Papers by Keyword: Coercivity

Abstract: The curling reversal mode is discussed, with emphasis on the assumptions of the model, many of them unrealistic. The curling mode assumes the Bloch approximation, where a cosine function is approximated by a polynomial of second order. It is argued that if the curling model takes place, this could affect the anisotropy field, and then the anisotropy field would be function of the grain size, but this is not observed. Exchange effects may affect hysteresis curve, as discussed in the SW-CLC model.

Abstract: Anisotropic bulk nanocrystal Nd-lean Nd-Fe-B magnets were prepared by hot-pressing the mixture of Nd_11.5Fe_81.5Nb₁B₆ melt-spun ribbons and pure Zn powder and subsequent hot-deforming. The effects of deformation on the magnetic properties and microstructure were also studied. The magnetic properties increased significantly for hot-deformed (HD) samples due to the formation of good c-axis texture with increasing deformation. The remanence M_r and maximal energy product (BH)_m increased and reached their maximal values at 65% deformation due to the increasing orientation. On the other hand, the grains increased and were elongated normal to the press direction gradually with increasing deformation. The variation trend was similar to that of traditional Nd-Fe-B magnets. However, there was an obvious improvement for coercivity in the initial stage of hot-deformation resulting from the adequate Zn diffusion into grain-boundary. When the deformation was larger than 65%, the magnetic properties were deteriorated due to abnormal grain growth. In order to improve further the coercivity Hci, the samples with Dy addition were prepared. The coercivity of 12300 Oe could be obtained for anisotropic bulk Nd_9.5Dy₂Fe_81.5Nb₁B₆.

Abstract: A microsecond pulsed laser beam was used to local magnetic domain modification of electrical grain oriented silicon steel. It was carried out using three different laser pulse regimes: a single pulse laser regime, a multipulse laser regime and a multipulse laser regime with modulation of laser pulses. The laser processing variables were pulse energy and and number of pulses. The samples were tested for nanohardness and coercivity before and after laser treatment. Light optical microscopy, scanning electron microscopy and magnetic force microscopy were used to observe the cross-sectional profile, surface of the samples, and magnetic domain visualization, respectively. The local laser treatment of grain oriented silicon steel surface has been studied in terms of its influence on the magnetic domains and coercivity. It was found that laser-modified samples showed coercivity improvement in comparison to the non-treated samples. The most significant improvement in coercivity was obtained in the modulated multipulse regime and negligible improvement in the single pulse laser regime. Three main effects responsible for the observed improvement were identified, namely: magnetic domain refinement, influence of number of laser pulses and shape of laser HAZ profile. The present work highlights on differences in the magnetic domain structure, microstructure of the laser modified material and basic electromagnetic and mechanical properties. In present study, the pulse laser surface processing was presented as a useful energy efficient alternative to other techniques e.g. mechanical scribing, electrical discharge scribing, plasma jet scribing, etc. The refined magnetic domains in electrosteels are responsible for the observed low coercivity, which indicates perspective application of the investigated laser modified steels in the power transformer cores with lower core losses.

Abstract: Liquid-liquid phase separation (LLPS) in the rapidly solidified Fe₇₁Cu₁₀P₁₀B₉ alloy under different casting conditions is investigated based on the XRD, DSC, and SEM measurements. It is found that during rapid solidification process, Cu-rich globules precipitated in the matrix which mainly consists of α-Fe and Fe₃B_0.82P_0.18 crystals. With increasing cooling rate, LLPS becomes weaker, leading to less precipitation of Cu-rich globules, while the microstructure of the matrix became finer. Magnetic measurements show that the saturation induction and the coercivity of the present samples increase first and then decrease with increasing cooling rate. The corresponding mechanisms related to magnetic performance are also discussed details.

Abstract: In the present investigation, FeCo/Cu core shell nanoparticles were prepared by coating a Cu layer over FeCo alloy nanoparticles through displacement reaction. X-ray diffraction studies confirmed the presence of FeCo and Cu phases in the sample. The grain size and lattice strains of the core shell nanostructures were evaluated from the x-ray profiles by using single line profile analysis technique. The effect of annealing temperature on the magnetic properties of the core shell nanoparticles was studied by using a vibrating sample magnetometer. The results showed that the magnetic properties improve significantly after annealing the compacts of core shell nanoparticles under a magnetic field. Enhancement in magnetization was observed in the compacts with the increase in annealing temperature. Highest saturation magnetization value of 56 emu/g was recorded in the sample which was annealed at 600°C. It has been also found that the blocking temperature of the core shell nanoparticles increases with the increase in annealing temperature.

Abstract: The Alnico magnets (also called Ticonal) have coercivity attributed to shape anisotropy. The conditions for appearing this type of anisotropy are discussed, and the principal is the existence of elongated grains parallel to the easy magnetization axis. A very important detail is that the conditions assumed in the Stoner Wohlfarth model have to be obeyed. The assumptions are i) ferromagnetic phase with single domain particle size and ii) grains without magnetic interaction between them, magnetically insulated by a grain boundary paramagnetic phase. In Alnico magnets, the coercivity mechanism is coherent rotation of single domain size particles.

Abstract: The Stoner-Wohlfarth hysteresis model can describe very well the hysteresis curves since the assumptions of the model are compatible with the studied samples. These conditions are (i) single domain particle size and (ii) inexistence of interactions between the particles. These conditions are obtained in Nd₂Fe₁₄B magnets with nanocrystalline grain size below 200 nm and with high Nd, which promotes a paramagnetic phase at the grain boundaries. In the present study, both isotropic and anisotropic NdFeB magnets are modeled.

Abstract: [(Fe_0.5Co_0.5)_0.75B_0.2Si_0.05]₉₆Nb₄}_100-x,y(Gd_x, Cu_y) (x = 0.5, 1 and 2), (y = 0.5) alloys with different diameters were prepared by copper mold casting. The structure, the thermal and the magnetic properties were studied by X-ray diffraction, differential scanning calorimetry and vibrating sample magnetometry, respectively. Minor Cu addition completely changes the crystallization behavior, and also the glass-forming ability decreases because of the decrease in the supercooled liquid region. However, the magnetic properties are significantly improved upon annealing because of the precipitation of (Fe,Co) phase. In case of [(Fe_0.5Co_0.5)_0.75B_0.2Si_0.05]₉₆Nb₄}_100-x (Gd_x) glass, Gd addition does not change the crystallization behavior. In turn, it increases the supercooled liquid region when compared with [(Fe_0.5Co_0.5)_0.75B_0.2Si_0.05]₉₆Nb_4, but even then the glass-forming ability is reduced (critical glassy diameter d_c < 1.5 mm). The saturation magnetization of the glass is also decreased with increasing Gd content.

Abstract: Hybrid bonded magnet Ba-Ferrite/NdFeB with 5% wt Epoxy Resin (ER) as polymer binder hsa been developed with variations in BaFe₁₂O₁₉ to NdFeB weight ratio. The variation of the BaO6Fe₂O₃ : Nd-Fe-B weight ratio are 90%:10%; 80%:20%; 70%:30% and 60%:40%. The magnetic particle consist of Ba-Ferrite and NdFeB were mixed until homogenize and compacted by using hydraulic press machine with 8 Tonf force to form a disc shape sample. The disc sample was dried using vacuum dryer with 10 mm bar pressure at 80°C for one hour before being magnetized using impulse magnetizer. The best %wt composition ratio of Ba-Ferrite/NdFeB is 70%/30% and 60%/40%. The hybrid bonded magnetic properties at the best %wt composition ratio are: bulk density = 4.28-4.43 g/cm³, FM = 1057-1121 Gauss, Br = 3.46-3.70 kG, Hc = 3.25-3.70 kOe, and BHmax = 1.60-1.70 MGOe.

Abstract: The magnetic and electrical properties of (Co/Ge)_nfilms are experimentally studied. It is established that at the Co/Ge interfacean intermediate magnetic layer forms. Twophases of cobalt, one is a face-centered cubic phase and the other ispresumably a Co–Ge alloy with a weakly ferromagnetic order, have been found toexist. A “dead” layer no more than 2 nm in thickness is formed at the interface.This layer affects the magnetic behavior andmagnetoresistive effect in the investigated structures.