Magnetic properties of strongly disordered electronic systems

We present a unified, global perspective on the magnetic properties of strongly disordered electronic systems, with special emphasis on the case where the ground state is metallic. We review the arguments for the instability of the disordered Fermi liquid state towards the formation of local magnetic moments, and argue that their singular low temperature thermodynamics are the ``quantum Griffiths'' precursors of the quantum phase transition to a metallic spin glass; the local moment formation is therefore not directly related to the metal-insulator transition. We also review the the mean-field theory of the disordered Fermi liquid to metallic spin glass transition and describe the separate regime of ``non-Fermi liquid'' behavior at higher temperatures near the quantum critical point. The relationship to experimental results on doped semiconductors and heavy-fermion compounds is noted.Comment: 25 pages; Contribution to the Royal Society Discussion Meeting on "The Metal-Non Metal Transition in Macroscopic and Microscopic Systems", March 5-6, 199

Structures and Electromagnetic Properties of New Metal-Ordered Manganites; RBaMn_{2}O_{6} (R = Y and Rare Earth Elements)

New metal-ordered manganites RBaMn_{2}O_{6} have been synthesized and investigated in the structures and electromagnetic properties. RBaMn_{2}O_{6} can be classified into three groups from the structural and electromagnetic properties. The first group (R = La, Pr and Nd) has a metallic ferromagnetic transition, followed by an A-type antiferromagnetic transition in PrBaMn_{2}O_{6}. The second group (R = Sm, Eu and Gd) exhibits a charge-order transition, followed by an antiferromagnetic long range ordering. The third group (R = Tb, Dy and Ho) shows successive three phase transitions, the structural, charge/orbital-order and magnetic transitions, as observed in YBaMn_{2}O_{6}. Comparing to the metal-disordered manganites (R^{3+}_{0.5}A^{2+}_{0.5})MnO_{3}, two remarkable features can be recognized in RBaMn_{2}O_{6}; (1) relatively high charge-order transition temperature and (2) the presence of structural transition above the charge-order temperature in the third group. We propose a possible orbital ordering at the structural transition, that is a possible freezing of the orbital, charge and spin degrees of freedom at the independent temperatures in the third group. These features are closely related to the peculiar structure that the MnO_{2} square-lattice is sandwiched by the rock-salt layers of two kinds, RO and BaO with extremely different lattice-sizes.Comment: 5 pages, 4 figures, submitted to J. Phys. Soc. Jp

Organically templated three-dimensional open-framework metal selenites with a diamondoid network

Three-dimensional open-framework metal selenites of the formula, [H2N(CH2)4NH2]0.5[M(HSeO3)(Se2O5)] (M=Zn, Co or Ni), containing both selenite and diselenite units and intersecting 10-membered channels, have been prepared hydrothermally in the presence of piperazine. The compounds are isomorphous and crystallize in the P-1 space group. The structure is built up of MO6 dimers along with Se2O5 and HSeO3 units, the first two forming sheets, which are connected by the selenite units to yield a three-dimensional non-interpenetrating diamondoid network. The 10-membered channels have an unusual structure, being formed by 4-membered (M2Se2) and 6-membered (M2Se4) rings

Lattice Green's function for crystals containing a planar interface

Flexible boundary condition methods couple an isolated defect to a harmonically responding medium through the bulk lattice Green's function; in the case of an interface, interfacial lattice Green's functions. We present a method to compute the lattice Green's function for a planar interface with arbitrary atomic interactions suited for the study of line defect/interface interactions. The interface is coupled to two different semi-infinite bulk regions, and the Green's function for interface-interface, bulk-interface and bulk-bulk interactions are computed individually. The elastic bicrystal Green's function and the bulk lattice Green's function give the interaction between bulk regions. We make use of partial Fourier transforms to treat in-plane periodicity. Direct inversion of the force constant matrix in the partial Fourier space provides the interface terms. The general method makes no assumptions about the atomic interactions or crystal orientations. We simulate a screw dislocation interacting with a $(10\bar{1}2)$ twin boundary in Ti using flexible boundary conditions and compare with traditional fixed boundary conditions results. Flexible boundary conditions give the correct core structure with significantly less atoms required to relax by energy minimization. This highlights the applicability of flexible boundary conditions methods to modeling defect/interface interactions by \textit{ab initio} methods

Interplay of 4f-3d Magnetism and Ferroelectricity in DyFeO3

DyFeO3 exhibits a weak ferromagnetism (TNFe ~ 645 K) that disappears below a spin-reorientation (Morin) transition at TSRFe ~ 50 K. It is also known that applied magnetic field induces ferroelectricity at the magnetic ordering temperature of Dy-ions (TNDy ~ 4.5 K). Here, we show that the ferroelectricity exists in the weak ferromagnetic state (TSRFe < T < TN,C) without applying magnetic field, indicating the crucial role of weak ferromagnetism in inducing ferroelectricity. 57Fe M\"ossbauer studies show that hyperfine field (Bhf) deviates from mean field-like behaviour that is observed in the weak ferromagnetic state and decreases below the onset of spin-reorientation transition (80 K), implying that the Bhf above TSR had additional contribution from Dy-ions due to induced magnetization by the weak ferromagnetic moment of Fe-sublattice and below TSR, this contribution decreases due to collinear ordering of Fe-sublattice. These results clearly demonstrate the presence of magnetic interactions between Dy(4f) and Fe(3d) and their correlation with ferroelectricity in the weak ferromagnetic state of DyFeO3.Comment: 5 pages, 6 figures, published in EP

Beneficiation studies on beach placer sample for steel making industries

Beneficiation studies were carried out on the Talashil beach placer sample of South Maharastra Coast, India. The sample contains magnetite, ilmenite, rutile, hematite, goethite and chromite as opaque minerals in the sample. The total heavy minerals fraction reaches 53.8 % by weight whereas the total magnetic minerals are 56.9%. It is observed that the 2nd stage DHIMS magnetic fraction contains 65.2 % Fe2O3 with an over all yield of 37.8 % and a 86 % recovery from a containing 26.8 % Fe2O3 feed. This product can be used in the pellet feed for steel making after suitable blending with high-grade iron ore fines

Field-induced Polar Order at the N\'eel Temperature of Chromium in Rare-earth Orthochromites: Interplay of Rare-earth and Cr Magnetism

We report field-induced switchable polarization (P = 0.2 ~ 0.8 microC/cm2) below the N\'eel temperature of chromium (TN Cr) in weakly ferromagnetic rareearth orthochromites, RCrO3 (R=rareearth) but only when the rareearth ion is magnetic. Intriguingly, the polarization in ErCrO3 (TC ~ 133 K) disappears at a spin reorientation (Morin) transition (TSR ~ 22 K) below which the weak ferromagnetism associated with the Cr sublattice also disappears, demonstrating the crucial role of weak ferromagnetism in inducing the polar order. Further, the polarization (P) is strongly influenced by applied magnetic field, indicating a strong magneto electric effect. We suggest that the polar order occurs in RCrO3, due to the combined effect of poling field that breaks the symmetry and the exchange field on R ion from Cr sublattice stabilizes the polar state. We propose that a similar mechanism could work in the isostructural rareearth orthoferrites, RFeO3 as well.Comment: 31 pages (Manuscript(6 figures)+supplemental information(8 figures)

The eigenspectra of Indian musical drums

In a family of drums used in the Indian subcontinent, the circular drum head is made of material of non-uniform density. Remarkably, and in contrast to a circular membrane of uniform density, the low eigenmodes of the non-uniform membrane are harmonic. In this work we model the drum head by a non-uniform membrane whose density varies smoothly between two prescribed values. Using a Fourier-Chebyshev spectral collocation method we obtain the eigenmodes and eigenvalues of the drum head. For a suitable choice of parameters, which we find by optimising a cost function, the eigenspectra obtained from our model are in excellent agreement with experimental values. Our model and the numerical method should find application in numerical sound synthesis