Effect of nanocrystallization on giant magnetoimpedance effect of Fe-based microwires

We studied giant magnetoimpedance (GMI) effect and magnetic properties of Fe_70.8Cu₁Nb_3.1Si_14.5B_10.6 and Fe_71.8Cu₁Nb_3.1Si₁₅B_9.1 Finemet microwires. We observed that GMI effect and magnetic softness of glass-coated microwires produced by the Taylor–Ulitovski technique can be tailored either controlling magnetoelastic anisotropy of as-cast FeCuNbSiB microwires, and/or controlling their structure by heat treatment or by changing the fabrication conditions. High GMI effect has been observed in as-prepared Fe-rich and heat treated microwires with nanocrystalline structure.     

Magnetic property and magnetocaloric effect in TmCoAl compound

A large reversible magnetocaloric effect accompanied by a second order magnetic phase transition from paramagnetic (PM) to ferromagnetic (FM) has been observed in TmCoAl intermetallic compound. For the magnetic field change of 5 T, the maximum value of magnetic entropy change (−ΔS_M^max) and the value of refrigerant capacity (RC) are evaluated to be 18.2 J/kg K and 211 J/kg, respectively. In particular, a large −ΔS_M^max (10.2 J/kg K) is achieved at 7.5 K under a low magnetic field change from 0 to 2 T with no thermal hysteresis and magnetic hysteresis loss. The large reversible magnetocaloric effect (both the large −ΔS_M and the high RC) indicates that TmCoAl is one of a promising material for magnetic refrigeration in low temperature.     

Specific heat and magnetocaloric effect of the Mn5Ge3 ferromagnet

The specific heat C_p(T) of the Mn₅Ge₃ ferromagnet has been studied in a wide temperature range of 2–400 K. The ferromagnetic ordering has been confirmed at T_C = 297.4 K, however the behaviour of C_p(T,H) in small magnetic fields suggests a presence of a weak antiparallel component in the magnetic structure. The combination of the C_p(T,H) and magnetization M(T,H) measurements enabled the determination of not only the isothermal magnetic entropy change ΔS_M but the adiabatic temperature change ΔT_ad as well. It has been also found that the mechanical milling process leads only to a moderate drop of the magnetocaloric effect. The reduction of the grain size by 50% decreases the relative cooling power by 28%.     

Thermoelectric properties of novel skutterudites with didymium: DDy(Fe1−xCox)4Sb12 and DDy(Fe1−xNix)4Sb12

In order to improve the thermoelectric properties via efficient phonon scattering Didymium (DD), a mixture of Pr and Nd, was used as a new filler in ternary skutterudites (Fe_1−xCo_x)₄Sb₁₂ and (Fe_1−xNi_x)₄Sb₁₂. DD-filling levels have been determined from combined data of X-ray powder diffraction and electron microprobe analyses (EMPA). Thermoelectric properties have been characterized by measurements of electrical resistivity, thermopower and thermal conductivity in the temperature range from 4.3 to 800 K. The effect of nanostructuring in DD_0.4Fe₂Co₂Sb₁₂ was elucidated from a comparison of both micro-powder (ground in a WC-mortar, 10 μm) and nano-powder (ball-milled, 150 nm), both hot pressed under identical conditions. The figure of merit ZT depends on the Fe/Co and Ni/Co-contents, respectively, reaching ZT > 1. At low temperatures the nanostructured material exhibits a higher thermoelectric figure of merit. The Vickers hardness was measured for all samples being higher for the nanostructured material.     

Magnetostriction effect of Co substitution in the Nd6Fe13Si intermetallic compound

We studied the magnetostriction of Nd₆Fe_13−xCo_xSi (x = 0, 1) intermetallic compounds with tetragonal Nd₆Fe₁₃Si-type structure, using the strain gauge method in the temperature range of 77–600 K under applied magnetic fields up to 1.5 T. The anisotropic magnetostriction (Δλ) versus temperature of the studied samples has shown almost similar field-dependence behavior. Below the spin reorientation temperature (T_SR), Δλ changes its sign from positive to negative value at an applied threshold field which increases with decreasing temperature. This behavior may originate from the reduction of the magnetocrystalline anisotropy with temperature. It is also observed that absolute value of Δλ increases by Co substitution. On the other hand, the volume magnetostriction (ΔV/V) versus field shows different behavior. The ΔV/V curves of Nd₆Fe₁₂CoSi tend to have a nearly quadratic dependence on applied field near magnetic ordering temperature as expected for the parastrictive behavior. The temperature dependence of magnetostriction values is discussed based on the magnetostriction relation of the tetragonal structure to determine the signs of some of magnetostriction constants for these polycrystalline compounds.     

Structure,magnetic properties and giant magnetocaloric effect of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy

Tb₄Gd₁Si_2.035Ge_1.935Mn_0.03 alloy was prepared by arc melting followed by annealing at 1193 K for 168 h. Structural characterizations reveal that monoclinic (Tb, Gd)₅Si₂Ge₂-type phase, secondary phase with orthorhombic 5-4 type structure and hexagonal 5-3 type structure coexist in the alloy. The paramagnetic Curie temperature (θ_p) is 120 K, indicating that the dominant exchange interaction is ferromagnetic or ferrimagnetic. That the thermal hysteresis of 13 K between heating and cooling and the negative slopes of Arrott plots derived from M–H curves between 116 K and 170 K confirm a typical first-order magnetic transition from ferromagnetism to paramagnetism occurs. The maximum magnetic entropy changes of the Tb₄Gd₁Si_2.05Ge_1.95Mn_0.03 alloy for magnetic field changes of 0–1 T, 0–2 T, 0–3 T, 0–4 T and 0–5 T are about 3.3, 8.6, 14.0, 18.9 and 22.4 J/kg K, respectively. And the effective refrigeration capacity (RC_eff) value is 231 J/kg with a subtracted magnetic hysteresis loss of 30 J/kg for a magnetic field change from 0 to 5 T. Large −ΔS_M and RC_eff suggest that Tb₄Gd₁Si_2.035Ge_1.935Mn_0.03 alloy is an attractive potential magnetocaloric material working in the vicinity of 143 K.     

Microstructural evolution and phase transition dependent on annealing temperature and carbon content for LaFe11.5Si1.5Cx compounds prepared by arc-melting

Three different parts of LaFe_11.5Si_1.5C_x(x = 0, 0.1, 0.2, 1.0) ingots prepared by arc-melting were determined. The region of the sample close to the surface towards the arc was marked as part 1 and the region of the sample close to the water jacketed copper crucible was marked as part 3. Without affecting the microstructure of part 1, carbon doping could increase the amount of 1:13 phase of the LaFe_11.5Si_1.5C_x(x = 0, 0.1, 0.2) ingots, which mainly existed in the part 3. The DSC curves of part 1 of LaFe_11.5Si_1.5C_x(x = 0, 0.2) ingots show a peak at 1445 K moved to 1402 K after carbon doping. The eutectoid reaction: LaFeSi → 1:13 phase + La₅Si₃ is believed to be around the peak. After annealing at 1353 K, 1443 K and 1473 K for 6 h, the microstructure of LaFe_11.5Si_1.5C_x(x = 0, 0.1, 0.2) compounds was investigated. Too high annealing temperature could induce the formation of La₅Si₃ phase and even the growth of α-Fe. The best annealing temperature should be slightly lower than the eutectoid reaction temperature, even if larger atomic diffusion rate can be obtained at higher temperature. Carbon doping could reduce the eutectoid reaction temperature. Energy can be saved for a low annealing temperature. In addition, carbon doping can accelerate the formation of the 1:13 phase by improving the nucleation rate.     

Formation of quasicrystals in Zr–Pd–(Cu) melt spun ribbons and mechanically milled powders

Amorphous Zr₇₀Pd₃₀ and Zr₇₀Pd₂₀Cu₁₀ alloys were prepared by mechanical milling and melt spinnng to compare their devitrification behaviors. The devitrification of mechanically milled Zr₇₀Pd₃₀ and Zr₇₀Pd₂₀Cu₁₀ powders occurs via a single-step, first-order transformation to a stable Zr₂Pd tetragonal structure. This is in sharp contrast to the devitrification of the same amorphous alloys prepared by melt spinning, in which a primary meta-stable quasicrystalline phase forms. Since the mechanical milling process does not involve direct liquid phase formation of an amorphous structure, it is inferred that the short-range order in the solid state derived amorphous powder is different from that in the melt spun ribbon. During mechanical milling of an amorphous melt spun ribbon, crystallization of the quasicrystalline phase appears to precede disordering into an amorphous structure having an different short range order. Deformation of an amorphous melt spun ribbon by repetitive rolling at ambient temperature crystallizes the meta-stable quasicrystalline phase.     

Magnetic ordering and magnetic properties of ErAuxNi1−xIn (0 ≤ x ≤ 1) solid solution

The ErAu_xNi_1−xIn (0 ≤ x ≤ 1) quasiternary compounds crystallize in the hexagonal layered crystal structure of ZrNiAl-type. ErAuIn was reported to be an antiferromagnet with T_N = 3 K and magnetic moments having triangular arrangement within the basal plane (the magnetic order is described by the propagation vector ). On the contrary ErNiIn is a ferromagnet with T_C = 9 K and magnetic moments pointing along the c-axis. The magnetic ordering in ErAu_xNi_1−xIn (0 < x < 1) solid solution, has been investigated by neutron diffractometry in the temperature range between 1.5 and 15 K. Moreover, bulk magnetic measurements have been carried out in the range 1.72–400 K. All alloys of intermediate composition were found to be antiferromagnets with T_N between 4.6 and 7 K. Below 2 K their magnetic order is described by the propagation vector and magnetic moments are aligned along the c-axis. However, for alloys with 0.2 ≤ x ≤ 0.7 the propagation vector was found to turn into with increasing temperature.     

Isothermal oxidation behavior of FeCo–2V intermetallic alloy

Isothermal oxidation behavior and the nature of oxide layer formed during oxidation of FeCo–2V alloy were characterized in the temperature range of 500–600 °C. Oxidation kinetics of the alloy follows a parabolic rate law. SEM and XRD studies indicate the formation of an iron rich outer oxide layer and an inner solute rich layer containing cobalt and vanadium rich oxides. The oxidation mechanism of the FeCo–2V alloy is similar to that of low alloy steels. During the initial stages, preferential oxidation of iron and cobalt occurs at the alloy surface and leads to the formation of a solute rich inner layer. Continued oxidation occurs through oxidation of iron and cobalt at the outer layer and internal oxidation of inner layer. The iron rich oxide layer formed at the surface on oxidation of FeCo alloy is semi-conducting in nature and may not provide the necessary insulating barrier required at the surface to minimize eddy current losses during A.C. applications.     

Effect of Co addition on the martensitic transformation and magnetocaloric effect of Ni–Mn–Al ferromagnetic shape memory alloys

A series of Ni_50−xCo_xMn₃₂Al₁₈ (x = 3, 4, 5, 6, 7, and 8) alloys were prepared by the arc melting method. The martensitic transformation (MT) shifts to a lower temperature with increasing Co concentration and can be tuned to occur from a ferromagnetic austenite to a weak-magnetic martensite in the range of 6 ≤ x ≤ 8. The field-induced metamagnetic behavior was realized in Ni₄₂Co₈Mn₃₂Al₁₈ sample in which a large magnetic entropy change of 7.7 J/kg K and an effective refrigerant capacity value of 112 J/kg were obtained under the field of 60 kOe. The large magnetocaloric effect and adjustable MT temperature suggest that Ni–Co–Mn–Al alloys should have promising potential as magnetic refrigerants.     

Magnetic and magnetocaloric properties in Cu-doped high Mn content Mn50Ni40−xCuxSn10 Heusler alloys

The effects of Cu substitution on the phase transitions and magnetocaloric effect of Mn₅₀Ni_40−xCu_xSn₁₀ Heusler alloys were investigated. With the increase of Cu content, the martensitic transformation (MT) temperature shifts substantially towards lower temperature, while the Curie temperature of austenite remains almost unchanged. The reverse MT temperature decreases from 180 to 171 K for Mn₅₀Ni₃₉Cu₁Sn₁₀ alloy as the magnetic field increases from 1 to 30 kOe. Under an applied magnetic field of 30 kOe, the maximum values of magnetic field induced entropy changes are 19.6, 28.9, and 14.2 J/kg K for x = 0, 1, and 2, respectively. The effective refrigerant capacities and hysteresis losses for these alloys were discussed in this paper.     

First-principle study of magnetic,elastic and thermal properties of full Heusler Co2MnSi

In this work, first principles calculation of structural, electronic magnetic and elastic properties of the half-metallic ferromagnetic Heusler compound Co₂MnSi are presented. We have applied the full-potential linearized augmented plane waves plus local orbitals (FP-L/APW+lo) method based on the density functional theory (DFT). For the exchange and correlation potential generalized-gradient approximation (GGA) is used. The computed equilibrium lattice parameters agree well with the available theoretical and experimental data. Elastic constants and their pressure dependence are also calculated. The calculated total magnetization of 5 μ_B is in excellent agreement with recent experiments. We also presented the thermal effects using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. Temperature and pressure effects on the structural parameters, heat capacities, entropy, thermal expansion coefficient, and Debye temperatures are determined from the non-equilibrium Gibbs functions.     

The effect of melt-spinning processing parameters on crystal structure and magnetic properties in Fe–Mn–Ga alloys

We have succeeded to fabricate body-centered cubic (bcc) single phase of Fe–Mn–Ga alloys using melt-spinning technique. Heusler type L2₁ structure of Fe₂MnGa alloy are predicted to have half-metallic properties, however bulk Fe₂MnGa alloys crystallize into face-centered cubic (fcc) lattice with small admixture of bcc phase. By changing either ejection temperature or rotation speed of melt-spinning processing parameters, fcc or bcc lattice can be obtained from same precursor ingot. For stoichiometric Fe₂MnGa as-spun alloy, super-lattice diffraction peaks indicative of L2₁ structure are observed from XRD measurements. The as-spun bcc alloys transform into ferromagnetic hexagonal lattice by thermal annealing.     

Investigation of the thermal and magnetic properties of Fe61Co10Zr2 Fe61Co10Zr2.5Hf2.5Me2W2B20 (Me = Y, Nb, W, Ti, Mo, Ni) bulk amorphous alloys obtained by an induction suction method

The microstructure, magnetic properties and thermal stability of Fe₆₁Co₁₀Zr_2.5Hf_2.5Me₂W₂B₂₀ (Me = Y, Nb, W, Ti, Mo, Ni) alloys were investigated. The samples were obtained by an induction suction method as 0.5 mm thickness plates. The microstructure was examined using X-ray diffraction and Mössbauer spectroscopy. It was shown that the investigated samples have amorphous structure throughout the volumes of the samples. The magnetic properties were measured using a Vibrating Sample Magnetometer. The investigated alloys are soft magnetic materials with low coercivity field (from 5.8 A/m to 54 A/m) and high saturation of the magnetization (from 0.87 T to 1.26 T). The studies of thermal stability were performed using a differential scanning calorimeter. It was shown that the addition of respective atoms led to changes of Curie temperature in the range from 497 to 587 K, depending on the composition of the alloys.     

Crystal structural transformation accompanied by magnetic transition in MnCo1−xFexGe alloys

The effects of Fe-doping on the crystal structure and martensitic transformation (MT) temperature in MnCoGe alloy have been investigated by using x-ray diffraction, calorimetry and magnetic measurements. Substitution of Fe for Co atoms can stabilize the parent phase and significantly lower the MT temperature of the MnCoGe alloy. By tuning the Fe content, the magnetostructural transition from paramagnetic parent phase (i.e. austenite) with a Ni₂In-type hexagonal structure to ferromagnetic TiNiSi-type martensite can be realized in a temperature window determined by the Curie temperature of the austenite and that of the martensite. A large difference in magnetization between the austenite and martensite, accompanied by the magnetostructural coupling, gives rise to the magnetic-field-induced temperature shift of MT, which makes the MnCo_1−xFe_xGe alloys being a new kind of potential magnetic functional materials used as the magnetic-field-driven actuator or magnetic refrigeration material.     

Structural,half-metallic and elastic properties of the half-Heusler compounds NiMnM (M = Sb,As and Si) and IrMnAs from first-principles calculations

The structural, half-metallic and elastic properties of the half-Heusler compounds NiMnM (M = Sb, As and Si) and IrMnAs were investigated using first-principles calculations within the generalized gradient approximation (GGA) based on density function theory (DFT). The most stable lattice configurations about site occupancy are (Ni)_4a(Mn)_4c(Sb)_4d, (Ni)_4a(Mn)_4c(As)_4d, (Ni)_4a(Mn)_4c(Si)_4d and (Ir)_4a(Mn)_4c(As)_4d, respectively, and the exchange of elements in Wyckoff position 4c and 4d results in an identical (symmetry-related) phase. The half-Heusler compounds show half-metallic ferromagnetism with a half-metallic gap of 0.168 eV, 0.298 eV, 0.302 eV and 0.109 eV, respectively, and the total magnetic moments (M_tot) are 4.00 μ_B, 4.00 μ_B, 3.00 μ_B and 3.00 μ_B per formula unit, respectively, which agree well with the Slater–Pauling rule based on the relationship of valence electrons. The compound (Ir)_4a(Mn)_4c(As)_4d with half-metallic ferromagnetic character was reported for the first time. The individual elastic constants, shear modulus, Young's moduli, ratio B/G and Poisson's ratio were also calculated. The compounds are ductile based on the ratio B/G. The Debye temperatures derived from the average sound velocity (ν_m) are 327 K, 332 K, 434 K and 255 K, respectively. The predicted Debye temperature for NiMnSb agrees well with the available experimental value, and the Debye temperatures for the rest three compounds were reported for the first time.     

Effect of Dy2O3 intergranular addition on thermal stability and corrosion resistance of Nd–Fe–B magnets

Sintered Nd–Fe–B magnets with and without Dy₂O₃ were prepared by powder blending method. The temperature-dependent magnetic properties, thermal stability, microstructure and corrosion resistance of sintered magnets were investigated. The temperature-dependent magnetic properties revealed that with intergranular addition of Dy₂O₃, the reversible temperature coefficients β and α in the range of 293–373 K were both lowered, indicating that the thermal stability was effectively improved. This was also verified by the decreased irreversible flux loss (h_irr) and the increased maximum operating temperature (MOT). Moreover, the electrochemical and accelerated corrosion results clearly evidenced that the corrosion resistance of Nd–Fe–B magnet was also modified by addition of Dy₂O₃. Furthermore, the related mechanisms on improved thermal stability and corrosion resistance were systematically discussed.     

Single crystal study on UNi0.5Sb2

Single crystals of UNi_1−xSb₂ have been grown from an Sb-rich melt and studied by means of X-ray diffraction, magnetic and electrical transport measurements. Crystal structure refinements indicated significant deficiency on the transition metal sites in the tetragonal HfCuSi₂-type unit cell, yielding the actual composition UNi_0.5Sb₂. The single crystals studied order antiferromagnetically below T_N=161 K and exhibit another phase transition at T_t=60 K, presumably caused by a spin-reorientation. No crystal structure distortion could be detected at 10 K. Above T_N the electrical resistivity is dominated by a Kondo effect, whereas at lower temperatures it shows a behavior characteristic of antiferromagnets. The overall magnetic and electrical transport properties of UNi_0.5Sb₂ are highly anisotropic both in the ordered and paramagnetic states.