Rie Y. Umetsu

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.

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.

Phase Separation and Magnetic Properties of Half-metal-type Co2Cr1-xFexAl Alloys

The phase stability and magnetic properties of the half-metal-type Co2Cr1−xFexAl alloy system were investigated. It was found that the occurrence of two-phase separation is unavoidable in a concentration range of less than x=0.4, leading to deviation of the saturation magnetic moments from the generalized Slater–Pauling line. The L21-type phase becomes stable in a concentration range of more than x=0.7, where no half-metallic behaviors are present. Consequently, it is concluded that the most favorable concentration for applications to spintronic devices is located around x=0.4 in Co2Cr1−xFexAl alloys having the B2-type phase.

Magnetic field-induced reverse transformation in B2-type NiCoMnAl shape memory alloys

Effects of the addition of Co to the NiMnAl alloy on the martensitic and magnetic properties were investigated. The magnetic properties of the parent (P) phase drastically changed from paramagnetic to ferromagnetic, while the magnetization in the martensite (M) phase slightly decreased with increasing Co composition. Consequently, in the Ni40Co10Mn33Al17 alloy with the B2 ordered structure, the martensitic transformation from a ferromagnetic P to a weak magnetic M phase was obtained. The martensitic transformation temperatures in this alloy decreased by about 30K with the application of a magnetic field of 7T and a metamagnetic phase transformation was confirmed.

Magnetic properties and band structures of half-metal-type Co2CrGa Heusler alloy

The saturation magnetic moment Ms and the Curie temperature TC of the half-metal-type Co2CrGa Heusler alloy have been investigated. The value of Ms at 4.2K for the L21-type Co2CrGa alloy is 3.01μB∕f.u., in agreement with the generalized Slater-Pauling line and the theoretical calculation. The Curie temperature TC and the phase-transformation temperature Tt from the L21 to B2-type structure are 495 and 1050K, respectively. The band calculations disclose that the spin polarization is also relatively high in the B2-type structure, although its estimated TC is about 100K lower than that of the L21-type structure.

Martensitic transformation and magnetic field-induced strain in Fe–Mn–Ga shape memory alloy

Martensitic and magnetic properties of Fe–Mn–Ga single and polycrystalline alloys were investigated. It was found that Fe–Mn–Ga alloys exhibit martensitic transformation from the paramagnetic L21 Heusler parent phase to the ferromagnetic L10 martensite phase. The martensitic transformation temperatures increased by about 20 K by the application of a magnetic field of 7 T, and a metamagnetic phase transition was observed. In addition, a magnetic field-induced strain of 0.6% associated with magnetic field-induced forward transformation was confirmed in an Fe43Mn28Ga29 single crystal.

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.

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.

Half-metallic properties of Co 2 ( Cr 1 − x Fe x ) Ga Heusler alloys

Magnetic and half-metallic properties of

The effect of Co substitution on the magnetic properties of the Heusler alloy Ni50Mn33Sn17

{\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Ga}