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Herein, fluid oil joule annealing modulation (FOJA) for improving the giant magnetoimpedance (GMI) property of CoFe‐based microwires with the microstructure is adopted, and magnetic domain structure is systematically investigated. Compared with general joule annealing (JA) method, a much larger DC current can be used. The maximum GMI value is 440.8% in the 250 mA FOJA‐treated sample and the maximum monotonically increasing magnetic field is 12 Oe in 400 mA FOJA‐treated sample. In the 100–400 mA FOJA‐treated samples, there is a nearly linear increase and decrease relationship between GMI and the magnetic field. In addition, the FOJA method can control the radial stress release and helical magnetic domains of the microwire by changing the Joule heat transfer and the circumferential magnetic field and then tune the GMI performance of the microwires. The remarkable features, including a continuous and clear linear‐sensitive response field Hex of 0–95 Oe, quick response sensitivity, large GMI ratio, and excellent tunability, make the FOJA‐tailored microwires promising candidate material for miniaturized GMI sensors. [ABSTRACT FROM AUTHOR]

High-quality Gd60Co15Al25 microwires with an average diameter of 40 µm were successfully fabricated by the melt-extraction method. The as-cast microwires undergo a second-order paramagnetic to ferromagnetic (PM-FM) transition at ∼100 K. Large values of the magnetic entropy change (−Δ SM ∼9.73 J kg−1 K−1) and the refrigerant capacity (RC ∼732 J kg−1) are achieved for a field change of 5 T. A careful analysis of critical exponents near the PM-FM transition indicates the significant effects of structural disorder on the long-range ferromagnetic interaction and the magnetocaloric response of the microwires. The excellent magnetocaloric properties make the Gd60Co15Al25 microwires very promising for use in magnetic refrigerators operating in the liquid nitrogen temperature range. [ABSTRACT FROM AUTHOR]

Ni50Mn25Ga25- xFe x (1 ≤ x ≤ 6) microwires with a diameter of 30-50 µm were prepared by melt-extraction. The effects of Fe doping on the martensite transformation temperature, Curie point and magnetic properties before and after chemical ordering treatments were investigated. Experimental results show that Ni50Mn25Ga25- xFe x microwires exhibit typical thermoelastic martensite transformation and magnetic transition. Transformation temperatures of the microwires increase with Fe content with a rate of 27.3 K/at%. Thermal hysteresis during martensite transformation is lowered after Fe doping. The Curie point of Ni50Mn25Ga25- xFe x (1 ≤ x ≤ 6) microwires keeps stable at 390 K, which is 25 K higher compared with Ni2MnGa ternary alloy. After chemical ordering heat treatment, the microwires exhibit a typical ferromagnetic transition and a higher saturation magnetization at low field compared to as-extracted microwires. [ABSTRACT FROM AUTHOR]

We report a systematic study of a new class of melt-extracted Ni49.7Mn25.0Ga19.8Fe5.5 polycrystalline microwires in terms of fabrication, microstructural characterization, and evaluation of shape memory effects (SMEs). The as-extracted microwire has enhanced ductility due to its fine grains with diameters of 1-3 µm. The total strain in its martensite state reached 2.14% as loading at 350 MPa and 100% plastic deformation strain recovery was achieved after heating process. During thermo-cycling tests, a fully recoverable transformation strain of 1.5% was obtained under a tensile bias-stress of 468 MPa. It can be therefore concluded that Fe-doped microwires display the least strain dependence on stress compared with other alloys such as NiTi and NiMnGa, and could be useful in devices required constant stress output. [ABSTRACT FROM AUTHOR]

We report here on a comprehensive study of an alternatingcurrent Joule annealing (ACJA) method for optimizing the giant magnetoimpedance (GMI) effect of Co-rich amorphous wires for sensor applications. Experimental results indicated that the proper ACJA treatment can drastically improve the GMI property of as-cast wire. At 10 MHz, the maximum GMI ratio [ΔZ/Z0]max of ACJA-ed wire increase to 205.38%, which is nearly 3.3 times that of 62.84% for as-cast wire, and the maxima field response sensitivity ξmax of ACJA-ed wire increases to 345.90%/Oe by more than twice that of 170.92%/Oe for as-cast wire. With an increase of AC amplitude, a remarkable dependence of GMI performance appears on the microstructural evolution such as the structural relaxation, the degree of local order, as well as circular magnetic domains. Importantly, the atomic order orientation incorporating HRTEM observation and circumferential permeability were effectively increased by the co-action of thermal activation energy and magnetic field energy during ACJA, further to enhance the magnetic properties to some extent. Based on both the numerical calculation of transient temperature rise and GMI behavior, we propose ACJA as an efficient annealing technique to achieve simultaneous best performance of [ΔZ/Z0]max and ξmax for potentially magnetic sensor applications in detecting weak magnetic fields. [ABSTRACT FROM AUTHOR]

In this work, we report a comprehensive investigation on mechanical and giant magneto-impedance (GMI) properties of melt-extracted Co-rich amorphous microwires with the nominal composition of Co69.25Fe4.25Si13B13.5-xNbx (x=0, 1, 2, 3, 4). Results show that the tensile strength and GMI properties gradually increase with the substitution of B by Nb at first, and then decrease with a further Nb substitution. The meltextracted microwires with the substitution of B by 3% Nb exhibit a maximum tensile strength σm of 4000±50 MPa owing to the formation of nanosized crystals in the amorphous matrix. In addition, the substitution of B by 1% Nb assures an excellent GMI property. At 5 MHz, the maximum GMI ratio [ΔZ/Zmax]max for the melt-extracted microwires reaches as high as 450±10% versus 380±10% for traditional CoFeSiB microwires and the field-response sensitivity increases from 350±10% Oe-1 to 700±10% Oe-1 for 1% Nb substitution. It is thus concluded that both mechanical and GMI properties of the melt-extracted Co-based amorphous microwires can be greatly improved by properly adding Nb, and this is promising for many engineering applications, including high-resolution magnetic, stress and biomedical sensors. [ABSTRACT FROM AUTHOR]

A new magnetocaloric composite composed of three types of soft ferromagnetic Gd‐based microwires with equal mass ratio and close magnetocaloric performances is designed. The Curie temperatures (TC) of these three wires are ≈97, ≈100, and ≈101 K, respectively. However, the magnetization versus temperature curve of the composite sample shows almost a single ferromagnetic–paramagnetic transition at ≈97 K. For a field change of µ0ΔH = 5 T, the maximum entropy change (−ΔSMmax) and refrigerant capacity (RC) of this composite are calculated to be ≈9.98 J kg−1 K−1 and ≈665 J kg−1, respectively. There is a good agreement between the calculated −ΔSM (T) curve and the experimental data, indicating weak magnetic interactions among different wire compositions. Additionally, the calculated universal master curves of the composite and its Gd‐Al‐Co components are fitted very well, which reveals the second‐order type of the magnetic transition for the composite. The designed composite integrates all magnetocaloric properties of its components, suggesting that the Gd‐based amorphous wires with close magnetocaloric properties can be designed as an excellent refrigerant for use in active magnetic refrigerators. [ABSTRACT FROM AUTHOR]

The images on the inside back cover refer to the article by Dawei Xing et al. (pp. 1905–1910). They show the critical behavior for melt‐extracted Gd‐based microwires. The authors investigated Gd60Co15Al25 wires with an average diameter of 40 μm that underwent a second‐order paramagnetic to ferromagnetic (PM–FM) transition at ∼100 K. A careful analysis of critical exponents near the PM–FM transition was performed. The n value (related to the magnetic state of the specimen) of the investigated wires is ∼0.743 calculated from the magnetic field dependence of the maximum magnetic entropy change (ΔSm), which deviates from the value of n = 2/3 (∼0.67) based on the mean field approach. The critical exponents of β, γ, and δ for the present wires are obtained by the modified Arrott plots (MAP) method based on the scaling theory and the values are 0.432, 1.244, and 3.351, respectively. All these values also significantly deviate from those obtained according to the mean field theory (β = ∼0.5, y = ∼1, and δ = ∼3). Scaling plots obtained by using the critical exponents and the experimental data demonstrate that the critical exponents obtained for the present microwires are precise and reliable. This study indicates that the Gd60Co15Al25 microwires are excellent magnetic refrigerants for active magnetic refrigeration in the liquid nitrogen temperature range. [ABSTRACT FROM AUTHOR]