Tetsuya Shoji

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.

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.

First order reversal curve studies of permanent magnets

First order reversal curve (FORC) diagrams are a useful tool to analyze the magnetization processes in magnetic materials. FORC diagrams are computed from measured first order reversal curves on sintered Nd2Fe14B magnets. It is shown that the FORC diagram simplifies if the first order reversal curves a desheared using the macroscopic demagnetizing field given by the geometry of the sample. Furthermore the resulting FORC diagram is almost identical to the FORC diagram measured for a thin platelet of the same material. This opens the possibility to compare experimental FORC diagrams with FORC diagrams computed by micromagnetic simulations.

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.

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.

Spin reorientation transition and hard magnetic properties of MnBi intermetallic compound

The effects of mechanical grinding (MG) on the crystallite size, the spin reorientation transition temperature (TSR) and the hard magnetic properties in melt-spun low temperature phase (LTP) MnBi have been investigated in order to understand the origin of magnetic hardening induced by MG. The room-temperature coercive field (μ0Hcj) is enhanced dramatically from 0.08 T before MG to 1.5 T after MG for 43.2 ks while TSR is concurrently suppressed from 110 to 38 K. The coercive force exhibits positive temperature dependence approximately 50–60 K above TSR and the lowered TSR after MG could result in magnetic hardening at room temperature. The room-temperature coercive force of LTP-MnBi is highly dependent on the crystallite size (D) and is found to be described phenomenologically by the following relationship: μ0Hcj = μ0Ha(δ/D)n, where μ0Ha is ∼ 4 T, the Bloch wall width δ is 7 nm, and the exponent n is approximately 0.7. Our results suggest that the grain refinement is the primary origin of the hardening eff...

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.