP). For a magnetic field change of 0–7 T, the values of maximal −ΔS_M, RC and RCP are 25.1 J/kg K, 527 J/kg and 732 J/kg for Ho₂Cu₂Cd, respectively.     

Structure and magnetic properties of strontium ferrite anisotropic powder with nanocrystalline structure

The phase composition, nanocrystallite size, lattice microstrain and particles morphology of a SrFe₁₂O₁₉ powder subjected to milling and subsequent annealing were studied by various methods. The investigations showed that the high-temperature annealing of the preliminarily milled powder resulted in the increase in the coercive force (μ₀Н_сi) of the SrFe₁₂O₁₉ powder up to 0.4 T owing to the formation of nanocrystalline structure (D ∼ 10³ nm) with low lattice microstrains. However, the annealed powder cannot be textured in an applied magnetic field because of random orientations of the crystallites in powder particles. A processing technique, which includes the low-temperature annealing of powder in an applied magnetic field, was suggested. It allowed us to produce the anisotropic powder of the strontium ferrite with the nanocrystalline structure that ensures the high coercive force of the powder (∼0.4 T) and possibility of the powder texturing in the magnetic field. The prepared samples textured in a magnetic field exhibit the higher both remanence (by a factor of 1.4) and energy product (by a factor of 2.1) as compared to those of isotropic SrFe₁₂O₁₉ samples.     

Phase field study of grain boundary effects on spinodal decomposition

We have developed a phase field model of a polycrystalline alloy by combining the Cahn–Hilliard model [J Chem Phys 28 (1958) 258] with a model of polycrystals due to Fan and Chen [Acta Mater 45 (1997) 3297]. We have used this model to study grain boundary (GB) effects on spinodal decomposition (SD) in two-dimensional (2D) systems. In binary A–B systems with constant atomic mobility, when the GB-energy (γ_α) of the A-rich α phase is lower than that (γ_β) of the B-rich β phase, decomposition starts by enriching the GB with species A, setting off a composition wave that produces alternating α and β bands near the GB. Simultaneously, the grain interiors undergo normal SD. Thus, when decomposition ends, GB-bands coexist with grain interiors with spinodal microstructure. The number of GB bands is rationalized in terms of (γ_β−γ_α) and the rate of SD in the grain interior. Further, during decomposition, grain growth is effectively suppressed.     

α″ phase-assisted nucleation to obtain ultrafine α precipitates for designing high-strength near-β titanium alloys

The diffusion-multiple method was used to determine the composition of Ti–6Al–4V–xMo–yZr alloy (0.45<x<12, 0.5<y<14, wt.%), which can obtain an ultrafine α phase. Results show that Ti–6Al–4V–5Mo–7Zr alloy can obtain an ultrafine α phase by using the α″ phase assisted nucleation. The bimodal microstructure obtained with the heat-treatment process can confer the alloy with a good balance between the strength and plasticity. The deformation mechanism is the dislocation slip and the twinning in the primary α phase. The strengthening mechanism is α/β interface strengthening. The interface of (0001)_α/(110)_β has a platform–step structure, whereas / interface is flat with no steps.     

Megahertz magneto-dielectric properties of nanosized NiZnCo ferrite from CTAB-assisted hydrothermal process

A simple hydrothermal route with cetyltrimethyl ammonium bromide (CTAB) was proposed for directly synthesizing single-crystalline NiZn ferrite at 160 °C. X-ray diffraction patterns and micrographs indicate that products consist of spinel ferrite nanogranulars. The dielectric constant of NiZnCo ferrite is about 11 and the imaginary part of complex permittivity is 1.3. The saturation magnetization (M_s) of NiZn ferrite improves from 0.041 to 0.074 A·m²/g for Ni_0.49Zn_0.5Co_0.01Fe_1.98O₄. The dielectric contant of NiZn ferrite increases from 7 to 11 with a cobalt stoichiometry of 0.01. The real part μ′ of complex permeability for NiZnCo ferrite reaches 3 at 1 GHz. The imaginary part μ′′ of NiZnCo ferrite has a value higher than 1.2 within 0.7?3.0 GHz. Through the incorporation of the magnetic fillers, the low dielectric constant of the composites may meet the requirements of impedance matching to achieve the maximal absorption of the electromagnetic energy in megahertz frequency range.     

The influence of the magnetic field configuration on plasma parameters and microstructure of niobium nitride films

Niobium nitride (NbN) coatings have a variety of interesting properties such as high chemical inertness, excellent mechanical properties, high electrical conductivity, high melting point, and a superconducting transition temperature around 16 K. We have investigated the effects of magnetic field configuration on the plasma characteristic (electron temperature, plasma density, the ion-to-metal flux ratio J_i/J_a and energy parameter E_p) and the microstructure of NbN films grown with a variable magnetron system. The coatings were deposited under identical deposition conditions but with varying the configuration of the magnetic field in the magnetron. The plasma characteristics were determined by planar and cylindrical Langmuir probes for the different magnetic field configurations. The film microstructure and composition were analyzed by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The film hardness and Young's Modulus were measured by Nanoindentation.The variation of the magnetic field with respect to the unbalance state showed that the field changed from a minimum of 3.6 to a maximum of 4.6 mT at the substrate position (5 cm away from target) while in the target center the corresponding values were 49.0 to 98.0 mT, respectively. The lower magnetic field at the target resulted in higher J_i/J_a ratios, plasma densities and potentials. These characteristics resulted in changes in the value of E_p and as this increased the preferred crystalline orientation changed from [200] to [111] and the hardness and Young Modulus increased to 40 GPa and 430 GPa, respectively.     

Effects of the doping element on crystal structure and magnetic properties of Sm(Co,M)7 compounds (M=Si,Cu, Ti,Zr, and Hf)

The crystal structure and magnetic properties of Sm(Co,M)₇ (M=Si, Cu, Ti, Zr, and Hf) compounds with the TbCu₇-type structure have been investigated by means of X-ray powder diffraction and magnetic measurements. For the Sm(Co,M)₇ compounds, the formation of the compound is dominated by the enthalpy of formation of MCo₇, atomic radius ratio of Sm to (Co, M) and the electronic configuration of the doping element M, while the site occupation of M is determined by the enthalpies of solution of M in liquid Sm and Co as well as by the electronegativity difference among M, Sm and Co. However, the Sm(Co,Cu)₇ compounds exhibit different stabilizing mechanism, which may be due to the large mutual solubility between Co and Cu in the Sm(Co,Cu)₇. The stabilizing element Si and Cu prefer to occupy the 3g site, whereas Ti, Zr and Hf have preference to occupy the 2e site. Both the saturation magnetization and Curie temperature of the Sm(Co,M)₇ compounds decrease with increasing M content, while the magnetic anisotropy increases with increasing M content. The magnetic anisotropy field is related closely to the content and site occupation of the doping element M, which agrees with the theoretical analysis based on the individual site contribution model.