Grain oriented NiMnSn and NiMnIn Heusler alloys ribbons produced by melt spinning: Martensitic transformation and magnetic properties

We outline the microstructural, martensitic transformation and magnetic properties of Heusler alloys with starting compositions Ni₅₀Mn₃₇Sn₁₃, Ni₅₀Mn₃₆In₁₄, and Mn₅₀Ni₄₀In₁₀, produced by melt spinning. The ribbons were obtained in argon environment at a high wheel linear speed of 48 m s⁻¹ (typical dimensions: 1.2-2.0 mm in width, 4-12 mm in length, and 7-12 μm in thickness). EDS microanalysis showed that the resulting average elemental chemical composition is slightly shifted with respect to the starting one. Ribbons are fully crystalline and tend to show a highly ordered columnar-like microstructure with grains running through the entire ribbon thickness; the larger dimension of the grains is perpendicular to the ribbon plane. As-spun alloys were single-phase with ferromagnetic bcc L2₁ austenite as high-temperature parent phase. At low temperatures austenite transforms into a structurally modulated martensite with a lattice symmetry that depends on the system (7 M orthorhombic for Ni₅₀Mn₃₇Sn₁₃, 10 M monoclinic for Ni₅₀Mn₃₆In₁₄, and 14 M monoclinic for Mn₅₀Ni₄₀In₁₀). Magnetization isotherms measured in the temperature interval where martensite thermally transforms into austenite confirmed the occurrence of field-induced reverse martensitic transition in the alloys studied.     

Hard magnetic properties of melt-spun Co82Zr18−xTix alloys

The phases and magnetic properties of Co-Zr-Ti melt-spun ribbons were studied by XRD analysis and magnetic measurements. The optimal magnetic properties of Ms=59.0 emu/g, Mr=4.0 kG, Hc=2.9 kOe, and (BH)_max=3.0 MGOe were obtained in Co₈₂Zr₁₄Ti₄ ribbons produced at a wheel speed of 30 m/s. In this work, we found that Ti was one of the few large atomic radius elements, which could improve hard magnetic properties of Co-Zr alloy.     

Structure and magnetic properties of melt-spun Tb0.27Dy0.73Fex ribbons

The structure and magnetic properties of the melt-spun ribbons of Tb_0.27Dy_0.73Fe_x alloy are investigated as a function of various wheel speeds during melt-quenching using a single-roll technique. It is found that Tb_0.27Dy_0.73Fe_x alloy is difficult to be fabricated as amorphous state by using the melt-quenching method. X-ray diffractions show that all these ribbons for x=1.7−2.0 are the MgCu₂-type phase at the wheel speed of 45 m s⁻¹. For Tb_0.27Dy_0.73Fe_x alloy, the high wheel speed is beneficial to eliminate the RFe₃ phase and form the perfect MgCu₂-type phase. Compared with the bulk of Tb_0.27Dy_0.73Fe_1.95, these ribbons exhibit higher intrinsic coercivity value and their saturation magnetizations increase as well. The magnetostriction of Tb_0.27Dy_0.73Fe_1.95 composite with 4% epoxy resin is 640×10⁻⁶ at 900 kA m⁻¹.     

Ultrafine Fe-Co nanowires: Fabrication and heat treatment influence on the structure and magnetic properties

Ultrafine nanowires of Fe-Co with a diameter around 15 nm have been fabricated by electrodeposition method using anodic porous alumina as a template. The alloy nanowires were in the form of arrays and consisting of polycrystalline structures. They showed obvious shape anisotropy parallel to the axis of nanowires and the perpendicular coercivity (Hc_⊥) was found to be 2576.8 Oe which is higher than any coercivity value reported in the literature. The effects of critical factors such as heat treatment and temperature of annealing on the structure and magnetic properties of the ultrafine nanowire arrays were studied and discussed.     

Thermal stability,magnetic and magnetocaloric properties of Gd55Co35M10 (M = Si,Zr and Nb) melt-spun ribbons

The thermal stability, magnetic and magnetocaloric properties of Gd₅₅Co₃₅M₁₀ (M = Si, Zr and Nb) melts-pun ribbons were studied. The relatively high reduced glass transition temperature (T_x1/T_m > 0.60) and low melting point (T_m) resulted in excellent glass forming ability (GFA). The Curie temperatures (T_C) of melt-spun amorphous ribbons Gd₅₅Co₃₅M₁₀ for M = Si, Zr and Nb were 166, 148 and 173 K, respectively. For a magnetic field change of 2 T, the values of maximum magnetic entropy change (−ΔS_M)^max for Gd₅₅Co₃₅Si₁₀, Gd₅₅Co₃₅Zr₁₀ and Gd₅₅Co₃₅Nb₁₀ were found to be 2.86, 4.28 and 4.05 J kg⁻¹K⁻¹, while the refrigeration capacity (RC) values were 154, 274 and 174 J kg^–1, respectively. The RC_FWHM values of amorphous alloys Gd₅₅Co₃₅M₁₀ (M = Si, Zr and Nb) are comparable to or larger than that of LaFe_11.6Si_1.4 crystalline alloy. Large values of (−ΔS_M)^max and RC along with good thermal stability make Gd₅₅Co₃₅M₁₀ (M = Si, Zr and Nb) amorphous alloys be potential materials for magnetic cooling operating in a wide temperature range from 150 to 175 K, e.g., as part of a gas liquefaction process.     

ZnO纳米线紫外探测器的制备和快速响应性能的研究

一维ZnO纳米结构由于具有比表面积大、室温下具有大激子结合能等特点而受到广泛关注. 但是如何实现纳米结构的器件一直是目前研究的一个挑战. 文章通过水热方法, 在玻璃衬底上实现了ZnO纳米线横向生长, 并制备出基于ZnO纳米线的金属-半导体-金属紫外探测器. 测量结果显示器件在365 nm处探测器的响应度达到5 A/W, 并且制备的探测器在空气中对紫外光照具有快速的响应, 其上升时间约4 s, 下降时间约5 s, 这与ZnO纳米线中的氧空位吸附和脱附水分子相关.     

Magnetic properties of cobalt nanowires: Study by polarized SANS

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic cobalt nanowires embedded into Al₂O₃ matrix have been studied using polarized small-angle neutron scattering. A comprehensive analysis of contributions to the scattering intensity was carried out, including nonmagnetic (nuclear) contribution, magnetic contribution depending on the magnetic field, and nuclear-magnetic interference indicating the correlation between the magnetic and nuclear structures. Experiments have revealed an anomalously low value of the magnetic contribution as compared to the nuclear one. This behavior is interpreted in terms of low coherence of the magnetic structure caused by the anisotropy of Co crystallites as compared with the large coherency of nuclear structure of nanowires.     

Effect of Al substitution for B on magnetic and structural properties of Co-based melt-spun ribbons

This paper reports structural and magnetic properties of rapidly quenched Co₆₄Fe₄Ni₂B_19−xSi₈Cr₃Al_x (x=0, 1, 2, 3) amorphous ribbons prepared by the single roller melt spinning process. Thermal analysis of the ribbons shows that the replacement of B by Al causes a decrease in the crystallization temperature. Structural studies of the samples have been carried out by transmission electron microscopy and X-ray diffraction. With optimum amount of Al in the alloy, the as-cast material has better soft magnetic properties. The highest maximum permeability (3.55×10⁵), saturation magnetization (523.7 mT) and the lowest coercivity (0.8174 A/m) were obtained in the sample with x=2.     

Effects of Nb substitution for Zr on the phases,microstructure and magnetic properties of Co80Zr18−xNbxB2 melt-spun ribbons

The phases, microstructure, and magnetic properties of Co₈₀Zr_18−xNb_xB₂ (x=1, 2, 3, and 4) melt-spun ribbons were investigated. The small substitution of Nb for Zr in the Co–Zr–B melt-spun ribbons resulted in the improvement of magnetic properties, especially the coercivity. The main effect of added Nb on the coercovity of Co–Zr–Nb–B melt-spun ribbons, originated from modification of the grain size of Co₁₁Zr₂ phase. The coercivity of the Co–Zr–Nb–B melt-spun ribbons depends on the annealing temperature. The optimal magnetic properties of H_c=5.1 kOe, and (BH)_max=3.4 MGOe were obtained in the Co₈₀Zr₁₅Nb₃B₂ melt-spun ribbons annealed at 600 °C for 3 min.     

纳米晶复合Pr2Fe14B/α-Fe快淬薄带的织构与磁性

采用快淬方法制备了纳米晶复合Pr₂Fe₁₄B/α-Fe永磁薄带，研究了不同淬火速率对薄带织构和磁性的影响.通过改善快淬工艺，使得薄带中Pr₂Fe₁₄B相的晶粒在薄带的自由面形成显著的织构，Pr₂Fe₁₄B相晶粒易轴沿垂直于带面方向取向.分析了快淬凝固过程中Pr₂Fe₁₄B相的晶粒取向过程和机理，以及晶粒的大小和薄带结构的均匀性对薄带磁性的影响.对自由面有显著取向的薄带，进行酸蚀和打磨减薄处理，去除贴辊面未取向的部分，剩余部分为具有Pr₂Fe₁₄B相晶粒取向的各向异性薄带，Pr₂Fe₁₄B相取向使薄带的剩磁得到增强，矫顽力也有所提高. 关键词： 快淬 2Fe₁₄B/α-Fe永磁薄带')" href="#">纳米晶复合Pr₂Fe₁₄B/α-Fe永磁薄带 织构 磁性能     

Fabrication and magnetic properties of metallic nanowires via AAO templates

Metallic (Ni, Co, Cu and Fe) nanowires were fabricated by electrodeposition into anodic aluminum oxide (AAO) template. In this work, we have studied the effect of the electrode potential on the microstructure and magnetic properties of nanowires. Transmission electron microscopy (TEM) results showed that the metal nanowires were single-crystal. Cu and Ni nanowires had the same orientation along the [2 2 0] direction, while Co had a preferred orientation along the [1 0 0] direction. Fe nanowires had a preferred orientation along the [2 0 0] direction. The growth mechanisms are probably due to the competition growth of the adjacent grains and the confinement of growth in the nano-sized hole of the AAO template. Results showed that single crystal or highly textured nanowires had better magnetic properties compared with that of polycrystal nanowires in terms of coercivity and squareness.     

Phase composition, microstructures and magnetic properties of melt-spun Nd1.2Fe10.5Mo1.5 ribbons and their nitrides

The effect of wheel speed on the phase compositions and microstructures of melt-spun Nd_1.2Fe_10.5Mo_1.5 ribbon was investigated. It is found that the Nd(Fe,Mo)₁₂ phase can be obtained at the wheel speed of 10 m/s, and TbCu₇-type Nd(Fe,Mo)₇ phase is formed with the wheel speed higher than 10 m/s. The amorphous phase is achieved at 65 m/s. The average grain size of phases decreases linearly with increasing wheel speed. The Nd(Fe,Mo)₁₂N_1.0 nitride obtained from annealed ribbons quenched at 65 m/s shows a coercivity much higher than that from the ribbons quenched at 10 m/s, which is due to the smaller grain size in the former ribbons.     

Successive phase transformation in ferromagnetic shape memory alloys Co37Ni34Al39 melt-spun ribbons

The martensitic transformation in Co37Ni34Al29 ribbon is characterized in detail by means of in-situ thermostatic x-ray diffraction and magnetic measurements.The results show a structural transition from the body-centred cubic to martensite with a tetragonal structure during cooling.Comparison between the results of the diffraction intensity with the magnetic susceptibility measurements indicates that the martensitic transformation takes place in several different steps during cooling from 273 to 163 K.During heating from 313 to 873 K,the peak width becomes very wide and the intensity turns very low.The γ-phase(face-centred cubic structure) emerges and increases gradually with temperature increasing from 873 to 1073 K.     

Magnetic properties of one-dimensional embedded nickel nanostructures in gold nanowires

Magnetic nanostructures of nickel embedded in gold were successfully fabricated by electrochemical deposition in porous alumina templates. Structural characterization of the samples confirmed the formation of pure phase, crystalline multi-segmented Au-Ni-Au nanowires. Magnetic characterization of the wires reveals that ferromagnetism arises as a result of Ni embedded in Au segments. An interesting behavior of coercivity was observed that showed a rapid decrease of coercivity for smaller Ni segments while a monotonic decrease was found for the larger segments. Finally, the saturation magnetization of the wires exhibited a slower increase for smaller Ni segments while a sharp increase was observed for larger Ni segments.     

Double magnetic entropy change peaks and high refrigerant capacity in Gd1-xHoxNi compounds in the melt-spun form

Gd_1-xHo_xNi melt-spun ribbons were fabricated by a single-roller melt spinning method. All the compounds crystallize in an orthorhombic CrB-type structure. The Curie temperature (T_C) was tuned between 46 and 99 K by varying the concentration of Gd and Ho. A spin reorientation (SRO) transition is observed around 13 K. Different from T_C, the SRO transition temperature is almost invariable for all compounds. Two peaks of magnetic entropy change (ΔS_M) were found. One at the higher temperature range was originated from the paramagnet-ferromagnet phase transition and the other at the lower temperature range was caused by the SRO transition. The maximum of ΔS_M around T_C is almost same. The other maximum of ΔS_M around SRO transition, however, had significantly positive relationship with x. It reached a maximum about 8.2 J kg⁻¹ K⁻¹ for x = 0.8. Thus double large ΔS_M peaks were obtained in Gd_1-xHo_xNi melt-spun ribbons with the high Ho concentration. And the refrigerant capacity power reached a maximum of 622 J kg⁻¹ for x = 0.6. Gd_1-xHo_xNi ribbons could be good candidate for magnetic refrigerant working in the low temperature especially near the liquid nitrogen temperature range.     

Influence of crystallization treatment on structure,magnetic properties and magnetocaloric effect of Gd71Ni29 melt-spun ribbons

The influence of crystallization treatment on the structure, magnetic properties and magnetocaloric effect of Gd₇₁Ni₂₉ melt-spun ribbons has been investigated in detail. Annealing of the melt-spun samples at 610 K for 30 min, a majority phase with a Fe₃C-type orthorhombic structure (space group, Pnma) and a minority phase with a CrB-type orthorhombic structure (space group, Cmcm) were obtained in the amorphous matrix. The amorphous melt-spun ribbons undergo a second-order ferromagnetic to paramagnetic phase transition at 122 K. For the annealed samples, two magnetic phase transitions caused by amorphous matrix and Gd₃Ni phases occur at 82 and 100 K, respectively. The maximum magnetic entropy change (–ΔS_M)^max is 9.0 J/(kgˑK) (5T) at 122 K for the melt-spun ribbons. The values of (–ΔS_M)^max in annealed ribbons are 1.0 and 5.7 J/(kgˑK), corresponding to the two adjacent magnetic transitions.     

Impacts of phase transformation on the thermoelectric properties of ZnO nanowires

Impacts of phase transformation from regular wurtzite phase to a graphitic polymorph hexagonal phase under tensile loading, on the thermoelectric properties of Zinc oxide nanowires (ZnO NWs), which is oriented in the [0001] direction, are studied by combining the first-principles simulation with one-dimensional (1D) Boltzmann transport equation (BTE). Our results show that this phase transformation has greatly influenced the thermoelectric properties of the ZnO NWs. We also find that the largest value of figure of merit ZT achieved for hexagonal phase (H phase) is larger than that for wurtzite phase (W phase) in the temperature range (200 K-1000 K), which means that hexagonal phase may become the optimal choice for ZnO nanowires in thermoelectric applications.     

Electronic and magnetic properties of single-wall GeC nanotubes filled with iron nanowires

Under GGA, the structural, electronic and magnetic properties of single-wall (8, 8) GeC nanotubes filled with iron Fe_n nanowires (n = 5, 9, 13 and 21) have been investigated systematically using the first-principles PAW potential within DFT. We find that the initial shapes of the Fe₅@(8, 8), Fe₉@(8, 8) and Fe₁₃@(8, 8) systems are preserved without any visible changes after optimization. But for the Fe₂₁@(8, 8) system, the initial shapes are distorted largely for both nanowire and nanotube. The binding processes of Fe_n@(8, 8) systems are exothermic, and Fe₅@(8, 8) system is the most stable structure. The pristine (8, 8) GeCNT is nonmagnetic and direct semiconductor with a wide band gap of about 2.65 eV. Projected densities of states onto different shell Fe atoms show that the separation between the bonding and antibonding d states is reduced as going from the core Fe atom to the outermost shell Fe atom. The spin polarization of the Fe_n@(8, 8) systems and free-standing nanowires are higher than that in bulk Fe. And the spin polarization generally decreases with the number n of the Fe atoms increasing for both the Fe_n@(8, 8) systems and free-standing nanowires. Both the largest spin polarization value itself and not more decrease with respect to value of free-standing Fe₅ nanowire suggest the Fe₅@(8, 8) system could be of interest for the use in electron spin injection. The magnetism is mainly confined within the inner Fe nanowire for these combined systems. More importantly, the Fe₅ nanowire encapsulated inside (8, 8) GeCNT is under the protection of the GeCNT to prevent from oxidation, thus may stably exist in atmosphere for long time and can be expected to have potential applications in building nanodevices.     

Orientation-controlled synthesis and magnetism of single crystalline Co nanowires

Orientation control and the magnetic properties of single crystalline Co nanowires fabricated by electrodeposition have been systematically investigated. It is found that the orientation of Co nanowires can be effectively controlled by varying either the current density or the pore diameter of AAO templates. Lower current density or small diameter is favorable for forming the (1 0 0) texture, while higher current values or larger diameter leads to the emergence and enhancement of (1 1 0) texture of Co nanowires. The mechanism for the manipulated growth characterization is discussed in detail. The orientation of Co nanowires has a significant influence on the magnetic properties, resulting from the competition between the magneto-crystalline and shape anisotropy of Co nanowires. This work offers a simple method to manipulate the orientation and magnetic properties of nanowires for future applications.