W.C. Chang

High performance α-Fe/Nd2Fe14B-type nanocomposites

The α-Fe/R2Fe14B-type exchange-coupled NdFeB nanocomposites with a coercivity (iHc) of more than 11 kOe and energy product, (BH)max, of more than 16 MGOe have been successfully obtained on melt spun (Nd0.95La0.05)7.5+xFebalCr2B10 alloy powders, where x ranged from 3 to 3.5. It was found that a slight substitution of Cr for Fe suppresses the formation of the R2Fe23B3 and Fe3B phases during crystallization and results in the formation of an α-Fe/R2Fe14B mixture. Increasing the total rare earth content was found to enhance the remanence, and the iHc of (Nd0.95La0.05)7.5+xFe80.5−xCr2B10 ribbons was also increased drastically with increasing values of x. A Br of 9.2–9.7 kG, iHc of 11.1–13.2 kOe, and (BH)max of 16.5–18.0 MGOe have been obtained on (Nd0.95La0.05)7.5+xFe80.5−xCr2B10 (x=3–3.5).

Magnetic property enhancement of melt-spun Pr2Fe23B3 ribbons with dilute Ti substitution

The phase evolution and magnetic properties of Pr2Fe23B3 and Pr2(Fe0.975Ti0.025)23B3 melt-spun ribbons have been investigated. For Pr2Fe23B3 spun at 25 m/s, the magnetically hard Pr2Fe14B phase coexists with α-Fe, Pr2Fe23B3, and Fe3B when annealed at temperatures above 700 °C, while merely two phases, namely, Pr2Fe14B and α-Fe, coexist if annealed above 800 °C. Because of the excessive annealing, these ribbons exhibit large grain sizes that lead to low Br and iHc values. A slight substitution of Ti for Fe in Pr2(Fe0.975Ti0.025)23B3 suppresses the formation of metastable Pr2Fe23B3 phase and leads to the presence of large amount of Pr2Fe14B and α-Fe phases of fine grain sizes in the matrix even at low annealing temperature (i.e., TH=650 °C). As a result, the magnetic properties improve remarkably from Br=6.7 kG, iHc=1.5 kOe and (BH)max=1.7 MG Oe of the ternary Pr2Fe23B3 to Br=9.9 kG, iHc=5.7 kOe and (BH)max=15.0 MGOe of the Pr2(Fe0.975Ti0.025)23B3.

The effects of La-substitution on the microstructure and magnetic properties of nanocomposite NdFeB melt spun ribbons

Abstract The magnetic properties and microstructural evolution of (Nd 1− x La x ) 9.5 Fe 85.5 B 5 (x = 0, 0.05, 0.10and0.15) melt spun ribbons have been investigated. It was found that La is effective in refining the grain size of these ribbons. La-substitution is also effective in yielding excellent magnetic properties even when the alloy is melt spun at a lower wheel speed. The magnetic properties of the ribbons are determined by two major factors: (1) the magnetic dilution and (2) the enhancement of exchange coupling between Nd 2 Fe 14 B and α-Fe phases of fine grains. B r of 9.9 kG, i H c of 6.2 kOe and (BH) max = 15.5MGOe have been obtained in the alloy ribbons with x = 0.10 when directly quenched at a wheel speed of 18 m/s.

The effects of refractory metals on the magnetic properties of /spl alpha/-Fe/R/sub 2/Fe/sub 14/B-type nanocomposites

The phase transformations and magnetic properties of rare earth lean and boron rich (Nd/sub 0.95/La/sub 0.05/)/sub x/Fe/sub bal./M/sub 2/B/sub 10.5/, where x=9.5 or 11 and M=Cr, Ti, Nb, V, Mo, Zr, Nf, Ta, Mn or W, melt spun ribbons have been investigated. Two magnetic phases, namely /spl alpha/-Fe and R/sub 2/Fe/sub 14/B, were found in ribbons studied. A third magnetic phase, the R/sub 2/Fe/sub 23/B/sub 3/, was also detected in (Nd/sub 0.95/La/sub 0.05/)/sub 9.5/Fe/sub 78/M/sub 2/B/sub 10.5/ (M=Mo and Mn). Remanence (B/sub r/) and coercivity (/sub i/H/sub c/) values in the range of 8.0 to 9.1 kG and 9.5 to 18.9 kOe, respectively, have been achieved in nanocomposites with merely two magnetic phases. Among compositions studied, the Ti and W-substitutions were found to be most effective in increasing the B/sub r/ and /sub i/H/sub c/, respectively. For a fixed refractory metal substitution, namely, M=Cr, Ti or Nb, an increase in the total rare earth concentration resulted in nanocomposites of small grain sizes and a high volume fraction of the R/sub 2/Fe/sub 14/B phase. The combination of fine grain size and high volume fraction of R/sub 2/Fe/sub 14/B phase led to an increase in the /sub i/H/sub c/ and maximum energy product, (BH)/sub max/ of nanocomposites studied. A B/sub r/ of 9.1 kG, /sub i/H/sub c/ of 16.7 kOe and (BH)/sub max/ of 16.8 MGOe have been obtained on (Nd/sub 0.95/La/sub 0.05/)/sub 11/Fe/sub 76.5/Ti/sub 2/B/sub 10.5/.

Effect of Au cap layer on the magnetic properties and the microstructure for FePt thin films

We have investigated the effect of interfacial diffusion on the magnetic properties of Au∕Fe51Pt49 bilayer thin films. The samples were prepared in two stages. First, an ordered Fe51Pt49 continuous thin film was sputtered on a quartz substrate. Then an aurum cap layer was deposited onto the Fe51Pt49 film at room temperature, followed by a postannealing at 300–800°C to promote the interfacial diffusion. A high coercivity of 23.5kOe was achieved in the isotropic polycrystalline FePt-based film. Transmission electron micrographs indicate that the bilayer sample exhibits Ll0 ordered FePt grains with a diameter of about 20nm, which were partially isolated by Au phase. We consider that the isolation of FePtLl0 grains and the grain refining effect can be two contributory factors for the enhanced coercivity.

Mössbauer studies of melt-spun Pr2Fe14B ribbons

Melt-spun ribbons of tetragonal Pr2Fe14B exhibit favorable hard magnet characteristics. Technically relevant materials based on this compound, however, generally contain a certain amount of soft magnetic α-Fe or Fe3−xBx for remanence enhancement through exchange coupling. The nominal off-stoichiometric compositions lead to metallurgical complications, which are not easily resolvable by standard phase identification techniques such as x-ray diffraction and thermal magnetic analysis. As a viable alternative, 57Fe-Mossbauer spectroscopy can be used to delineate individual Fe sites. To provide a basis for such an approach, this report gives Mossbauer parameters including hyperfine magnetic field, isomer shift, and quadrupole splitting as obtained from a single-phase melt-spun Pr2Fe14B ribbon.

Alloying effect on the magnetic properties of RFeB-type bulk magnets

Composition dependence on the magnetic properties of directly casted cylindrical RFeBM-type magnets with diameters from 0.7 mm to 1.5 mm has been studied. In order to refine the microstructure for improving the magnetic properties of bulk magnets with various sizes, the following ways were adopted in the alloys: (1) changing the boron concentration, (2) substituting different kinds of refractory elements for Fe, (3) adding a slight carbon and finally, (4) adopting multi-component refractory elements. The experimental results showed that the Nd9.5Fe72.5Ti3B15 bulk magnets with a diameter of 0.7 mm demonstrated the optimal magnetic properties of Br = 6.5 kG, iHc = 10.3 kOe and (BH)max = 8.7 MG Oe, while the highest iHc of 16.2 kOe was achieved for the Pr9.5Fe71.5Nb4B15 magnet. Meanwhile, the diameter of the magnets could be increased to 0.9 mm preserving the attractive magnetic properties by the substitution of 0.5 at% Zr for Ti in the Nd9.5Fe72.5Ti3B15 magnet. Finally, the directly casted Nd9.5Fe72.5−yTi2.5Zr0.5CryB15−xCx (x = 0.25–1; y = 0–3) magnets with a larger diameter of 1.3 mm exhibited attractive magnetic properties of Br = 5.3–6.1 kG, iHc = 7.2–12.5 kOe and (BH)max = 6.5–7.2 MG Oe.

The effect of boron and rare earth contents on the magnetic properties of La and Cr substituted α-Fe/R2Fe14B-type nanocomposites

The effect of phase transformations on the magnetic properties of rare earth lean (Nd0.95La0.05)9.5Fe82.5−xCr2B6+x (x=0 to 4.5) and (Nd0.95La0.05)7.5+yFe80.5−yCr2B10 (y=0 to 4) melt spun ribbons has been investigated. The phase mixture, after optimum thermal processing, was found to be strongly dependent upon the rare earth and boron contents. Two magnetic phases, namely α-Fe and R2Fe14B, were found in (Nd0.95La0.05)9.5Fe82.5−xCr2B6+x alloy ribbons with x ranging from 0 to 4.5. For a fixed rare earth content, increases in the boron concentration resulted in a higher volume fraction of the R2Fe14B phase, which led to an increase in the intrinsic coercive force from 7.1 kOe for x=0 to 12.6 kOe for x=4.2. A Br=9.6 kG, iHc=9.5 kOe, and (BH)max=15.5 MGOe have been obtained in the alloy ribbons with x=4.5. On the other hand, the increase in the total rare earth content, or y, was found to suppress the formation of the metastable Fe3B and/or R2Fe23B3 phases and to yield an α-Fe/R2Fe14B mixture for y>1. This incr...

Microstructure, martensitic transitions, magnetocaloric, and exchange bias properties in Fe-doped Ni-Mn-Sn melt-spun ribbons

The effects of Fe substitution for Ni on microstructure, phase transformations, magnetocaloric effect, and exchange-bias behavior of the Ni46−xFexMn43Sn11 (x = 0–3) alloy ribbons have been investigated. The free surface of as-spun Fe-doped ribbons shows the granular microstructure containing multiple shapes (the tree leaf-like, small columnar grain, etc.), while the ordered columnar grains are observed in fracture cross-section. The martensitic structural transition temperature (TM) of as-annealed ribbons decreases from 240 K for x = 0 to 185 K for x = 3 due to the decrease in valence electron concentration, while the Curie temperature of the austenitic phase remains almost unchanged (TC = 275 K). The positive values of magnetic entropy changes (+ΔSM), around TM, are 21.0, 29.1, 24.1, and 14.8 J/kg K for x = 0–3, respectively, while the negative −ΔSM values vary in 3.0–3.5 J/kg K range around TC, under a field change of 0–5 T. The values of exchange-bias field (HE) at 10 K change in the range of 469 to 53...

Magnetic properties enhancement of nanocomposite Nd9.5Fe81.5B9 melt–spun ribbons by La and Cr substitutions

The phase transformation and magnetic properties of nanocomposite (Nd0.95La0.05)9.5(Fe1−xCrx)81.5B9, where x=0, 0.033, 0.067, and 0.1, melt–spun ribbons have been studied. For Cr-free ribbons (i.e, x=0,) a slight substitution of La for Nd decreases the crystallization temperature of the over-quenched ribbons. Moreover, the (Nd, La)3Fe62B14 and (Nd, La)2Fe23B3 phases were found to coexist with (Nd, La)2Fe14B/Fe3B/α-Fe phases in the crystallized (Nd0.95La0.05)9.5Fe81.5B9 ribbons. The crystallization temperatures of (Nd0.95La0.05)9.5Fe1−xCrx)81.5B9 were found to increase with Cr concentration. The intrinsic coercivity of (Nd0.95La0.05)9.5(Fe1−xCrx)81.5B9 increased from 6.2 kOe for x=0 to 10.7 kOe for x=0.1: Transmission electron microscopy-energy dispersion analytical x-ray analyses indicated that the increase in intrinsic coercivity may be associated with the increase in Cr concentration around grain boundaries. A sample with Br=8.3 kG, iH0=9.1 kOe, and (BH)max=12.5 MGOe was obtained on (Nd0.95La0.05)9.5(Fe...