Tadamiki Hihara

Crystal phases and Yb valence transition in Ybx>In1−x>Cu2

Abstract Metallurgical, crystallographic and magnetic investigations have been made in Yb x In 1- x Cu 2 with x = 0.4 and 0.5, which exhibits the temperature-induced Yb valence phase transition. Electron probe X-ray microanalysis reveals that the phase exhibiting the valence transition is YbInCu 4 ( x = 0.5) and the sample of x = 0.4 is primarily in the two phases of YbInCu 4 and InCu 2 . The crystal structure of YbInCu 4 is found to be of the C15b type one, in which Yb ions are ordered. In well-annealed samples the susceptibility behaves as that for the free Yb 3+ ion above T v = 40 K, below which it remains nearly constant. The 115 In Knight shift shows a drastic change from -2.4 to +0.2% at 40K with lowering the temperature. The experimental results are consistent with a formation of nonmagnetic valence fluctuating states below T v .

Neutron and X-Ray Diffraction Studies of a Valence Fluctuating Compound YbInCu4

The crystal structure of YbInCu 4 has been refined by the Rietveld analysis of TOF neutron powder diffraction data. This compound has a cubic AuBe 5 (C15b, F\bar43m)-type structure with the lattice constant a =7.1575(6) A, and Yb ions are completely ordered at the 4c site. X-ray powder diffraction data have also been taken as a function of temperature. The lattice constant shows an abrupt change of 0.15% at the temperature of Yb valence phase transition (about 40 K) without any change in the crystal symmetry.

Nuclear Magnetic Resonances of Mn55 and P31 in Ferromagnetic and Antiferromagnetic MnP

The zero-field nuclear resonances of Mn 55 and P 31 have been observed in both the ferromagnetic and antiferromagnetic (screw-spin) states of MnP. In the ferromagnetic state remarkably asymmetric resonance lines are observed. It is shown by using single crystal specimens as well as powder ones that all the resonances are enhanced by domain wall motion. The line shapes are analysed by taking account of the variation in the hyperfine field, electric quadrupole splitting and enhancement factor of the r f field across the domain wall. In the screw-spin state weak resonance lines having asymmetric shapes similar to those in the ferromagnetic state are observed. The line shapes are also interpreted on the basis of the screw-spin arrangement obtained by the neutron deffraction study. Anomalous changes in the resonance frequencies occur at 47°K, which corresponds to the temperature of the ferro-antiferromagnetic transition.

ESR and Magnetic Susceptibility in Y2BaCuO5

ESR and magnetic susceptibility measurements were made in insulating Y2BaCuO5. A uniaxial type of the ESR spectrum was observed at room temperature and was found to be due to Cu2+ ions by an intensity measurement. The g values parallel and perpendicular to the symmetry axis are 2.24±0.02 and 2.06±0.02, respectively. Susceptibility measurements showed that the Curie constant is (0.22±0.1) emuK/mol and the paramagnetic Curie temperature is (-8±1) K. The small value of the Curie constant suggests that the sample contains some oxygen deficiency.

Pressure-induced magnetic transition in EuSe

Abstract The weak field a.c. susceptibility of EuSe was measured as functions of temperature from 1.5 to 15 K and of hydrostatic pressure up to 15 kbar. The Neel temperature did not change with pressure, and a new pressure-induced ferromagnetic phase appeared in the region above 4.6 K and above 4.5 kbar (triple point).

Effect of absorbed hydrogen on the magnetic properties of y(Fe1−xMnx)2 compounds

Abstract Measurements of the magnetization and the Mossbauer effect were made on Y(Fe 1− x Mn x ) 2 compounds and their hydrides. All the compounds were found to absorb 3.7–4.0 hydrogen atoms per formula unit. The hydrogen absorption is accompanied by a large expansion of about 8% in the lattice constant without a change in the crystal structure of the cubic Laves phase (C15). The Curie temperature decreases linearly with x and ferromagnetism disappears at x = 0.7 for the host compounds. The Curie temperatures of the hydrides are nearly one-half those of the corresponding host compounds; the value of the Curie temperature decreases drastically beyond x = 0.2 and ferromagnetism disappears at x ≈ 0.4. The magnetic moment decreases linearly with x for the host compounds. Whereas the magnetic moment in each hydride is larger than that in the corresponding host compound for x ⩽ 0.2, beyond x = 0.2 it decreases rapidly with x . The hyperfine field of 57 Fe is found to be significantly larger in the hydride phases than in the corresponding host compounds for x ⩽ 0.2, indicating that hydrogen absorption leads to an increase in the iron moment in this system. From the composition dependence of the hyperfine field and the magnetic moment, it seems likely that the magnetic moment of manganese atoms is enhanced by hydriding. The spin-glass-like magnetism observed in the hydrides with x ⩾ 0.4 is thought to originate with competing coupling tendencies between the magnetic atoms.

ESR Study of Hyperfine Coupling Constants of Eu2+ in Alkaline Earth Chalcogenides –Transferred Hyperfine Field in Eu-Chalcogenides

The hyperfine (hf) fields at isolated Eu 2+ ions in alkaline earth chalcogenides are measured by ESR method to make a separation between the core polarization and transferred contributions to the hf field at the Eu nucleus in Eu-chalcogenides. The Eu hf fields, which are assumed to be equal to the core polarization fields in Eu-chalcogenides, are (-)299, (-)300, (-)292 and (-)283 kOe in SrO, SrS, SrSe and SrTe, respectively. Using NMR data, the transferred hf fields at Eu nuclei are estimated as -8, -32, -38 and -30 kOe in the ferromagnetic states of EuO, EuS, EuSe and EuTe, respectively. The variation of the transferred hf field from EuO to EuTe is correlated to the exchange mechanism involving 5d and 6s electron states.

Nuclear magnetic relaxation of 1H in high-Tc superconductor YBa2Cu3O6.9Hx (x = 0.18 and 0.63)

Abstract Spin-lattice relaxation time T1 (at 29 and 58 MHz) and T1ϱ of 1H have been measured in YBa2Cu3O6.9Hx (x = 0.18 and 0.63) between 300 and 77 K. In the fresh sample with x = 0.18, T1 has a minimum near 260 K, which is attributed to the diffusion of hydrogen in the oxygen vacancies in the Cu(1)-O plane with an activation energy of Ea = 0.29 eV. For the sample with x = 0.63, another diffusion mode with Ea = 0.25 eV exists at lower temperature around 167 K. This mode might be due to the diffusion in the interstitial site in the Y-plane. But all of these modes disappeared in the temperature dependence of T1 after keeping the samples in ampoules in three months, and no evidence of diffusion was found either in T1ϱ measurements. The temperature dependence of T1 for x = 0.18 showed an abrupt change at T c ≊ 90 K , suggesting the effect of superconductivity.

Magnetic characteristics of pseudo-binary intermetallics Gd(Mn1−xCox)2

Abstract Magnetic, thermal expansion and NMR measurements have been made on the C-15 type Gd(Mn1−xCox)2. A Gd canted and Mn antiferromagnetic structure seems to appear in GdMn2 at low temperatures and persists up to x = 0.2. Collinear ferrimagnetism is established for x>0.3 where the Mn and Co moments couple antiparallel to the Gd moments.

NMR study of YbCdCu4 and YbTlCu4

Abstract In the C15b-type compound YbCdCu 4 the nuclear spin-lattice relaxation rates T 1 −1 for 113 Cd and 63 Cu (NQR) suggest a gradual change with temperature of 4f electron states from itinerant to localized ones. In YbTlCu 4 a nearly temperature-independent 205 Tl Knight shift and T 1 T = constant law for 205 Tl and 63 Cu indicate the itinerancy of 4f electrons.