Magnetic Ordering In Materials Research Articles

Magnetic Neutron Diffraction of Quasi-Two-Dimensional Magnets

This study is devoted to magnetic neutron diffraction analysis (elastic neutron scattering yielding information about spin ordering in magnetic materials). Experimental approaches to magnetism are intensely developed mainly due to the discovery of new functional materials with unusual properties. Nevertheless, magnetic neutron diffraction analysis is a unique method, which makes it possible to directly determine magnetic structures (i.e., values and mutual orientations of magnetic moments and their orientations relative to the crystallographic axes). Neutron diffraction proved to be a good instrument for studying both simple commensurate magnetic structures in bulk samples and more complex noncollinear spiral magnets. Magnetic neutron diffraction is widely applied for studying magnetic structures in nanoscale multilayer film materials. However, bulk samples with a layered crystal structure, which can be considered as natural quasi-two-dimensional objects due to the weakness of interlayer couplings, are of greater interest for physics of low-dimensional magnetism. Our neutron diffraction studies on quasi-two-dimensional magnets, transition-metal oxides A3M2SbO6 (A = Li or Na, M = Co or Ni) and rare-earth intermetallic compounds La1–xTbx(Ni1–xMnx)2Si2, are briefly considered.

Magnon–fluxon interaction in a ferromagnet/superconductor heterostructure

Ferromagnetism and superconductivity are most fundamental phenomena in condensed matter physics. Entailing opposite spin orders, they share an important conceptual similarity: Disturbances in magnetic ordering in magnetic materials can propagate in the form of spin waves (magnons) while magnetic fields penetrate superconductors as a lattice of magnetic flux quanta (fluxons). Despite a rich choice of wave and quantum phenomena predicted, magnon-fluxon coupling has not been observed experimentally so far. Here, we clearly evidence the interaction of spin waves with a flux lattice in ferromagnet/superconductor Py/Nb bilayers. We demonstrate that, in this system, the magnon frequency spectrum exhibits a Bloch-like band structure which can be tuned by the biasing magnetic field. Furthermore, we observe Doppler shifts in the frequency spectra of spin waves scattered on a flux lattice moving under the action of a transport current in the superconductor.

A two-dimensional spin liquid in quantum kagome ice

Actively sought since the turn of the century, two-dimensional quantum spin liquids (QSLs) are exotic phases of matter where magnetic moments remain disordered even at zero temperature. Despite ongoing searches, QSLs remain elusive, due to a lack of concrete knowledge of the microscopic mechanisms that inhibit magnetic order in materials. Here we study a model for a broad class of frustrated magnetic rare-earth pyrochlore materials called quantum spin ices. When subject to an external magnetic field along the [111] crystallographic direction, the resulting interactions contain a mix of geometric frustration and quantum fluctuations in decoupled two-dimensional kagome planes. Using quantum Monte Carlo simulations, we identify a set of interactions sufficient to promote a groundstate with no magnetic long-range order, and a gap to excitations, consistent with a Z2 spin liquid phase. This suggests an experimental procedure to search for two-dimensional QSLs within a class of pyrochlore quantum spin ice materials.

Seeing magnetism on an atomic level

Magnetic Microscopy Electrons tunneling from the magnetized tip of a scanning tunneling microscope into the surface of a material can reveal the material's magnetic structure. Although researchers have used the technique on simple nanostructures, they've had trouble preparing the tip in just the right way to visualize the magnetic order in more exotic materials. Enayat et al. used a tip with a magnetic cluster on its apex to reveal patterns of magnetic ordering in the material Fe1+yTe, which becomes superconducting by substituting Te with Se atoms. The researchers prepared the tip by simply picking up atoms from the surface of the material. Science , this issue p. [653][1] [1]: /lookup/doi/10.1126/science.1251682

Electronic Structures in LaTiO<sub>3</sub>/LaAlO<sub>3</sub> Multilayers

We demonstrate the existence of a hidden degree of freedom controlling the orbitalordering in [LaTiO1/[LaAlO5 multilayers with comprehensive density-functional theorycalculations. The orbitals of two-dimensional (2D) 3d1state of Ti atoms in the multilayers alwayscontain large dxy components, which is unexpected from crystal field theory (first Jahn-Tellerdistortion). The competition between first and second Jahn-Teller distortion induces variousmagnetic properties. Thus, transition-metal oxides/non-transition-metal oxides multilayers mayprovide an important direction to manipulate the spin and orbital ordering in magnetic materials.

Neutron scattering applied to materials problems

Neutron scattering techniques for studying materials have been applied for more than 50 years, in part led by Clifford G. Shull and Bertram N. Brockhouse, who shared the 1994 Nobel Prize in physics. The award was given for their developments in studying both the structure and dynamics of materials. The application of neutron scattering to materials has received significantly greater attention in the last few years, due to the construction of the Spallation Neutron Source (SNS) at the Oak Ridge National laboratory. The SNS will provide unprecedented access to a wide variety of instruments designed for materials research. The idea for this series of articles originated during the 2005 TMS Annual Meeting. Two symposia were devoted to neutron scattering: Neutron Scattering in Materials Research; and Neutron Diffraction Characterization of Mechanical Behavior. the goal of these articles is to introduce techniques for studying materials using neutrons, particularly to answer what are considered traditional materials problems. The first article discusses structure and phase analysis. Neutrons may be used for diffraction, similar to x-rays. However, in certain circumstances, they bring particular advantages. For example, x-rays have difficulty 'seeing' light elements, particularly when heavier elements are present, whereas neutrons may scatter effectively from elementsmore » such as hydrogen and its isotopes. The scattering strength of neutrons is sensitive to the isotopic composition. This can be used to examine the influence of a particular element. Neutrons also interact magnetically, allowing their use for studying magnetic order in materials. Because neutrons are highly penetrating, in-situ investigations under special sample environments (e.g., temperature, magnetic field, high pressure) have become routine. The second article describes applications for studying residual stress and mechanical deformation. neutrons are better able to penetrate engineering components compared to x-rays so neutron diffraction is used to examine the spatial distribution of stress and texture. In-situ loading studies have allowed for a sensitive, nondestructive study of the deformation mechanisms. More recently, neutrons have been used to analyze the dislocation and subgrain structures due to deformation. neutrons are also well suited for the study of composite materials, as diffraction peaks from each phase are separated. The author describes the fundamentals and applications of inelastic neutron scattering. By measuring not only the change in wave vector, but also the change in energy, neutrons can probe the dynamics in materials. This is used for examining elastic properties, magnetic excitations, and other dynamics that affect both the response of materials to applied driving forces and the thermodynamic properties of materials. The last article discusses facilities. The user facilitates are almost as varied as the applications, with many different instruments at each major facility. The article discusses the various facilities and instruments, as well as how to apply for access to these facilities.« less

Magnetic Ordering in Materials with Singlet Crystal-Field Ground State. II. Behavior in the Ordered State or in an Applied Field

Magnetic Ordering in Materials with Singlet Crystal-Field Ground State. II. Behavior in the Ordered State or in an Applied Field

Magnetic Ordering in Materials with Singlet Crystal-Field Ground State. II. Behavior in the Ordered State or in an Applied Field

Magnetic Ordering in Materials with Singlet Crystal-Field Ground State. II. Behavior in the Ordered State or in an Applied Field

Collective Excitations and Magnetic Ordering in Materials with Singlet Crystal-Field Ground State

The low-lying magnetic excited states ("Frenkel spin excitons") in rare-earth compounds with a singlet crystal-field ground state are studied. We consider the effect of exchange interaction between the ionic moments as it increases from zero towards the critical value necessary for magnetic ordering at zero temperature. For simplicity, we consider the two-level system where the first-excited crystal-field state is also a singlet. By employing a pseudospin formalism, we avoid many of the fundamental difficulties encountered in previous treatments which introduce fermion operators for each of the crystal-field states. Applying techniques similar to those used in treating standard spin problems such as the Heinsenberg ferromagnet, we obtain an improved collective-excitation spectrum with a gap which decreases as the exchange increases. The critical value of exchange for magnetic ordering found is substantially greater than that obtained from molecular-field theory, and also is somewhat greater than that found with the constant-coupling approximation.