Akira Manabe

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.

Element-Specific Magnetic Domain Imaging of (Nd, Dy)-Fe-B Sintered Magnets Using Scanning Transmission X-Ray Microscopy

We demonstrate an element-specific observation of magnetic domains in thermally demagnetized Nd-Fe-B and (Nd, Dy)-Fe-B sintered magnets using scanning transmission X-ray microscopy (STXM). Clear chemical and magnetic contrast images with the 30-nm spatial resolution were taken by STXM. Both maze-like magnetic domains and stripe magnetic domains with their widths of 200-300 nm are observed in both Nd-Fe-B and (Nd, Dy)-Fe-B sintered magnets. In both sintered magnets, multidomain structures are mostly formed within each grain-that is, magnetic domains are likely to be terminated at the grain boundaries. Stripe domains are originated from the grains with the (001)-axis misoriented to the sample normal. From the comparison between chemical and magnetic images, it is found that no clear magnetic domain is observed in Nd-rich phase at grain boundary triple points. Furthermore, it is also found that the interface between Nd2Fe14B phase and Nd-rich phase is chemically abrupt. Similar magnetic domain patterns are observed in Nd-Fe-B and (Nd, Dy)-Fe-B sintered magnets.

Influence of defect thickness on the angular dependence of coercivity in rare-earth permanent magnets

The coercive field and angular dependence of the coercive field of single-grain Nd2Fe14B permanent magnets are computed using finite element micromagnetics. It is shown that the thickness of surface defects plays a critical role in determining the reversal process. For small defect thicknesses reversal is heavily driven by nucleation, whereas with increasing defect thickness domain wall de-pinning becomes more important. This change results in an observable shift between two well-known behavioral models. A similar trend is observed in experimental measurements of bulk samples, where an Nd-Cu infiltration process has been used to enhance coercivity by modifying the grain boundaries. When account is taken of the imperfect grain alignment of real magnets, the single-grain computed results appears to closely match experimental behaviour.

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.

Interface Properties of SiO2/4H-SiC(0001) with Large Off-Angles Formed by N2O Oxidation

In this study, we have investigated N2O oxidation of various off-angled 4H-SiC (0001) epilayers and characterized the properties of MOS interfaces. The oxide thickness almost linearly increases with increasing off-angle. Oxidation on highly off-angled (0001) 4H-SiC is faster than that on 8o off-axis (0001). The off-angle dependence of Dit is very small for the MOS capacitors in the off-angle range from 8o to 30o. The depth profiles of carbon and nitrogen atoms near the MOS interface on 15o off-axis 4H-SiC(0001) are similar to those on 8o off-axis (0001).

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.