Phase diagram and magnetocaloric effect of CoMnGe_{1-x}Sn_{x} alloys

We propose the phase diagram of a new pseudo-ternary compound, CoMnGe_{1-x}Sn_{x}, in the range x less than or equal to 0.1. Our phase diagram is a result of magnetic and calometric measurements. We demonstrate the appearance of a hysteretic magnetostructural phase transition in the range x=0.04 to x=0.055, similar to that observed in CoMnGe under hydrostatic pressure. From magnetisation measurements, we show that the isothermal entropy change associated with the magnetostructural transition can be as high as 4.5 J/(K kg) in a field of 1 Tesla. However, the large thermal hysteresis in this transition (~20 K) will limit its straightforward use in a magnetocaloric device.Comment: 12 pages, 5 figure

Spin Frustration and Orbital Order in Vanadium Spinels

We present the results of our theoretical study on the effects of geometrical frustration and the interplay between spin and orbital degrees of freedom in vanadium spinel oxides $A$V$_2$O$_4$ ($A$ = Zn, Mg or Cd). Introducing an effective spin-orbital-lattice coupled model in the strong correlation limit and performing Monte Carlo simulation for the model, we propose a reduced spin Hamiltonian in the orbital ordered phase to capture the stabilization mechanism of the antiferromagnetic order. Orbital order drastically reduces spin frustration by introducing spatial anisotropy in the spin exchange interactions, and the reduced spin model can be regarded as weakly-coupled one-dimensional antiferromagnetic chains. The critical exponent estimated by finite-size scaling analysis shows that the magnetic transition belongs to the three-dimensional Heisenberg universality class. Frustration remaining in the mean-field level is reduced by thermal fluctuations to stabilize a collinear ordering.Comment: 4 pages, 4 figures, proceedings submitted to SPQS200

Orbital and spin chains in ZnV2O4

Our powder inelastic neutron scattering data indicate that \zvo is a system of spin chains that are three dimensionally tangled in the cubic phase above 50 K due to randomly occupied $t_{2g}$ orbitals of V$^{3+}$ ($3d^2$) ions. Below 50 K in the tetragonal phase, the chains become straight due to antiferro-orbital ordering. This is evidenced by the characteristic wave vector dependence of the magnetic structure factor that changes from symmetric to asymmetric at the cubic-to-tetragonal transition

Spin-Peierls phases in pyrochlore antiferromagnets

In the highly frustrated pyrochlore magnet spins form a lattice of corner sharing tetrahedra. We show that the tetrahedral ``molecule'' at the heart of this structure undergoes a Jahn-Teller distortion when lattice motion is coupled to the antiferromagnetism. We extend this analysis to the full pyrochlore lattice by means of Landau theory and argue that it should exhibit spin-Peierls phases with bond order but no spin order. We find a range of Neel phases, with collinear, coplanar and noncoplanar order. While collinear Neel phases are easiest to generate microscopically, we also exhibit an interaction that gives rise to a coplanar state instead.Comment: REVTeX 4, 14 pages, 12 figures (best viewed in color

Spin Driven Jahn-Teller Distortion in a Pyrochlore system

The ground-state properties of the spin-1 antiferromagnetic Heisenberg model on the corner-sharing tetrahedra, pyrochlore lattice, is investigated. By breaking up each spin into a pair of 1/2-spins, the problem is reduced to the equivalent one of the spin-1/2 tetrahedral network in analogy with the valence bond solid state in one dimension. The twofold degeneracy of the spin-singlets of a tetrahedron is lifted by a Jahn-Teller mechanism, leading to a cubic to tetragonal structural transition. It is proposed that the present mechanism is responsible for the phase transition observed in the spin-1 spinel compounds ZnV$_2$O$_4$ and MgV$_2$O$_4$.Comment: 4 pages, 3 eps figures, REVTeX, to appear in Phys. Rev. Let

White noise excited vibrations of viscoelastic shallow shells

The paper presents the results of the analysis of randomly excited vibrations of viscoelestic shallow shells. The parameter of interest is the dispersion of velocity normal to the element surface. The choice of parameter is motivated by estimation of the noise generated by a vibrating panel.Drgania lepkosprężystych powłok małowyniosłych poddanych wymuszeniu losowemu typu białego szumu W artykule omówiono rezultaty analiz drgań powłok małowyniosłych wykonanych z materiału lepkosprężystego i poddanych wymuszeniu losowemu typu białego szumu. Jako parametr opisujący drgania przyjęto dyspersję składowej prędkości normalnej do powierzchni środkowej powłoki. Wybór tego parametru jest uzasadniony faktem, iż jego wartość może być miarą wartości mocy akustycznej promieniowanej przez drgający element

Magnetic and Transport Properties of (Fe1−x\text{}_{1-x}Nix\text{}_{x})2\text{}_{2}P in View of KKR-CPA Results

We present results of the electronic structure study for the solid solution (Fe$\text{}_{1-x}$Ni$\text{}_{x}$)$\text{}_{2}$P (space group P6̅2m). The samples were synthesized for x=0.0 up to 0.15. From the electrical conductivity measurements on these materials, it was verified that resistivity increases with increasing temperature up to the Curie point T$\text{}_{C}$ and changes the slope at T$\text{}_{C}$ decreasing when temperature is raised above T$\text{}_{C}$. To get insight into the possible origin of such observations, we carried out the Korringa-Kohn-Rostoker-coherent potential approximation electronic structure calculations for (Fe$\text{}_{1-x}$Ni$\text{}_{x}$)$\text{}_{2}$P in the range of concentration x=0.0 up to 1.0. It was found that alloying with Ni in Fe$\text{}_{2}$ P causes vanishing of spin band splitting and non-rigid band behaving of electronic valence states near the Fermi level. The polarization factor for electrons at E$\text{}_{F}$ changes in (Fe$\text{}_{1-x}$Ni$\text{}_{x}$)$\text{}_{2}$P with increasing x, and interestingly, it reaches maximum at x=0.1, closely to the concentration (x=0.08), when the largest value of T$\text{}_{C}$ and a kink in temperature dependent resistivity were detected

Kinetic characterisation of catalysts for methanol synthesis

The results of activity studies of four catalysts in methanol synthesis have been presented. A standard industrial catalyst TMC-3/1 was compared with two methanol catalysts promoted by the addition of magnesium and one promoted by zirconium. The kinetic analysis of the experimental results shows that the Cu/Zn/Al/Mg/1 catalyst was the least active. Although TMC-3/1 and Cu/Zn/Al/Mg/2 catalysts were characterised by a higher activity, the most active catalyst system was Cu/Zn/Al/Zr. The activity calculated for zirconium doped catalyst under operating conditions was approximately 30% higher that of TMC-3/1catalyst. The experimental data were used to identify the rate equations of two types – one purely empirical power rate equation and the other one - the Vanden Bussche & Froment kinetic model of methanol synthesis. The Cu/ZnO/Al2O3catalyst modified with zirconium has the highest application potential in methanol synthesis

Magnetoelastic Properties of MnRhAs1−x\text{}_{1-x}px\text{}_{x} and Band Structure Results from KKR-CPA

The magnetoelastic phase transitions were observed in MnRhAs$\text{}_{1-x}$P$\text{}_{x}$ system (space group P6̅2m) by X-ray measurements (80 K < T < 300 K). For x = 0.33 and x = 0.4 contents an abrupt change of both lattice parameters and of the elementary cell volume at the metamagnetic transition was detected. The Korringa-Kohn-Rostoker electronic structure calculations performed for MnRhP compound show significant variations of densities of states at the Fermi level, while changing lattice data. The Korringa-Kohn- Rostoker-coherent potential approximation studies of disordered MnRhAs$\text{}_{1-x}$P$\text{}_{x}$ system (x=0.6, 0.8, 1.0) were done at the ferromagnetic region. From our calculations we observe moving of bands near E$\text{}_{F}$, while substituting with isoelectronic metalloid, likely as while changing the crystal data in MnRhP

Neutron Diffraction Studies of MnFeAsy\text{}_{y}P1−y\text{}_{1-y} System

Magnetic structures of the MnFeAs$\text{}_{y}$P$\text{}_{1-y}$ system were examined by means of the neutron diffraction technique in the 1.5-300 K temperature range. Atomic ordering in the metal sublattices, temperature dependence of the incommensurate (y = 0.2 and y = 0.275) and of the ferromagnetic (y = 0.275, y = 0.3, y = 0.5) ordering is discussed