FORC analysis of magnetically soft microparticles embedded in a polymeric elastic environment

First-order reversal curve (FORC) analysis allows one to investigate composite magnetic materials by decomposing the magnetic response of a whole sample into individual responses of the elementary objects comprising the sample. In this work, we apply this technique to analysing silicone elastomer composites reinforced with ferromagnetic microparticles possessing low intrinsic coercivity. Even though the material of such particles does not demonstrate significant magnetic hysteresis, the soft matrix of the elastomers allows for the translational mobility of the particles and enables their magnetomechanical hysteresis which renders into a wasp-waisted major magnetization loop of the whole sample. It is demonstrated that the FORC diagrams of the composites contain characteristic wing features arising from the collective hysteretic magnetization of the magnetically soft particles. The influence of the matrix elasticity and particle concentration on the shape of the wing feature is investigated, and an approach to interpreting experimental FORC diagrams of the magnetically soft magnetoactive elastomers is proposed. The experimental data are in qualitative agreement with the results of the simulation of the particle magnetization process obtained using a model comprised of two magnetically soft particles embedded in an elastic environment.

Electro-discharge sintering of nanocrystalline NdFeB magnets: process parameters, microstructure, and the resulting magnetic properties

This study investigates the compaction of nanocrystalline NdFeB magnet powder by electro-discharge sintering (EDS). On this account, process parameters, microstructure, and the associated magnetic properties of the EDS-densified nanocrystalline NdFeB specimens were investigated by varying the discharge energy EEDS and compression load pEDS. Although optimized process parameters could be evaluated, three different microstructures (fully densified zone, insufficiently densified zone, and melted zone) are present in the EDS-compacted specimens. Thereby, volume fractions of these formed three different microstructures determine the resulting mechanical and magnetic properties of the specimens. For all specimens, the intrinsic coercivity Hc,J deteriorates with increasing discharge energy, as the generated Joule heat leads to microstructural changes (grain growth, dissolution of magnetic phases), which reduces the magnetic properties. The compression load has less influence on the coercivity Hc,J, as it only affects the initial resistance of the pre-compacted powder loose. The residual induction Br deteriorates with increasing the discharge energy due to microstructural changes. An increase in the compression load pEDS results in an increase in the specimens’ density and thus promotes the residual induction Br.

PrFeB Based Alloys Obtained by Melt Spinning for the Production of Permanent Magnets

Rare earth permanent magnets are essential components in many fields of technology due to their excellent magnetic properties. There are some techniques used in the manufacture of permanent rare earth magnets: the powder metallurgy to obtain anisotropic HD sintered permanent magnets and the melt spinning and HDDR processes to obtain isotropic and anisotropic bonded permanent magnets. In this work, the influence of the melt spinning parameters on the microstructural and magnetic properties of the Pr14FebalCo16B6 alloy was studied. The alloy was melted and rapidly cooled at 9.9 x 105°C/s. The parameters used in the process were: wheel speed of 15 m/s and 20 m/s and ejection pressure of 25.3 kPa and 50.7 kPa. Ribbons and/or flakes of 30 μm thickness and width until 5 mm were obtained. Results show that the melt spinning alloys are nanocrystalline and that the parameters of the process influence the microstructure and their magnetic properties. Mean crystallite size up to 38.5 nm and intrinsic coercivity (iHc) up to 254 kA/m were obtained.

Antisymmetric linear magnetoresistance and the planar Hall effect

The phenomena of antisymmetric magnetoresistance and the planar Hall effect are deeply entwined with ferromagnetism. The intrinsic magnetization of the ordered state permits these unusual and rarely observed manifestations of Onsager’s theorem when time reversal symmetry is broken at zero applied field. Here we study two classes of ferromagnetic materials, rare-earth magnets with high intrinsic coercivity and antiferromagnetic pyrochlores with strongly-pinned ferromagnetic domain walls, which both exhibit antisymmetric magnetoresistive behavior. By mapping out the peculiar angular variation of the antisymmetric galvanomagnetic response with respect to the relative alignments of the magnetization, magnetic field, and electrical current, we experimentally distinguish two distinct underlying microscopic mechanisms: namely, spin-dependent scattering of a Zeeman-shifted Fermi surface and anomalous electron velocities. Our work demonstrates that the anomalous electron velocity physics typically associated with the anomalous Hall effect is prevalent beyond the ρxy(Hz) channel, and should be understood as a part of the general galvanomagnetic behavior.