Keizo Endo

Magnetic Studies of Clb-Compounds CuMnSb, PdMnSb and Cu1-x (Ni or Pd)x MnSb

Magnetic properties of manganese compounds with C1 b -type crystal structure are studied. CuMnSb (lattice parameter a =6.09 5 A) is antiferromagnetic with a Neel temperature of 55°K and an asymptotic Curie temperature of -160°K. The paramagnetic Bohr magneton number per Mn is 5.4 7 . PdMnSb ( a =6.24 6 A) is ferromagnetic with a Curie temperature of 500°K and the magnetic moment per Mn is 3.9 5 µ B . Magnetic studies are also carried out for the compounds in which Ni or Pd are substituted for Cu in CuMnSb, resulting in a conclusion that the concentration of conduction electrons is very important in determining the magnetic interaction. Qualitative interpretation of experimental results is given based on the theory of resonance scattering by Friedel, Caroli and Blandin.

Magnetic Properties of Clb Compounds; CoVSb, CoTiSb and NiTiSb

Experimentally CoVSb is found to be a ferromagnetic compound with a ferromagnetic and a paramagnetic Curie temperature of 58 K and 75 K, respectively. The saturation magnetic moment per formula at 0 K is 0.18µ B . and the paramagnetic moment per formula 1.26µ B . The relations of the Curie temperature and magnetic moment between ferromagnetic and paramagnetic states are very different from that of usual ferromagnetic materials. CoTiSb and NiTiSb are found to be temperature independent paramagnetic compounds with the susceptibility per gram of 5×10 -7 and 6×10 -7 emu/g, respectively. It should be a noticeable fact that CoVSb has a spontaneous magnetic moment while CoTiSb and NiTiSb have not.

Hyperfine Fields at Tin Site in Heusler Alloys: Co2TiSn, Co2ZrSn and Co2HfSn

Hyperfine fields at Sn in new ferromagnetic Heusler alloys Co 2 TiSn, Co 2 ZrSn and Co 2 HfSn are studied by means of Mossbauer effect and NMR. The magnitudes and the signs of the hyperfine fields obtained from Mossbauer measurement are +81.6, +88.1 and +106.0 kG at 80 K for Co 2 TiSn, Co 2 ZrSn and Co 2 HfSn respectively. NMR results for Sn nucleus agree with those of Mossbauer effect except for high frequency resonance lines which are probably due to ferromagnetic Co clusters. Moreover a strong NMR signal of Co 59 in Co 2 TiSn at correct lattice sites is found to be +22.2 MHz at 77 K. The results are compared with other Heusler alloys and are discussed about the origin of the hyperfine field at Sn in the alloys based on Blandin and Campbell theory of the spin polarization of conduction electron.

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.

Anomalous High Field Magnetization in Sc(Co1-xAlx)2

Anomalous magnetization curves observed under the strong pulsed magnetic fields up to 520 kOe are understood with itinerant electron metamagnetism. From the results of Sc(Co 1- x Al x ) 2 the critical field of the transition and the spontaneous moment after the transition of ScCo 2 can be estimated to be a few megagauss and about 0.4 µ B /Co, respectively. These are consistent with recent theoretical results.

Exchange Enhanced Pauli Paramagnetism of ScCo2

The temperature dependence of magnetic susceptibility, χ, of a cubic Laves phase compound ScCo 2 was measured in the temperature range 4.2–1018 K and found to have a broad maximum at about 600 K. The temperature dependent Knight shift, K , of Sc was obtained from NMR experiment below room temperature. The K vs χ plot gives a straight line resulting in -41 kOe/ µ B as a transferred hyperfine field from Co. It was concluded that ScCo 2 is an exchange enhanced Pauli paramagnet of the same kind of YCo 2 and LuCo 2 .

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.

The Metamagnetic Transition in Pseudobinary Laves Phase Compounds Lu(Co1-xAlx)2

Magnetization measurements have been carried out for cubic Laves phase compounds Lu(Co 1- x Al x ) 2 . Stable ferromagnets at 4.2 K are found at the compositions of x =0.12 and 0.17, where the maximum moment and Curie temperature are 0.65 µ B per Co atom and 147 K, respectively. Anomalous magnetization curves with large hysteresis are found at x =0.08. This is understood by considering the metamagnetic transition between a paramagnetic and a ferromagnetic state which is the same as that of x =0.12. The transition is 1st order at temperatures below ∼65 K and is 2nd order above 65 K. It is found that a weak itinerant ferromagnetism, like that found in Y(Co 1- x Al x ) 2 and Sc(Co 1- x Al x ) 2 , is not clearly observed in this compound series.

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.