Juichiro Arai

Muon Spin Relaxation Study of Partially Disordered State in Triangular-Lattice Antiferromagnet: Ca3Co2O6

A muon spin relaxation (µSR) experiment has been performed in zero-field (ZF) and longitudinal-field (LF) conditions for a Ca 3 Co 2 O 6 sample which has an antiferromagnetic triangular lattice on the a b -plane with Ising ferromagnetic chains along the c -axis. At 150 K, far above the magnetic transition temperature T c1 (=25 K) determined from magnetization data, exponential-like relaxation curves are observed in ZF-µSR time spectra, suggesting the development of intrachain ferromagnetic coupling even far above T c1 . Below the temperature T c2 (∼12 K) where magnetization is saturated, the tail of the relaxation curve reaches 1/3 the value of the amplitude in ZF-µSR spectra and the decoupling phenomenon in LF-µSR, that is, a further rise of the tail due to the applied LF, can be observed, implying the appearance of the static internal magnetic field. However, between T c1 and T c2 , the above-mentioned phenomenon cannot be observed. This indicates that the internal magnetic field at each muon site is no...

The Mössbauer Effect and the Anomalous Properties of the Invar Alloys

In order to investigate the cause of anomalous properties of the γ-phase iron-nickel alloys with Invar compositions, the Mossbauer effect spectra of these alloys were measured as a function of temperature. Especially, the influences of phase mixture (α+γ), cold-work and external magnetic field were examined. From these measurements, it was concluded that there exist transition states between the α and γ phases, and they cause a diversity of hyperfine fields in the Invar-type alloy. Many hyperfine fields thus produced make the Invar alloy an assembly of various magnetic transition points, which, as a result, gives rise to the anomalies of physical properties.

Influence of Fe, Co, Ni and Zn substitution on critical temperature and structural change of GdBa2Cu3O7

Abstract Differences in T c -suppression and structural change between GdBa 2 Cu 3 O 7 and YBa 2 Cu 3 O 7 with substitution of Fe, Co, Ni, and Zn for Cu are investigated. Substitution effects of these elements on T c and lattice structure are different in Gd- and Y-compounds. With Fe- and Co-substitution, T c s of Gd-compounds are suppressed more than those of Y-compounds at the same concentration of dopant. Ni-substitution for 10% of Cu atom leads to T c ∽35 K in the Gd-compound. Furthermore, by substitution of Ni and Zn with x > 0.1, an orthorhombic to tetragonal phase transition is brought about in the Gd-compound, although oxygen content does not change in both compounds. T c -suppression and structural change in Gd-compounds are discussed.

Antiferromagnetic coupling between Fe and Cu moments in superconductor YBa2(Cu1−xFex)3Oy

Abstract Superconductivity and magnetic properties are investigated in YBa 2 (Cu 1−x Fe x ) 3 O y with x =0−0.19 and y =6−7 through DC magnetic susceptibility and electrical resistivity. With increase of Fe contents and oxygen deficiency, T c decreases and, furthermore, their normal-state conductivity also decreases markedly, of which temperature dependence is approximated by In T , suggesting a weakly localized state in two dimensional systems. The susceptibility of all samples obeys the modified Curie-Weiss law. The effective Bohr magneton number P eff per Fe calculated from Curie constants decreases from 5.5 to 3.1 with an increase of Fe concentration up to x =0.19. Oxygen deficiency depresses these P eff to a minimum value of about 1.5 but more deficiency such as y =6 raises them up to about 3.0. These results indicate that Cu spins are weakly frozen around Fe atoms through weak localization due to Fe impurity and oxygen deficiency, and that their moments antiferromagnetically couple with the Fe moment locally without long range magnetic order.

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.

Superconductivity and Magnetic Properties of YBa2(Cu1-xFex)3O7-y

Superconductivity and magnetic properties of YBa 2 (Cu 1- x Fe x ) 3 O 7- y with x =0-0.1 are investigated by the measurements of static magnetic susceptibility, electrical resistance, weight change and lattice parameters. Substitution of Fe atoms for Cu atoms causes orthorhombic to tetragonal phase transition at about x =0.035 and depresses critical temperature T c from 95 K to 32 K at x =0.1. The effective magnetic moment per Fe, about 5.5 µ B below x =0.01 decreases to 3.6 µ B at x =0.1. The removal of oxygen atoms in the specimens of superconducting tetragonal phase with x =0.04 by quenching brings about remarkable expansion of c -axis and decreases T c from 74 K to below 4.2 K. The effective moment per Fe was found to decrease from 4.4 µ B to 3.3 µ B with increase of oxygen deficiency due to quenching. Causes of reduced iron effective moments due to increase of iron concentration and oxygen deficiency are discussed in consideration of superexchange interaction between Fe and Cu spins.

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.

Mössbauer Spectra of the Fe-Ni Invar Alloys under the Influence of Non-Hydrostatic Stress

The Fe-Ni Invar alloys under the action of non-hydrostatic stress were studied by means of the Mossbauer spectroscopy. The unidirectional tension or shearing stress induces broadening of the spectrum. According to our calculation, this behavior implies a change in the distribution of hyperfine fields in the alloy. Furthermore, the irreversible alloys with strong internal-stress caused by many transformation-cycles \(\alpha{\rightleftarrows}\gamma\) were examined; a paramagnetic-like peak is developed from the ferromagnetic peaks of the α-phase alloy, whereas ferromagnetic peaks are born from the paramagnetic-like spectrum of the γ-phase alloy. There should be some distorted or intermaediate structures between b.c.c. and f.c.c. lattices, to which the anomalous properties of these alloys may be attributed.

Anomalous magnetic behaviors of Fe-Ni invar alloys deposited by sputtering

Abstract The relation between interatomic distance and hyperfine field is examined for Fe-Ni Invar films produced by dc sputtering. The hyperfine field of sputter-deposited films is much larger than that of bulk alloys, which is interpreted by a change in 3d band configuration by incorporation of Ar atoms.

Anomalous Decrease of Thermal Expansion Due to Inhomogeneity in Fe-Ni Invar Alloy

In order to investigate the effect of inhomogeneity on the Invar anomaly, the Mossbauer effect, the temperature dependence of magnetization and the thermal expansion were measured for an Invar alloy with various degrees of inhomogeneity which was introduced by heating a foil of 30.3% Ni-Fe, electroplated with nickel, at 1000°C for various periods of time. An analysis of the peculiar shapes of the Mossbauer spectra and the magnetization vs temperature curves reveals that there exists a very large fluctuation of magnetic moment and Curie temperature in the specimen. Furthermore, the thermal expansion is negative or zero in a wide temperature range and exhibits anisotropy. These observations indicate that the inhomogeneity enhances the Invar anomaly. These results are discussed on the basis of the thermodynamical relation between the thermal expansion coefficient and the internal stress piled up in the specimen.