Norio Umeyama

Synthesis and Magnetic Properties of NiSe, NiTe, CoSe, and CoTe

Polycrystalline NiSe, NiTe, CoSe, and CoTe have been synthesized by two methods, i.e., (1) the solid-state reaction in an evacuated quartz ampoule (QA) and (2) the high-pressure (HP) technique (HP) in a Au capsule using a cubic anvil cell. All the obtained samples have the nickel arsenide-type hexagonal crystal structure, except CoTe (HP), and their estimated lattice parameters, a and c, show almost the same values within 0.8% in comparison between samples prepared by the QA method and those by the HP method. On the other hand, a distinct difference between samples prepared by QA and HP processes is found in the magnetization of NiSe and NiTe. In particular, for NiTe, the temperature dependence of magnetization indicates two transition temperatures at about 20 and 130 K in samples prepared by the HP process. However, samples of NiTex (x = 0.5, 0.66, 0.82, 1, 1.22, and 2) and NiTe containing Au prepared by QA process do not show similar magnetic orders in the temperature range between 2 and 300 K. Also, samples of NiSe (HP) show magnetic transition at ~20 K, while those of NiSe (QA) show a magnetic inflection point at ~10 K. To the best of our knowledge the remarkable difference in magnetic properties caused by the method of synthesis has not been reported. In this paper, we present comparative experimental results of magnetic, electric, and specific heat measurements of samples prepared by the two methods. A possible explanation for such a substantial difference in magnetic interaction will be discussed.

Quasi-Two-Dimensional Fermi-Liquid State in Sr2RhO4-δ

Single crystals of layered perovskite Sr 2 RhO 4-δ (δ=0.0 and 0.1) are successfully grown by the floating-zone method. Stoichiometric single crystals (Sr 2 RhO 4.0 ) are obtained by O 2 -annealing the as-grown crystals (Sr 2 RhO 3.9 ). Sr 2 RhO 4.0 and Sr 2 RhO 3.9 show quasi-two-dimensional Fermi-liquid behavior at low temperatures, whereas there are large differences in the anisotropy of electrical resistivity ρ c (3 K)/ρ a b (3 K) and Wilson ratio R w between Sr 2 RhO 4.0 and Sr 2 RhO 3.9 : ρ c (3 K)/ρ a b (3 K) = 2400 (19000) and R w =3.8 (6.4) for Sr 2 RhO 4.0 (Sr 2 RhO 3.9 ). The differences observed between the temperature dependence of the in-plane electrical resistivity ( T <15 K), magnetic susceptibility ( T <300 K) and specific heat ( T <10 K) of Sr 2 RhO 4.0 and Sr 2 RhO 3.9 are mainly derived from those between the density of states and band structure near the corresponding Fermi level. This indicates that the changes in these physical properties, which are accompanied by oxygen defects in th...

Magnetism of the Single Crystal Mg-V-O Spinel

In MgV2O4 with normal spinel structure (AB2O4), there is magnetic frustration of V3+ (3d2+) ions located on B-sites, if an antiferromagnetic interaction among B-site spins is essential for its magnetism. The excess Mg substituted materials such as Mg1+xV2−xO4, may construct isolated magnetic triangular lattices, because some of B-sites forming magnetic tetrahedral lattice are substituted by non-magnetic Mg ions, while the normal spinel MgV2O4 contains tetrahedral magnetic ion networks. Here, we have succeeded in growing single crystals of Mg–V–O system spinel oxide by conventional floating-zone method. The valence numbers and abundance ratio of V ions in the single crystal were estimated by wavelength-dispersive X-ray fluorescence analysis and X-ray photoelectron spectroscopy analysis. Although the composition ratio was Mg:V = 1:2, a small amount of Mg and V ion partly replace each other between A- and B- sites in this crystal. The composition formula is described as (Mg1−xVx)(V2−xMgx)O4. The single crystal shows magnetic transition at around 12 K and large negative antiferromagnetic Weiss temperature about −840 K, and the frustration parameter f = 70 estimated by those temperatures. This result suggests the existence of strong magnetic frustration in our material.

Specific Heat of Layered Ruthenates Sr2Ru1−xZrxO4

We have measured specific heat (Cp) for polycrystalline single‐phased Sr2Ru1−xZrxO4 (0.0 ⩽ x ⩽ 0.8). The value of Cp at low temperatures is reduced with increasing x. Both of the Debye temperature (Θ) and the electronic specific heat coefficient (γ) estimated from the data decreased with increasing x for x < 0.4. The smaller γ towards x = 0.4 seems to blend into the insulating transport property for x ⩾ 0.4. We argue that the decrease of Θ with increasing x stems from the increased unit‐cell volume.

An in-line MOSFET process with photomask fabrication process in a minimal fab

This paper describes a cleanroom-less device fabrication process of a minimal fab including an in-line photomask fabrication process, where half-inch wafers and half-inch photomasks are transferred in airtight containers and loaded into fabrication tools to prevent invasion of air-borne particles, gases and UV light from outside. Photomasks are fabricated using a maskless exposure system in the minimal fab. We have fabricated two types of MOSFET using the photomasks or maskless exposure system, and found that the two types of MOSFET have the almost same electric performance with the density of interface states (Dit) of the order of ID10 states/cm2.

Development of a half-inch wafer for minimal fab process

Specifications and fabrication process suitable for a small wafer with the diameter of half-inch, which is used for a minimal fab, is presented. We beveled wafer edge by rapping and polishing in order to clean the edge and to suppress the strong surface tension at the edge. To show the crystallographic orientation of the wafer, we introduced laser marking process. By the processes, we have formed silicon wafer that is suitable for fabricating electronic devices. Formed MOS capacitor on the wafer revealed that the density of interface states (Dit) was 7.8×1010 states/cm2 as the average of 37 wafers.

Frustration effects in the spinel oxide Ge(Fe1−xCox)2O4

We studied magnetic properties of Ge-based spinel oxides Ge(Fe1−xCox)2O4 to verify the frustration effect. We discovered that the spin-glass-like behavior in Ge(Fe1−xCox)2O4 exists in 0 ≤ x ≤ 0.9, and coexists with the antiferromagnetic order in 0.1 ≤ x ≤ 0.9. Taking into account the coexistence of spin molecular excitations (frustration effect) and antiferromagnetic order in GeCo2O4 (x = 1), the spin-glass-like behavior coexisting with the antiferromagnetic order in Ge(Fe1−xCox)2O4 might be attributed to the freezing of the spin molecules by the substitution of Fe for Co.

Synthesis and Electronic Properties of TlFe2Se2-δ

TlFe2Se2-δ was synthesized by a self-flux method with excess amount of selenium. Elemental analysis using energy-dispersive X-ray spectroscopy estimates the deficiency δ at about 0.5. The magnetic susceptibility reveals an antiferromagnetic transition at TN ≈ 475 K. The resistivity exhibits a semiconducting behavior with an activation energy, EA = 0.04 eV. There is a clear anomaly in the resistivity at TN, indicating a strong interplay between magnetism and electric conduction.

Single crystal growth of a layered perovskite V oxide Sr4V3O10 with an FZ method under controlled ρ(O2)

We have successfully grown single crystals of a layered compound Sr4V3O10 with a floating-zone method under controlled O2 partial pressure (p(O2)). We realized that the adjustment of p(O2) less than 10-28 atm was indispensable for the crystal growth, and achieved that growth condition by an oxygen pump system. Sr4V3O10 is a member of the Ruddlesden-Popper type series of Srn+1VnO3n+1 with n = 3. The crystal structure was indexed in a tetragonal space group I 4/mmm. The unit cell dimensions of Sr4V3O10 were found to be a = 3.858 A and c = 27.93 A. We have measured electrical resistivity from 30mK – 300K and magnetic susceptibility from 1.8K – 300K. The resistivity of Sr4V3O10 showed that it is a metallic conductor from 300 K to 30 mK. The magnetic susceptibility of the sample shows the Curie-Weiss type temperature dependence, χ = χo + C/(T-Θ). The values of paramters were obtained as χo = 6.93 x 10-5 emu/mole, Θ = -8.05 K, and C = 0.186 emuK/mole.

Convergent beam electron diffraction study on ge-based oxide spinels

Transition metal oxides with spinel crystal structure exhibit intriguing and non trivial magnetic phenomena owing to magnetic frustration between spins having antiferromagnetic coupling interaction on triangle or kagome lattice. GeCo2O4(GCO) and GeNi2O4(GNO), which belong to above category, are very rare normal spinels containing Ge ion. Both reveal antiferromagnetic-like phase transitions at 20 K and 12 K, respectively. According to previous neutron and x-ray diffraction measurements, GNO keeps its cubic structural symmetry down to 2 K which is not natural because such a magnetic transition tends to associate with symmetry breaking structural transitions. In order to know whether the structural transition or symmetry change occur or not at the magnetic transition in detail, convergent beam electron diffraction measurements is employed for the compounds.