Noritsugu Sakuma

Conversion of Anisotropically Phase-Segregated Pd/γ-Fe2O3 Nanoparticles into Exchange-Coupled fct-FePd/α-Fe Nanocomposite Magnets

Exchange-coupled fct-FePd/alpha-Fe nanocomposite magnets were fabricated by converting anisotropically phase-segregated Pd/gamma-Fe2O3 nanoparticles via the interfacial atom diffusion. The magnetically hard fct-FePd phases formed by the interdiffusion between alpha-Fe and fcc-Pd phases nearly preserve their sizes at the nanometer scale because they are surrounded by the alpha-Fe matrix. The VSM measurements reveal that the exchange coupling between the soft and hard phases has been realized.

Exchange coupling interaction in L10-FePd/α-Fe nanocomposite magnets with large maximum energy products.

Nanocomposite magnets (NCMs) consisting of hard and soft magnetic phases are expected to be instrumental in overcoming the current theoretical limit of magnet performance. In this study, structural analyses were performed on L1(0)-FePd/α-Fe NCMs with various hard/soft volume fractions, which were formed by annealing Pd/γ-Fe(2)O(3) heterostructured nanoparticles and pure Pd nanoparticles. The sample with a hard/soft volume ratio of 82/18 formed by annealing at 773 K had the largest maximum energy product (BH(max) = 10.3 MGOe). In such a sample, the interface between the hard and soft phases was coherent and the phase sizes were optimized, both of which effectively induced exchange coupling. This exchange coupling was directly observed by visualizing the magnetic interaction between the hard and soft phases using a first-order reversal curve diagram, which is a valuable tool to improve the magnetic properties of NCMs.

Grain-size dependent demagnetizing factors in permanent magnets

The coercive field of permanent magnets decreases with increasing grain size. The grain size dependence of coercivity is explained by a size dependent demagnetizing factor. In Dy free Nd2Fe14B magnets, the size dependent demagnetizing factor ranges from 0.2 for a grain size of 55 nm to 1.22 for a grain size of 8300 nm. The comparison of experimental data with micromagnetic simulations suggests that the grain size dependence of the coercive field in hard magnets is due to the non-uniform magnetostatic field in polyhedral grains.

High energy product in Battenberg structured magnets

Multiphase nano-structured permanent magnets show a high thermal stability of remanence and a high energy product while the amount of rare-earth elements is reduced. Non-zero temperature micromagnetic simulations show that a temperature coefficient of remanence of −0.073%/K and that an energy product greater than 400 kJ/m3 can be achieved at a temperature of 450 K in a magnet containing around 40 volume percent Fe65Co35 embedded in a hard magnetic matrix.

A (Nd, Zr)(Fe, Co)11.5Ti0.5Nx compound as a permanent magnet material

We studied NdFe11TiNx compounds as permanent magnet materials. The (Nd0.7,Zr0.3)(Fe0.75Co0.25)11.5Ti0.5N0.52 powder that contained a limited amount of the α-(Fe, Co) phase shows fairly good magnetic properties, such as a saturation polarization (Js) of 1.68 T and an anisotropic field (Ha) of 2.88 (Law of approach to saturation) – 4.0 MA/m (Intersection of magnetization curves). Both properties are comparable to those of the Nd2Fe14B phase.

(Sm,Zr)(Fe,Co)11.0-11.5Ti1.0-0.5 compounds as new permanent magnet materials

We investigated (Sm,Zr)(Fe,Co)11.0-11.5Ti1.0-0.5 compounds as permanent magnet materials. Good magnetic properties were observed in (Sm0.8Zr0.2)(Fe0.75Co0.25)11.5Ti0.5powder containing a limited amount of the α-(Fe, Co) phase, including saturationpolarization (Js) of 1.63 T, an anisotropic field (Ha) of 5.90 MA/m at room temperature, and a Curie temperature (Tc) of about 880 K. Notably, Js and Ha remained above 1.5 T and 3.70 MA/m, respectively, even at 473 K. The high-temperature magnetic properties of (Sm0.8Zr0.2)(Fe0.75Co0.25)11.5Ti0.5 were superior to those of Nd2Fe14B.

Coercivity enhancement in Ce-Fe-B based magnets by core-shell grain structuring

Ce-based R2Fe14B (R= rare-earth) nano-structured permanent magnets consisting of (Ce,Nd)2Fe14B core-shell grains separated by a non-magnetic grain boundary phase, in which the relative amount of Nd to Ce is higher in the shell of the magnetic grain than in its core, were fabricated by Nd-Cu infiltration into (Ce,Nd)2Fe14B hot-deformed magnets. The coercivity values of infiltrated core-shell structured magnets are superior to those of as-hot-deformed magnets with the same overall Nd content. This is attributed to the higher value of magnetocrystalline anisotropy of the shell phase in the core-shell structured infiltrated magnets compared to the homogeneous R2Fe14B grains of the as-hot-deformed magnets, and to magnetic isolation of R2Fe14B grains by the infiltrated grain boundary phase. First order reversal curve (FORC) diagrams suggest that the higher anisotropy shell suppresses initial magnetization reversal at the edges and corners of the R2Fe14B grains.

Nonlinear conjugate gradient methods in micromagnetics

Conjugate gradient methods for energy minimization in micromagnetics are compared. The comparison of analytic results with numerical simulation shows that standard conjugate gradient method may fail to produce correct results. A method that restricts the step length in the line search is introduced, in order to avoid this problem. When the step length in the line search is controlled, conjugate gradient techniques are a fast and reliable way to compute the hysteresis properties of permanent magnets. The method is applied to investigate demagnetizing effects in NdFe12 based permanent magnets. The reduction of the coercive field by demagnetizing effects is μ0ΔH = 1.4 T at 450 K.

On the limits of coercivity in permanent magnets

The maximum coercivity that can be achieved for a given hard magnetic alloy is estimated by computing the energy barrier for the nucleation of a reversed domain in an idealized microstructure without any structural defects and without any soft magnetic secondary phases. For Sm1–zZrz(Fe1–yCoy)12–xTix based alloys, which are considered an alternative to Nd2Fe14B magnets with a lower rare-earth content, the coercive field of a small magnetic cube is reduced to 60% of the anisotropy field at room temperature and to 50% of the anisotropy field at elevated temperature (473 K). This decrease of the coercive field is caused by misorientation, demagnetizing fields, and thermal fluctuations.

Influence of Zr substitution on the stabilization of ThMn12-type (Nd1−αZrα)(Fe0.75Co0.25)11.25Ti0.75N1.2−1.4 (α = 0–0.3) compounds

The influence of Zr substitution in ThMn12 compounds was investigated using strip casting alloys. It was found that Zr substitution stabilized (Nd1−αZrα)(Fe0.75Co0.25)11.25Ti0.75N1.2−1.4 (α = 0–0.3) compounds. Specifically, a reduction in the lattice constant along the a-axis was observed. Energy-dispersive X-ray spectroscopy mapping combined with Cs-corrected scanning transmission electron microscopy indicated that Zr atoms preferentially occupied Nd 2a sites. Both the magnetic anisotropy field and saturationpolarization were maximum at Zr substitution ratio α = 0.1. The (Nd1−αZrα)(Fe0.75Co0.25)11.25Ti0.75N1.2−1.4 (α = 0–0.3) compounds displayed higher saturationpolarization than Nd2Fe14B at high temperatures.