Kiyoshi Ooiwa

Antiferromagnetism in a Heusler Alloy Fe2VSi

Antiferromagnetism has been found in a Heusler alloy Fe2VSi from measurements of electrical resistivity, magnetic susceptibility and Mossbauer spectroscopy of 57Fe. The Neel and Weiss temperatures are 123 K and -212 K, respectively. The magnetic moment of an Fe atom estimated from Mossbauer spectrum splitting is 0.22 µ B at 15 K. The effective moment determined from Curie-Weiss susceptibility above 123 K is 1.87 µ B . The results are discussed in relation to a half-metallic antiferromagnet.

Anomalous giant magnetoresistance in Heusler-type alloys Fe2+xV1-xGa

Electrical resistance, magnetoresistance and magnetization were measured for Heusler-type alloys, Fe 2+ x V 1- x Ga. An anomalous giant magnetoresistance (GMR) near 50% was observed for Fe 2 VGa which was a weak and inhomogeneous ferromagnet. For alloys with x >0, which show clear ferromagnetism, the electrical resistance showed an anomaly below the Curie temperature. For alloys with x <0, the temperature dependence of the resistance indicated behavior like that of a semiconductor. The energy gap was tentatively estimated to be 0.03 eV for x =-0.10. The origin of the present GMR is discussed in terms of the metal-insulator transition arising from ordering of the magnetic moment.

A structural phase transition and magnetic properties in a Heusler alloy Ni2MnGa

Abstract Magnetization and NMR measurements have been made for a ferromagnetic Heusler alloy Ni 2 MnGa. The following results were obtained, (1) a magnetic moment on the Ni site was determined for the first time, (2) the magnitude of Ni moment which is induced by the molecular field from local Mn moments depends on structure and also on temperature. (3) These results are consistent with recent theoretical results discussing the band Jahn-Teller effect.

GMR in Heusler type alloys Fe2+xV1-xAl

Electrical resistivity (ρ), magnetoresistance (MR) and magnetization (σ) for Heusler type alloys Fe 2+ x V 1- x Al with -0.10≦ x ≦0.20 were measured. A giant magnetoresistance (GMR) of about 40% was observed for the weakly inhomogeneous ferromagnet Fe 2.1 V 0.9 Al at 4.2 K in 130 kOe. In the ferromagnetic alloys ( x ≧0.10) the ρ showed a maximum near the Curie temperature ( T C ) and the temperature coefficient of resistivity (TCR) was negative above T C . For the paramagnetic alloys ( x ≦0.05) TCR was negative in the entire temperature ( T ) range examined. In the low T range, the alloys with x ≦0.10 exhibited variable-range hopping (VRH) conduction behavior, ln ρ∝ T -1/4 . The magnetism of the present alloys was explained on the basis of the concept of magnetic clusters. The origin of the present GMR was discussed in relation to the Anderson localization of carriers in alloys having atomic and magnetic disorders.

Anomalous GMR in Fe2+xV1-xGa and Fe2+yV1-yAl

Abstract Negative giant magnetoresistance (GMR) has been obtained in Fe2+xV1-xGa and Fe2-yV1-yAl: 48% for Fe2VGa and 30% for Fe2.1V0.9Al. These alloys show semiconductor-like electrical resistance for x 0 reveal resistance maximum near the Curie temperature. These results indicate that the metal-insulator transition is ascribed to magnetic ordering.

The Transition from Nonmagnetic State to Ferromagnetic State of Co in a Pseudobinary Alloy CoTi1-xAlx

The coexistence of ferromagnetic and nonmagnetic Co is found in a pseudobinary alloy CoTi 1- x Al x by means of magnetization measurement and NMR of Co 59 . The dependence of the component of the ferromagnetic Co on external field and on composition x result in a critical field H c and a critical composition x c where the magnetic state of Co in the alloy takes place transition from nonmagnetic state to ferromagnetic one or vice versa . The nonmagnetic state of Co at x =0.6 is identified with the strongly paramagnetic and exchange enhanced state with Stoner factor of 24.

Anomalous transport and magnetic properties in Fe2+xV1-xSi in relation to the origin of GMR

Measurements of electrical resistivity (ρ), magnetoresistance (MR), magnetization (σ) and Mossbauer spectroscopy are performed for Heusler type alloys Fe 2+ x V 1- x Si. At x =0.20 an anomalous peak of residual resistivity and the giant MR (GMR) of -27% in maximum are first observed. The magnetization and Mossbauer measurements show that the moment of a Fe atom in the alloy is determined from the number of other Fe atoms sited at 1st neighbor (local environment model) and the alloy with x =0.20 is ferromagnetic microscopically but not macroscopically. All these results are compared and discussed with the recent theoretical results and other experimental works. The origin of GMR and other anomalies around x =0.20 can be explained qualitatively from the combination of the matrix with low charge carrier concentration and the magnetic cluster determined by the local environment model.

Hall Effect in Heusler Alloys Fe2+xV1-xAl and Fe2+xV1-xGa

We have investigated the field dependence of the Hall resistivity for Fe 2+ x V 1- x Al ( x =0, 0.1, 0.2) and Fe 2+ x V 1- x Ga ( x =-0.1, 0.0, 0.1). The carrier concentration of nonmetallic Fe 2 V...

Nuclear magnetic relaxation in Heusler alloys Fe2VGa and Fe2VAl

Abstract NMR experiments were performed for 51 V and 27 Al in Fe 2 VGa and Fe 2 VAl. NMR spectrum of 51 V in Fe 2 VGa consists of two lines. The Knight shift of 51 V in Fe 2 VGa is almost zero for the one, and is about + 0.6% for the other, and independent of temperature. The temperature dependence of the relaxation rate, 1/ T 1 , does not follow the Korringa relation. The values of 1/ T 1 are less than that of pure Cu by a factor of 100 at 300 K and a factor of 10 at 4.2 K. These results suggest that these alloys are not metallic but are semiconductors with small energy gaps.

Structural phase transitions and magnetism in Ni2Mn1−xVxGa and (Co1−yNiy)2NbSn

Abstract The structural phase transition from cubic L21 to tetragonal structure is studied by magnetization measurements and NMR for 55Mn. The transition is suppressed with increasing x and y, and disappears at x = 0.1 and y = 0.4. The decrease of the Ni moment, which is determined for the first time in Ni2Mn1−xVxGa, and of the Co moment in (Co1−yNiy)2NbSn is closely related to the suppression of the transition. This result is consistent with recent theoretical work.