V. O. Garlea

Oak Ridge National Laboratory, Oak Ridge, Florida, United States

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Publications (60)128.56 Total impact

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    ABSTRACT: We present a study of the crystal structure and physical properties of single crystals of a new Fe-based ternary compound, Zr2−xFe4Si16−y(x = 0.81, y = 6.06). Zr1.19Fe4Si9.94 is a layered compound, where stoichiometric β-FeSi2-derived slabs are separated by Zr-Si planes with substantial numbers of vacancies. High resolution transmission electron microscopy (HRTEM) experiments show that these Zr-Si layers consist of 3.5 nm domains where the Zr and Si vacancies are ordered within a supercell sixteen times the volume of the stoichiometric cell. Within these domains, the occupancies of the Zr and Si sites obey symmetry rules that permit only certain compositions, none of which by themselves reproduce the average composition found in x-ray diffraction experiments. Magnetic susceptibility and magnetization measurements reveal a small but appreciable number of magnetic moments that remain freely fluctuating to 1.8 K, while neutron diffraction confirms the absence of bulk magnetic order with a moment of 0.2μB or larger down to 1.5 K. Electrical resistivity measurements find that Zr1.19Fe4Si9.94 is metallic, and the modest value of the Sommerfeld coefficient of the specific heat γ = C/T suggests that quasi-particle masses are not particularly strongly enhanced. The onset of superconductivity at Tc 6 K results in a partial resistive transition and a small Meissner signal, although a bulk-like transition is found in the specific heat. Sharp peaks in the ac susceptibility signal the interplay of the normal skin depth and the London penetration depth, typical of a system in which nano-sized superconducting grains are separated by a non-superconducting host. Ultra low field differential magnetic susceptibility measurements reveal the presence of a surprisingly large number of trace magnetic and superconducting phases, suggesting that the Zr-Fe-Si ternary system could be a potentially rich source of new bulk superconductors.
    Journal of Physics Condensed Matter 08/2014; 26(37):376002. · 2.22 Impact Factor
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    ABSTRACT: The AR$_2$O$_4$ family (R = rare earth) have recently been attracting interest as a new series of frustrated magnets, with the magnetic R atoms forming zigzag chains running along the $c$-axis. We have investigated polycrystalline BaNd$_2$O$_4$ with a combination of magnetization, heat capacity, and neutron powder diffraction (NPD) measurements. Magnetic Bragg peaks are observed below $T_N$ $=$ 1.7 K, and they can be indexed with a propagation vector of $\vec{k}$ $=$ (0 1/2 1/2). The signal from magnetic diffraction is well described by long-range ordering from only one of the two types of Nd zigzag chains, with collinear up-up-down-down intrachain spin configurations. Furthermore, low temperature magnetization and heat capacity measurements reveal two field-induced spin transitions at 2.5 T and 4 T for $T$ $=$ 0.46 K. The high field phase is paramagnetic, while the intermediate field state may arise from a spin transition of the long-range ordered Nd chains, resulting in an up-up-down intrachain spin configuration. The proposed intermediate field state is consistent with the magnetic structure determined in zero field for these chains by NPD, as both phases are predicted for the classical Ising chain model with nearest neighbor and next nearest neighbor interactions.
    07/2014;
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    ABSTRACT: We describe why Ising spin chains with competing interactions in $\rm SrHo_2O_4$ segregate into ordered and disordered ensembles at low $T$. Using elastic neutron scattering, magnetization, and specific heat measurements, the two distinct spin chains are inferred to have N\'eel ($\uparrow\downarrow\uparrow\downarrow$) and double-N\'eel ($\uparrow\uparrow\downarrow\downarrow$) ground states respectively. Below $T_\mathrm{N}=0.68(2)$ K, the N\'eel chains develop three dimensional (3D) long range order (LRO), which causes the double-N\'eel chains to freeze into a disordered incommensurate state at $T_\mathrm{S}= 0.52(2)$ K. $\rm SrHo_2O_4$ distills an important feature of incommensurate low dimensional magnetism: kinetically trapped topological defects in a quasi$-d-$dimensional spin system can preclude order in $d+1$ dimensions.
    07/2014;
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    ABSTRACT: Neutron and X-ray diffraction, magnetic susceptibility, and specific heat measurements have been used to investigate the magnetic and structural phase transitions of the spinel system Fe1+xCr2-xO4(0.0<=x<=1.0). The temperature versus Fe concentration (x) phase diagram features two magnetically ordered states and four structural states below 420 K. The complexity of the phase diagram is closely related to the change in the spin and orbital degrees of freedom induced by substitution of Fe ions for Cr ions. The systematic change in the crystal structure is explained by the combined effects of Jahn-Teller distortion, spin-lattice interaction, Fe2+-Fe3+ hopping, and disorder among Fe2+, Fe3+, and Cr3+ ions.
    04/2014; 89(13).
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    ABSTRACT: The spinel vanadates have become a model family for exploring orbital order on the frustrated pyrochlore lattice, and recent debate has focused on the symmetry of local crystal fields at the cation sites. Here, we present neutron scattering measurements of the magnetic excitation spectrum in $\mathrm{FeV_2O_4}$, a recent example of a ferrimagnetic spinel vanadate which is available in single crystal form. We report the existence of two emergent magnon modes at low temperatures, which draw strong parallels with the closely related material, $\mathrm{MnV_2O_4}$. We were able to reproduce the essential elements of both the magnetic ordering pattern and the dispersion of the inelastic modes with semi- classical spin wave calculations, using a minimal model that implies a sizeable single-ion anisotropy on the vanadium sublattice. Taking into account the direction of ordered spins, we associate this anisotropy with the large trigonal distortion of $\mathrm{VO_6}$ octahedra, previously observed via neutron powder diffraction measurements. We further report on the spin gap, which is an order-of-magnitude larger than that observed in $\mathrm{MnV_2O_4}$. By looking at the overall temperature dependence, we were able to show that the gap magnitude is largely associated with the ferro-orbital order known to exist on the iron sublattice, but the contribution to the gap from the vanadium sublattice is in fact comparable to what is reported in the Mn compound. This reinforces the conclusion that the spin canting transition is associated with the ordering of vanadium orbitals in this system, and closer analysis indicates closely related physics underlying orbital transitions in $\mathrm{FeV_2O_4}$ and $\mathrm{MnV_2O_4}$.
    03/2014;
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    ABSTRACT: Results of magnetic field and temperature dependent neutron diffraction and magnetization measurements on oxy-arsenate Rb$_{2}$Fe$_{2}$O(AsO$_{4}$)$_{2}$ are reported. The crystal structure of this compound contains pseudo-one-dimensional [Fe$_{2}$O$_{6}$]$^\infty$ sawtooth-like chains, formed by corner sharing isosceles triangles of $Fe^{3+}$ ions occupying two nonequivalent crystallographic sites. The chains extend infinitely along the crystallographic $b$-axis and are structurally confined from one another via diamagnetic (AsO$_{4}$)$^{3-}$ units along the $a$-axis, and Rb$^+$ cations along the $c$-axis direction. Neutron diffraction measurements indicate the onset of a long range antiferromagnetic order below approximately 25 K. The magnetic structure consists of ferrimagnetic chains which are antiferromagnetically coupled with each other. Within each chain, one of the two Fe sites carries a moment which lies along the \emph{b}-axis, while the second site bears a canted moment in the opposite direction. Externally applied magnetic field induces a transition to a ferrimagnetic state, in which the coupling between the sawtooth chains becomes ferromagnetic. Magnetization measurements performed on optically-aligned single crystals reveal evidence for an uncompensated magnetization at low magnetic fields that could emerge from to a phase-segregated state with ferrimagnetic inclusions or from antiferromagnetic domain walls. The observed magnetic states and the competition between them is expected to arise from strongly frustrated interactions within the sawtooth chains and relatively weak coupling between them.
    01/2014;
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    ABSTRACT: We report magnetic, transport, and neutron diffraction measurements as well as a doping study of the V-phase compound Zr4Fe4Si7. This compound exhibits collinear antiferromagnetic order below TN=98±1 K with a staggered moment of 0.57(3)μB/Fe as T→ 0. The magnetic order can be quenched with Co substitution to the Fe site, but even then a 1.5μB/Fe paramagnetic moment remains. The resistivity and heat capacity of Zr4Fe4Si7 are Fermi-liquid-like below 16 and 7 K, respectively, and reveal correlations on the scale of those observed in superconducting Fe pnictides and chalcogenides. Electronic structure calculations overestimate the ordered moment, suggesting the importance of dynamical effects. The existence of magnetic order, electronic correlations, and spin fluctuations make Zr4Fe4Si7 distinct from the majority of Fe-Si compounds, fostering comparison instead with the parent compounds of Fe-based superconductors.
    Physical Review B 08/2013; 88(8). · 3.66 Impact Factor
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    ABSTRACT: Simultaneous Rietveld refinements of X-ray and neutron powder diffraction patterns were applied to study the effect of Fe substitution on the crystal structure properties of the Ca5−x Fe x (PO4)3OH system (0 ≤ x ≤ 0.3). From variations of the Ca(1) and Ca(2) site occupancies and modifications of interatomic distances with x, it is inferred that Fe substitutes at both crystallographic sites with a preference at the Ca(2) site. Such partiality is attributed to similar geometries of the sixfold coordinated Fe with the sevenfold coordinated Ca(2). The expected overall decrease of the lattice constants in the iron-substituted samples is followed by an increasing trend with x that is explained in terms of local lattice distortions. Hematite forms as a secondary phase starting at x = 0.1 up to 3.7 wt% for x = 0.3. Transmission electron microscopy reveals a nanosystem consisting of 15–65 nm rods and spheres, while hematite nanoparticles are distinguishable for x ≥ 0.1. A transition of the diamagnetic hydroxyapatite to paramagnetic Fe-hydroxyapatite was found from magnetic measurements, while the antiferromagnetic hematite develops hysteresis loops for x > 0.1.
    Journal of Materials Science 05/2013; 48(9):3535. · 2.31 Impact Factor
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    ABSTRACT: We report a comprehensive study of dc susceptibility, specific heat, neutron diffraction, and inelastic neutron scattering measurements on polycrystalline Ba3(Cr1-xVx)2O8 samples, where x=0, 0.06, 0.15, and 0.53. A Jahn-Teller structure transition occurs for x=0, 0.06, and 0.15 samples and the transition temperature is reduced upon vanadium substitution from 70(2) K at x=0 to 60(2) K at x=0.06 and 0.15. The structure becomes less distorted as x increases and such transition disappears at x=0.53. The observed magnetic excitation spectrum indicates that the singlet ground state remains unaltered and spin gap energy \Delta=1.3(1) meV is identical within the instrument resolution for all x. In addition, the dispersion bandwidth W decreases with increase of x. At x=0.53, W is reduced to 1.4(1) meV from 2.0(1) meV at x=0.
    Physical Review B 04/2013; 87:144427. · 3.66 Impact Factor
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    ABSTRACT: We report the structural and magnetic properties of a new class of cobaltates with the chemical formula (BaSr)4-xLa2xCo4O15 (x = 0, 0.5 and 1). These compounds crystallize in a hexagonal structure in which cobalt ions are distributed among two distinct crystallographic sites with different oxygen coordination. Three Co-O tetrahedra and one octahedron are linked by shared oxygen atoms to form Co4O15 clusters, which are packed together into a honeycomb-like network. Partial substitution of Sr and/or Ba atoms by La allows one to adjust the degree of Co valence mixing, but all compositions remain subject to a random distribution of charge. Magnetic susceptibility together with neutron scattering measurements reveal that all studied specimens are characterized by competing ferro- and antiferro-magnetic exchange interactions that give rise to a three dimensional Heisenberg spin-glass state. Neutron spectroscopy shows a clear trend of slowing down of spin-dynamics upon increasing La concentration, suggesting a reduction in charge randomness in the doped samples.
    Physical Review B 09/2012; 86(9). · 3.66 Impact Factor
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    ABSTRACT: We present a neutron and x-ray scattering investigation of Ca3LiOsO6, a 5d material predicted to host magnetic ordering solely through an extended superexchange pathway involving two anions. This contrasts with investigations of extended superexchange interactions that have been largely limited to low-dimensional 3d systems involving both superexchange and extended superexchange. Despite the apparent one-dimensional nature and triangular units of magnetic osmium ions in Ca3LiOsO6, the onset of magnetic correlations has been observed at a high temperature of 117 K in bulk measurements. We experimentally determine the magnetically ordered structure and show it to be long range and three dimensional. Our results support the model of extended superexchange interaction.
    Physical Review B 06/2012; 86(5). · 3.66 Impact Factor
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    ABSTRACT: Magnetic order in the thermally quenched photomagnetic Prussian blue analogue coordination polymer K0.27Co[Fe(CN)6]0.73[D2O6]0.27 1.42D2O has been studied down to 4 K with unpolarized and polarized neutron powder diffraction as a function of applied magnetic field. Analysis of the data allows the onsite coherent magnetization of the Co and Fe spins to be established. Specifically, magnetic fields of 1 T and 4 T induce moments parallel to the applied field, and the sample behaves as a ferromagnet with a wandering axis.
    Physical review. B, Condensed matter 06/2012; 86(5). · 3.66 Impact Factor
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    ABSTRACT: 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 135K and V3+ orbitals order at 60K 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 temperatures obey `ice rules' more commonly associated with the frustrated rare-earth pyrochlore systems.
    Physical review. B, Condensed matter 04/2012; · 3.66 Impact Factor
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    ABSTRACT: We report a sequence of continuous phase transformations in iron telluride, Fe1+yTe (y~0.1), 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 = 70 K, a temperature where the discontinuous first order magneto-structural phase transition is found in systems with small y (< 0.06), the reduction of the lattice symmetry in Fe1.1Te only occurs at Ts = 63 K. Below TN = 57.5 K the long-range magnetic order develops, whose incommensurate wave vector Qm varies with temperature. Finally, at Tm ~ 45 K the system enters the low-T phase, where Qm is locked at (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.
    Physical Review B 02/2012; 85:085105. · 3.66 Impact Factor
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    ABSTRACT: 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 1D lattice). 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 NaOsO3 is coincident with the onset of long-range commensurate three dimensional magnetic order. Whilst candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT. We discuss our results in the light of recent reports of a Mott spin-orbit insulating state in other 5d oxides.
    Physical Review Letters 02/2012; 108(25). · 7.73 Impact Factor
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    ABSTRACT: The bi-layer perovskite, Sr3Ru2O7, has sparked a lot of interest because of the quantum critical behavior---related to a metamagnetic (magnetic field-tuned) phase transition. One of the key issues is related to the magnetism in the system. Here we report an investigation of the effects on magnetism resulting from chemical substitution in this compound. Our neutron scattering investigation reveals an unusual E-type antiferromagnetic (AFM) structure induced by Mn-substitution in the ground state of Sr3(Ru1-xMnx)2O7 (x = 0.16). The AFM structure exhibits a long-range order in ab-plane but almost only a single bilayer-thickness correlation along the c-direction, thus characterizing the system as a quasi-two-dimensional antiferromagnet while the AFM order parameter shows almost three-dimensional-like scaling character as T approaches TN (˜ 82 K). The magnetic moments are aligned along the c-axis with an upper limit of ˜ 0.70 μB/Ru site. The induced AFM order most likely results from the enhancement of super-exchange interactions rather than from structural distortions or from freezing of electronic instabilities due to the nesting character of Fermi surface in the parent compound.
    02/2012;
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    ABSTRACT: There is ongoing interest in the study of Ce-based compounds that are derivatives of certain structure types (e.g., ThCr2Si2and PbClF), as they are often associated with correlated electron phenomena. We will report results for a new system that falls into this category, CeRu2Al2B. This compound crystallizes in a filled variant of the layered/tetragonal CeMg2Si2 structure. Contrary to what is often observed for Ce-based compounds, we find pronounced local moment behavior of the Ce ions, resulting in complicated magnetic ordering at strikingly high temperatures: i.e., antiferromagnetism (AFM) at TN = 14.1 K followed by ferromagnetism (FM) at TC = 12.8 K, which is first order in character. We also find a temperature-magnetic field phase diagram that consists of three distinct ordered phases: (1) AFM, (2) spin reoriented, and (3) FM. Since this type of behavior is unusual for Ce-based compounds, we will discuss prospects for suppressing the ordered state toward T = 0 in order to produce a Doniach-like phase diagram, which may provide a route towards a FM quantum critical point.
    Physical review. B, Condensed matter 02/2012; · 3.66 Impact Factor
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    ABSTRACT: We will report results for single crystals of a new Ce-based local moment ferromagnet, CeRu2Ga2B. Electrical resistivity, magnetization, and magnetic susceptibility measurements reveal ferromagnetism and hysteresis around TC = 16.1 K, while specific heat measurements uncover a huge anomaly at the phase transition, which results in a large discontinuity in the magnetic entropy (δSmag = 1.7 J/mol-K). Taken together, these data show that CeRu2Ga2B undergoes a first order ferromagnetic phase transition at a surprisingly high temperature. Since this type of behavior is unusual for Ce-based compounds, we additionally undertook an effort to tune the magnetic state using pressure. We will present electrical resistivity measurements under applied pressures and the resulting temperature-pressure phase diagram, with an emphasis on implications for possible nearness to a ferromagnetic quantum critical point.
    02/2012;
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    ABSTRACT: The vanadium spinels, AV2O4, with divalent cations on the diamond sublattice are model magnetic systems for the study of interacting orbital, lattice and spin degrees of freedom. Studies of both systems with diamagnetic (e.g. Zn^2+, Cd^2+, Mg^2+) and spin-only (e.g. Mn^2+) cations on the A-site sublattice have revealed multiple phase transitions and ground state properties heavily influenced by V^3+ orbital degrees-of-freedom. I will report on neutron powder diffraction measurements of another spinel system, FeV2O4, which additionally has two-fold orbitally degenerate Fe^2+ cations on the A-site sublattice. Previous x-ray and Mossbauer studies have reported four structural phase transitions in this material and at least one magnetic transition. Our data confirm the existence of three structural transitions and reveal distortions of local polyhedra with important implications for orbital order. We confirm the existence of hypothesized collinear antiferromagnetism below a temperature TN1=110K and further identify a second magnetic transition at TN2=60K where V^3+ moments cant away from the Fe^2+ spin direction to form a 2-in-2-out spin structure on the pyrochlore sublatice. I will discuss these observations in the context of recent predictions for orbital order in vanadate spinels.
    02/2012;
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    ABSTRACT: Multi-substituted hydroxyapatite (HAp) with crystallite size 4-130 nm is the major mineral phase in physiological apatites. Substitutions at all ionic sites affect their physicochemical properties. Fe is one of the minor substitutions at the Ca sites of HAp. It is important because it reduces the solubility of HAp, functioning as a cavities preventive agent, whereas Fe overload leads to a decreased mechanical strength and osteoporosis. Powder x-ray and neutron diffraction methods as well as energy-filtered transmission electron microscopy were used to study the effect of Fe substitution on the crystal structure properties of the Ca(5-x)Fex(PO4)3OH systems. Single phase HAp is identified in systems with x <= 0.1. Hematite is formed for higher x. Simultaneous Rietveld refinement of the x-ray and neutron diffraction patterns reveals an unexpected increase of the a-lattice constant. It is attributed to the increase of the Ca1-O3 and Ca2-O1 interatomic distances indicating a local lattice relaxation. Fe substitutes in both Ca1 and Ca2 sites with a preference to the Ca2 site and an occupancy up to 0.05 for x=0.3. Magnetic measurements reveal a transition from the diamagnetic state of the HAp to the paramagnetic of the Fe-doped systems.
    02/2012;