V. O. Garlea

Magnetic and Orbital Ordering in the Spinel MnV2O4

Neutron inelastic scattering and diffraction techniques have been used to study the MnV2O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states, the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.

Magnetically Driven Metal-Insulator Transition in NaOsO3

The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials. Various mechanisms producing MITs have been extensively considered, including the Mott (electron localization via Coulomb repulsion), Anderson (localization via disorder), and Peierls (localization via distortion of a periodic one-dimensional lattice) mechanisms. One additional route to a MIT proposed by Slater, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention. Using neutron and x-ray scattering we show that the MIT in NaOsO(3) is coincident with the onset of long-range commensurate three dimensional magnetic order. While candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT.

Magnetic order and ice rules in the multiferroic spinel FeV 2 O 4

We present a neutron-diffraction study of FeV2O4, which is rare in exhibiting spin and orbital degrees of freedom on both cation sublattices of the spinel structure. Our data confirm the existence of three structural phase transitions previously identified with x-ray powder diffraction and reveal that the lower two transitions are associated with sequential collinear and canted ferrimagnetic transitions involving both cation sites. Through consideration of local crystal and spin symmetry, we further conclude that Fe2+ cations are ferro-orbitally ordered below 135 K and V3+ orbitals order at 60 K, in accordance with predictions for vanadium spinels with large trigonal distortions and strong spin-orbit coupling. Intriguingly, the direction of ordered vanadium spins at low temperature obey ice rules more commonly associated with the frustrated rare-earth pyrochlore systems.

Lattice dynamics and phonon softening in Ni-Mn-Al Heusler alloys

Inelastic and elastic neutron scattering have been used to study a single crystal of the Ni{sub 54}Mn{sub 23}Al{sub 23} Heusler alloy over a broad temperature range. The paper reports the experimental determination of the low-lying phonon dispersion curves for this alloy system. We find that the frequencies of the TA2 modes are relatively low. This branch exhibits an anomaly (dip) at a wave number {xi}{sub 0}=1/3{approx}0.33, which softens with decreasing temperature. Associated with this anomalous dip at {xi}{sub 0}, an elastic central peak scattering is also present. We have also observed satellites due to the magnetic ordering.

Excitations from a Bose-Einstein Condensate of Magnons in Coupled Spin Ladders

The weakly coupled quasi-one-dimensional spin ladder compound (CH3)2CHNH3CuCl3 is studied by neutron scattering in magnetic fields exceeding the critical field of Bose-Einstein condensation of magnons. Commensurate long-range order and the associated Goldstone mode are detected and found to be similar to those in reference to spin-dimer materials. However, for the upper two massive magnon branches, the observed behavior is totally different, culminating in a drastic collapse of excitation bandwidth beyond the transition point.

Hydrothermal Synthesis and Characterization of Novel Brackebuschite-Type Transition Metal Vanadates: Ba2M(VO4)2(OH), M = V3+, Mn3+, and Fe3+, with Interesting Jahn–Teller and Spin-Liquid Behavior

A new series of transition metal vanadates, namely, Ba2M(VO4)2(OH) (M = V(3+), Mn(3+), and Fe(3+)), was synthesized as large single crystals hydrothermally in 5 M NaOH solution at 580 °C and 1 kbar. This new series of compounds is structurally reminiscent of the brackebuschite mineral type. The structure of Ba2V(VO4)2(OH) is monoclinic in space group P21/m, a = 7.8783(2) Å, b = 6.1369(1) Å, c = 9.1836(2) Å, β = 113.07(3)°, V = 408.51(2) Å(3). The other structures are similar and consist of one-dimensional trans edge-shared distorted octahedral chains running along the b-axis. The vanadate groups bridge across edges of their tetrahedra. Structural analysis of the Ba2Mn(VO4)2(OH) analogue yielded a new understanding of the Jahn-Teller effect in this structure type. Raman and infrared spectra were investigated to observe the fundamental vanadate and hydroxide vibrational modes. Single-crystal temperature-dependent magnetic studies on Ba2V(VO4)2(OH) reveal a broad feature over a wide temperature range with maximum at ∼100 K indicating that an energy gap could exist between the antiferromagnetic singlet ground state and excited triplet states, making it potentially of interest for quantum magnetism studies.

Continuous magnetic and structural phase transitions in Fe1+yTe

We report a sequence of continuous phase transformations in iron telluride, Fe{sub 1+y}Te with y=0.10(1), which is observed by combining neutron diffraction, magnetic susceptibility, and specific-heat measurements on single-crystal samples. While a gradual increase of magnetic scattering near the wave vector (0.5,0,0.5) is seen below T {approx} 70 K, a temperature where the discontinuous first-order magnetostructural phase transition is found in systems with small y ({approx}<0.06), the reduction of the lattice symmetry in Fe{sub 1.1}Te only occurs at T{sub s} {approx} 63 K. Below T{sub N} {approx} 57.5 K, the long-range magnetic order develops, the incommensurate wave vector Q{sub m} of which varies with temperature. Finally, at T{sub m} {approx}< 45 K, the system enters the low-T phase, where Q{sub m} is locked at {approx}(0.48,0,0.5). We conclude that these instabilities are weak compared to the strength of the underlying interactions, and we suggest that the impact of the Fe interstitials on the transitions can be treated with random-field models.

Structure-property coupling in Sr3(Ru1−xMnx)2O7

Layered ruthenates are prototype materials with strong structure-property correlations. We report the structural and physical properties of double-layered perovskite Sr3(Ru1−xMnx)2O7 single crystals with 0≤x≤0.7. Single crystal x-ray diffraction refinements reveal that Mn doping on the Ru site leads to the shrinkage of unit-cell volume and disappearance of (Ru/Mn)O6 octahedron rotation when x>0.16, while the crystal structure remains tetragonal. Correspondingly, the electronic and magnetic properties change with x. The electrical resistivity reveals metallic character (dρ/dT>0) at high temperatures but insulating behavior (dρ/dT<0) below a characteristic temperature TMIT. Interestingly, TMIT is different from TM, at which magnetic susceptibility reaches maximum. TMIT monotonically increases with increasing x while TM shows non-monotonic dependence with x. The difference between TMIT and TM (TMIT>TM) becomes larger when x>0.16. The constructed phase diagram consists of five distinct regions, demonstrating that the physical properties of such a system can easily be tuned by chemical doping.

Ferro-Orbital Ordering Transition in Iron Telluride Fe1+yTe

Fe(1+y)Te with y≲0.05 exhibits a first-order phase transition on cooling to a state with a lowered structural symmetry, bicollinear antiferromagnetic order, and metallic conductivity, dρ/dT>0. Here, we study samples with y=0.09(1), where the frustration effects of the interstitial Fe decouple different orders, leading to a sequence of transitions. While the lattice distortion is closely followed by incommensurate magnetic order, the development of bicollinear order and metallic electronic coherence is uniquely associated with a separate hysteretic first-order transition, at a markedly lower temperature, to a phase with dramatically enhanced bond-order wave (BOW) order. The BOW state suggests ferro-orbital ordering, where electronic delocalization in ferromagnetic zigzag chains decreases local spin and results in metallic transport.

Crystal and magnetic structures and physical properties of a new pyroxene NaMnGe2O6 synthesized under high pressure.

A new pyroxene compound, NaMnGe(2)O(6), has been synthesized at 3 GPa and 800 °C and fully characterized by X-ray single-crystal diffraction, neutron powder diffraction, and measurements of magnetization and specific heat. NaMnGe(2)O(6) crystallizes into a monoclinic C2/c structure with unit-cell parameters a = 9.859(2) Å, b = 8.7507(18) Å, c = 5.5724(11) Å, and β = 105.64(3)° at 153 K. A cooperative Jahn-Teller distortion is formed by an ordering of the longest Mn-O bonds between two neighboring octahedra along the chain direction. This feature distinguishes NaMnGe(2)O(6) from other pyroxene compounds without Jahn-Teller active cations and suggests that the Jahn-Teller distortion competes with the intrinsic local distortion in the pyroxene structure. No orbital order-disorder transition has been found up to 750 K. Like other alkali-metal pyroxenes with S > (1)/(2), NaMnGe(2)O(6) (S = 2) was found to undergo a long-range antiferromagnetic (AF) ordering at T(N) = 7 K due to intrachain and interchain exchange interactions. Due to the peculiar structural features and the corresponding magnetic coupling, the weak AF spin ordering gives way to a ferromagnetic-like state at a sufficiently high magnetic field. Specific-heat measurements demonstrated that a large portion of the magnetic entropy, >60%, has been removed above T(N) as a result of strong spin correlations within the quasi-one-dimensional Mn(3+)-spin chains. The Reitveld refinement of neutron powder diffraction data gives a commensurate magnetic structure defined by k = [0 0 0.5] with Mn moments aligned mainly along the c-axis with a small component along both a- and b-axes.