Tapan Chatterji

Publications of Tapan Chatterji

• Magnetoelastic effects in Jahn-Teller distorted CrF₂ and CuF₂ studied by neutron powder diffraction.

  Authors: Tapan Chatterji, Thomas C Hansen

  Journal of physics. Condensed matter : an Institute of Physics journal. 07/2011; 23(27):276007.

  We have studied the temperature dependence of the crystal and magnetic structures of the Jahn-Teller distorted transition metal difluorides CrF₂ and CuF₂ by neutron powder diffraction in theWe have studied the temperature dependence of the crystal and magnetic structures of the Jahn-Teller distorted transition metal difluorides CrF₂ and CuF₂ by neutron powder diffraction in the temperature range 2-280 K. The lattice parameters and the unit cell volume show magnetoelastic effects below the Néel temperature. The lattice strain due to the magnetostriction effect couples with the square of the order parameter of the antiferromagnetic phase transition. We also investigated the temperature dependence of the Jahn-Teller distortion, which does not show any significant effect at the antiferromagnetic phase transition but increases linearly with increasing temperature for CrF₂, and remains almost independent of temperature in CuF₂. The magnitude of magnetovolume effect seems to increase with the low temperature saturated magnetic moment of the transition metal ions but the correlation is not at all perfect.

• Structural and thermal properties of LaMnO3 from neutron diffraction and first principles studies.

  Authors: Urszula D Wdowik, Bachir Ouladdiaf, Tapan Chatterji

  Journal of physics. Condensed matter : an Institute of Physics journal. 06/2011; 23(24):245402.

  Neutron diffraction experiments have been performed on powder samples of LaMnO(3) below and above the Jahn-Teller transition temperature of 750 K. Experimental investigations are assisted by densityNeutron diffraction experiments have been performed on powder samples of LaMnO(3) below and above the Jahn-Teller transition temperature of 750 K. Experimental investigations are assisted by density functional theory calculations. Theoretical studies are carried out for the orbitally ordered state of LaMnO(3) which allows one to compare the behavior of the orbitally ordered and disordered structures as a function of temperature. The temperature dependences of the structural parameters characterizing the Jahn-Teller distortions are reported and discussed. A gradual departure of the experimental data from theoretical predictions is observed above 650 K. In this range of temperatures, anions surrounding the Jahn-Teller active cations perform more isotropic thermal motion. The onset of structural phase transition induces a reduction of the crystal volume by about 0.4% which follows from the structural transformations yielding more regular oxygen octahedra formed above the phase transformation. It is found that above the Jahn-Teller transition the distortions of the MnO(6) octahedra are not completely removed. The non-vanishing distortions are accompanied by the lifted degeneracy of the Mn e(g) states. Weak residual distortions can be assigned to the short-range orbital order that persists within a local scale but it seems quenched on average giving rise to a disappearance of the long-range order coherency of the Jahn-Teller effect.

• Magnetoelastic effects in Jahn-Teller distorted CrF$_2$ and CuF$_2$ studied by neutron powder diffraction

  Authors: Tapan Chatterji, Thomas C Hansen

  06/2011;

  We have studied the temperature dependence of crystal and magnetic structures of the Jahn-Teller distorted transition metal difluorides CrF$_2$ and CuF$_2$ by neutron powder diffraction in theWe have studied the temperature dependence of crystal and magnetic structures of the Jahn-Teller distorted transition metal difluorides CrF$_2$ and CuF$_2$ by neutron powder diffraction in the temperature range 2-280 K. The lattice parameters and the unit cell volume show magnetoelastic effects below the N\'eel temperature. The lattice strain due to the magnetostriction effect couples with the square of the order parameter of the antiferromagnetic phase transition. We also investigated the temperature dependence of the Jahn-Teller distortion which does not show any significant effect at the antiferromagnetic phase transition but increases linearly with increasing temperature for CrF$_2$ and remains almost independent of temperature in CuF$_2$. The magnitude of magnetovolume effect seems to increase with the low temperature saturated magnetic moment of the transition metal ions but the correlation is not at all perfect.

• Negative thermal expansion in ZnF$_2$

  Authors: Tapan Chatterji, Mohamed Zbiri, Thomas C Hansen

  04/2011;

  We have investigated temperature dependence of the lattice parameters and the unit cell volume of ZnF$_2$ by neutron diffraction and have discovered negative thermal expansion (NTE) at lowWe have investigated temperature dependence of the lattice parameters and the unit cell volume of ZnF$_2$ by neutron diffraction and have discovered negative thermal expansion (NTE) at low temperature. To understand why this simple compound exhibits NTE we performed first principle calculations. These calculations reproduce qualitatively the experimental temperature dependence of volume.

• Metamagnetism and soliton excitations in the modulated ferromagnetic Ising chain CoV2O6

  Authors: Simon A. J. Kimber, Hannu Mutka, Tapan Chatterji, Tommy Hoffman, Paul F Henry, Heloisa N Bordallo, Dimitri N. Argyriou, J Paul Attfield

  08/2010;

  We report a combination of physical property and neutron scattering measurements for polycrystalline samples of the one-dimensional spin chain compound CoV2O6. Heat capacity measurements show that anWe report a combination of physical property and neutron scattering measurements for polycrystalline samples of the one-dimensional spin chain compound CoV2O6. Heat capacity measurements show that an effective S = 1/2 state is found at low temperatures and that magnetic fluctuations persist up to 6.Tn. Above Tn = 6.3 K, measurements of the magnetic susceptibility as a function of T and H show that the nearest neighbour exchange is ferromagnetic. In the ordered state, we have discovered a crossover from a metamagnet with strong fluctuations between 5 K and Tn to a state with a 1/3 magnetisation plateau at 2 < T < 5 K. We use neutron powder diffraction measurements to show that the AFM state has incommensurate long range order and inelastic time of flight neutron scattering to examine the magnetic fluctuations as a function of temperature. Above Tn, we find two broad bands between 3.5 and 5 meV and thermally activated low energy features which correspond to transitions within these bands. These features show that the excitations are deconfined solitons rather than the static spin reversals predicted for a uniform FM Ising spin chain. Below Tn, we find a ladder of states due to the confining effect of the internal field. A region of weak confinement below Tn, but above 5 K, is identified which may correspond to a crossover between 2D and 3D magnetic ordering.

• Magnetic lattice dynamics of the oxygen-free FeAs pnictides: how sensitive are phonons to magnetic ordering?

  Authors: Mohamed Zbiri, Ranjan Mittal, Stéphane Rols, Yixi Su, Yinguo Xiao, Helmut Schober, Samrath L Chaplot, Mark R Johnson, Tapan Chatterji, Yasunori Inoue, Satoru Matsuishi, Hideo Hosono, Thomas Brueckel

  Journal of physics. Condensed matter : an Institute of Physics journal. 08/2010; 22(31):315701.

  To shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperatureTo shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperature orthorhombic parent compounds, which present a spin density wave. The study covers both the 122 (AFe(2)As(2); A = Ca, Sr, Ba) and 1111 (AFeAsF; A = Ca, Sr) phases. We extend our recent work on the Ca (122 and 1111) and Ba (122) cases by treating, computationally and experimentally, the 122 and 1111 Sr compounds. The effect of magnetic ordering is investigated through detailed non-magnetic and magnetic lattice dynamical calculations. The comparison of the experimental and calculated phonon spectra shows that the magnetic interactions/ordering have to be included in order to reproduce well the measured density of states. This highlights a spin-correlated phonon behavior which is more pronounced than the apparently weak electron-phonon coupling estimated in these materials. Furthermore, there is no noticeable difference between phonon spectra of the 122 Ba and Sr, whereas there are substantial differences when comparing these to CaFe(2)As(2) originating from different aspects of structure and bonding.

• Magnetoelastic effect in MF2 (M = Mn, Fe, Ni) investigated by neutron powder diffraction.

  Authors: Tapan Chatterji, Gail N Iles, Bachir Ouladdiaf, Thomas C Hansen

  Journal of physics. Condensed matter : an Institute of Physics journal. 08/2010; 22(31):316001.

  We have investigated the magnetoelastic effects in MF(2) (M = Mn, Fe, Ni) associated with the antiferromagnetic phase transition temperature T(N) by neutron powder diffraction. The temperatureWe have investigated the magnetoelastic effects in MF(2) (M = Mn, Fe, Ni) associated with the antiferromagnetic phase transition temperature T(N) by neutron powder diffraction. The temperature variation of the lattice parameters and the unit cell volume has been determined accurately with small temperature steps. From the temperature variation of the lattice parameters a, c and V the lattice strains Δa, Δc and ΔV associated with the antiferromagnetic phase transition have been extracted. Rietveld refinement of the crystal and magnetic structures from the diffraction data at low temperature gave a magnetic moment of 5.12 ± 0.09 μ(B), 4.05 ± 0.05 μ(B) and 1.99 ± 0.05 μ(B) per Mn, Fe and Ni ions, respectively. The lattice strains Δa, Δc and ΔV couple linearly with the intensity of the 100 magnetic reflection, which is proportional to square of the order parameter of the antiferromagnetic phase transition. The volume strains in MF(2) (M = Mn, Fe, Co, Ni) due to the magnetostriction vary smoothly along the transition metal series and seem to be correlated with the strength of the exchange interaction and the moments of the magnetic ions.

• The magnetoelastic effect in CoF2 investigated by means of neutron powder diffraction.

  Authors: Tapan Chatterji, Bachir Ouladdiaf, Thomas C Hansen

  Journal of physics. Condensed matter : an Institute of Physics journal. 03/2010; 22(9):096001.

  We have investigated the magnetoelastic effects in CoF(2) associated with the antiferromagnetic phase transition temperature T(N)≈39 K by means of neutron powder diffraction. The temperatureWe have investigated the magnetoelastic effects in CoF(2) associated with the antiferromagnetic phase transition temperature T(N)≈39 K by means of neutron powder diffraction. The temperature variation of the lattice parameters and the unit cell volume has been determined accurately with small temperature steps. From the temperature variation of the lattice parameter c we extracted the lattice strain Δc associated with the antiferromagnetic phase transition. Rietveld refinement of the crystal and magnetic structure from the diffraction data at 2.2 K gave a magnetic moment of 2.57 ± 0.02 μ(B) per Co ion. We determined the temperature variation of the intensity of the 100 magnetic Bragg reflection, which is proportional to the square of the order parameter of the phase transition. We established that the lattice strain Δc couples linearly with the square of the order parameter of the antiferromagnetic phase transition in CoF(2).

• Similarities between structural distortions under pressure and chemical doping in superconducting BaFe2As2

  Authors: Simon A. J. Kimber, Andreas Kreyssig, Yu-Zhong Zhang, Harald O. Jeschke, Roser Valenti, Fabiano Yokaichiya, Estelle Colombier, Jiaqiang Yan, Thomas C Hansen, Tapan Chatterji, Robert J. McQueeney, Paul C. Canfield, Alan I. Goldman, Dimitri N. Argyriou

  12/2009;

  The discovery of a new family of high Tc materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent,The discovery of a new family of high Tc materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent, for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked, however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal to orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behavior under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.

• Non-stoichiometry and the magnetic structure of Sr2CrO3FeAs

  Authors: Marcus Tegel, Franziska Hummel, Yixi Su, Tapan Chatterji, Michela Brunelli, Dirk Johrendt

  11/2009;

  The iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to previous X-rayThe iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to previous X-ray crystallographic studies, a mixed occupancy of chromium and iron was found within the FeAs4/4 layer (93+/-1% Fe : 7+/-1% Cr). We suggest that the partial Cr-doping at the Fe site is the reason for the absence of a spin-density wave anomaly and superconductivity in this compound. Additional experiments via neutron polarization analysis revealed short-range spin correlations below ~100 K and long-range antiferromagnetic ordering below T_N = 36 K with a magnetic propagation vector of q = (1/2, 1/2, 0). The Cr3+ ions form a collinear magnetic structure of the C-type in the magnetic space group C_Pmma' (a' = a-b, b'=a+b, c'=c), where Cr3+-ions occupy the 4g (0, 1/4, z) Wyckoff position. The magnetic moments are aligned along the orthorhombic a'-axis. At 3.5 K, an ordered magnetic moment of 2.75+/-0.05 mu_B for the Cr3+-sublattice was refined. Comment: 6 pages, 6 figures

• Anomalous hyperfine interaction in CoF(2) investigated by high resolution neutron spectroscopy.

  Authors: Tapan Chatterji, G J Schneider

  Journal of physics. Condensed matter : an Institute of Physics journal. 10/2009; 21(43):436008.

  We investigated the low energy excitations in CoF(2) in the µeV range with a back-scattering neutron spectrometer. The energy scans on a CoF(2) powder sample revealed inelastic peaks at E = 0.728 ±We investigated the low energy excitations in CoF(2) in the µeV range with a back-scattering neutron spectrometer. The energy scans on a CoF(2) powder sample revealed inelastic peaks at E = 0.728 ± 0.008 µeV at T = 3.46 K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and finally merge with the elastic peak at the electronic magnetic ordering temperature T(N)≈37 K. We interpret the inelastic peaks to be due to the transition between the hyperfine-split nuclear level of the (59)Co isotopes with spin I = 7/2. We have shown that the energy of the inelastic peak or the hyperfine splitting in CoF(2) can be treated as an order parameter of the antiferromagnetic phase transition and yields the critical exponent β = 0.313 ± 0.007, consistent with the neutron diffraction results and also the three-dimensional Ising character of the magnetic system. The determined hyperfine splitting in CoF(2) deviates from the linear relationship between the ordered electronic magnetic moment and the hyperfine splitting in Co, Co-P amorphous alloys and CoO, presumably due to the presence of an unquenched orbital moment.

• Neutron diffraction investigation of the magnetic structure and magnetoelastic effects in NdMnO(3).

  Authors: Tapan Chatterji, Bachir Ouladdiaf, Dipten Bhattacharya

  Journal of physics. Condensed matter : an Institute of Physics journal. 07/2009; 21(30):306001.

  We investigated the evolution of the magnetic structure and magnetoelastic effects of NdMnO(3) by neutron powder diffraction. We confirmed the A-type antiferromagnetic (AF) structure of NdMnO(3)We investigated the evolution of the magnetic structure and magnetoelastic effects of NdMnO(3) by neutron powder diffraction. We confirmed the A-type antiferromagnetic (AF) structure of NdMnO(3) below T(N)≈82 K with magnetic moments parallel to the b axis of the orthorhombic crystal structure (space group Pbnm). We found that the magnetic moments of Nd order below about T(N)(Nd)≈20 K in a ferromagnetic structure with moments parallel to the c axis. At the same temperature the magnetic moments of Mn develop a ferromagnetic component parallel to the c axis. We found strong magnetoelastic effects associated with the AF transition at T(N)≈82 K. The effect is very prominent for the b lattice parameter. All three lattice parameters and therefore the unit cell volume contracts at T(N)≈82 K. We treated quantitatively the magnetoelastic effect of the b lattice parameter and found that the extra change in the lattice parameter Δb due to the magnetoelastic effect is proportional to the magnetic moment of the Mn ion. We also determined the critical exponent of the AF phase transition to be β = 0.296 ± 0.008.

• Similarities between structural distortions under pressure and chemical doping in superconducting BaFe(2)As(2).

  Authors: Simon A. J. Kimber, Andreas Kreyssig, Yuzhong Z Zhang, Harald O. Jeschke, Roser Valentí, Fabiano Yokaichiya, Estelle Colombier, Jiaqiang Yan, Thomas C Hansen, Tapan Chatterji, Robert J. McQueeney, Paul C. Canfield, Alan I. Goldman, Dimitri N. Argyriou

  Nature materials. 06/2009;

  The discovery of a new family of high-T(C) materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are nowThe discovery of a new family of high-T(C) materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe(2)As(2), namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe(2)As(2).

• Direct evidence for the Nd magnetic ordering in NdMnO(3) from the hyperfine field splitting of Nd nuclear levels.

  Authors: Tapan Chatterji, G J Schneider, L van Eijck, B Frick, Dipten Bhattacharya

  Journal of physics. Condensed matter : an Institute of Physics journal. 03/2009; 21(12):126003.

  We investigated the low energy excitations in NdMnO(3) in the µeV range by a backscattering neutron spectrometer. The energy spectra on polycrystalline NdMnO(3) samples revealed inelastic peaks at EWe investigated the low energy excitations in NdMnO(3) in the µeV range by a backscattering neutron spectrometer. The energy spectra on polycrystalline NdMnO(3) samples revealed inelastic peaks at E = 2.15 ± 0.01 µeV at T = 2 K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and tend to merge with the elastic peak at the electronic magnetic ordering temperature of Nd, T(Nd)≈20 K. However, at temperatures higher than T(Nd)≈20 K the energy of the inelastic peak decreases at a much slower rate and remains finite up to T = 55 K, the highest temperature investigated. We interpret the inelastic peaks to be due to the transition between the hyperfine-split nuclear level of the (143)Nd and (145)Nd isotopes with spin I = 7/2 caused by the magnetic ordering of Nd electronic moment below T(Nd)≈20 K. We ascribe the finite energy of the inelastic peak and its much smaller temperature dependence at T>20 K to be due to the polarization of the Nd magnetic moment by the field of Mn moments that order below T(N)≈78 K.

• Orbital order–disorder transition in La1−xBaxMnO3 in the low-doping region

  Authors: Tapan Chatterji, Bachir Ouladdiaf, P. Mandal, B. Ghosh

  Solid State Communications.

  We have investigated the Jahn–Teller transition accompanied by orbital order–disorder transition in La1−xBaxMnO3 by high temperature powder neutron diffraction in the low-doping region (0≤x≤0.1). TheWe have investigated the Jahn–Teller transition accompanied by orbital order–disorder transition in La1−xBaxMnO3 by high temperature powder neutron diffraction in the low-doping region (0≤x≤0.1). The Jahn–Teller transition temperature which is about for the stoichiometric LaMnO3 decreases drastically on doping with Ba. We have shown that the previously identified metrically cubic orthorhombic O-phase for x=0 is metrically orthorhombic and the orthorhombicity increases with doping. The Jahn–Teller transition is accompanied only by the partial reduction of the distortion of the MnO6 octahedra. From the distortion of the MnO6 octahedra we have determined the orbital mixing coefficients. Negative thermal expansion has been observed in stoichiometric LaMnO3 in a temperature range of about 75 K just below The negative thermal expansion has also been observed in La1−xBaxMnO3 below TJT but the effect is reduced with doping.