Wataru Ito

Bio: Wataru Ito is an academic researcher from Tohoku University. The author has contributed to research in topics: Magnetic shape-memory alloy & Diffusionless transformation. The author has an hindex of 26, co-authored 55 publications receiving 4164 citations.

Magnetic-field-induced shape recovery by reverse phase transformation.

Abstract: Large magnetic-field-induced strains1 have been observed in Heusler alloys with a body-centred cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field1,2,3. These materials have attracted considerable attention as potential magnetic actuator materials. Here we report the magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy. Stresses of over 100 MPa are generated in the material on the application of a magnetic field of 70 kOe; such stress levels are approximately 50 times larger than that generated in a previous ferromagnetic shape-memory alloy4. We observed 3 per cent deformation and almost full recovery of the original shape of the alloy. We attribute this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase at 298 K in the Ni45Co5Mn36.7In13.3 single crystal.

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

Abstract: 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.

Martensitic and Magnetic Transformation Behaviors in Heusler-Type NiMnIn and NiCoMnIn Metamagnetic Shape Memory Alloys

Abstract: Martensitic and magnetic transformation behaviors of Ni50MnIn, Ni45Co5MnIn, and Ni42.5Co7.5MnIn Heusler alloys were investigated by differential scanning calorimetry (DSC), vibrating sample magnetometry (VSM), and transmission electron microscopy (TEM). The martensitic transformation starting temperature (M s ) decreases with increasing In composition, while the Curie temperatures (T c ) of the parent phase are almost independent in each alloy series. On the other hand, the addition of Co resulted in a decrease of the M s and an increase of the T c , and the degree of the decline of M s was accelerated by magnetic transformation of the parent phase. The M s temperature change induced by the magnetic field was also confirmed. It was found that the degree of M s change is strongly related to the entropy change by the martensitic transformation, which shows a correlation with T c -M s . These behaviors can be qualitatively explained on the basis of thermodynamic considerations.

Observation of large magnetoresistance of magnetic Heusler alloy Ni50Mn36Sn14 in high magnetic fields

Abstract: The magnetic and electrical properties on magnetic Heusler alloy Ni50Mn36Sn14 were studied in magnetic fields up to 18T in 4.2–270K temperature range. It was found that at the vicinity of 160K the resistivity jump of 46% is accompanied by the magnetic phase transition. Furthermore, the large magnetoresistance effect of 50% by the magnetic field induced magnetic phase transition was observed.

Effect of magnetic field on martensitic transition of Ni46Mn41In13 Heusler alloy

Abstract: Magnetic and martensitic transition behaviors of a Ni46Mn41In13 Heusler alloy were investigated by differential scanning calorimetry and vibrating sample magnetometry. A unique martensitic transition from the ferromagnetic austenite 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 positive magnetic entropy change, which reached 13J∕kgK at 9T, was observed to accompany reverse martensitic transition. This alloy shows promise as a metamagnetic shape memory alloy with magnetic-field-induced shape memory effect and as a magnetocaloric material.

Giant magnetocaloric effect driven by structural transitions

Abstract: Magnetic cooling could be a radically different energy solution that could replace conventional vapour compression refrigeration in the future. It is now shown that a Heusler-type magnetocaloric alloy exhibits a remarkable cooling capability due to the effect of a sharp structural transformation at a specific temperature. The finding may be of relevance beyond Heusler alloys and represents an important step towards the implementation of cooling systems based on magnetocaloric materials.

The Magnetocaloric Effect and Magnetic Refrigeration Near Room Temperature: Materials and Models

Abstract: In the past 20 years, there has been a surge in research on the magnetocaloric response of materials, due mainly to the possibility of applying this effect for magnetic refrigeration close to room temperature. This review is devoted to the main families of materials suitable for this application and to the procedures proposed to predict their response. Apart from the possible technological applications, we also discuss the use of magnetocaloric characterization to gain fundamental insight into the nature of the underlying phase transition.

Magnetocaloric effect and its relation to shape-memory properties in ferromagnetic Heusler alloys

Abstract: Magnetic Heusler alloys which undergo a martensitic transition display interesting functional properties. In the present review, we survey the magnetocaloric effects of Ni-Mn-based Heusler alloys and discuss their relation with the magnetic shape-memory and magnetic superelasticity reported in these materials. We show that all these effects are a consequence of a strong coupling between structure and magnetism which enables a magnetic field to rearrange martensitic variants as well as to provide the possibility to induce the martensitic transition. These two features are respectively controlled by the magnetic anisotropy of the martensitic phase and by the difference in magnetic moments between the structural phases. The relevance of each of these contributions to the magnetocaloric properties is analysed.