Asaya Fujita

Promising ferromagnetic Ni–Co–Al shape memory alloy system

A system of ferromagnetic β phase Ni–Co–Al alloys with an ordered B2 structure that exhibits the shape memory effect has been developed. The alloys of this system within the composition range Ni (30–45 at. %) Co–(27–32 at. %) Al, undergo a paramagnetic/ferromagnetic transition as well as a thermoelastic martensitic transformation from the β to the β′(L10) phase. The Curie and the martensitic start temperatures in the β phase can be controlled independently to fall within the range of 120–420 K. The specimens from some of the alloys undergoing martensitic transformation from ferromagnetic β phase to ferromagnetic β′ phase are accompanied by the shape memory effect. These ferromagnetic shape memory alloys hold great promise as new smart materials.

Metamagnetic shape memory effect in a Heusler-type Ni43Co7Mn39Sn11 polycrystalline alloy

Shape memory and magnetic properties of a Ni43Co7Mn39Sn11 Heusler polycrystalline alloy were investigated by differential scanning calorimetry, the sample extraction method, and the three-terminal capacitance method. A unique martensitic transformation from the ferromagnetic parent phase to the antiferromagneticlike martensite phase was detected and magnetic-field-induced “reverse” transition was confirmed in a high magnetic field. In addition, a large magnetic-field-induced shape recovery strain of about 1.0% was observed to accompany reverse martensitic transformation, and the metamagnetic shape memory effect, which was firstly reported in a Ni45Co5Mn36.7In13.3 Heusler single crystal, was confirmed in a polycrystalline specimen.

Itinerant electron metamagnetic transition in La(FexSi1−x)13 intermetallic compounds

A first-order transition above the Curie temperature for ferromagnetic La(FexSi1−x)13 (x=0.86 and 0.88) compounds has been confirmed by applying a magnetic field. The magnetic state changes from the paramagnetic to the ferromagnetic state and the transition field increases with temperature, indicating an itinerant electron metamagnetic (IEM) transition. The IEM transition is broad in x=0.86 and becomes clearer in x=0.88, which takes a negative slope of the Arrott plot. The volume change just above the Curie temperature for x=0.88 is huge, about 1.5%, which is caused by a large magnetic moment induced by the IEM transition.

Magnetic properties and large magnetic-field-induced strains in off-stoichiometric Ni–Mn–Al Heusler alloys

Magnetic properties and magnetic-field-induced strains (MFIS) have been investigated for off-stoichiometric Ni–Mn–Al Heusler alloys with an ordered L21 structure. A clear martensitic transformation in Ni53Mn25Al22 alloy was revealed below the Curie temperature. In the polycrystalline specimen, an irreversible relative change due to the MFIS was confirmed between the martensite start and finish temperatures Ms and Mf, and a maximum relative length change ΔL/L|7T of about −100 ppm was observed at just above Mf. On the other hand, a large irreversible relative length change of about 1000 ppm has been demonstrated in the magnetic field of 7 T for a single crystal cut from the polycrystalline specimen. A delay of the response of strains against the magnetic field was also confirmed.

Kinetic arrest of martensitic transformation in the NiCoMnIn metamagnetic shape memory alloy

Magnetic and electrical resistivity changes due to a martensitic transformation in large magnetic fields were investigated in a NiCoMnIn alloy. The transformation is interrupted at about 150K during field cooling and does not proceed with further cooling. The obtained two-phase condition is frozen at low temperatures and zero field heating releases this condition, inducing a “forward” transformation. These unusual phenomena can be explained by an abnormal behavior in the transformation entropy change and an extremely low mobility of the phase interfaces detected at low temperatures.

Giant isotropic magnetostriction of itinerant-electron metamagnetic La(Fe0.88Si0.12)13Hy compounds

La(FexSi1−x)13 compounds exhibit an itinerant-electron metamagnetic (IEM) transition above Curie temperature TC. The IEM transition in the compound with x=0.88 is accompanied by a giant volume change. From a practical viewpoint, TC was controlled by hydrogen absorption in order to obtain such a giant volume magnetostriction at room temperature. For the La(Fe0.88Si0.12)13H1.0 compound, the IEM transition occurs above TC=278 K, and a significant isotropic linear magnetostriction of about 0.3% at 7 T is induced in the vicinity of room temperature. This large magnetostriction is attributed to the giant volume magnetostriction of about 1% by the IEM transition.

Giant barocaloric effect enhanced by the frustration of the antiferromagnetic phase in Mn3GaN

First-order phase transitions are accompanied by a latent heat. Consequently, manipulating them by means of an external field causes a caloric effect. Although transitions from antiferromagnetic to paramagnetic states are not controlled by a magnetic field, a large barocaloric effect is expected when strong cross-correlations between the volume and magnetic order occur. Here we examine how geometric frustration in itinerant antiferromagnetic compounds can enhance the barocaloric effect. We study the thermodynamic behaviour of the frustrated antiferromagnet Mn3GaN, and report an entropy change of 22.3 J kg(-1) K(-1) that is concomitant with a hydrostatic pressure change of 139 MPa. Furthermore, the calculated value of the adiabatic temperature change reaches 5 K by depressurization of 93 MPa. The giant barocaloric effect in Mn3GaN is caused by a frustration-driven enhancement of the ratio of volume change against the pressure coefficient of the Néel temperature. This mechanism for enhancing the barocaloric effect can form the basis for a new class of materials for solid-state refrigerants.

Magnetic-field-induced strain of Fe-Ni-Ga in single-variant state

The magnetic-field-induced strain (MFIS) and the magnetocrystalline anisotropy in Fe19.3Ni54.2Ga26.5 ferromagnetic shape memory alloy have been investigated in a single-variant state. From the magnetization curves, the magnetocrystalline anisotropy constant K in the single crystal Fe19.3Ni54.2Ga26.5 β′ martensite phase is estimated to be 1.8×106 erg/cm3 at 5 K. In the single-variant martensite phase, the reversible MFIS of 0.02% is observed, and the value of K is reduced with increasing temperature. On the other hand, the magnitude of MFIS increases up to 100 K, and then decreases with increasing temperature. Finally, no MFIS is observed above 150 K. From these data, the condition of K for the MFIS can be confirmed at low temperatures.

Mössbauer study on martensite phase in Ni50Mn36.5Fe0.557Sn13 metamagnetic shape memory alloy

Magnetic and differential scanning calorimetric measurements and Mossbauer examination were carried out to clarify the magnetic features of Ni50Mn36.5Fe0.557Sn13. The magnetic field cooling effects were observed in the thermomagnetization curves below 235K and the Curie temperature of the parent phase was near the martensitic transformation temperature. The Mossbauer spectra taken from the parent+martensite two-phase state at 312K and from the martensite single-phase state at 264K were both singlets, showing a typical paramagnetic feature. On the other hand, the Mossbauer spectra taken from the martensite phase at 199 and 80K were complicated, including some magnetic components.

Stress-assisted magnetic-field-induced strain in Ni–Fe–Ga–Co ferromagnetic shape memory alloys

To obtain a large strain for Ni–Fe–Ga ferromagnetic shape memory alloys, the Curie temperature was increased by adding Co, and the magnetic-field-induced strain (MFIS) has been investigated under static stresses. The magnetocrystalline anisotropy constant K is increased by the addition of Co, and Ni49Fe18Ga27Co6 alloy gives a relatively large value of 1.15×106erg∕cm3 at 300K. From the stress-strain curves for this alloy, the twinning stress is estimated to be 8–9MPa. Consequently, the Ni49Fe18Ga27Co6 alloy exhibits a large MFIS of about 8.5% at room temperature under a static compressive stress of about 8MPa.