Frustration induced disordered magnetism in Ba3RuTi2O9

The title compound Ba3RuTi2O9 crystallizes with a hexagonal unit cell. It contains layers of edge shared triangular network of Ru4+ (S=1) ions. Magnetic susceptibility chi(T) and heat capacity data show no long range magnetic ordering down to 1.8K. A Curie-Weiss (CW) fitting of chi(T) yields a large antiferromagnetic CW temperature theta_CW=-166K. However, in low field, a splitting of zero field cooled (ZFC) and field cooled (FC) chi(T) is observed below ~30K. Our measurements suggest that Ba3RuTi2O9 is a highly frustrated system but only a small fraction of the spins in this system undergo a transition to a frozen magnetic state below ~30K.Comment: 5 pages, 6 figures (accepted in EPJB

Existence Of The Magnetorotational Instability

By posing and solving a global axisymmetric eigenvalue problem on an infinite domain with modes vanishing at zero and infinity for a differentially rotating MHD plasma, the conditions for the occurrence of a purely growing low-frequency mode known as the magnetorotational instability (MRI) are mapped. It is shown that the MRI criterion drawn from the "local dispersion relation" is at best inadequate and may even be misleading. The physics of the MRI is rather nuanced. It is dictated by the details of the radial profile of the rotation velocity Omega(r) and not just by the sign and the magnitude of its gradient, Omega'. The salient features of the class of profiles for which the MRI-like eigenmodes may occur are given along with the eigenspectrum. For a variety of other profiles, it is shown that an unstable magnetorotational mode is not a valid eigensolution.Institute for Fusion Studie

A S=1/2 vanadium-based geometrically frustrated spinel system Li2ZnV3O8

We report the synthesis and characterization of Li2ZnV3O8, which is a new Zn-doped LiV2O4 system containing only tetravalent vanadium. A Curie-Weiss susceptibility with a Curie-Weiss temperature of CW ~214 K suggests the presence of strong antiferromagnetic correlations in this system. We have observed a splitting between the zero-field cooled ZFC and field cooled FC susceptibility curves below 6 K. A peak is present in the ZFC curve around 3.5 K suggestive of spin-freezing . Similarly, a broad hump is also seen in the inferred magnetic heat capacity around 9 K. The consequent entropy change is only about 8% of the value expected for an ordered S = 1=2 system. This reduction indicates continued presence of large disorder in the system in spite of the large CW, which might result from strong geometric frustration in the system. We did not find any temperature T dependence in our 7Li nuclear magnetic resonance NMR shift down to 6 K (an abrupt change in the shift takes place below 6 K) though considerable T-dependence has been found in literature for LiV2O4- undoped or with other Zn/Ti contents. Consistent with the above observation, the 7Li nuclear spin-lattice relaxation rate 1/T1 is relatively small and nearly T-independent except a small increase close to the freezing temperature, once again, small compared to undoped or 10% Zn or 20% Ti-doped LiV2O4.Comment: 7 pages, 8 figures, accepted in JPCM (Journal of Physics condensed matter

Tunable temperature induced magnetization jump in a GdVO3 single crystal

We report a novel feature of the temperature induced magnetization jump observed along the a-axis of the GdVO3 single crystal at temperature TM = 0.8 K. Below TM, the compound shows no coercivity and remanent magnetization indicating a homogenous antiferromagnetic structure. However, we will demonstrate that the magnetic state below TM is indeed history dependent and it shows up in different jumps in the magnetization only when warming the sample through TM. Such a magnetic memory effect is highly unusual and suggesting different domain arrangements in the supposedly homogenous antiferromagnetic phase of the compound.Comment: 17 pages, 8 Figure

Semi-empirical model for prediction of unsteady forces on an airfoil with application to flutter

A semi-empirical model is described for predicting unsteady aerodynamic forces on arbitrary airfoils under mildly stalled and unstalled conditions. Aerodynamic forces are modeled using second order ordinary differential equations for lift and moment with airfoil motion as the input. This model is simultaneously integrated with structural dynamics equations to determine flutter characteristics for a two degrees-of-freedom system. Results for a number of cases are presented to demonstrate the suitability of this model to predict flutter. Comparison is made to the flutter characteristics determined by a Navier-Stokes solver and also the classical incompressible potential flow theory