MAGNETIC--FIELD--CONTROLLED SHAPE MEMORY EFFECT AND SUPERELASTICITY OF Ni53.2Mn22.6Ga24.2 SINGLE CRYSTAL

证实了Ni_53.2Mn_22.6Ga_24.2单晶发生的两步马氏体相变行为是完全热弹性的. 在磁场作用下, 该材料的马氏体相变和中间马氏体相变展现出相同的应变特征, 且具有磁控双向形状记忆效应. 磁场下应力--应变特性的测量结果表明, 磁场不但对压应力诱发马氏体相变过程中变体重取向所需应力的大小有影响, 而且使原来不可逆的形变成为可逆, 这种磁控超弹性特性预示了该合金用作磁控超弹性元器件材料的可能性.     

Influence of anisotropy, the latent heat and the thermal history of alloy on martensitic transformation strain in Ni3Ta single crystal

Transformation characteristics in the single crystal of Ni₃Ta shape memory alloy were studied by the dilatation measurement in the temperature range of room temperature up to 500 °C. The transformation strains were positive in the direction of the b-axes and the c-axes and negative in the direction of a-axes. The martensitic phase transformation takes place without volume change of the sample. Thermal diffusivity of the single crystal measured in two directions b-axes and a-axes was higher than that for polycrystalline material.The latent heat of the martensitic phase transformation influences the temperature distribution inside sample. Absorption (releasing) of the latent heat during heating (cooling) leads to cooling (heating) of the sample in place where the phase transformation takes place. This decrease (increase) of the temperature in the interface between both phases leads to stopping of the phase transformation. This effect is visible on the temperature dependence of the dilatation characteristics. The martensitic phase transformation in Ni₃Ta single crystal took place with hysteresis of 30 °C. This hysteresis changes depending on the thermal history of the sample. Hysteretic behaviour of the Ni₃Ta single crystal was analyzed and compared with behaviour of Ni_53.6Mn_27.1Ga_19.3 alloy where no hysteresis was found.     

Ni46Mn35Ga19单晶大的磁熵变和磁控形状记忆效应

采用多种测量手段,对制备出的转变温度在室温以上的Ni46Mn35Ga19单晶的物性进行表征。电阻和交流磁化率结果表明,该材料的磁转变和马氏体相变是同时发生的。定温磁化强度测量得出材料磁熵变的增长速率高达9.0J/kg·K·T,在1274kA/m(1.6T)的磁场下,磁熵变达13.8J/kg·K。此外,应变测量表明,伴随该磁熵变,材料展现出应变量分别高达–0.89%和–1.90%的自发和磁控双向形状记忆效应。     

The influence of chemical disorder enhancement on the martensitic transformation of the Ni50Mn36Sn14 Heusler-type alloy

The effect of chemical disorder over the martensitic phase transformation of the Ni₅₀Mn₃₆Sn₁₄ Heusler-type alloy was systematically investigated by performing X-ray diffractometry (DRX), DC magnetization and ⁵⁷Fe-doping and ¹¹⁹Sn-Mössbauer spectroscopy measurements. DRX patterns are characteristics of a L2₁-type chemically disordered structure, where the presence of this disorder was first evaluated by analyzing the relative intensity of the (1 1 1) DRX reflection, which varies in the case of Fe-doped and practically disappears for the milled samples. In consequence, the magnetic properties of Fe-doped well-milled samples related to the martensitic phase transformation change substantially. 300 K ⁵⁷Fe-Mössbauer spectroscopy data suggest that the changes in the magnetic properties related to the martensitic transformation are intrinsically correlated to the ferromagnetic and paramagnetic fractions, which are respectively associated with Fe atoms replacing Mn- and Sn-sites. In the case of milled samples, the drastic reduction of alloy magnetization was explained by the increase of the number of Mn atoms in the shell regions, which have a reduced magnetic moment comparatively to those in the grain cores. The magnetization change and the temperature transition in the martensitic transformation are governed by the grain core. The initial magnetic properties and martensitic transformation can be recovered by a subsequent annealing on the milled sample.     

Transport phenomena in the La<Subscript>0.72</Subscript>Ba<Subscript>0.28</Subscript>MnO<Subscript>3</Subscript> single crystal

Results of an experimental study of the temperature dependences of the magnetization M, electrical resistivity, magnetoresistance, thermo-and magnetothermo-emf, and Hall effect of the La_0.72Ba_0.28MnO₃ single crystal are presented. An analysis of the temperature dependences of kinetic properties shows that, at low temperatures, electrons are principal charge carriers in La_0.72Ba_0.28MnO₃ and the metallic conduction takes place. As the temperature increases to T ≈ 145 K, the sign of the ordinary Hall coefficient reverses; this indicates the change in the type of the majority charge carriers. Within a certain temperature range which lies substantially below the Curie temperature (T _C ), a metal-semiconductor transition occurs. Near the Curie temperature and within the paramagnetic range, the manganite under study is a semiconductor; the conduction is mainly effected by holes activated to the mobility edge. The critical behavior of the resistance and magnetoresistance is discussed.     

Effect of magnetic field on the morphology and fine structure of low-temperature martensite phase in a ferromagnetic Ni<Subscript>2.08</Subscript>Mn<Subscript>0.96</Subscript>Ga<Subscript>0.96</Subscript> alloy

The morphology and fine structure of high-temperature austenitic and low-temperature martensitic phases in a ferromagnetic Ni_2.08Mn_0.96Ga_0.96 alloy and the effect of magnetic field on the peculiarities of the martensite formation have been studied. The Ni_2.08Mn_0.96Ga_0.96 alloy in the initial cast and annealed states was found to undergo martensitic and magnetic phase transformations at temperatures M _s = 280 K, M _f = 265 K, A _s = 280 K, A _f = 295 K, and T _C = 375 K, respectively. Above M _s, the alloy is in a metastable premartensitic state, which leads to a characteristic diffuse scattering and tweed contrast when studying by electron microscopy. An applied magnetic field of up to 600 kA/m was found to affect the martensitic transformation in the alloy. The field application leads to changes in the morphology and fine structure of martensite due to orientation along the field direction of the magnetizations of the energetically advantageous (in terms of the directions of the magnetization vectors) c domains of the existing orientation variants of the martensite crystals having a packet pairwise-twinned morphology. The martensitic structure of the Ni_2.08Mn_0.96Ga_0.96 alloy already formed previously during cooling is not affected by an external magnetic field with a strength of up to 600 kA/m.     

Direct measurement of large reversible magnetic-field-induced strain in Ni–Co–Mn–In metamagnetic shape memory alloys

Direct measurements of reversible magnetic-field-induced strain (MFIS) on a single crystalline Ni₄₅Co₅Mn_36.5In_13.5 metamagnetic shape memory alloy were attained via magnetic-field-induced martensitic transformation under different stress levels and at various temperatures. This was achieved using a custom-designed micro-magneto-thermo-mechanical testing system capable of applying constant stress while measuring strain and magnetization simultaneously on the samples, which can fit into conventional superconducting magnets. MFIS levels are reported as a function of temperature, magnetic field and external bias stress. It was necessary to apply an external bias stress in these materials to detect a notable MFIS because a magnetic field does not favor a specific martensite variant resulting in no shape change even though magnetic field leads to reversible martensitic transformation. Fully recoverable transformation strains up to 3.10% were detected under repeated field applications in the presence of different compressive stress levels up to 125 MPa. The bias stress opposes the field-induced martensite-to-austenite phase transformation and causes the critical field for the transformation to increase at a given temperature in accordance with the Clausius Clapeyron relationship. The effect of the bias stress on the kinetic arrest of austenite is also explored.     

Grain size effect on the martensitic transformation of Ni50Mn25Ga17Cu8 high-temperature shape memory alloy

Single crystal, coarse-grained and fine-grained polycrystals of Ni₅₀Mn₂₅Ga₁₇Cu₈ high-temperature shape memory alloy were prepared. Grain size effect on the martensitic transformation was investigated. Compared with the single crystal, the martensitic transformation temperatures were slightly decreased by coarse grains, but were greatly decreased by fine grains. Strong grain size dependence of the kinetics of the martensitic transformation was monitored. The average activation energy of the transformation required for the fine-grained polycrystal was nearly four times of that for the single crystal.     

Martensitic and magnetic transformation of Co41Ni32Al24Sb3 and Co41Ni32Al27 alloys

The martensitic transformation and magnetic property of Co41Ni32Al27 and Co41Ni32Al24Sb3 alloys were investigated by optical microscopy（OM）, scanning electric microscopy（SEM）, energy dispersive X-ray spectroscopy（EDS）, X-ray diffractometry （XRD）, differential scanning calorimeter analysis（DSC） and vibration sample magnetometer（VSM） methods. The results show that martensitic crystal structure of Co41Ni32Al24Sb3 alloy is still Llo type. Both martensitic transformation temperature Tm and Curie point Tc are in linear relation to quenching temperature. Tm increases by 9 K and Tc increases by 7.5 K for every 10 K increasing in quenched temperature. Quenched from same temperature, Tm of Co41Ni32Al24Sb3 alloy is higher than that of Co41Ni32Al27 alloy by 76 K, meanwhile Tc is higher by 18 K. The melting point of Co-Ni-Al alloy is decreased by the Sb addition, eutectic structure appears in Co41Ni32Al24Sb3 alloy annealed at l 573 K, which indicates that the alloy is partially melted, whereas Co41Ni32Al27 alloy can be annealed at 1 623 K without melted. The martensitic transformation temperature range of Co41Ni32Al24Sb3 alloy is 22-29 K, only half that of Co41Ni32Al27 alloy. This is a very important result to benefit the achievement of large magnetic field induced strain on Co-Ni-Al based alloy. The results of Tm and Tc were explained by total average s＋d electron concentration and magnetic valence number Zm respectively.     

Pyroelectric and electrocaloric effect of 〈1 1 1〉-oriented 0.9PMN-0.1PT single crystal

In this paper, the polarization vs. electric field hysteresis loops of 〈1 1 1〉-oriented 0.9PbMg_1/3Nb_2/3O₃-0.1PbTiO₃ (0.9PMN-0.1PT) single crystal at different temperatures (20-110 °C) were measured. The adiabatic temperature change ΔT of 〈1 1 1〉-oriented 0.9PMN-0.1PT single crystal due to the application or withdraw of electric field were calculated through the thermodynamic relation. The largest temperature change ΔT achieves ∼1 K with only a change of 40 kV/cm electric field, the mechanism of the electrocaloric effect (ECE) is discussed for 0.9PMN-0.1PT crystal. The pyroelectric coefficient of 0.9PMN-0.1PT under bias field was calculated according to the data of hysteresis loop. The result shows that 0.9PMN-0.1PT have large pyroelectric coefficient under bias field, the largest (∂P/∂T)_E value achieves −0.5 μC/cm² K.     

Martensitic transformation and anomalies in resistivity of (Ti–50Ni)1−xCx (x = 0.1, 0.5 at.%) shape memory alloys

Phase transformation of solid solution (Ti–50Ni)_1−xC_x (x = 0.1, 0.5 at.%) alloys have been studied by using differential scanning calorimetry, physical property measurement system and optical microscope. The transformation temperature decreases due to the existence of titanium carbide (TiC) particles compared with that of near-equiatomic Ti–Ni shape memory alloy. The resistivity vs. temperature curves show hysteresis. Thermoelastic martensitic transformation occurred in two alloys despite the difference in TiC content. Nevertheless, the resistivity results show different martensitic transformation routes. A one-step B2 → B19′ transformation occurred in the low TiC content alloy and an R transformation appeared in another alloy, suggesting that the martensitic transformation routes depended on the TiC content. The cumulative effect of the TiC particles causes the local stress field and lattice distortion to restrain the transformation of the B19′. On the other hand, the TiC content has an effect on the temperature coefficient of electrical resistivity (TCR) of alloys. The Ti–Ni–0.5C alloy shows a negative TCR in the range 100–300 K during which transformation occurs. Another alloy shows the opposite result. The cause of the negative TCR is briefly discussed.     

Magnetoresistance of the La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal

The dependence of the resistance ρ of the La_0.7Ca_0.3MnO₃ single crystal on the temperature (in a range of 77 < T < 410 K) and magnetic field H is studied. The dependence of the magnetoresistance Δρ/ρ of the ferromagnetic phase on the field is shown to be determined by the competition of two mechanisms. In low magnetic fields, the magnetoresistance is positive Δρ/ρ > 0 and is determined by changes in the resistance with changing magnetization orientation with respect to the crystallographic axes; in high magnetic fields, the magnetoresistance is negative Δρ/ρ < 0, since it is the suppression of spin fluctuations in the magnetic field that plays the principal role. The phase transition from the ferromagnetic to paramagnetic state is a first-order transition close to the second-order one. In the transition range, the magnetoresistance is determined by the resistivity in the zero field ρ(T) and by the shift of the transition temperature T _C(H) in the magnetic field. In the paramagnetic state, the resistivity ρ(T) has an activation character; similarly to the magnetoresistance of other lanthanum manganites, the magnetoresistance of this single crystal is controlled by a change in the activation energy in the magnetic field.     

Formation and properties of SrB6 single crystals synthesized under high pressure and temperature

Pure SrB₆ single crystals are synthesized under high pressure (5 GPa) and temperature (1300 °C). The structure and morphology of the SrB₆ single crystals are characterized by X-ray diffraction and field emission scanning electron microscope. The lattice constant of the SrB₆ crystals with a space group of Pm-3m is a = 4.1975 Å. The dependence of electric resistivity and Hall coefficient on temperatures from 2 to 300 K show that the SrB₆ single crystals are conductive materials with semi-metallic behavior and can be considered as electronic current carriers. The results of the band structure calculation show that the conduction and valence bands meet at the X point at the Fermi Level, which is consistent with the experimentally measured conducting behavior of SrB₆ single crystals. The total and partial density of states show that the states at the Fermi level come from the 2p orbitals of the B atoms and the 4d orbital of the Sr atom. The magnetization measurement shows the diamagnetic nature of the SrB₆. The nanoindentation measurement indicates that the hardness of SrB₆ reached 33 GPa.     

EFFECT OF MAGNETIC FIELD ON MARTENSITIC TRANSFORMATION IN Fe-21Ni-4Mn ALLOY

The pulsed magnetic field induced martensitic transformation with isothermal and athermal kinetics in Fe-2Ni-4Mn(wt-%)alloy has been studied by means of magnetization measurements,optical microscopy and thermodymical analyses.It is shown that there exits a critical magnetic intensity for induing martensitic transformation at a given temperature above Ms.The critical magnetic field increases linearly with increasing ΔT= T-M_S.The magnetic field strongly promotes the athermal martensitic transforamtion and restrains the isothermal one.The entropy change ΔS for athermal transformation at Ms is 4.13 J/mol· K.The effect of magnetic field on martensitic transformation in Fe-21Ni-4Mn alloy is main- ly due to Zeeman effect.Lath,plate and butterfly martensities were observed under magnetic field.     

Magnetism in PrRhSn studied on a single crystal

We have grown a single crystal of PrRhSn, analyzed the structure by X-ray diffraction methods and measured the specific heat, ac susceptibility, magnetization, and electrical resistivity as functions of temperature and magnetic field. Also we have calculated the electronic structure of this compound by ab initio methods. Ferromagnetism below T_C = 2.9 K characterized by strong uniaxial anisotropy (an anisotropy field 65 T) has been confirmed. Pronounced crystal field (CF) effects were observed on the temperature dependences of the magnetic susceptibility, electrical resistivity and specific heat. The calculations revealed that besides the stable Pr magnetic moment owing to the localized 4f-electrons a small magnetic moment of at most 0.2μ_B is induced at the Rh site (and 0.1μ_B at the Sn site) due to the polarized Rh 4d-electron states (Sn 5p-states) hybridizing with the Pr 5d-electron states, i.e. the Rh and Sn moments play some role in the total balance of the magnetic moments in this compound. This result is in agreement with the experimentally determined saturated magnetic moment of PrRhSn, which is about 0.3μ_B larger than the ordered moment value expected for the Pr³⁺ free ion.     

Martensitic transformations and magnetic-field-induced strains in Ni50Mn50−x Gax alloys

Concentration dependences of the temperatures of forward and reverse martensitic transformations in Ni₅₀Mn_50?x Ga_x alloys (x = 19–25) and features of the jumpwise elongation ε induced by magnetic field H in a single crystal of the Ni₅₀Mn_28.5Ga_21.5 alloy have been studied. A single-variant state of martensite in the single crystal was formed by compression under the action of both a reference magnetic field and mechanical loading. It has been shown that when employing uniaxial mechanical compression, several large jumps (whose nature is associated with the appearance of structural defects hindering the displacement of boundaries of martensite twins) arise in curves of the single-crystal elongation induced by an applied perpendicular magnetic field H _⊥.     

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-field-induced martensitic transformations in Ni47 − x Mn42 + x In11 alloys (with 0 ≤ x ≤ 2)

Magnetic properties and martensitic transformations in the Ni_{47 ? x} Mn_{42 + x} In₁₁ alloys (with 0 ≤ x ≤ 2) have been studied. The magnetic-field-induced martensitic transformation was found to be observed for all the alloys. The critical temperatures of magnetic and structural phase transformations, temperature dependences of spontaneous magnetization of austenite and martensite, and the critical field, at which the martensitic transformation occurs, have been determined based on magnetic measurements performed for the alloys under study. The spontaneous magnetization of the alloys in the martensitic state has been shown to be lower than that in the magnetic-field-induced austenitic state by a factor of six.     

Volume Change During Intermartensitic Transformations in Ni-Mn-Ga Alloy

Characteristics of phase transitions in Ni₅₄Mn₂₄Ga₂₂ alloy were studied at different hydrostatic pressures to shed light on the nature and mechanisms of intermartensitic transformations. The temperature dependence of resistivity of the alloy was used to find characteristic temperatures of martensitic and intermartensitic transformations as a function of hydrostatic pressure. The latent heat of these transformations was determined by differential scanning calorimetry. The transformation volume effects were calculated using Clausius-Clapeyron equation. They make up 0.082% for martensitic and 0.024% for intermartensitic transformations.     

Grain size dependent magnetization, electrical resistivity and magnetoresistance in mechanically milled La0.67Sr0.33MnO3

We have studied magnetization, ac susceptibility, resistivity and magnetoresistance in mechanically milled La_0.67Sr_0.33MnO₃. The material with grain size micron to nanometer scale has stabilized in rhombohedral crystal structure with space group R3C. We have found various grain size effects, e.g., decrease of ferromagnetic moment, increase of surface spin disorder, and appearance of insulator/semiconductor type resistivity. In addition to these conventional features, we have identified a magnetic anomaly at 45 K in bulk sample. Ferromagnetic to paramagnetic transition temperature (T_C) is above room temperature for all samples. The samples are typical soft ferromagnet that transformed from multi-domain state to single domain state in nanocrystalline samples. The remarkable observation is that low temperature freezing of ferromagnetic domains/clusters does not follow the conventional spin glass features. Experimental results clearly showed the enhancement of high field magnetoresistance in nanocrystalline samples below 200 K, whereas low field magnetoresistance gradually decreases above 200 K and almost absent at 300 K. We have discussed few more magnetic and electrical changes, highly relevant to the progress of nanomaterial research in ferromagnetic manganites.