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New determination of the magnetic anisotropy from single crystals of ( Fe 1 - x Co x ) 2 B alloys are presented. The anomalous temperature dependence of the anisotropy constant is discussed using the standard Callen-Callen theory, which is shown to be insufficient to explain the experimental results. A more material specific study using first-principles calculations with disordered moments approach gives a much more consistent interpretation of the experimental data. Since the intrinsic properties of the alloys with x = 0.3 - 0.35 are promising for permanent magnets applications, initial investigation of the extrinsic properties are described, in particular the crystallization of melt spun ribbons with Cu, Al, and Ti additions. Previous attempts at developing a significant hysteresis have been unsuccessful in this system. Our melt-spinning experiment indicates that this system shows rapid crystallization.

The Stoner-Wohlfarth is the most used model of magnetic hysteresis, but its computation is time-consuming. We use machine learning to approximate piecewise this model by easy-to-compute analytic functions. Our parametrization is suitable for fast quantitative evaluations and fitting experimental data, which we exemplify., Comment: 5 pages, 4 figures

SiO2 coated Fe3O4 submicrometer spherical particles (a conducting core/insulating shell configuration) are fabricated using a hydrothermal method and are loaded at 10 and 20 vol % into a bisphenol E cyanate ester matrix for synthesis of multifunctional composites. The dielectric constant of the resulting composites is found to be enhanced over a wide frequency and temperature range while the low dielectric loss tangent of the neat cyanate ester polymer is largely preserved up to 160 °C due to the insulating SiO2 coating on individual conductive Fe3O4 submicrometer spheres. These composites also demonstrate high dielectric breakdown strengths at room temperature. Dynamic mechanical analysis indicates that the storage modulus of the composite with a 20 vol % filler loading is twice as high as that of neat resin, but the glass transition temperature considerably decreases with increasing filler content. Magnetic measurements reveal a large saturation magnetization and negligibly low coercivity and remanent magnetization in these composites.

A simple mean field model for an exchange-coupled permanent magnet containing a distribution of coercivities provides a qualitative description for the origin of time-independent hysteresis in the recoil loop and demonstrates that the area of the recoil loop is not a valid metric for the interaction strength. The conclusions of this model may be applied to two-phase exchange-coupled “spring magnets” as well as to crystallographically single-phase material where the distribution in coercivities arise from variations in microstructures. When the mean field interaction with the magnetization of the hard component exceeds the coercivity of the soft component, the interaction field serves as an effective bias field shifting the major loop of the soft component into the second quadrant of the magnetization curve with respect to the applied field. The area enclosed in the recoil is demonstrated to be a function of the mean field interaction strength, the distribution of coercivities, the volume fraction of the soft component, and the field from which the recoil is measured. The area of the recoil loop is expected to approach zero for both weak and strong interactions.

The Nd–Ni system has been investigated via experiments and thermodynamic modeling. In the experimental part, alloys across the entire composition range were prepared by arc melting pure Nd and Ni slugs and annealing the alloys at 500 °C ( 70 at.% Ni) for 2 weeks. The annealed alloys were then subjected to differential scanning calorimetry and differential thermal analysis (DTA) measurements to determine the invariant reaction temperatures. Except for the peritectic reaction liquid + Nd 7 Ni 3 ↔ Nd 3 Ni and the polymorphous phase transformation of Nd 2 Ni 7 found in this study, all the other invariant reactions are confirmed to be the same types but with significant different temperatures as reported results. In the modeling part, the available phase equilibrium and thermodynamic data in the Nd–Ni system were analyzed by using thermodynamic models for the Gibbs energies of individual phases. An optimal set of thermodynamic parameters were obtained using WinPhad software. The calculated phase equilibria and thermodynamic properties from the model parameters were compared to the corresponding experimental data and good agreement was obtained.

The magnetic properties of monodisperse Fe nanoparticles with over 4 orders of magnitude difference in concentration are studied by a combination of ordinary and remanent hysteresis loops, zero field cooled magnetization as a function of temperature, and magnetic relaxation rates. We compare the behavior of dilute dispersions with different concentrations, dispersions, and arrays made from the same particles, and nanoparticle arrays with different particle sizes and separations. The results are related to theoretical predictions and are used to create a unified picture of magnetostatic interactions within the assemblies.

For a uniaxial nanocrystalline magnetic material, the determination of the saturation magnetization, M s , requires measurements of the magnetization at fields which exceed the anisotropy field. For a typical RE–Tm compound, where RE=rare earth and Tm=transition metal, this may require fields above 7 T if the approach to saturation law is used. However for an isotropic material composed of a random distribution of non-interacting uniaxial grains, both M s and the anisotropy filed, H a , may be determined by fitting the Stoner–Wohlfarth (SW) model (Philos. Trans. Roy. Soc. 240 (1948) 599) to the reversible part of the demagnetization curve in the first quadrant. Furthermore, using the mean field interaction model of Callen, Liu and Cullen [2], a quantitative measure of the interaction strength for interacting particles may be determined. In conjunction with an analytical fit to the first quadrant demagnetization curve of the SW model, this allows M s , H a and the mean field interaction constant of a nanocrystalline magnet to be determined from measurements below 5 T. Furthermore, comparison of the model solution for the reversible magnetization with experimental data in the 2nd and 3rd quadrants allows the accurate determination of the switching field distribution. In many cases the hysteresis loop may be accurately described by a normal distribution of switching fields.

Neutron and synchrotron resonant x-ray magnetic scattering (RXMS) complemented by heat capacity and resistivity measurements reveal the evolution of the magnetic structures of Fe and Ce sublattices in a CeFeAsO single crystal. The RXMS of magnetic reflections at the Ce ${L}_{\mathrm{II}}$ edge shows a magnetic transition that is specific to the Ce antiferromagnetic long-range ordering at ${T}_{\mathrm{Ce}}\ensuremath{\approx}$ 4 K with short-range Ce ordering above ${T}_{\mathrm{Ce}}$, whereas neutron diffraction measurements of a few magnetic reflections indicate a transition at ${T}^{*}\ensuremath{\approx}$ 12 K with an unusual order parameter. Detailed order-parameter measurements on several magnetic reflections by neutrons show a weak anomaly at 4 K that we associate with the Ce ordering. The successive transitions at ${T}_{\mathrm{Ce}}$ and ${T}^{*}$ can also be clearly identified by two anomalies in heat capacity and resistivity measurements. The higher transition temperature at ${T}^{*}\ensuremath{\approx}$ 12 K is mainly ascribed to Fe spin reorientation transition, below which Fe spins rotate uniformly and gradually in the $ab$ plane. The Fe spin reorientation transition and short-range Ce ordering above ${T}_{\mathrm{Ce}}$ reflect the strong Fe-Ce couplings prior to long-range ordering of the Ce. The evolution of the intricate magnetic structures in CeFeAsO going through ${T}^{*}$ and ${T}_{\mathrm{Ce}}$ is proposed.

Single-crystal synchrotron x-ray diffraction studies of CeFeAsO reveal strong anisotropy in the charge correlation lengths along or perpendicular to the in-plane antiferromagnetic (AFM) wave-vector at low temperatures, indicating an anisotropic two-dimensional magnetoelastic coupling. The high-resolution setup allows to distinctly monitor each of the twin domains by virtue of a finite misfit angle between them that follows the order parameter. In addition, we find that the in-plane correlations, above the orthorhombic (O)-to-tetragonal (T) transition, are shorter than those in each of the domains in the AFM phase, indicating a distribution of the in-plane lattice constants. This strongly suggests that the phase above the structural O-to-T transition is virtually T with strong O-T fluctuations that are probably induced by spin fluctuations., Comment: 5 pages, 4 Figures, Rapid Communications

Current Nd 2 Fe 14 B magnet alloys exhibit excellent room-temperature magnetic properties and they are well suited for applications with operating temperatures ≤ 120°C, due in part to their low Curie temperature of ∼310°C. The poor temperature stability of these rare earth (RE) permanent magnet alloys above 120°C limits their current performance in existing motors and their potential application in advanced drive-motor designs. Consequently, it is necessary to find other compositions to improve the thermal stability of RE 2 Fe 14 B magnets. A systematic study was conducted by melt spinning on the magnetic properties of a series of isotropic nanocrystalline magnet alloys where a yttrium (Y)+dysprosium (Dy) mixture replaced neodymium (Nd) or praseodymium (Pr) as the dominant RE constituent in mixed rare earth (MRE) 2 Fe 14 B (MRE = Y+Dy+Nd). The most recent results have shown that the Y+Dy-based MRE 2 Fe 14 B alloy can result in isotropic bonded magnets with superior magnetic properties in competitive commercial isotropic bonded magnets above ∼30°C and well beyond 200°C by a judicious combination of Y, Dy, and Nd, along with a minor cobalt substitution for iron. Gasatomized powders of these advanced magnet alloys have also demonstrated improved magnetic strength over existing commercial spherical powders in a fine-powder-size range that is suitable for injection molding of bonded isotropic magnets.

Elastic and inelastic neutron-scattering studies of $\text{Li}({\text{Ni}}_{1\ensuremath{-}x}{\text{Fe}}_{x}){\text{PO}}_{4}$ single crystals reveal anomalous spin-wave dispersions along the crystallographic direction parallel to the characteristic wave vector of the magnetic incommensurate phase. The anomalous spin-wave dispersion (magnetic soft mode) indicates the instability of the Ising-type ground state that eventually evolves into the incommensurate phase as the temperature is raised. The pure ${\text{LiNiPO}}_{4}$ system $(x=0)$ undergoes a first-order magnetic phase transition from a long-range incommensurate phase to an antiferromagnetic (AFM) ground state at ${T}_{N}=20.8\text{ }\text{K}$. At 20% Fe concentrations, although the AFM ground state is to a large extent preserved as that of the pure system, the phase transition is second order, and the incommensurate phase is completely suppressed. Analysis of the dispersion curves using a Heisenberg spin Hamiltonian that includes interplane and in-plane nearest- and next-nearest-neighbor couplings reveals frustration due to strong competing interactions between nearest- and next-nearest-neighbor sites, consistent with the observed incommensurate structure. The Fe substitution only slightly lowers the extent of the frustration, sufficient to suppress the incommensurate phase. An energy gap in the dispersion curves gradually decreases with the increase in Fe content from $\ensuremath{\sim}2\text{ }\text{meV}$ for the pure system $(x=0)$ to $\ensuremath{\sim}0.9\text{ }\text{meV}$ for $x=0.2$.

Variation of crystal‐lattice parameters with temperature was studied by means of X‐ray diffraction technique on the perovskite‐like structures with Ba‐ substitution for light rare earth. Anisotropic negative thermal expansion was observed and explained within the “membrane effect” model proving its lattice origin.

Pr2Co14B permanent magnet powders were prepared by mechanical milling of an arc-melted ingot. X-ray diffraction analysis revealed the presence of the 2:14:1 phase after 1 h of milling which transformed into an amorphous phase with additional milling time. Increasing the milling time also lowered the intrinsic coercivity while the saturation magnetization increased up to 105 emu/g. Differential scanning calorimetry measurements revealed a crystallization temperature of around 560 °C. Upon annealing 30 h of as-milled amorphous powders between 500 and 900 °C, we observed the precipitation of the 2:14:1 phase. The optimum post-milling annealing temperature was 600 °C with an intrinsic coercivity of 7 kOe and maximum energy product of 6 MGOe.

The effect of titanium carbide (TiC) addition and Pr substitution in nanocrystalline Nd-Fe-B based magnetic materials has been investigated. Rapidly solidified (Pr,Nd)-Fe-B alloys were prepared by melt spinning. The compositions studied were (PrxNd1−x)9.5Fe84.5B6 (x=0, 0.1, 0.25, 0.5, 0.75, and 1) with the addition of 3at.% TiC. Varying the wheel speed from 15to30m∕s, the best magnetic properties were found for samples prepared at 20m∕s. All samples were nanocrystalline in the as-cast state. The average coercive field ranged from 10.0to12.6kOe and energy product from 7.8to9.0MGOe. The improvement in the coercive field compared to similar Nd based alloys was about 30%. Rietveld analysis of x-ray data for the Pr9.5Fe84.5B6 sample revealed that this material consists of approximately 70wt.% of 2:14:1 phase and 30wt.% of α‐Fe. This result was confirmed by Mossbauer data analysis and is in agreement with compositions expected from the ternary phase diagram.