Effect of Mn addition on the structural and magnetic properties of Fe–Pd ferromagnetic shape memory alloys

The effect of Mn addition on the structural and magnetic properties of Fe–Pd ferromagnetic shape memory alloys is investigated. In particular, a complete characterization of the influence of the partial substitution of Fe by Mn has been performed on Fe_69.4?xPd_30.6Mn_x (x = 0, 1, 2.5 and 5) alloys. The substitution of 1% Fe by Mn fully inhibits the undesirable irreversible face-centered tetragonal to body-centered tetragonal transformation without decreasing the face-centered cubic to face-centered tetragonal temperature. In addition, the substitution of 2.5% Fe by Mn gives rise to the highest thermoelastic transformation temperature observed to date in the Fe–Pd system, probably due to an increase in the valence electron concentration. The magnetocaloric effect has been evaluated in this alloy system for the first time. Nevertheless, the low values obtained suggest that the Fe–Pd alloys are not good candidates for magnetic refrigeration applications.     

Martensitic transformation and magnetic properties in Ni–Fe–Ga–Co magnetic shape memory alloys

Ni–Fe–Ga–Co is a promising system for magnetic shape memory alloy applications, due to its good ductility, mobile twin boundaries and high transformation temperatures. Unlike previous studies which focused on compositions with a Ga content of 27 at.%, here the martensitic transformation and magnetic properties over a large composition range of Ni_54−xFe₂₀Ga₂₆Co_x, Ni_54−xFe₁₉Ga₂₇Co_x, Ni_56−xFe₁₇Ga₂₇Co_x and Ni_54−xFe₁₈Ga₂₈Co_x (x = 0, 2, 4) are investigated. The martensitic transformation temperature T_m and the Curie temperature T_c can be tailored in a wide range by changing composition and heat treatment. A coupling of martensitic and magnetic transformations at ∼90 °C is found for Ni₅₂Fe₁₇Ga₂₇Co₄. Additionally, the effect of thermal cycling on the martensitic transformation of single- and two-phase Ni–Fe–Ga–Co alloys is discussed. Furthermore, an intermediate face-centered cubic phase induced by powderization and transformed into a body-centered cubic phase by aging is reported. The saturation magnetization is significantly decreased by powderization, while recovered by the subsequent aging.     

Effects of rotation speed on microstructure and transition temperatures in Ni–Fe–Al melt-spun ribbons

The influence of quenching rate on the magnetic and martensitic properties of Ni–Fe–Al β phase alloys, with Al content less than 25 at.%, was investigated through melt-spinning. Rapidly solidified ribbons with different rotation speeds were fabricated for comparison. Microstructure observation and X-ray diffraction measurements revealed that lower quenching rate produced a β + γ two phase structure, whereas higher rate could produce a single β phase in a certain range of composition. The solid solution range of the β phase can be extended to lower Al content range by preventing the precipitation of the γ phase with an appropriate quenching rate. In Ni_75?xFe_xAl₂₅ ferromagnetic shape memory alloys, the magnetic transition temperatures (T_C) are limited to below 250 K in conventional processing, which is a disadvantage for practical applications. In the ribbons with lower Al content and higher quenching rate, both T_C and martensitic transformation temperatures (T_M) became simultaneously around room temperature. The T_M and T_C showed a linear dependence with average electron concentration e/a and the magnetic valence Z_m, respectively.     

Low core loss of Fe85Si2B8P4Cu1 nanocrystalline alloys with high Bs and B800

The Fe–Si–B–P–Cu nanocrystalline alloys exhibit high saturation magnetic flux density (B_s) as well as good soft magnetic properties such as low coercivity, high effective permeability and low magnetostriction after nanocrystallization. In this paper, the Fe₈₅Si₂B₈P₄Cu₁ alloy has been newly developed. On the viewpoint of magnetic softness, the Fe₈₅Si₂B₈P₄Cu₁ nanocrystalline alloy reveals low core loss (W) at a commercially frequency of 50 Hz in the maximum induction (B_m) range of up to 1.75 T, and the W in the B_m range of less than 1.8 T is smaller than that of the highest-graded oriented Si-steel due to high magnetic flux density at 800 A/m (B₈₀₀) of above 1.8 T and excellent magnetic softness originated from much higher Fe content and uniform nanocrystalline structure with small magnetostriction. The electrical resistivity (ρ) is relative higher than Si-steels. Thus the Fe–Si–B–P–Cu alloys are attractive for applying to magnetic parts such as motors, transducers, choke-coils and so-forth.     

Fe–B–P–Cu nanocrystalline soft magnetic alloys with high Bs

The effects of the addition of Cu on the crystallization processes, nanostructures and soft magnetic properties for the Fe_80.8–84.8B_8–10P_6–8Cu_1.2 alloys were investigated. The Fe–B–P–Cu alloys show two separated distinct exothermic peaks upon heating due to the addition of Cu. Furthermore, the interval temperature between each one for the Fe_82.8B₉P₇Cu_1.2 alloy is 103 K, and the first and second exothermic peaks result from the phase transition from amorphous to α-Fe and then to Fe₃(B,P), respectively. A uniform nanocrystalline structure composed of α-Fe grains with a 17 nm diameter was realized by annealing just above the first exothermic peak, and this nanocrystalline alloy exhibits high B_s of 1.70 T and low H_c of 4.9 A/m. Therefore, the nanocrystalline Fe–B–P–Cu soft magnetic alloy with high B_s and low H_c has a large industrial advantage due to miniaturization, high efficiency and low material cost of electric devices.     

Precipitation in Fe–15Cr–1Nb alloys after oxygenation

The effect of O on the phase relations at 950 °C in Fe–15Cr–1Nb alloys is experimentally investigated. Fe–15Cr–1Nb alloys are oxygenated by subjecting high-purity Fe–15Cr–1Nb to an O atmosphere at 600 °C. Both the high-purity and the oxygenated Fe–15Cr–1Nb alloys are heat treated for up to 500 h at 950 °C, quenched and investigated by scanning electron microscopy, transmission electron microscopy and electron probe microanalysis. The results show that Fe₂Nb is in equilibrium with α (Fe, Cr) with 0.29 at.% Nb in solid solution in the pure Fe–15Cr–1Nb alloy. The presence of a small amount of O induces the precipitation of a Fe₆Nb₆O_x phase with a cubic crystal structure and lattice parameter 1.13 nm, thereby decreasing the Nb in solid solution in α (Fe, Cr) with increasing O content.     

Effect of ternary alloying elements on the shape memory behavior of Ti–Ta alloys

The effect of ternary alloying elements (X = V, Cr, Fe, Zr, Hf, Mo, Sn, Al) on the shape memory behavior of Ti–30Ta–X alloys was investigated. All the alloying elements decreased the martensitic transformation temperatures. The decrease in the martensitic transformation start (M_s) temperature due to alloying was affected by the atomic size and number of valence electrons of the alloying element. A larger number of valence electrons and a smaller atomic radius of an alloying element decreased the M_s more strongly. The effect of the alloying elements on suppressing the aging effect on the shape memory behavior was also investigated. It was found that the additions of Sn and Al to Ti–Ta were effective in suppressing the effect of aging on the shape memory behavior, since they strongly suppress the formation of ω phase during aging treatment. For this reason the Ti–30Ta–1Al and Ti–30Ta–1Sn alloys exhibited a stable high-temperature shape memory effect during thermal cycling.     

Effect of solute segregation on the strength of nanocrystalline alloys: Inverse Hall–Petch relation

We have used a high-energy ball mill to prepare single-phased nanocrystalline Fe, Fe₉₀Ni₁₀, Fe₈₅Al₄Si₁₁, Ni₉₉Fe₁ and Ni₉₀Fe₁₀ powders. We then increased their grain sizes by annealing. We found that a low-temperature anneal (T < 0.4 T_m) softens the elemental nanocrystalline Fe but hardens both the body-centered cubic iron- and face-centered cubic nickel-based solid solutions, leading in these alloys to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of solute segregation to the grain boundaries of the nanocrystalline alloys. Our analysis can also explain the inverse Hall–Petch relationship found in previous studies during the thermal anneal of ball-milled nanocrystalline Fe (containing ∼1.5 at.% impurities) and electrodeposited nanocrystalline Ni (containing ∼1.0 at.% impurities).     

Shape memory behavior of Ti–Ta and its potential as a high-temperature shape memory alloy

In this study, the effect of Ta content on shape memory behavior of Ti–Ta alloys was investigated. The shape memory effect was confirmed in Ti–(30–40)Ta alloys. The martensitic transformation start temperature (M_s) decreased by 30 K per 1 at.% Ta. The amount of ω phase formed during aging decreased with increasing Ta. A stable high-temperature shape memory effect was confirmed for Ti–32Ta (M_s = 440 K) during thermal cycling between 173 and 513 K. On the other hand, the high-temperature shape memory effect of Ti–22Nb, which has a similar M_s to Ti–32Ta, exhibited poor stability due to the large amount of ω phase formed during thermal cycling. It is suggested that Ti–Ta is an attractive candidate for the development of novel high-temperature shape memory alloys.     

Correlation between composition and phase transformation temperatures in Ni–Mn–Ga–Co ferromagnetic shape memory alloys

The effect of Co addition on the phase transformation temperatures (martensitic and Curie point) and crystal structure of Ni–Mn–Ga–Co shape memory alloys has been investigated on (Ni_50.26Mn_27.30Ga_22.44)_100−xCo_x (x = 0, 2, 4, 6) alloys as well as on alloys having different Ni/Mn/Ga ratios and a fixed amount of Co. Alloying by Co affects the martensitic transformation temperature and the transformation enthalpy change mainly through the change on the valence electron concentration (e/a), but the transformation entropy is almost unaffected. On the other hand, the composition (analyzed through the e/a ratio) shows a different influence on the Curie temperature depending on the crystallographic phase (austenite or martensite) in which the magnetic ordering takes place. It is also reported that in Ni–Mn–Ga–Co alloys the Curie temperature of the martensitic phase is lower than that of the austenitic phase, opposite to what occurs in ternary Ni–Mn–Ga alloys.     

Ferromagnetism and its photo-induced effect in 2D iron mixed-valence complex coupled with photochromic spiropyran

Iron mixed-valence complex, (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto = C₂O₂S₂), shows a spin-entropy driven phase transition called charge transfer phase transition around 120 K and a ferromagnetic transition at 6.5 K. These phase transitions remarkably depend on the hexagonal ring size in two-dimensional honeycomb network structure of [Fe^IIFe^III(dto)₃]_∞. In order to control the magnetic properties and the electronic state in the dto-bridged iron mixed-valence system by means of photo-irradiation, we have synthesized a photo-sensitive organic–inorganic hybrid system, (SP)[Fe^IIFe^III(dto)₃] (SP = spiropyran), and investigated the photo-induced effect on the magnetic properties by the measurements of magnetic susceptibility, ESR, IR, UV–vis and ⁵⁷Fe Mössbauer spectra. Upon UV irradiation at 337 nm, a broad absorption band between 500 nm and 600 nm appears and continuously increases with the photo-irradiation time, which implies that the UV irradiation changes the structure of spiropyran from closed form to open one in solid state. The photochromism in spiropyran changes the ferromagnetic transition temperature and the coercive force.     

Magnetic properties and magnetocaloric effect of Nd(Mn1−xFex)2Ge2 compounds

Polycrystalline Nd(Mn_1?xFe_x)₂Ge₂ (0 ≤ x ≤ 1) compounds have been prepared by arc-melting. X-ray powder diffraction analysis shows that all samples crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. The substitution of Fe for Mn results in decreases of the lattice constants a, c and the unit-cell volume V. Magnetic properties and magnetocaloric effect of the compounds Nd(Mn_0.9Fe_0.1)₂Ge₂ and Nd(Mn_0.8Fe_0.2)₂Ge₂ have been studied by magnetic measurements. A spin reorientation transition at about 188 K is observed for Nd(Mn_0.9Fe_0.1)₂Ge₂. The Nd(Mn_0.8Fe_0.2)₂Ge₂ compound shows more complicated magnetic behavior, which is characterized by four magnetic ordering states below room temperature. The maximum values of the magnetic entropy change are 2.35 and 1.84 J kg^?1 K^?1 for x = 0.1 and 0.2, respectively, under the applied field changing from 0 to 5 T. The relative cooling powers of Nd(Mn_0.9Fe_0.1)₂Ge₂ and Nd(Mn_0.8Fe_0.2)₂Ge₂ are 145.9 and 133.8 J kg^?1 under an applied field change of 5 T.     

Novel FePBNbCr glassy alloys “SENNTIX” with good soft-magnetic properties for high efficiency commercial inductor cores

According to a recent study, Fe-based glassy alloys are expected good soft-magnetic properties such as high saturation magnetization and lower coercive force. We focused on Fe-based glassy alloys and have succeeded in developing novel glassy Fe_97?x?yP_xB_yNb₂Cr₁ (x = 5–13, y = 7–15) alloys for an inductor material. The glassy alloy series of Fe_97?x?yP_xB_yNb₂Cr₁ (x = 5–13, y = 7–15) have high glass-forming ability with the large critical thickness of 110–150 μm and high B_s of 1.25–1.35 T. The glassy alloy powder with chemical composition Fe₇₇P_10.5B_9.5Nb₂Cr₁ exhibits an excellent spherical particle shape related to the lower melting point and liquid phase point. In addition, Fe–P–B–Nb–Cr powder/resin composite core has much lower core loss of 653–881 kW/m³, which is approximately 1/3 lower than the conventional amorphous Fe–Si–B–Cr powder/resin composite core and 1/4 lower than the conventional crystalline Fe–Si–Cr powder/resin composite core due to the lower coercive force of 2.5–3.1 A/m. Based on above results, the glassy Fe₇₇P_10.5B_9.5Nb₂Cr₁ alloy powder enable to achieve ultra-high efficient and high quality products in a commercial inductor. In fact, the surface mounted inductor using Fe–P–B–Nb–Cr powder/resin exhibits the high efficiency of approximately 2.0% compared with the conventional inductors made of the crystalline Fe–Si–Cr powder/resin composite core.     

Phase transformation of Cu precipitates from bcc to fcc in Fe–3Si–2Cu alloy

Phase transition of Cu precipitates during aging of an Fe–3Si–2Cu alloy was studied by transmission electron microscopy. The precipitation of 3–5-nm-sized body-centered cubic (bcc) Cu in ferrite matrix was confirmed by high-angle annular dark-field scanning transmission electron microscopy imaging. The bcc Cu precipitates transformed to 9R Cu as they grew. Many 9R Cu precipitates were twinned, but untwinned 9R Cu particles were also observed. The 9R Cu transformed to twinned face-centered cubic (fcc) Cu by the glide of ±a/3 [1 0 0]_9R Shockley-type partial dislocations. Formation of the 3R structure previously reported could not be confirmed in this study. Finally, twins in fcc Cu precipitates disappeared to form stable fcc Cu particles. The importance of electron beam-orientation-dependent moiré fringes in the correct identification of Cu structure is discussed in detail.     

An investigation of the Al–Pd–Rh phase diagram between 50 and 100 at% Al

Partial 1100, 1000, 900 and 790 °C isothermal sections of the Al–Pd–Rh phase diagram were studied. The isostructural binary AlPd and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. The Al–Pd and Al–Rh ε-phases form another continuous range. The C–Al₅Rh₂ phase dissolves up to 13 at% Pd, Al₉Rh₂ and Al₇Rh₃ are extended up to 3 at% Pd. Two ternary phases: cubic C₂ (a=1.5483 nm) and hexagonal C₃ (a=1.09159, c=1.3386 nm) were revealed. The former extends along about 65 at% Al from 4 to 27 at% Pd.     

The effect of Co doping on the magnetic entropy changes in Ni44−xCoxMn45Sn11 alloys

A series of Ni_44?xCo_xMn₄₅Sn₁₁ (x = 0, 1, and 2) ferromagnetic shape memory alloys (FSMAs) were prepared by arc melting method. The martensitic transition (MT) and Curie temperatures vary obviously with Co addition. With the increasing temperature, the magnetization increases from a weak-magnetic martensite to a ferromagnetic austenite, for x = 0 and 1. But in the case of x = 2, the magnetization increases from a ferromagnetic martensite to another ferromagnetic austenite. Under an applied magnetic field of 10 kOe, the peak values of magnetic entropy changes are 10.1, 14.1, and 6.2 J/(kg K), for x = 0, 1, and 2, respectively. The magnetic phase transition near the martensitic transition temperatures and the field-induced metamagnetism should account for the large magnetic entropy changes.     

Cr effects on magnetic and corrosion properties of Fe–Co–Si–B–Nb–Cr bulk glassy alloys with high glass-forming ability

Effects of the Cr addition on glass formation, magnetic and corrosion properties of {[(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05]_0.96Nb_0.04}_100−xCr_x (x = 1, 2, 3, 4 at.%) alloys have been investigated. It was found that the addition of Cr element slightly decreases the glass-forming ability (GFA), but is very effective in increasing corrosion resistance and improving soft magnetic properties for this Fe–Co–B–Si–Nb bulk glassy alloy within the composition range examined. The Fe–Co–B–Si–Nb–Cr alloys exhibit high GFA. Full glassy rods with diameters up to 4 mm can be synthesized by copper mold casting. The Fe-based bulk glassy alloys (BGAs) exhibit a high saturation magnetization of 0.81–0.98 T as well as excellent soft magnetic properties, i.e., extremely low coercive force of 0.6–1.6 A/m and super-high initial permeability of 26,400–34,100. Furthermore, corrosion measurements show that corrosion rate and corrosion current density of these Fe-based BGAs in 0.5 M NaCl solution decrease from 7.0 × 10⁻¹ to 1.6 × 10⁻³ mm/year and 3.9 × 10⁻⁶ to 8.7 × 10⁻⁷ A/cm², respectively, with increasing Cr content from 0 to 4 at.%. The success of synthesizing the new Fe-based BGAs exhibiting simultaneously high GFA as well as excellent good soft magnetic properties combined with high saturation magnetization and enhanced corrosion resistance allows us to expect future progress as a new type of soft magnetic materials.     

Roles of interface roughness and internal stress in magnetic anisotropy of CoPt/AlN multilayer films

Magnetic anisotropy of CoPt/AlN multilayer films has been studied by systematically varying the nominal thickness of CoPt layers, t_CoPt (1–10 nm), and the annealing temperature, T_a (300–500 °C). The as-deposited films show in-plane magnetic anisotropy in the full range of t_CoPt, whereas the annealed films show perpendicular magnetic anisotropy (PMA) within small t_CoPt but change to in-plane magnetic anisotropy when t_CoPt is over a certain thickness. The critical thickness for such anisotropic transformations increases as the T_a increases. The maximum PMA obtained in this work is 1.13 × 10⁷ erg cm^?3. The interface roughness was analyzed by cross-sectional high-resolution electron microscopy and X-ray reflectivity using an abrupt interface model with a Debye exponent shape. The internal stress was analyzed by X-ray diffraction using an equal biaxial stress model. The results show that the CoPt/AlN interface roughness decreases from 0.385 nm to 0.158 nm and the internal stress increases from ?2.36 GPa (compressive) to 1.73 GPa (tensile), as the T_a increases to 500 °C. The roles of the interface roughness and the internal stress in the magnetic anisotropy of CoPt/AlN multilayer films are studied.     

Three-dimensional atom probe study of Fe–B-based nanocrystalline soft magnetic materials

Solute clustering and partitioning in new Fe–B-based soft magnetic materials with high saturation magnetic flux density (B_s), (Fe_0.85B_0.15)_100?xCu_x (x = 0.0, 1.0, and 1.5) and Fe_82.65Cu_1.35Si_yB_16?y (y = 0.0, 2.0, and 5.0) melt-spun alloys, were investigated by three-dimensional atom probe and transmission electron microscopy. Although Cu clusters form after annealing in all the samples, it was found that only the clusters of 4–6 nm can serve as heterogeneous nucleation sites for α-Fe. While annealing the Si-free alloys at 410 °C led to the precipitation of Fe₃B, only α-Fe nanocrystals were observed in the Si-containing alloys. Lorenz TEM observation indicated the Fe₃B particles pin magnetic domain walls. The Fe_82.65Cu_1.35Si_yB_16?y alloy with y = 2.0 crystallized by annealing at 400 °C exhibited optimal nanocrsytal/amorphous microstructure without the precipitation of Fe₃B, which led to the lowest coercivity while keeping a high B_s ～1.85 T.