Phase transitions and thermal expansion in Ni<Subscript>51–<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>36 + <Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>13</Subscript> alloys

Thermal expansion and structural and magnetic phase transitions in alloys of the Ni–Mn–Sn system have been investigated. The spontaneous martensitic transformation in Ni_51–xMn_{36 + x}Sn₁₃ (0 ≤ x ≤ 3) alloys is found to be accompanied by high jumps in the temperature dependences of the linear thermal expansion. The relative change in the linear sizes of these alloys at the martensitic transformation is ~1.5 × 10^–3. There are no anomalies in the magnetic-ordering temperature range in the temperature dependences of the coefficient of linear thermal expansion. The differences in the behavior of linear thermal expansion at the martensitic transformation in Ni_51–xMn_{36 + x}Sn₁₃ (0 ≤ x ≤ 3) and Ni₄₇Mn₄₀Sn₁₃(x = 4) alloys have been established.     

Effect of cobalt doping on thermoelastic martensitic transformations and physical properties of magnetic shape memory alloys Ni50 − x Co x Mn29Ga21

The magnetic and thermoelastic martensitic transformations and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of multicomponent magnetic shape memory alloys Ni_{50 ? x} Co _x Mn₂₉Ga₂₁ (x = 0, 1, 2, 3, 10 at %) have been investigated. The critical temperatures of thermoelastic martensitic transformation and magnetic transitions have been determined. It has been found that the alloy with 10 at % Co undergoes a martensitic transformation in the temperature range of 6–10 K.     

Effect of Mn incorporation for Ni on the properties of melt spun off-stoichiometric compositions of NiMnGa alloys

The investigation addresses the effect of Mn incorporation for Ni on the properties of a series of Ni_77−xMn_xGa₂₃ (x=22-29; at%) ferromagnetic shape memory alloys prepared in the form of ribbons by a melt spinning technique. Phase transformation studies in these ribbons by differential scanning calorimetry revealed that austenitic start and martensitic start temperatures decreased with the increase in Mn content. The Curie temperature (T_C) of these alloys determined from thermal variation of magnetisations was found to rise with increasing Mn content. The martensitic transformation temperatures were above T_C in low Mn containing (x=22 and 23) alloys. Morphology observed through transmission electron microscopy manifested complex martensitic features in the alloy with x=22 while x=29 had an austenitic phase. The alloys with intermediate Mn content (x=24, 25) had overlapping magnetic and martensitic transformations close to room temperature. The thermal lag between austenitic and martensitic characteristic temperatures in these alloys has been corroborated to their structural state. X-ray diffraction indicated a predominant martensite phase and austenite phase in low and high Mn containing alloys respectively. In-situ diffraction studies during thermal cycle indicate martensite-austenite transformations.     

Effect of alloying on the structure and phase transformations in Ni-Mn alloys alloyed by titanium

The crystal structure of NiMn alloy alloyed by titanium in a wide range of temperatures and compositions has been investigated using resistivity measurements, transmission electron microscopy, electron diffraction, and X ray diffraction. It is found that alloying by titanium not only decreases the martensitic transformation temperature but also changes the martensite crystal structure. The martensitic transformation temperatures are determined and the diagram of martensitic transformations for Ni₅₀Mn_{50 ? x} Ti _x alloys is constructed.     

The effect of Si content on the martensitic transfor-mation temperature of Ni55.5e18Ga26.5-xSix alloys

This paper investigates the effects of substitution of Si for Ga on the martensitic transformation behaviours in Ni-Fe-Ga alloys by using optical metallographic microscope and differential scanning calorimetry (DSC) methods. The structure type of Ni_55.5Fe₁₈Ga_26.5-xSi_x alloys is determined by x-ray diffraction (XRD), and the XRD patterns show the microstructure of Ni-Fe-Ga-Si alloys transformed from body-centred tetragonal martensite (with Si content x = 0) to body-centred cubic austenite (with x = 2) at room temperature. The martensitic transformation temperatures of the Ni_55.5Fe₁₈Ga_26.5-xSi_x alloys decrease almost linearly with increasing Si content in the Si content range of x ≤ 3. Thermal treatment also plays an important role on martensitic transformation temperatures in the Ni-Fe-Ga-Si alloy. The valence electronic concentrations, size factor, L2₁ degree of order and strength of parent phase influence the martensitic transformation temperatures of the Ni-Fe-Ga-Si alloys. An understanding of the relationship between martensitic transformation temperatures and Si content will be significant for designing an appropriate Ni-Fe-Ga-Si alloy for a specific application at a given temperature.     

Si掺杂的铁磁形状记忆合金Co50Ni21Ga29Six的物性研究

成功生长了Co₅₀Ni₂₁Ga₂₉:Si(x=1,2)单晶样品,对其磁性,马氏体相变及其相关性质进行了细致的测量.发现掺Si成分的单晶具有非常迅速的马氏体相变行为、2.5％的大相变应变、大于100 ppm的磁感生应变和4.5％的相变电阻.进一步研究指出,在CoNiGa合金中掺入适量Si元素,能够降低材料的马氏体相变温度,减小相变热滞后,提高材料的居里温度,并使得磁性原子的磁矩有所降低.尤其重要的是Si元素的添加能够增大材料马氏体的磁晶各向异性能,改善马氏体变体的迁移特性,从而获得更大的磁感生应变. 关键词： 铁磁形状记忆合金 Heusler合金 50Ni₂₁Ga₂₉Si_x')" href="#">Co₅₀Ni₂₁Ga₂₉Si_x     

Structural and magnetic phase formation in nanophase brass—iron electron compounds

Starting with Cu0.65Zn0.35 with an e/a ratio of 1.35 we studied the phase formation in nanophase (Cu_0.65Zn_0.35)_1?x Fe _x alloys in the concentration range 0.1 ≤x ≤0.7 to see the effect of altering the electron concentration. The evolution of bcc phase from the fcc phase as a function of Fe concentration was investigated by Mössbauer spectroscopy and X-ray diffraction. The grain size, lattice parameters, and average hyperfine magnetic field distributions were estimated for the nanophase alloys. The fcc phase was observed to persist up to 40 atomic per cent Fe substitutions, a mixed (fcc + bcc) phase region up to 70 atomic per cent Fe and bcc phase beyond 70 atomic per cent Fe. The magnetic state of the alloys changed from nonmagnetic forx ≤0.3 to magnetically ordered state at room temperature forx ≤0.33, which lies in the fcc phase region. The fcc phase alloys of Fe with non-magnetic metals have very low magnetic transition temperatures. However, in this system the room temperature state is unusually magnetic     

The large low-field magnetic entropy changes in Ni43Mn46Sn11−xSbx alloys

A series of Ni₄₃Mn₄₆Sn_11−xSb_x (x=0, 1, and 3) alloys were prepared by an arc melting method. The martensitic transition shifts to higher temperature with the increasing Sb content. The isothermal magnetization curves and Arrott plots around martensitic transition temperatures show a typical metamagnetic behavior. Under a low applied magnetic field of 10 kOe, large magnetic entropy changes around the martensitic transition temperature are 10.4, 8.9, and 7.3 J/kg K, for x=0, 1, and 3, respectively. The origin of the large magnetic entropy changes and potential application for Ni₄₃Mn₄₆Sn_11−xSb_x alloys as working substances in magnetic refrigeration are discussed.     

Martensitic transformation and magnetocaloric properties of Sn doping Mn-Ni-Ga alloys

The effects of Sn addition on phase transformation behavior and magnetocaloric properties of Mn₅₀Ni₂₅Ga_25−xSn_x (x=0, 0.1, 0.5, 1 and 2 at%) alloys were investigated in this work. The results show that the addition of Sn reduces the structural transformation temperatures. It is found that the second phase exists in the austenite matrix of the as-casted alloys at room temperature. After being annealed at 1073 K for 48 h, the precipitates totally soluted into the matrix. Magnetization measurements indicate that the saturation magnetizations of the alloys increase significantly with increase in Sn contents. In addition, the ΔM/ΔS obviously increases with increase in the Sn contents, implying the higher efficiency shift of the martensitic transformation temperature under the magnetic field.     

Crystal structure and physical properties of magnetic shape memory alloys Ni50 − x Cu x Mn29Ga21

The phase composition, crystal structure, and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of the stoichiometric alloy Ni₅₀Mn₂₅Ga₂₅ and nonstoichiometric alloys Ni_{50 ? x} Cu _x Mn₂₉Ga₂₁ (x = 0, 1, 2) with the thermoelastic martensitic transformation have been investigated. The influence of the chemical composition on the transformations and physical properties of the alloys has been determined.     

多晶Mn1-xCux(0.1≤x≤0.3)合金的磁诱发应变

采用X射线衍射分析、显微形貌观察、差示扫描量热法、标准电阻应变计法等实验方法,研究了室温下多晶Mn_1-xCu_x(0.1≤x≤0.3,原子分数)合金在低磁场中的磁诱发应变性能.结果表明,Mn_1-xCu_x合金经过长时间的固溶处理,在冷却过程中会出现fcc(γ)→fct(γ’)马氏体相变,形成(γ+γ 关键词： 磁诱发应变 MnCu合金 马氏体相变     

The influence of Al substitution on the phase transitions and magnetocaloric effect in Ni43Mn46Sn11−xAlx alloys

The effects of Al substitution on the phase transitions and magnetocaloric effect of Ni₄₃Mn₄₆Sn_11−xAl_x (x=0-2) ferromagnetic shape memory alloys were investigated by X-ray diffraction and magnetization measurements. With the increase of Al content, the cell volume decreases due to the smaller radius of Al, and the martensitic transformation temperature increases rapidly, while the Curie temperature of austenitic phase shows a small increase. A large positive and a negative magnetic entropy change were observed near the first-order martensitic transition and the second-order magnetic transition, respectively. The magnetic entropy changes, hysteresis behavior, and refrigerant capacity near the two transitions are compared.     

Amorphous Fe–Mg alloy thin films: magnetic properties and atomic vibrational dynamics

Amorphous (a-) Fe _x Mg_1?x alloys are interesting materials for the investigation of non-Debye-like low-energy vibrational excitations. We have prepared a-Fe _x Mg_1?x alloy thin films (0.3 ≤ × ≤0.7) by vapour quenching. The amorphous state was confirmed by conversion electron Mössbauer spectroscopy between 4.2–300 K, and the x- and temperature-dependence of the isomer shift and hyperfine magnetic field was measured. For x= 0.6 and 0.7, magnetic ordering occurs below ~150 K. The atomic vibrational density of states, g(E), was determined by nuclear resonant inelastic scattering, providing clear evidence for the non-Debye-like low-energy vibrational excitations.     

Influence of Cr on microstructure and elastocaloric effect in Ni–Mn–In–Co–Cr polycrystalline alloys

Ni–Mn-based metamagnetic shape memory alloys have been proposed as potential elastocaloric refrigerants. The intrinsic brittleness of the alloys has limited their cooling application. Introducing a soft second phase is an effective way to reduce the brittleness. From the viewpoint of application, the effect of second phase on elastocaloric effect should be illustrated. In this paper, we have investigated the microstructure, martensitic transformation and elastocaloric effect of Ni₄₅Mn_37-xIn₁₃Co₅Cr_x ($x=0,1\text{ and }2$) polycrystalline alloys. Single-phase and precipitates-containing microstructures are obtained for the undoped and doped alloys, respectively. The precipitates in Cr-doped alloys enhances the fracture strength but significantly hinders the martensitic transformation. Balancing the fracture strength and martensitic transformation, the Ni₄₅Mn₃₆In₁₃Co₅Cr alloy with small amount of precipitates along grain boundaries exhibits large cooling effects of 4–6 K in the temperature range of 317–353 K.     

Determination of the normal and anomalous hall effect coefficients in ferromagnetic Ni50Mn35In15 − x Si x Heusler alloys at the martensitic transformation

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni₅₀Mn₃₅In_{15 ? x} Si _x (x = 1.0, 3.0, 4.0) Heusler alloys are studied at T = 80-320 K. The martensitic transformation in these alloys occurs at T = 220?C280 K from the high-temperature ferromagnetic austenite phase into the low-temperature martensite phase having a substantially lower magnetization. A method is proposed to determine the normal and anomalous Hall effect coefficients in the presence of magnetoresistance and a possible magnetization dependence of these coefficients. The resistivity of the alloys increases jumpwise during the martensitic transformation, reaches 150?C200 ??? cm, and is almost temperature-independent. The normal Hall effect coefficient is negative, is higher than that of nickel by an order of magnitude at T = 80 K, decreases monotonically with increasing temperature, approaches zero in austenite, and does not undergo sharp changes in the vicinity of the martensitic transformation. At x = 3, a normal Hall effect nonlinear in magnetization is detected in the immediate vicinity of the martensitic transformation. The temperature dependences of the anomalous Hall effect coefficient in both martensite and austenite and, especially, in the vicinity of the martensitic transformation cannot be described in terms of the skew scattering, the side jump, and the Karplus-Lutinger mechanisms from the anomalous Hall effect theory. The possible causes of this behavior of the magnetotransport properties in Heusler alloys are discussed.     

Large roomtemperature magnetocaloric effect with negligible magnetic hysteresis losses in Mn1−xVxCoGe alloys

The magnetic and magnetocaloric properties have been investigated in a series of Mn_1−xV_xCoGe (x=0.01, 0.02, 0.03, and 0.05) alloys. The substitution of V for Mn reduces the structural transformation temperature of MnCoGe alloy effectively and results in a second-order magnetic transition in Mn_0.95V_0.05CoGe alloys. Large room temperature magnetocaloric effect and almost zero magnetic hysteresis losses are simultaneously achieved in the alloys with x=0.01, 0.02, and 0.03. The reasons for the negligible magnetic hysteresis losses and the potential application for the roomtemperature magnetic refrigeration are discussed.     

Phase transformation and magnetic properties of SmCo7−xBx alloys prepared by mechanical alloying

The phase transformation and magnetic properties of SmCo_7−xB_x (x=0, 0.2, 0.5 and 1) alloys prepared by mechanical alloying have been investigated systematically. The coercivities of the alloys without B increase with increasing annealing temperature, as a consequence of complete crystallization of TbCu₇-type phase. The substitution of B for Co is favorable to the formation of Th₂Zn₁₇- and CaCu₅-type phases when annealed at 650°C, accompanied by the enhancement of the coercivities. Increasing the annealing temperature causes the formation of soft magnetic phase Sm₂Co₁₄B in the B-substituted alloys. In the alloys with x=0.5 and 1 annealed at 850°C, the major phase is Sm₂Co₁₄B, which degrades the magnetic properties sharply. A remanence enhancement has been observed in the SmCo_7−xB_x alloys due to the exchange coupling of the nanoscale structure.     

Model of colossal magnetostriction in the martensite phase of Ni-Mn-Ga alloys

A model is proposed for describing the experimentally observed martensite and magnetic domain structures in Heusler ferromagnetic alloys Ni_2+x Mn_1?x Ga. On the basis of this model, the field dependences of magnetization and deformation of the alloys are calculated numerically and an expression for the maximum attainable strains induced by external magnetic fields in these alloys is derived. It is shown that for small values of the effective elastic modulus and demagnetizing factor of alloys, the strains induced by the magnetic field may attain maximum possible values of approximately 5%, which are determined by lattice distortions as a result of the martensite transition in fields of about 1 kOe.     

Magnetocaloric effects in Ni-Mn-X based Heusler alloys with X=Ga, Sb, In

The Mn-based Heusler alloys encompass a rich collection of useful materials from highly spin-polarized systems to shape memory alloys to magnetocaloric materials. In this work we have summarized our studies of magnetostructural transitions from paramagnetic austenite to ferromagnetic martesite phases at TMC in Ni₂MnGa-based alloys (Ni₂Mn_0.75Cu_0.25-xCo_xGa, Ni₂Mn_0.70Cu_0.30Ga_0.95Ge_0.05, Ni₂Mn_1-xCu_xGa, Ni_2+xMn_1-xGa, and Ni₂Mn_0.75-xCu_xGa), and martensitic transitions from the ferromagnetic austenite to the martesite state in off-stoichiometric Ni-Mn-(In/Sb) Heusler alloys. The phase transition temperatures and respective magnetic entropy changes (ΔS) depend on composition in these systems and have been determined from magnetization measurements in the temperature interval 5-400 K, and in magnetic fields up to 5 T. It is shown that, depending on the composition and doping scheme the “giant” ΔS=40-60 J/(kgK) (for a field change of 5 T) can be observed in the temperature range (300-360 K) for the Ga-based alloys. The interplay between or coupling of the various transitions in Ni₂Mn(Mn,X) systems with X=Sb and In leads to exchange bias effects, giant magnetoresistance, and both inverse and “normal” magnetocaloric effects.     

Phase transitions and spin excitations in new multiferroics with modulated magnetic structure

Coexistence of an antiferromagnetic (modulated) structure and electric polarization has been revealed in single crystals of Eu_{1 ? x} Y_xMnO₃ (0.2 ≤ x ≤ 0.5) and Gd_{1 ? x} Y_xMnO₃ (0 ≤ x ≤ 0.2) manganites. Hence, these compounds can be considered as a new family of multiferroics. Various phase transitions, both spontaneous and induced by magnetic fields up to 250 kOe, accompanied by anomalies in magnetization, magnetostriction, permittivity, and electric polarization, have been found, and phase T-x diagrams have been constructed. In the submillimeter range (8–40 cm^?1), new spin excitations—electromagnons—have been revealed; they are excited by an electric field. It is established that suppression of the modulated structure by a magnetic field leads to the disappearance of electromagnons; this process is accompanied by significant changes in the permittivity in a wide frequency range.