V. O. Garlea

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

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Publications (117)331.13 Total impact

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    ABSTRACT: We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo2As2 (A = Eu, Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (<0.1 electron) change in the population of the 3d orbitals. The mixed valence state of Eu observed in the high-pressure (HP) form of EuCo2As2 exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo2As2, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo2As2, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer due to chemical compression of Eu sites in Ca0.9Eu0.1Co1.91As2 or direct electron doping in Ca0.85La0.15Co1.89As2. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in material's properties via involvement of strongly correlated electrons.
    No preview · Article · Feb 2016 · Journal of the American Chemical Society
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    ABSTRACT: The low energy spin excitation spectrum of the breathing pyrochlore Ba3Yb2Zn5O11 has been investigated with inelastic neutron scattering. Several nearly resolution limited modes with no observable dispersion are observed at 250 mK while, at elevated temperatures, transitions between excited levels become visible. To gain deeper insight, a theoretical model of isolated Yb3+ tetrahedra parametrized by four anisotropic exchange constants is constructed. The model reproduces the inelastic neutron scattering data, specific heat, and magnetic susceptibility with high fidelity. The fitted exchange parameters reveal a Heisenberg antiferromagnet with a very large Dzyaloshinskii-Moriya interaction. Using this model, we predict the appearance of an unusual octupolar paramagnet at low temperatures and speculate on the development of inter-tetrahedron correlations.
    No preview · Article · Jan 2016
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    ABSTRACT: Polycrystalline samples of the quarter-doped manganites $R_{0.75}$Ca$_{0.25}$MnO$_3$ ($R$ = Y, Tb, Dy, Ho, and Er) were studied by X-ray diffraction and AC/DC susceptibility measurements. All five samples are orthorhombic and exhibit similar magnetic properties: enhanced ferromagnetism below $T_1$ ($\sim80$ K) and a spin glass (SG) state below $T_{SG}$ ($\sim30$ K). With increasing $R^{3+}$ ionic size, both $T_1$ and $T_{SG}$ generally increase. The single crystal neutron diffraction results on Tb$_{0.75}$Ca$_{0.25}$MnO$_3$ revealed that the SG state is mainly composed of a short-range ordered version of a novel canted (i.e. noncollinear) antiferromagnetic spin state. Furthermore, calculations based on the double exchange model for quarter-doped manganites reveal that this new magnetic phase provides a transition state between the ferromagnetic state and the theoretically predicted spin-orthogonal stripe phase.
    No preview · Article · Jan 2016
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    P. Ari-Gur · V.O. Garlea · H. Cao · Y. Ge · I. Aaltio · S.P. Hannula · V. Koledov
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    ABSTRACT: Heusler alloys of Ni-Mn-Ga compositions demonstrate ferromagnetic shape memory effect in the martensitic state. The transformation temperature and the chemical order depend strongly on the composition. In the current work, the structure and chemical order of the martensitic phase of Ni1.91Mn1.29Ga0.8 were studied using neutron diffraction; the diffraction pattern was refined using the FullProf software. It was determined that the structural transition occurs around 330K. At room temperature, 300K, which is below the martensite transformation temperature, all the Bragg reflections can be described by a monoclinic lattice with a symmetry of space group P 1 2/m 1 and lattice constants of a = 4.23047(7) [Å], b = 5.58333(6) [Å], c = 21.0179(2) [Å], beta = 90.328(1). The chemical order is of critical importance in these alloys, and it was previously studied at 363K. Analysis of the neutron diffraction in the monoclinic phase shows that the chemical order is maintained during the martensitic transformation.
    Preview · Article · Dec 2015
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    Michael A. McGuire · V. Ovidiu Garlea
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    ABSTRACT: The magnetic properties of the layered oxypnictide LaMnAsO have been revisited using neutron scattering and magnetization measurements. The present measurements identify the N\'{e}el temperature $T_N$ = 360(1) K. Below $T_N$ the critical exponent describing the magnetic order parameter is $\beta$ = 0.33$-$0.35, consistent with a three dimensional Heisenberg model. Above this temperature, diffuse magnetic scattering indicative of short-range magnetic order is observed, and this scattering persists up to $T_{SRO}$ = 650(10) K. The magnetic susceptibility shows a weak anomaly at $T_{SRO}$ and no anomaly at $T_N$. Analysis of the diffuse scattering data using a reverse Monte Carlo algorithm indicates that above $T_N$ nearly two- dimensional, short-range magnetic order is present with a correlation length of 9.3(3) {\AA} within the Mn layers at 400 K. The inelastic scattering data reveal a spin-gap of 3.5 meV in the long-range ordered state, and strong, low-energy (quasi-elastic) magnetic excitations emerging in the short-range ordered state. Comparison with other related compounds correlates the distortion of the Mn coordination tetrahedra to the sign of the magnetic exchange along the layer-stacking direction, and suggests that short-range order above $T_N$ is a common feature in the magnetic behavior of layered Mn-based pnictides and oxypnictides.
    Preview · Article · Dec 2015
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    ABSTRACT: The structural and the magnetic properties of CeCu$_{6-x}$Ag$_x$ (0 $\leq$ $x$ $\leq$ 0.85) and CeCu$_{6-x}$Pd$_x$ (0 $\leq$ $x$ $\leq$ 0.4) have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), heat capacity, x-ray diffraction measurements and first principles calculations. The structural and magnetic phase diagrams of CeCu$_{6-x}$Ag$_x$ and CeCu$_{6-x}$Pd$_x$ as a function of Ag/Pd composition are reported. The end member, CeCu$_6$, undergoes a structural phase transition from an orthorhombic ($Pnma$) to a monoclinic ($P2_1/c$) phase at 240 K. In CeCu$_{6-x}$Ag$_x$, the structural phase transition temperature (${T_{s}}$) decreases linearly with Ag concentration and extrapolates to zero at $x_{S}$ $\approx$ 0.1. The structural transition in CeCu$_{6-x}$Pd$_x$ remains unperturbed with Pd substitution within the range of our study. The lattice constant $b$ slightly decreases with Ag/Pd doping, whereas, $a$ and $c$ increase with an overall increase in the unit cell volume. Both systems, CeCu$_{6-x}$Ag$_x$ and CeCu$_{6-x}$Pd$_x$, exhibit a magnetic quantum critical point (QCP), at $x$ $\approx$ 0.2 and $x$ $\approx$ 0.05 respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector ($\delta_1$ 0 $\delta_2$) where $\delta_1 \sim 0.62$, $\delta_2 \sim 0.25$, $x$ = 0.125 for CeCu$_{6-x}$Pd$_x$ and $\delta_1 \sim 0.64$, $\delta_2 \sim 0.3$, $x$ = 0.3 for CeCu$_{6-x}$Ag$_x$. The magnetic structure consists of an amplitude modulation of the Ce-moments which are aligned along the $c$-axis of the orthorhombic unit cell.
    Full-text · Article · Oct 2015 · Physical Review B
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    ABSTRACT: Large single crystals of Ba2V(VO4)2(OH) (I) and Sr2V(VO4)2(OH) (II) are hydrothermally synthesized from mixtures of BaO or SrO, V2O3, V2O5, and aqueous NaOH as a mineralizer solution (Ag ampules, autoclave, 1.2 kbar, 650 °C, 10 d).
    No preview · Article · Sep 2015 · ChemInform

  • No preview · Article · Sep 2015
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    ABSTRACT: Ternary intermetallics, A2Co12As7 (A=Ca, Y, Ce–Yb), have been synthesized by annealing mixtures of elements in molten Bi at 1223 K. The materials obtained crystallize in the P63/m variant of the Zr2Fe12P7 structure type. The unit cell volume shows a monotonic decrease with the increasing atomic number of the rare-earth metal, with the exception of Ce-, Eu-, and Yb-containing compounds. An examination of these outliers with X-ray absorption near edge structures (XANES) spectroscopy revealed mixed valence of Ce, Eu, and Yb, with the average oxidation states of +3.20(1), +2.47(5), and +2.91(1), respectively, at room temperature. Magnetic behavior of A2Co12As7 is generally characterized by ferromagnetic ordering of Co 3d moments at 100–140 K, followed by low-temperature ordering of rare-earth 4f moments. The 3d-4f magnetic coupling changes from antiferromagnetic for A=Pr–Sm to ferromagnetic for A=Ce and Eu–Yb. Polarized neutron scattering experiments were performed to support the postulated ferro- and ferrimagnetic ground states for Ce2Co12As7 and Nd2Co12As7, respectively.
    No preview · Article · Aug 2015
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    ABSTRACT: We determine the low-field ordered magnetic phases of the S=1 dimerized antiferromagnet Ba3Mn2O8 using single-crystal neutron diffraction. We find that for magnetic fields between μ0H=8.80T and 10.56T applied along the [11¯0] direction the system exhibits spin density wave order with incommensurate wave vectors of type (η,η,ε). For μ0H>10.56T, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the [hh0] direction. For both field-induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. The nature of these two transitions is fundamentally different: the low-field transition is a second-order transition to a spin density wave ground state, while the one at higher field, toward the spiral phase, is of first order. © 2015 American Physical Society.
    No preview · Article · Jul 2015 · Physical Review B
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    ABSTRACT: The ferrimagnetic spinel $CoV_2O_4$ has been a topic of intense recent interest, both as a frustrated insulator with unquenched orbital degeneracy and as a near-itinerant magnet which can be driven metallic with moderate applied pressure. Central outstanding questions include the number and form of magnetic transitions in this material, and the absence of any degeneracy breaking structural phase transition, contrary to all available models. To help address these questions, we have performed a series of neutron diffraction and inelastic scattering measurements on $CoV_2O_4$ powders with minimal cation site disorder. Our data indicate a near ideal cubic spinel structure at all temperatures, and a ferrimagnetic spin state below $T_N = 156 K$, consistent with previous reports. Significantly however, we also provide strong evidence for a weak ($\frac{\Delta a}{a} \sim 10^{-4}$), first order structural phase transition at $T^*$ = 90 K, the same temperature where spin canting is seen in recent single crystal measurements. This transition is characterized by a short-range distortion of oxygen octahedral positions, and a weak $\Delta\sim 1.25 meV$ spin gap is observed in low temperature inelastic data. Together, these findings provide strong support for the local orbital picture and the existence of an orbital glass state below $T^*$. We further rule out in our sample the presence of additional structural or magnetic transitions at lower temperatures, contrary to other studies.
    Full-text · Article · Jul 2015
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    ABSTRACT: 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.
    No preview · Article · Jul 2015 · Inorganic Chemistry
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    ABSTRACT: We have investigated polycrystalline samples of the zigzag chain system BaTb$_2$O$_4$ with a combination of magnetic susceptibility, heat capacity, neutron powder diffraction, and muon spin relaxation measurements. Despite the onset of Tb$^{3+}$ short-range antiferromagnetic correlations at $|\theta_{CW}|$ $=$ 18.5 K and a very large effective moment, our combined measurements indicate that BaTb$_2$O$_4$ remains paramagnetic down to 0.095 K. The magnetic properties of this material show striking similarities to the pyrochlore antiferromagnet Tb$_2$Ti$_2$O$_7$, and therefore we propose that BaTb$_2$O$_4$ is a new large moment spin liquid candidate.
    Full-text · Article · Feb 2015 · Physical Review B
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    ABSTRACT: We describe why Ising spin chains with competing interactions in ${\mathrm{SrHo}}_{2}{\mathrm{O}}_{4}$ segregate into ordered and disordered ensembles at low temperatures $(T)$. 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 ${T}_{\mathrm{N}}=0.68(2)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, the N\'eel chains develop three-dimensional long range order (LRO), which arrests further thermal equilibration of the double-N\'eel chains so they remain in a disordered incommensurate state for $T$ below ${T}_{\mathrm{S}}=0.52(2)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. ${\mathrm{SrHo}}_{2}{\mathrm{O}}_{4}$ distills an important feature of incommensurate low dimensional magnetism: kinetically trapped topological defects in a $\text{quasi}$-${}d$-${}\text{dimensional}$ spin system can preclude order in $d+1$ dimensions.
    No preview · Article · Feb 2015 · Physical Review B
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    ABSTRACT: HYSPEC is a high-intensity, direct-geometry time-of-flight spectrometer at the Spallation Neutron Source, optimized for measurement of excitations in small single-crystal specimens with optional polarization analysis capabilities. The incident neutron beam is monochromated using a Fermi chopper with short, straight blades, and is then vertically focused by Bragg scattering onto the sample position by either a highly oriented pyrolitic graphite (unpolarized) or a Heusler (polarized) crystal array. Neutrons are detected by a bank of 3He tubes that can be positioned over a wide range of scattering angles about the sample axis. HYSPEC entered the user program in February 2013 for unpolarized experiments, and is already experiencing a vibrant research program. Polarization analysis will be accomplished by using the Heusler crystal array to polarize the incident beam, and either a 3He spin filter or a supermirror wide-angle polarization analyser to analyse the scattered beam. The 3He spin filter employs the spin-exchange optical pumping technique. A 60- wide angle 3He cell that matches the detector coverage will be used for polarization analysis. The polarized gas in the post-sample wide angle cell is designed to be periodically and automatically refreshed with an adjustable pressure of polarized gas, optically pumped in a separate cell and then transferred to the wide angle cell. The supermirror analyser has 960 supermirror polarizers distributed over 60-, and has been characterized at the Swiss Spallation Neutron Source. The current status of the instrument and the development of its polarization analysis capabilities are presented.
    Preview · Article · Jan 2015 · The European Physical Journal Conferences
  • M. R. Koehler · V. O. Garlea · M.A. McGuire · L. Jia · V. Keppens
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    ABSTRACT: Tb6FeBi2 adopts a noncentrosymmetric crystal structure and orders ferromagnetically at T-C1 = 250 K with an additional magnetic transition at T-C2 = 60 K. The low temperature magnetoelastic response in this material is strong, and is enhanced by cobalt substitution. Here, the temperature dependence of the atomic and magnetic structure of Tb6Fe1-xCoxBi2 (x = 0, 0.125, 0.25, and 0.375) is reported from powder X-ray diffraction (XRD) and powder neutron diffraction (PND) measurements. Below the Neel temperature a ferrimagnetic ordering between the terbium and iron moments exists in all compounds studied. Related to the enhanced magnetostructural response, the Co-doped compounds undergo a crystallographic phase transition below about 60 K. This transition also involves a canting of the magnetic moments away from the c-axis. The structural transition is sluggish and not fully completed in the parent Tb6FeBi2 compound, where a mixture of monoclinic and hexagonal phases is identified below 60 K. The spin reorientation transition is discussed in terms of competing exchange interactions and magnetocrystalline anisotropies of the two Tb sites and Fe/Co sublattices.
    No preview · Article · Dec 2014 · Journal of Alloys and Compounds
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    ABSTRACT: The $A{R}_{2}{\text{O}}_{4}$ family ($R=\text{rare}$ earth) has 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 ${\mathrm{BaNd}}_{2}{\mathrm{O}}_{4}$ with a combination of magnetization, heat-capacity, and neutron powder diffraction measurements. Magnetic Bragg peaks are observed below ${T}_{N}=1.7\phantom{\rule{4pt}{0ex}}\mathrm{K}$, and they can be indexed with a propagation vector of $\stackrel{P\vec}{k}=(0,1/2,1/2)$. The signal from magnetic diffraction is well described by long-range ordering of only one of the two types of Nd zigzag chains, with collinear up-up-down-down intrachain spin configurations (double N\'eel state). Furthermore, low-temperature magnetization and heat-capacity measurements reveal two magnetic-field-induced spin transitions at 2.75 and 4 T for $T=0.46\phantom{\rule{4pt}{0ex}}\mathrm{K}$. The high-field phase is paramagnetic, while the intermediate-field state may arise from a spin transition of the long-range ordered Nd chains. One possible candidate for the field-induced ordered state corresponds to an up-up-down intrachain spin configuration, as predicted for a classical ${J}_{1}$-${}{J}_{2}$ Ising chain with a double N\'eel ground state in zero field.
    Full-text · Article · Oct 2014 · Physical Review B
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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.
    No preview · Article · Aug 2014 · Journal of Physics Condensed Matter
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    ABSTRACT: Quaternary phases La1–xNdxCo2P2 (x = 0, 0.12, 0.25, 0.37, 0.50, 0.63, 0.75, 0.88, 1.0) have been synthesized from Sn flux to investigate the origins of drastic differences in properties between ferromagnetic LaCo2P2 and antiferromagnetic NdCo2P2. Powder and single-crystal X-ray diffraction indicate that all La1–xNdxCo2P2 samples are isostructural and crystallize in the ThCr2Si2 structure type. The unit cell parameters and volume change non-linearly with the Nd content (x), with the x < 0.50 samples being closer to LaCo2P2 and the ones with x > 0.50 being closer to NdCo2P2. These structural differences are also reflected in the magnetic behavior. The samples with lower Nd content are characterized by ferromagnetic ordering in the Co sublattice with the TC increasing from 132 K for x = 0 to 262 K for x = 0.50, while the samples with higher Nd content exhibit suppressed magnetization in the Co sublattice and canted antiferromagnetic ordering with TC ~ 270 K. Refinement of neutron powder diffraction patterns for x = 0.50 and 0.75 reveals a gradual ordering of the Nd 4f moments under the influence of Co 3d moments below 100 K. At low temperatures and zero field, these samples exhibit antiferromagnetic ordering of both Nd and Co magnetic moments, but under applied field they demonstrate the stabilization of a ferrimagnetic state with antiparallel alignment of the 4f and 3d moments, as indicated by isothermal magnetization measurements. The re-entrant ferrimagnetic transition is also observed in samples with x > 0.50 if the temperature is lowered below 5 K. The occurrence of this low-temperature magnetic transition was confirmed by alternating-current susceptibility measurements.
    Full-text · Article · Jul 2014
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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.
    Full-text · Article · Jul 2014

Publication Stats

759 Citations
331.13 Total Impact Points

Institutions

  • 2007-2015
    • Oak Ridge National Laboratory
      • Neutron Scattering Science Division
      Oak Ridge, Florida, United States
  • 2010-2014
    • McMaster University
      • Department of Chemistry and Chemical Biology
      Hamilton, Ontario, Canada
  • 2013
    • Princeton University
      • Department of Chemistry
      Princeton, New Jersey, United States
  • 2006-2013
    • University Joseph Fourier - Grenoble 1
      • Institut Néel
      Grenoble, Rhône-Alpes, France
  • 2012
    • Los Alamos National Laboratory
      • Lujan Neutron Scattering Center
      Лос-Аламос, California, United States
  • 2005-2009
    • Iowa State University
      • • Ames Laboratory
      • • Department of Physics and Astronomy
      Ames, IA, United States