H. W. Chang

Magnetocaloric effect in Fe–Zr–B–M (M=Mn, Cr, and Co) amorphous systems

The magnetocaloric effect (MCE) of the amorphous Fe–Zr–B–M (M=Mn, Cr, and Co) ribbons has been investigated. The MCEs of the Fe90−xZr10Bx (x=5, 10, 15, and 20) ribbons are enhanced with small amounts of boron addition. Furthermore, the Curie temperature of the specimens can be decreased to be about room temperature with appropriate Mn and Cr substitutions, but the MCE performance of the specimens drops only slightly. It is also found that the magnetic entropy change of the Co-substitution series of Fe85−yZr10B5Coy ribbons almost remains constant although the Curie temperature is increased to be about 400K for y=5. Therefore, for the application of MCE refrigeration at above room temperature, the Fe85−yZr10B5Coy ribbons are preferred due to the constant MCE and the high refrigeration capacity of about 90J∕kg at the magnetic field change of 10kOe. Moreover, the field dependence of the magnetic entropy change exhibits power dependence for all the studied specimens. In the ferromagnetic range, the exponent is...

Exchange bias in sputtered FM/BiFeO3 thin films (FM = Fe and Co)

Magnetic properties of sputter-deposited ferromagnetic (FM)/BiFeO3 (BFO) films on Pt/Ti/SiO2/Si(100) substrate (FM = Co and Fe) have been investigated. Isotropic perovskite BFO single phase is obtained for 200-nm-thick BFO films deposited at 300–450 °C and BFO films at 400 °C with thickness of 50–400 nm. Large exchange bias field (HEB) of 308–400 Oe and coercivity (Hc) of 1201–3632 Oe at RT are obtained for polycrystalline Co/BFO bilayers. The roughened surface induced by high deposition temperature and increasing thickness of BFO layer enhances localized shape anisotropy of FM layer, resulting in the increase of Hc the improved crystallinity and roughened surface of BFO/Co interface might be responsible for the HEB enhancement. Additionally, comparison on the HEB in polycrystalline Co/BFO and Fe/BFO systems is also discussed.

Phase evolution and magnetic properties of boron enriched (Nd0.7Pr0.25La0.05)xFebal.Co10Ti2By (x=4.5–10.7; y=10.9–18.6) nanocomposites

Abstract Alloy ribbons of (Nd 0.7 Pr 0.25 La 0.05 ) x Fe bal. Co 10 Ti 2 B y , where ( x , y )=(10.7, 10.9), (10.3, 11.3), (9.7, 12.1), (8.4, 13.7), (7.1, 15.3), (5.8, 16.9) and (4.5, 18.6), were prepared by melt spinning method to investigate the phase evolution and key factors controlling their magnetic properties. X-ray diffraction and thermal magnetic analysis (TMA) revealed that the phase mixtures of these alloy ribbons changed in the sequence of α-Fe/R 2 Fe 14 B→R 2 Fe 23 B 3 /R 2 Fe 14 B→R 2 Fe 23 B 3 →Fe 3 B/R 2 Fe 23 B 3 /α-Fe. The amount of R 2 Fe 14 B phase is very critical in determining the B r , i H c and ( BH ) max obtained. In general, the B r , i H c and ( BH ) max decrease with increasing boron content and reach local minimum when only R 2 Fe 23 B 3 was present in ribbons, for example sample with a nominal composition of ( x , y )=(5.8, 16.9). The presence of a large amount R 2 Fe 23 B 3 , with the diminution of R 2 Fe 14 B, is responsible for degradation of thermal stability of the ribbons. In the meantime, the average grain size of individual phases also showed a crucial influence on the B r and ( BH ) max of ribbons studied.

Magnetic properties, Mössbauer and aftereffect studies of Pr10Fe90−xBx (x=5.88–11.5) nanocomposites

Magnetic properties, phase evolution and aftereffect of melt-spun Pr10Fe90−xBx (x=5.88–11.5) nanocomposite ribbons have been studied. Summarizing the results of thermal magnetic analysis and Mossbauer spectroscopy, two phases, namely Pr2Fe14B and α-Fe, were found for ribbons with x=5.88 and 7.5, while additional two metastable phases, Pr2Fe23B3 and Fe3B, prevailed for x=9. For x=10 and 11.5, only three of these phases: Pr2Fe14B, Pr2Fe23B3, and Fe3B, but not α-Fe were present. The optimal magnetic properties of Br=10.1 kG, iHc=6.4 kOe, and (BH)max=16.1 MGOe were achieved in the Pr10Fe84.12B5.88 ribbon, while iHc 8.71 kOe and (BH)max 14.7 MGOe were obtained in Pr10Fe80B10. Furthermore, Pr10Fe78.5B11.5 having the optimal iHc (9.33 kOe) exhibited minimum values in activation volume V=5.03×10−19 cm3, because it possessed the least volume fraction of the magnetically soft phases. Finally, confirmation of the absence of spin reorientation in Pr2Fe14B down to 5 K assures its advantage over Nd2Fe14B for low-temper...

Sputter-prepared BiFeO3(001) films on L10 FePt(001)/glass substrates

The preparation of BFO films by sputtering at a temperature as low as 450 °C on glass and commercial Pt/Ti/SiO2/Si(001) substrates have been studied. The underlayers with different orientations were prepared on the glass substrates including strongly textured Pt(111) and L10-FePt(001) induced by rapid thermal annealing process. Isotropic perovskite BFO grains with size of about 200 nm formed on the commercial substrates, showing larger surface roughness. Pt(111) suppresses BiFeO3 phase. Single phase perovskite BFO with strong (001) texture, reduced surface roughness and fine grain size was formed on the L10-FePt(001) buffer layer. Considerable enhancement of ferroelectric properties was achieved as compared to the films grown on commercial substrate.

Thermal stability and magnetocaloric effect of the Gd65Fe20Al15−xBx (x=0–7) glassy ribbons

The thermal stability, magnetocaloric effect, and refrigeration capacity (RC) of Gd-based Gd(65)Fe(20)Al(15-x)B(x) (x=0-7) glassy ribbons have been investigated. A relatively wide supercooled liquid region Delta T(x)(Delta T(x)=T(x)-T(g)) (50-80 K) and large reduced glass transition temperature T(rg)(T(rg)=T(g)/T(m)) (>0.63) are found in Gd(65)Fe(20)Al(15-x)B(x) glassy ribbons. The distinctive glass transition and sharp crystalline events as well as large values of Delta T(x) and T(rg) confirm the excellent glass forming ability of these alloys. The maximal magnetic entropy changes, -Delta S(M)(max), and RC values of the specimens are about 4.80-5.21 J/kg K and 700-800 J/kg under 50 kOe, respectively. These -Delta S(M)(max) values are comparable to or even higher than that of some reported bulk metallic glasses. Moreover, the larger RC values are due to the broad Delta S(M) peak (similar to 200 K), which is caused by the glassy structure. The large Delta S(M) and RC values make the Gd-based Gd(65)Fe(20)Al(15-x)B(x) glassy ribbons attractive candidates for magnetic refrigeration materials

Magnetic properties and crystal structure of melt-spun Sm(Co, M)7 (M = Al and Si) ribbons

Effect of Si and Al contents on the magnetic properties and crystal structure of melt-spun SmCo7-xMx (M = Al and Si) ribbons have been investigated. The Rietveld refinement results show that Al and Si prefer to occupy the 3g site in the TbCu7-type structure. The changes in both the magnetocrystalline anisotropy field and the microstructure with Si or Al substitution may account for the variation of magnetic properties. The coercivity of SmCo7−xSix ribbons is enhanced from 1.9 kOe for SmCo7 to 4.7 kOe for SmCo6.5Si0.5 and 2.6 kOe for SmCo6.95Al0.05, which is mainly because of the enhancement of magnetic anisotropy field by the Si and Al substitution for Co in 3g sites. Furthermore, in melt-spun SmCo7−xMx ribbons system, with increasing x, the grain size is slightly reduced for M = Si, but increased for M = Al. In addition, with further increasing x, the enhancement of coercivity is attributed to both the third element substitution for Co in 3g sites and grain refinement.Effect of Si and Al contents on the magnetic properties and crystal structure of melt-spun SmCo7-xMx (M = Al and Si) ribbons have been investigated. The Rietveld refinement results show that Al and Si prefer to occupy the 3g site in the TbCu7-type structure. The changes in both the magnetocrystalline anisotropy field and the microstructure with Si or Al substitution may account for the variation of magnetic properties. The coercivity of SmCo7−xSix ribbons is enhanced from 1.9 kOe for SmCo7 to 4.7 kOe for SmCo6.5Si0.5 and 2.6 kOe for SmCo6.95Al0.05, which is mainly because of the enhancement of magnetic anisotropy field by the Si and Al substitution for Co in 3g sites. Furthermore, in melt-spun SmCo7−xMx ribbons system, with increasing x, the grain size is slightly reduced for M = Si, but increased for M = Al. In addition, with further increasing x, the enhancement of coercivity is attributed to both the third element substitution for Co in 3g sites and grain refinement.

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/.

A study on the magnetic properties of melt spun Co-Hf-Zr-B nanocomposite ribbons

Magnetic properties of melt spun Co86.5Hf11.5-xZrxB2 (x = 0–5) ribbons have been investigated. For the ribbons spun at the wheel speed of 40 m/s, hard magnetic properties with high energy product ((BH)max) of 34.4–52.8 kJ/m3 and intrinsic coercivity (iHc) of 176–216 kA/m were obtained for x = 0–2, but soft magnetic behavior was observed for x = 3–5 due to the appearance of the amorphous phase. By annealing the ribbons with x = 3–5, hard magnetic properties were improved arisen from the formation of magnetically hard phase. The variation of magnetic properties for Co86.5Hf11.5−xZrxB2 ribbons was correlated to microstructure change. Proper Zr substitution for Hf was helpful in refining the grain size from 10–35 nm for x = 0 to 5–15 nm for x = 1, and thus improving the magnetic properties effectively. The optimal hard magnetic properties of Co86.5Hf10.5Zr1B2 ribbons might be originated from the fine magnetically hard Co11(Hf, Zr)2 phase, and the exchange coupling effect among grains and/or with the face-cent...