C.H. Chiu

Magnetic properties, phase evolution, and microstructure of melt spun SmCo7-xHfxCy (x=0-0.5; y=0-0.14) ribbons

Magnetic properties, phase evolution, and microstructure of melt spun SmCo7−xHfxCy (x=0–0.5; y=0–0.14) ribbons via direct quenching have been investigated. For SmCo7−xHfx series ribbons, proper volume fraction of Hf is effective to stabilize and increase the magnetocrystalline anisotropy field of the TbCu7-type structure. Accordingly, the optimal magnetic properties of Br=6.4kG, Hci=7.3kOe, and (BH)max=8.7MGOe could be obtained for x=0.1. Furthermore, a slight C addition to above alloys is helpful in refining the grain size of the ribbons from 100–400nm for C-free to 10–80nm for C-containing samples. As a result, the magnetic properties are further improved to Br=6.9kG, Hci=11.8kOe, and (BH)max=10.6MGOe for SmCo6.8Hf0.2C0.12 nanocomposite ribbons.

Magnetic properties, phase evolution, and coercivity mechanism of CoxZr98−xB2 (x=74−86) nanocomposites

Magnetic properties, phase evolution, and coercivity mechanism of melt-spun CoxZr98−xB2 (74⩽x⩽86) nanocomposites were studied systematically. From x-ray diffraction analysis, nearly single magnetic phase, Co23Zr6, is found in Co74Zr24B2 ribbon (x=74). With increasing Co content x, an additional phase, Co5Zr, is found for x=76; two phases, namely, fcc Co and Co5Zr, are found for x=78−84, but fcc Co is the only phase that can be detected in ribbon with x=86. The optimal magnetic properties of Br=5.13kG (σr=49emu∕g), Hci=4.1kOe, and (BH)max=5.1MGOe are obtained in Co80Zr18B2 ribbons quenched at Vs=45m∕s, which possesses Co5Zr and minor fcc-Co phases with nanoscale grain size (∼50 and ∼20nm, respectively). Strong exchange-coupling effect occurred between grains as evidenced by the Henkel plot, giving rise to the improvement of remanence and magnetic energy product. The coercivity of CoxZr98−xB2 nanocomposites is mainly governed by reverse domain nucleation mechanism.

Effect of boron on the magnetic properties and exchange-coupling effect of FePtB-type nanocomposite ribbons

Phase evolution and magnetic properties of melt-spun (Fe0.675Pt0.325)100−xBx (x=12–20) nanocomposite ribbons are investigated. For those ribbons spun at 45m∕s, followed with a 500°C isothermal annealing for 1–6h, the boron addition not only promotes disorder to order (γ→γ1) transformation but also leads to the formation of different kinds of Fe-borides. Pt exhibits a strong affinity to Fe during thermal processes in forming magnetically hard γ1(FePt) phase, leaving excess Fe to interact with B in forming Fe2B, Fe3B, or FeB. For x=12–18, all ribbons are composed of one hard phase, γ1-FePt, and three soft phases, γ-FePt, Fe2B, and Fe3B. But FeB also coexists for x=20. Through the exchange-coupling interaction between nanoscale γ1-FePt and multiple soft phases, record of magnetic properties of Br=9.4kG, Hci=7.5kOe, and (BH)max=14.0MGOe has been successfully developed in isotropic (Fe0.675Pt0.325)84B16 ribbons, where the reduced remanence ratio (σr∕σ12kOe) is as high as 0.86.

Magnetic property improvement of Pt-lean FePt∕Fe–B-type nanocomposites by Co substitution

Effects of Co content on the magnetic properties and microstructure of melt-spun [(Fe1−xCox)0.675Pt0.325]84B16 (x=0–0.5) and [(Fe1−yCoy)0.725Pt0.275]85B15 (y=0 and 0.3) nanocomposite ribbons have been investigated. The substitution of Co for Fe in [(Fe1−xCox)0.675Pt0.325]84B16 ribbons enhances the coercivity (Hci) from 7.5kOe for x=0to10kOe for x=0.3, due to the formation of ordered L10-(Fe,Co)Pt phase with higher anisotropy field. The effect of Co substitution for Fe in [(Fe1−yCoy)0.725Pt0.275]85B15 series ribbons is similar to that in [(Fe1−xCox)0.675Pt0.325]84B16 system. Interestingly, larger magnetization could be obtained by decreasing the boron and Pt content simultaneously. Moreover, L10-(Fe,Co)Pt phase provides [(Fe0.7Co0.3)0.725Pt0.275]85B15 ribbons sufficient high coercivity Hci=5.4kOe, resulting in a remarkable enhancement of energy product from 10.0MGOe for Co-free ribbons to 15.7MGOe for ribbons with y=0.3.

Improvement on the magnetic properties of Pr/sub 8.5/Fe/sub 81.5/B/sub 10/ nanocomposites by refractory elements substitution

The magnetic properties and phase evolution of Pr-lean and boron-enriched Pr/sub 8.5/Fe/sub 79.5/M/sub 2/B/sub 10/ (M = Cr, Nb, V, Ti, and Zr) melt-spun ribbons have been studied. Based on thermal magnetic analysis (TMA), a slight substitution of the selected refractory elements (Cr, Nb, V, Ti, and Zr) for Fe in Pr/sub 8.5/Fe/sub 79.5/M/sub 2/B/sub 10/ not only suppresses the formation of metastable Pr/sub 2/Fe/sub 23/B/sub 3/ and Fe/sub 3/B phases but also leads to the presence of large amount of Pr/sub 2/Fe/sub 14/B and /spl alpha/-Fe phases of fine grain size (<30 nm). Exchange coupling effect is found to exist in all nanocomposites, which is attributed to their finer grain size if suitable crystallization treatment is employed. As a result, the magnetic properties of the ribbons are improved remarkably. The optimum magnetic properties of B/sub r/=9.6 kG, /sub i/H/sub c/=8.5 kOe, and (BH)/sub max/=17.8 MGOe are achieved in Pr/sub 8.5/Fe/sub 79.5/Ti/sub 2/B/sub 10/, while the highest coercivity of /sub i/H/sub c/=9.9 kOe (B/sub r/=9.2 kG and (BH)/sub max/=16.9 MGOe) is obtained in Pr/sub 8.5/Fe/sub 79.5/Nb/sub 2/B/sub 10/.

Effect of boron content on the magnetic properties, phase evolution and microstructure of Pr/sub 9/Fe/sub 88.5-x/Ti/sub 2.5/B/sub x/ (x=7-15) nanocomposites

The effect of boron content on the magnetic properties, phase evolution and the exchange coupling effect of Pr/sub 9/Fe/sub 88.5-x/Ti/sub 2.5/B/sub x/ ribbons is investigated. By increasing the boron content, the optimal magnetic properties of B/sub r/=9.5 kG, /sub j/H/sub c/=10.8 kOe, and (BH)/sub max/=17.8 MGOe are obtained for Pr/sub 9/Fe/sub bal./Ti/sub 2.5/B/sub 11/. Further substituting 10 at% Co for Fe in the Pr/sub 9/Fe/sub bal./Ti/sub 2.5/B/sub 11/ alloy enhances the magnetic properties for Pr/sub 9/Fe/sub 67.5/Co/sub 10/Ti/sub 2.5/B/sub 11/. Based on thermal magnetic analysis of the optimized Pr/sub 9/Fe/sub 88.5-x/Ti/sub 2.5/B/sub x/ ribbons, three phases Pr/sub 2/Fe/sub 14/B, /spl alpha/-Fe and a small amount of Pr/sub 2/Fe/sub 23/B/sub 3/ phases prevailed in the ribbons with x=15. For x=7-13, only Pr/sub 2/Fe/sub 14/B and /spl alpha/-Fe phases were found. Increasing x increases the volume fraction of Pr/sub 2/Fe/sub 14/B phase and decreases the /spl alpha/-Fe phase, giving rise to the increment of /sub j/H/sub c/ and the decrement of B/sub r/. But a decrement of the volume fraction of total magnetically phases, i.e. /spl alpha/-Fe and Pr/sub 2/Fe/sub 23/B/sub 3/, results in the deterioration of both B/sub r/ and (BH)/sub max/ drastically as x=15.

Magnetic aftereffect and magnetic force microscopy studies of Fe–B∕FePt-type nanocomposite ribbons

Magnetic aftereffect and surface magnetic domain structure of (Fe0.675Pt0.325)100−xBx(x=12–20) nanocomposites have been investigated for correlating with their corresponding phases and magnetic properties. Exchange-coupling effect between grains is present for all the studied ribbons as evidenced by Henkel plot. The volume fraction of magnetically soft phases decreases with increasing B concentration from x=12 to x=18. It leads to the reduction in the activation volume of the reverse domain. Among all samples, the (Fe0.675Pt0.325)82B18 ribbon having the highest intrinsic coercivity (Hci) exhibits minimum value in activation volume V=1.1×10−18cm3, because it possesses the least volume fraction of the magnetic soft phases, i.e., Fe2B or Fe3B phases. Magnetic force microscopy studies reveal the magnetic domain structures of the ribbons, confirming the existence of strong exchange coupling between magnetic grains.

Magnetic properties, microstructure and phase evolution of (Ce1−xPrx)9.5Febal.CoyTi2B10 (x=0–1 and y=0, 2.5) nanocomposites

Magnetic properties, microstructure, and phase evolution of (Ce1−xPrx)9.5Febal.CoyTi2B10 (x=0–1 and y=0, 2.5) nanocomposites have been investigated. A progressive substitution of Pr for Ce enhances the remanence (Br), intrinsic coercivity (Hci), and maximum energy product [(BH)max] of the ribbons effectively. In addition, the Curie temperature of 2:14:1 phase also increases with increasing Pr concentration. For Co-free (Ce1−xPrx)9.5Febal.Ti2B10 ribbons, magnetically hard 2:14:1 phase and two soft phases, α-Fe and Fe3B, are found in the ribbons with x=0, 0.3, 0.4, and 0.7, while Fe3B phase disappears in the ribbons with x=1. On the other hand, the value of Br, (BH)max, temperature coefficient α, and Curie temperature of 2:14:1 and α-Fe are improved with a slight substitution of 2.5 at. % Co for Fe. The magnetic properties of Br=8.5kG, Hci=7.5kOe, (BH)max=13.5MGOe, α=−0.185%∕°C, and β=−0.54%∕°C can be achieved in (Ce0.5Pr0.5)9.5Febal.Co2.5Ti2B10 nanocomposites.

The effect of Ti and C on the phase evolution and magnetic properties of Pr9FebalTixB11-yCy (x=0-4, y=0-11) nanocomposites

Phase evolution, microstructure, and magnetic properties of melt-spun Pr9FebalTixB11−yCy (x=0, 2.5, and 4, y=0–11) ribbons have been investigated. For Pr9FebalTixB11−yCy series ribbons, addition of Ti suppresses the formation of metastable Pr2Fe23B3 phase and ensures the existence of a large amount of magnetically hard Pr2Fe14B phase in the ribbons. However, the increase of C substitution in the Pr9FebalTi2.5B11−yCy (y=0–5.5) ribbons diminishes the Br, Hci, and (BH)max monotonically, which comes from the increase of volume fraction of Pr2Fe17Cz and α-Fe phases and the rapid decrease of the 2:14:1 phase. Only in the higher Ti content Pr9FebalTi4B11−yCy ribbons, a slight C addition cannot only form the Pr2Fe14(B,C) phase but also refine the grain size, giving rise to the improvement of magnetic properties. The optimal properties of Br=9.4kG, Hci=11.1kOe, (BH)max=18.0MGOe, α=−0.207%∕°C, and β=−0.612%∕°C are achieved in Pr9FebalTi4B10.5C0.5 nanocomposites.

Coercivity enhancement of melt spun FePt ribbons by Au addition

The effect of Au content on the magnetic properties and microstructure of melt spun (FePt)100−xAux (x=0–40) ribbons have been investigated. X-ray diffraction and thermal magnetic analysis results indicate that Au-rich phase coexists with ordered L10-FePt(Au) phase in ternary FePtAu ribbons after an isothermal annealing. Meanwhile, the Curie temperature of L10-FePt(Au) phase in annealed (FePt)100−xAux (x=0–40) ribbons is almost unchanged, revealing that higher volume fraction of Au addition does not modify the composition of the L10-FePt(Au) phase. All the Au containing ribbons exhibit a completely isolated L10-FePt(Au) granular structure, and the grain size is effectively decreased with the increase of Au content. As a result, the intrinsic coercivity (Hci) of the ribbons increases substantially from 2.1kOe for binary FePt to 19.5kOe for (FePt)60Au40.