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.     

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 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.     

Fabricating Ni–Mn–Ga microtubes by diffusion of Mn and Ga into Ni tubes

Tubes of the ferromagnetic shape-memory alloy Ni–Mn–Ga of composition near the Ni₂MnGa Heusler phase can be used, alone or combined in structures, in magnetic actuators or magnetic refrigerators. However, fabrication of Ni–Mn–Ga tubes with sub-millimeter diameter by classical cold or hot drawing methods is hampered by the brittleness of the alloy. Here, we demonstrate a new process, where Ni–Mn–Ga tubes are fabricated by interdiffusion of Mn and Ga into drawn, ductile Ni tubes with 500 and 760 μm inner and outer diameters. After interdiffusion and homogenization of Mn and Ga at 1000 °C for 24–36 h, Ni–Mn–Ga tubes with ∼300 and ∼900 μm inner and outer diameters were obtained with homogeneous radial composition distribution, independently of the diffusion sequences (i.e., Mn and Ga diffused sequentially or simultaneously). Longitudinal composition was uniform over lengths of ∼1 mm, but variable over longer length due to incomplete process control. For two of the three diffusion sequences, a sizeable (20–80 μm) region exhibiting Kirkendall pores formed at the outer surface of the tubes. Magnetization values as high as ∼60 emu/g were measured, which is comparable to the magnetization of the Ni₂MnGa Heusler phase. X-ray diffraction on the tube with the highest magnetization confirmed the room-temperature structure as cubic austenite.     

Large reversible magnetocaloric effect in the RECoC2 (RE=Ho and Er) compounds

The crystal structure, magnetic properties and magnetocaloric effect (MCE) of HoCoC₂ and ErCoC₂ have been investigated. They both crystallize in orthorhombic CeNiC₂-type structure with Amm2 space group and a second-order paramagnetic to ferromagnetic phase transition around the Curie temperature T_C ∼11 K and ∼14 K occurred in them. Under the magnetic field change (ΔH) of 0–5 T, the maximal values of magnetic entropy change, refrigerant capacity and relative cooling power are 15.6 J/kg K, 183 J/kg, 242 J/kg for HoCoC₂ and 17.2 J/kg K, 243 J/kg, 375 J/kg for ErCoC₂, respectively.     

Magnetism and magnetocaloric effect in the ternary equiatomic REFeAl (RE = Er and Ho) compounds

Magnetic properties and magnetocaloric effect (MCE) in ErFeAl and HoFeAl intermetallic compounds have been studied systematically. Both compounds undergo a second order magnetic phase transition from paramagnetic to ferromagnetic state together with a probable spin reorientation transition at low temperature. A considerable reversible magnetocaloric effect was observed around its own Curie temperatures T_C ∼55 K and 80 K for ErFeAl and HoFeAl, respectively. For a magnetic field change of 0–7 T, the maximum values of magnetic entropy change (−ΔS_M^max) are found to be 8.3 and 9.9 J/kg K for RE = Er and Ho, respectively. The corresponding values of refrigerant capacity (RC) are evaluated to be 384 and 647 J/kg.     

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.     

Influence of Bridgman solidification on microstructures and magnetic behaviors of a non-equiatomic FeCoNiAlSi high-entropy alloy

The non-equiatomic FeCoNiAlSi alloy is prepared by the Bridgman solidification (BS) technique at different withdrawal velocities (V = 30, 100, and 200 μm/s). Various characterization techniques have been used to study the microstructure and crystal orientation. The morphological evolutions accompanying the crystal growth of the alloy prepared at different withdrawal velocities are nearly the same, from equiaxed grains to columnar crystals. The transition of coercivity is closely related to the local microstructure, while the saturation magnetization changes little at different sites. The coercivity can be significantly reduced from the equiaxed grain area to the columnar crystal area when the applied magnetic field direction is parallel to the crystal growth direction, no matter what is the withdrawal velocity. In addition, the alloy possesses magnetic anisotropy when the applied magnetic field is in different directions.     

Magnetic specific heat and magnetoresistance of URhSi

Specific heat of a single crystalline URhSi was measured by a relaxation method in a temperature range 0.3–25 K in magnetic fields up to 8 T applied along the two of the principal axes. The low-temperature specific heat exponentially decays with magnetic field. The decay is much faster in fields applied along the easy magnetization direction (the c-axis) than for the hard axis (the a-axis) case. A strong upturn in c_p/T versus T below 0.6 K that disappears with application of magnetic field is observed suggesting possible magnetic or superconducting phase transition at lower temperatures. The electrical resistivity in the vicinity of the ferromagnetic phase temperature is found to be reduced by more than 50% upon application of magnetic field of 8 T applied along the c-axis. URhSi represent an itinerant ferromagnetic system with influence of spin fluctuations.     

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.     

Effect of Co addition on martensitic phase transformation and magnetic properties of Mn50Ni40-xIn10Cox polycrystalline alloys

This study investigated the use of Co to enhance the magnetic driving force for inducing the martensitic transformation of Mn₅₀Ni_40-xIn₁₀Co_x alloys. These alloys present a martensitic transformation from a Hg₂CuTi-type austenite to a body centered tetragonal martensite, with a large lattice distortion of 15.7% elongation along the c direction and 8.2% contraction along a and b directions. The martensitic transformation temperatures, transformation enthalpy and entropy changes decreased with increasing the Co content in these alloys. The maximum magnetization of the austenite increased significantly, whereas that of the martensite changed much less prominently with increasing the Co substitution for Ni, leading to increase of the magnetic driving force for the transformation. The magnetization increase of the austenite is found to be due to (i) formation of ferromagnetically coupled Mn–Mn due to new atomic configuration in off-stoichiometric composition, (ii) magnetic moment contribution of Co and (iii) widening of the temperature window for magnetization of the austenite by lowering the temperature of the martensitic transformation. These findings clarify the effect of Co addition on martensitic transformation and magnetic properties in Mn-rich ferromagnetic shape memory alloys, and provide useful understanding for alloy design for magnetoactuation applications.     

Magnetic and transport properties in the Heusler series Ni2−xMn1+xSn affected by chemical disorder

Crystal structure, magnetic and transport characteristics of Ni_2−x Mn_1+x Sn Heusler series have been studied with the emphasis on chemical disorder effects. It is shown that the structure and the disorder character in these series can be predicted by using simple rules. Ni₂ MnSn is a ferromagnetic, congruent melting phase, which crystallizes cubic in the L2₁ structure type. By increasing x, Ni and Mn atoms randomly mix and occupy the heterocubic sites of the regular Heusler structure, and the magnetic structure becomes ferrimagnetic. The total magnetic moment m_sat decreases linearly in the range 0.2 ≤ x ≤ 1, while the Curie temperature T_C increases. At low Mn content (x < 0.2), the unit cell volume shows anomalous behavior, characterized by constant m_sat and T_C. Electrical resistivity, Seebeck coefficient, and thermal conductivity strongly depend on the amount of disorder, which increases with the Mn content. Results of first-principle calculations based on the coherent potential approximation (CPA) alloy theory for the magnetic and electrical properties are in reasonable agreement with the simple rules and all experimental data.     

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.     

Hot deformation behavior of Ni–Fe–Ga-based ferromagnetic shape memory alloy – A study using processing map

Ni–Fe–Ga-based alloys form a new class of ferromagnetic shape memory alloys (FSMAs) that show considerable formability because of the presence of a disordered fcc γ-phase. The current study explores the deformation processing of this alloy using an off-stoichiometric Ni₅₅Fe₁₉Ga₂₆ alloy that contains the ductile γ-phase. The hot deformation behavior of this alloy has been characterized on the basis of its flow stress variation obtained by isothermal constant true strain rate compression tests in the 1123–1323 K temperature range and strain rate range of 10⁻³–10 s⁻¹ and using a combination of constitutive modeling and processing map. The dynamic recrystallization (DRX) regime for thermomechanical processing has been identified for this Heusler alloy on the basis of the processing maps and the deformed microstructures. This alloy also shows evidence of dynamic strain-aging (DSA) effect which has not been reported so far for any Heusler FSMAs. Similar effect is also noticed in a Ni–Mn–Ga-based Heusler alloy which is devoid of any γ-phase.     

Magnetic properties and magnetocaloric effect in the aluminide RENiAl2 (RE = Ho and Er) compounds

Two single-phased aluminide RENiAl₂ (RE = Ho and Er) compounds have been synthesized by an arc-melting method. The magnetic properties and magnetocaloric effect (MCE) of HoNiAl₂ and ErNiAl₂ have been studied by magnetization measurements. A second-order paramagnetic (PM) to ferromagnetic (FM) phase transition together with a considerable reversible MCE were observed around the Curie temperature T_C ∼7.5 K and 5.0 K for HoNiAl₂ and ErNiAl₂, respectively. Under the magnetic field change (ΔH) of 0–5 T, the maximal values of magnetic entropy change, refrigerant capacity and relative cooling power are 14.0 J/kg K, 171 J/kg, 213 J/kg for HoNiAl₂ and 21.2 J/kg K, 267 J/kg, 357 J/kg for ErNiAl₂, respectively.     

Magnetic properties and tuneable magnetocaloric effect with large temperature span in GdCd1−xRux solid solutions

The magnetic properties and the magnetocaloric effect (MCE) in the GdCd_1−xRu_x (x = 0.1, 0.15, and 0.2) solid solutions have been systematically investigated. A large reversible MCE has been observed in GdCd_1−xRu_x accompanied by a second order magnetic phase transition from paramagnetic to ferromagnetic at T_C ∼ 149, 108, and 73 K for x = 0.1, 0.15, and 0.2, respectively. Under a field change from 0 to 7 T, the maximum values of magnetic entropy change (–ΔS_M^max) are 5.6, 7.8, and 11.0 J/kg K for x = 0.1, 0.15, and 0.2, respectively, the corresponding values of the relative cooling power (RCP) are 889, 852, and 828 J/kg. The considerable reversible MCE and large RCP values together with the tuneable T_M in a wide temperature range make the GdCd_1−xRu_x solid solutions considerable for active magnetic-refrigeration.     

The alignment,aggregation and magnetization behaviors in MnBi–Bi composites solidified under a high magnetic field

Bi–6 wt% Mn alloy is solidified under a high magnetic field and its microstructures and magnetic properties have been investigated. Microstructure results show that three kinds of morphology of MnBi phase appear in different temperature zones. In all these cases, the grains are orientated with the 〈001〉-crystal direction along the magnetic field direction and aggregated. Magnetic measurement shows a pronounced anisotropy in magnetization in directions normal and parallel to the fabrication field, resulting from this alignment. The effect of the magnetic field on the Mn_1.08Bi/MnBi (paramagnetic/ferromagnetic) phase transformation has been studied and the result shows that the phase transformation temperature T_C increases with the increase of the external magnetic field and under a field of 10 T, a typical increase of T_C is 20 °C during heating and 22 °C during cooling. The change in the morphology and in the magnetic properties of MnBi phase is discussed from the phase transformation and the crystal structure change in magnetic field.     

Magnetic properties and magnetocaloric effect of FeCrNbYB metallic glasses with high glass-forming ability

The influences of Cr addition on the Curie temperature (T_C), glass-forming ability (GFA), and magnetocaloric effect were investigated in FeCrNbYB metallic glasses. It was found that the addition of Cr element slightly decreases the GFA and saturation magnetization, whereas effectively modulates T_C. By the method of copper mold casting, bulk metallic glasses (BMGs) with critical diameters up to 5 mm can be obtained in Fe_68−xCr_xNb₄Y₆B₂₂ (x = 2–6) alloys. The resulting metallic glasses exhibit T_C of 271–367 K and excellent magnetocaloric properties, including magnetic entropy change of 0.76–1.05 J/kg K, and refrigerant capacity of 83–93 J/kg under a low field change of 1.5 T. In addition, they exhibit a wide supercooled liquid region of 116–135 K. The successful synthesis of the FeCrNbYB BMGs with near room-temperature magnetocaloric properties is encouraging for the future development of Fe-based BMGs as a new magnetic refrigerant in magnetic cooling system.     

Preliminary investigation on strong magnetic field treated NiAl–Cr(Mo)–Hf near eutectic alloy

The Ni-33Al-28Cr-6Mo-0.3Hf near eutectic alloy was treated in a 10 T strong magnetic field at 1073 K and 1173 K for 1.5 h respectively. Microstructure examination reveals that after the strong magnetic field treatment, most Ni₂AlHf Heusler particles have changed into Hf solid solution, and moreover Hf solid solution and Heusler particles along eutectic cell boundaries distribute more uniformly. In addition, the eutectic cellular microstructure of the alloy with strong magnetic field treatment at 1173 K is changed greatly. The eutectic cell has an obvious disintegration and Cr(Mo) plates become coarsening and spheroidizing. The compression tests show that room temperature compressive ductility of the alloys with strong magnetic field treatment improves significantly, compared with the heat-treated alloy.     

Structural and magnetic properties of Ni2CoSi: First-principles calculation and experimental realization

BCC Heusler phase Ni₂CoSi has been predicted to be a promising candidate to realize magnetic field induced martensitic transformation. We tried to prepare Ni₂CoSi single phase using different methods. Single phase Ni₂CoSi cannot be synthesized by arc-melting and annealing. Then we used mechanical alloying method to synthesize Ni₂CoSi. But a FCC phase rather than BCC was obtained after ball-milling. The lattice constant of FCC Ni₂CoSi is 3.52 Å and the Curie temperature is around 900 K. The saturation magnetization at 5 K is 2.44μ_B/f.u. This FCC phase is stable and no transition is observed when heating to 1173 K. The electronic structure and phase stability of the FCC and BCC Heusler phase have been investigated by first-principles calculations. The FCC Ni₂CoSi has lower total energy compared with BCC, agreeing with the experimental observation. But the calculated total moment is much smaller than the M_s at 5 K. This difference is related to the atomic disorder and was discussed by KKR-CPA calculation.