C. W. Shih

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...

Exchange bias in La0.7Sr0.3MnO3/NiO and LaMnO3/NiO interfaces

Bilayers of La0.7Sr0.3MnO3/NiO and LaMnO3/NiO were prepared and magnetic exchange coupling investigated in these bilayers, where the Curie temperature of the ferromagnetic (FM) layer is lower than the Neel temperature of the antiferromagnetic layer. After small-field cooling, the LSMO/NiO bilayer exhibits an exchange bias with field HEB = 60 Oe, whereas the LMO/NiO sample shows weak magnetic interaction (∼22 Oe). The unconventional exchange bias in LSMO/NiO bilayer vanishes as temperature rises above 50 K. The weak magnetic interaction at the LMO/NiO interface is due to a larger Hubbard parameter value and smaller transfer integral value in the Mott insulator LMO compared with that for the FM conductor LSMO. The valence states of Mn and Ni ions across the interfaces for LSMO/NiO and LMO/NiO have been studied using X-ray photoelectron spectroscopy. We speculate that the FM interaction between Ni2+ and Mn4+ gives rise to magnetic regions that pin the ferromagnetic LSMO layer.

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.

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.

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.

Phase evaluation, magnetic, and electric properties of Mn60+xGa40−x (x = 0–15) ribbons

Mn60+xGa40−x (x = 0, 5, 10, and 15) ribbons were prepared by the melt-spinning technique and subsequently by a heat treatment at 673 K for 1 h. The magnetic phases Mn8Ga5, Mn1.86Ga, D019-Mn3Ga, and D022-Mn3Ga are found to appear in these annealed melt-spun ribbons. The Curie temperature TC of the ribbons varies from 125 K to 185 K as x changes from 0 to 5. For x = 10, the hexagonal structure of the D019-Mn3Ga phase is distorted to an orthorhombic one below the phase transition temperature Td = 185 K. The optimized values for coercivity at room temperature are 0.13, 4.4, 8.1, and 7.7 kOe for the ribbons with x = 0, 5, 10, and 15, respectively. The resistance measurements indicate that the ribbons show a typical metallic behavior for x = 5–15.

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...

Study on the soft magnetic properties and high frequency characteristics of Co-M (M = Ti, Zr, and Hf) thin films

Soft magnetic properties and high frequency characteristics of Co100-xMx (M = Ti, Zr, and Hf) thin films have been investigated. All M-doped Co100-xMx films exhibit strong induced in-plane uniaxial magnetic anisotropy. With the increase of the x, the 4 pi M-s and coercivity along the hard and easy axes (H-ch and H-ce) are decreased, while H-k reaches the maximum value for the films having the composition to become the fully amorphous. Among them, less Hf or Zr content, i.e., x = 7, are sufficient in forming the Co100-xMx amorphous, in contrast, more Ti content, i.e., x = 15, should be needed. The optimal properties of 4 pi M-s = 11.0 kG, H-ce = 5.4 Oe, H-ch = 4.0 Oe, H-k = 320 Oe and f(FMR) = 5.2 GHz for Co93Hf7 films and 4 pi M-s = 11.7 kG, H-ce = 6.1 Oe, H-ch = 1.6 Oe, H-k = 266 Oe and f(FMR) = 4.9 GHz for Co93Zr7 films can be obtained

Exchange Bias Effect and Magnetic Properties in

The magnetic exchange coupling effect has been investigated in La0.7Sr0.3MnO3 (LSMO)-NiO nanocomposite films prepared by pulsed laser deposition on SrTiO3 (001) and (110) single crystal substrates. The films show an enhanced coercivity field HC = 560 Oe compared with the same thickness single pure LSMO thin film. The composite films exhibit a large exchange bias field HE = 140 Oe after applying a 4000 Oe field cooling. The values of the exchange bias field decrease with increasing temperature and become zero at about 45 K, which corresponds to the conventional exchange bias blocking temperature ( TB), regardless of epitaxial growth directions on (001) or (110) SrTiO3 substrates. We speculate that there may be magnetic interaction at the NiO and LSMO boundaries or charge transfer between the Ni and Mn ions, resulting in the formation of ferromagnetic interaction at the interface. The Curie temperature TC decreased from 340 K of the LSMO single film to 200 K of the nanocomposite film. The results demonstrate that the magnetic state of the composite film is directly related to the Mn-O-Mn and Mn-O-Ni spin interaction energy. In addition to this, the Mn-O-Ni at the boundary may also form the magnetic region that pins the ferromagnetic LSMO phase.

{\hbox{La}}_{0.7}{\hbox{Sr}}_{0.3}{\hbox{MnO}}_{3}\hbox{-}{\hbox{NiO}}

Magnetic properties and structure of sputter-prepared FePd thin films on the glass substrates by underlayering with refractory elements M (M = Nb, Mo, and W) have been studied. The structural analysis shows that FePd films have a (111) preferred orientation. All studied FePd films exhibit in-plane magnetic anisotropy. For single-layer FePd films, the coercivity (Hc) and magnetic energy product [(BH)max] are increased with annealing temperature (Ta) and reached the maximum value of 1.3 kOe and 3.8 MGOe in the sample annealing at 600 °C. With the further increase of Ta, magnetic properties drop rapidly. Dependence of magnetic hardening on Ta can be explained by dominant solid reactions in different region of Ta including ordering and grain growth. Interestingly, magnetic properties of L10-FePd are largely improved by underlayering with 5 nm-thick Nb, Mo, and W layers. Both the coercivity and energy product are significantly increased to 1.6 kOe and 4.9 MGOe for M = Nb, 2.5 kOe and 6.4 MGOe for M = Mo, and 3.3 kOe and 8.7 MGOe for M = W, respectively. Structural analysis suggests that this magnetic enhancement is related to both higher ordering degree and refined microstructure.