T. Kanomata

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.

The crystal structure and phase transitions of the magnetic shape memory compound Ni2MnGa

High resolution neutron powder diffraction and single crystal measurements on the ferromagnetic shape memory compound Ni2MnGa have been carried out. They enabled the sequence of transformations which take place when the unstressed, stoichiometric compound is cooled from 400 to 20 K to be established. For the first time the crystallographic structure of each of the phases which occur has been determined. At 400 K the compound has the cubic L21 structure, and orders ferromagnetically at TC ≈ 365 K. On cooling below ~ 260 K a super-structure, characterized by tripling of the repeat in one of the 110cubic directions, forms. This phase, known as the pre-martensitic phase, persists down to the structural phase transition at TM ≈ 200 K and can be described by an orthorhombic unit cell with lattice parameters aortho = 1/√2acubic, bortho = 3/√2acubic, cortho = acubic and space group Pnnm. Below TM the compound has a related orthorhombic super-cell with bortho ≈ 7/√2acubic, which can be described within the same space group. The new modulation appears abruptly at TM and remains stable down to at least 20 K.

Effect of magnetic field on martensitic transition of Ni46Mn41In13 Heusler alloy

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.

Effect of hydrostatic pressure on the Curie temperature of the Heusler alloys Ni2MnZ(Z = Al, Ga, In, Sn and Sb)

Abstract The pressure derivative of the Curie temperature d T c /d p of the Heusler alloys Ni 2 MnZ(Z = Al, Ga, In, Sn and Sb) has been obtained from the results of temperature dependence of initial permeability under pressure up to about 6 kbar. For all alloys the Curie temperatures increase linearly with increasing pressure at the rate of d T c /d p : +0.7 K/kbar for Ni 2 MnAl, +1.0 K/kbar for Ni 2 MnGa, +0.9 K/kbar for Ni 2 MnIn, +1.4 K/kbar for Ni 2 MnSn and +4.1 K/kbar for Ni 2 MnSb. On the basis of these results, the interatomic dependence of the exchange interaction for Heusler alloys is discussed. The magnetic susceptibilities of those alloys are also reported.

Atomic ordering and magnetic properties in the Ni45Co5Mn36.7In13.3 metamagnetic shape memory alloy

The effects of chemical order on the phase stability and magnetic properties of the metamagnetic shape memory alloy Ni45Co5Mn36.7In13.3 were investigated. Alloys quenched from the B2 and L21 phase regions were found to transform to the L10 and 6M martensite phases, respectively. For alloys quenched from the B2 region the martensitic transformation starting temperature is about 80 K higher than that for alloys quenched from the L21 region. The Curie temperature of the parent phase and the magnetization of the martensite phase were both lower for the alloy quenched from the B2 region than those for the alloy quenched from L21 region.

Magneto-volume effect of MnCo1−xGe(0⩽x⩽0.2)

Abstract The pressure effect on the Curie temperature TC and the thermal expansion for MnCoGe and MnCo0.8Ge are measured. The values of ∂TC/∂p are +3.9 K /kbar for MnCoGe and +1.12 K /kbar for MnCo0.8Ge. For both compounds, negative exchange striction is observed for the unit cell volume.

Vacancy induced structural and magnetic transition in MnCo1−xGe

The authors report ab initio total energy calculations on the first-order structural transition of the ferromagnetic MnCo1−xGe(0.00⩽x⩽0.25) intermetallic compound. They show that increasing Co vacancies induce a transition from an orthorhombic structure at 0⩽x⩽0.08 to a hexagonal structure at x>0.08. A concomitant high-to-low moment magnetic transition and a large magnetovolume effect occur due to the change of the symmetry and the resulting coupling distance between the magnetic atoms. These results provide an excellent account for the experimental results and reveal the crucial role of the Co vacancies in determining the relative structural stability and the magnetic properties of MnCo1−xGe.

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.

Reversibility and irreversibility of magnetocaloric effect in a metamagnetic shape memory alloy under cyclic action of a magnetic field

We have studied adiabatic temperature change ΔTad in a Ni50Mn36Co1Sn13 metamagnetic shape memory alloy. An irreversible character of ΔTad has been observed in the vicinity of the reverse martensitic transformation. In this region, cyclic application of the magnetic field converts comparatively large inverse magnetocaloric effect (MCE) with ΔTadmax=−0.8 K to a weaker conventional MCE (ΔTadmax=0.3 K). The crossover of ΔTad has been attributed to the irreversible character of the magnetic field-induced transformation and the closeness of the martensitic transformation and Curie temperature of the austenitic phase TCA.

Concentration dependence of magnetic moment in Ni50-xCoxMn50-yZy (Z=In,Sn) Heusler alloys

The concentration dependence of magnetic properties in the austenite phase of NiCoMnIn and NiCoMnSn Heusler alloys was systematically investigated by the magnetization measurements. The TC for Ni(50-x)CoxMn(50-y)Sny alloys linearly increases with increasing the Co content and fixed Sn content. Although showing a concave curve for Ni50Mn(50-y)Sny ternary alloys, the Sn content-dependence of magnetic moment per formula unit changes from the concave curve to a straight line when Co is substituted for Ni. On the other hand, the In content-dependence of the magnetic moment for Ni45Co5Mn(50-y)Iny alloys is perfectly linear as is that in the Ni50Mn(50-y)Iny ternary alloys.