P G Radaelli

Publications (112)205.56 Total impact

• Article: Giant Tunability of Ferroelectric Polarization in GdMn_{2}O_{5}.

  N Lee, C Vecchini, Y J Choi, L C Chapon, A Bombardi, P G Radaelli, S-W Cheong
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  ABSTRACT: Giant tunability of ferroelectric polarization (ΔP=5000 μC/m^{2}) in the multiferroic GdMn_{2}O_{5} with external magnetic fields is discovered. The detailed magnetic model from x-ray magnetic scattering results indicates that the Gd-Mn symmetric exchange striction plays a major role in the tunable ferroelectricity of GdMn_{2}O_{5}, which is in distinction from other compounds of the same family. Thus, the highly isotropic nature of Gd spins plays a key role in the giant magnetoelectric coupling in GdMn_{2}O_{5}. This finding provides a new handle in achieving enhanced magnetoelectric functionality.
  Physical Review Letters 03/2013; 110(13):137203. · 7.37 Impact Factor
• Article: Magneto-orbital helices as a route to coupling magnetism and ferroelectricity in multiferroic CaMn(7)O(12).

  N J Perks, R D Johnson, C Martin, L C Chapon, P G Radaelli
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  ABSTRACT: Orbital physics drives a rich phenomenology in transition-metal oxides, providing the microscopic underpinning for effects such as Colossal Magnetoresistance. In particular, magnetic and lattice degrees of freedom are coupled through orbital ordering, and it has long been hoped that this coupling could be exploited to create high-temperature multiferroics with large values of the electrical polarization. Here we report an unprecedented magneto-orbital texture in multiferroic CaMn(7)O(12), found to give rise to the largest magnetically induced ferroelectric polarization measured to date. X-ray diffraction characterization of the structural modulation in these 'magneto-orbital helices', and analysis of magnetic exchange shows that orbital order is crucial in stabilising a chiral magnetic structure, thus allowing for electric polarization. Additionally, the presence of a global structural rotation enables the coupling between this polarization and magnetic helicity required for multiferroicity. These novel principles open up the possibility of discovering new multiferroics with even larger polarization and higher transition temperatures.
  Nature Communications 12/2012; 3:1277. · 7.40 Impact Factor
• Article: Magneto-orbital helices and multiferroicity

  N. J. Perks, R. D. Johnson, C. Martin, L. C. Chapon, P. G. Radaelli
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  ABSTRACT: Orbitally ordered states evolving coincidentally with structural distortions and magnetic ordering provide a novel route to controlling electronic and magnetic properties of materials through external fields. We report an unprecedented magneto-orbital helix in CaMn7O12, found to give rise to the largest magnetically induced ferroelectric polarisation measured to date. Characterisation of the structural modulation using x-ray diffraction, and analysis of magnetic exchange shows that orbital order is crucial in stabilising a chiral magnetic structure, thus allowing for electric polarisation. Additionally, the presence of a global structural rotation enables the coupling between this polarisation and magnetic helicity required for multiferrocitiy. These novel principles open up the possibility of discovering new high-temperature multiferroics.
  08/2012;
• Source

  Available from: ArXiv

  Article: Spin waves and revised crystal structure of honeycomb iridate Na2IrO3.

  S K Choi, R Coldea, A N Kolmogorov, T Lancaster, I I Mazin, S J Blundell, P G Radaelli, Yogesh Singh, P Gegenwart, K R Choi, S-W Cheong, P J Baker, C Stock, J Taylor
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  ABSTRACT: We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order below T(N)=15.3  K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90° Ir-O-Ir bonds required for dominant Kitaev exchange.
  Physical Review Letters 03/2012; 108(12):127204. · 7.37 Impact Factor
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  Available from: ArXiv

  Article: Giant improper ferroelectricity in the ferroaxial magnet CaMn7O12.

  R D Johnson, L C Chapon, D D Khalyavin, P Manuel, P G Radaelli, C Martin
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  ABSTRACT: In rhombohedral CaMn7O12, an improper ferroelectric polarization of magnitude 2870  μC  m(-2) is induced by an incommensurate helical magnetic structure that evolves below T(N1)=90 K. The electric polarization was found to be constrained to the high symmetry threefold rotation axis of the crystal structure, perpendicular to the in-plane rotation of the magnetic moments. The multiferroicity is explained by the ferroaxial coupling mechanism, which in CaMn7O12 gives rise to the largest magnetically induced, electric polarization measured to date.
  Physical Review Letters 02/2012; 108(6):067201. · 7.37 Impact Factor
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  Available from: ArXiv

  Article: Cu3Nb2O8: a multiferroic with chiral coupling to the crystal structure.

  R D Johnson, Sunil Nair, L C Chapon, A Bombardi, C Vecchini, D Prabhakaran, A T Boothroyd, P G Radaelli
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  ABSTRACT: By combining bulk properties, neutron diffraction, and nonresonant x-ray diffraction measurements, we demonstrate that the new multiferroic Cu(3)Nb(2)O(8) becomes polar simultaneously with the appearance of generalized helicoidal magnetic ordering. The electrical polarization is oriented perpendicularly to the common plane of rotation of the spins-an observation that cannot be reconciled with the conventional theory developed for cycloidal multiferroics. Our results are consistent with coupling between a macroscopic structural rotation, which is allowed in the paramagnetic group, and magnetically induced structural chirality.
  Physical Review Letters 09/2011; 107(13):137205. · 7.37 Impact Factor
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  Available from: ArXiv

  Article: Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering.

  G L Pascut, R Coldea, P G Radaelli, A Bombardi, G Beutier, I I Mazin, M D Johannes, M Jansen
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  ABSTRACT: We report resonant x-ray scattering measurements on a single crystal of the orbitally degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below T(S)=365  K. We observe a strong resonant enhancement of the supercell reflections through the Ni K edge. The empirically extracted K-edge shift between the crystallographically distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of ∼1  eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.
  Physical Review Letters 04/2011; 106(15):157206. · 7.37 Impact Factor
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  Available from: ArXiv

  Article: Helical spin-waves, magnetic order, and fluctuations in the langasite compound Ba3NbFe3Si2O14

  C. Stock, L. C. Chapon, A. Schneidewind, Y Su, P G Radaelli, D. F. McMorrow, A Bombardi, N Lee, S-W Cheong
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  ABSTRACT: We have investigated the spin fluctuations in the langasite compound Ba3NbFe3Si2O14 in both the ordered state and as a function of temperature. The low temperature magnetic structure is defined by a spiral phase characterized by magnetic Bragg peaks at q=(0,0,tau ~ 1/7) onset at TN=27 K as previously reported by Marty et al. The nature of the fluctuations and temperature dependence of the order parameter is consistent with a classical second order phase transition for a two dimensional triangular antiferromagnet. We will show that the physical properties and energy scales including the ordering wavevector, Curie-Weiss temperature, and the spin-waves can be explained through the use of only symmetric exchange constants without the need for the Dzyaloshinskii-Moriya interaction. This is accomplished through a set of ``helical" exchange pathways along the c direction imposed by the chiral crystal structure and naturally explains the magnetic diffuse scattering which displays a strong vector chirality up to high temperatures well above the ordering temperature. This illustrates a strong coupling between magnetic and crystalline chirality in this compound. Comment: 16 pages, 16 figures, submitted to Physical Review B
  Physical Review B 07/2010; · 3.69 Impact Factor
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  Available from: Alexandros Lappas

  Article: One-Dimensional Magnetic Fluctuations in the Spin-2 Triangular Lattice α-NaMnO_ {2}

  C Stock, L C Chapon, O Adamopoulos, A Lappas, M Giot, J W Taylor, M A Green, C M Brown, P G Radaelli
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  ABSTRACT: The S=2 anisotropic triangular lattice α-NaMnO2 is studied by neutron inelastic scattering. Antiferromagnetic order occurs at T ≤ 45 K with opening of a spin gap. The spectral weight of the magnetic dynamics above the gap (∆ ≃ 7.5 meV) has been analysed by the single-mode approximation. Excellent agreement with the experiment is achieved when a dominant exchange interaction (|J|/kB ≃73 K), along the monoclinic b-axis and a sizeable easy-axis magnetic anisotropy (|D|/kB ≃3 K) are considered. Despite earlier suggestions for two-dimensional spin interactions, the dynamics illustrate strongly coupled antiferromagnetic S=2 chains and cancellation of the interchain exchange due to the lattice topology. α-NaMnO2 therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.
  08/2009;
• Article: One-dimensional magnetic fluctuations in the spin-2 triangular lattice alpha-NaMnO2.

  C Stock, L C Chapon, O Adamopoulos, A Lappas, M Giot, J W Taylor, M A Green, C M Brown, P G Radaelli
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  ABSTRACT: The S=2 anisotropic triangular lattice alpha-NaMnO2 is studied by neutron inelastic scattering. Antiferromagnetic order occurs at T< or =45 K with opening of a spin gap. The spectral weight of the magnetic dynamics above the gap (Delta approximately equal to 7.5 meV) has been analyzed by the single-mode approximation. Excellent agreement with the experiment is achieved when a dominant exchange interaction (|J|/k(B) approximately 73 K), along the monoclinic b axis and a sizable easy-axis magnetic anisotropy (|D|/k(B) approximately 3 K) are considered. Despite earlier suggestions for two-dimensional spin interactions, the dynamics illustrate strongly coupled antiferromagnetic S=2 chains and cancellation of the interchain exchange due to the lattice topology. alpha-NaMnO2 therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.
  Physical Review Letters 08/2009; 103(7):077202. · 7.37 Impact Factor
• Article: Magnetic correlations in the extended kagome YBaCo4O7 probed by single-crystal neutron scattering.

  P Manuel, L C Chapon, P G Radaelli, H Zheng, J F Mitchell
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  ABSTRACT: We have studied the frustrated system YBaCo4O7.0 generally described as an alternating stacking of kagome and triangular layers of magnetic ions on a trigonal lattice, by single-crystal neutron diffraction experiments above the Néel ordering transition. Experimental data reveal pronounced magnetic diffuse scattering, which is successfully modeled by direct Monte Carlo simulations. Long-range magnetic correlations are found along the c axis, due to the presence of corner-sharing bipyramids, creating quasi-one-dimensional order at finite temperature. In contrast, in the kagome layers (ab plane), the spin-spin correlation function, displaying a short-range 120 degrees configuration, decays rapidly as typically found in spin liquids. YBaCo4O7 experimentally realizes a new class of two-dimensional frustrated systems where the strong out-of-plane coupling does not lift the in-plane degeneracy, but instead acts as an external "field."
  Physical Review Letters 08/2009; 103(3):037202. · 7.37 Impact Factor
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  Available from: John F. Mitchell

  Article: Magnetic correlations in YBaCo4O7 probed by single-crystal neutron scattering

  P Manuel, L. C. Chapon, P G Radaelli, H Zheng, J. F. Mitchell
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  ABSTRACT: We have studied the frustrated system YBaCo4O7 generally described as an alternating stacking of Kagome and triangular layers of magnetic ions on a trigonal lattice, by single crystal neutron diffraction experiments above the Neel ordering transition. Experimental data reveals pronounced magnetic diffuse scattering, which is successfully modeled by direct Monte-Carlo simulations. Long-range magnetic correlations are found along the c-axis, due to the presence of corner-sharing bipyramids, creating quasi one-dimensional order at finite temperature. In contrast, in the Kagome layers ab-plane, the spin-spin correlation function -displaying a short-range 120 degrees configuration- decays rapidly as typically found in spin-liquids. YBaCo4O7 experimentally realizes a new class of two-dimensional frustrated systems where the strong out-of-plane coupling does not lift the in-plane degeneracy, but instead act as an external "field".
  04/2009;
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  Available from: ArXiv

  Article: Multiferroicity and spiral magnetism in FeVO$_4$ with quenched Fe orbital moments

  A Daoud-Aladine, B. Kundys, C Martin, P G Radaelli, P J Brown, C Simon, L. C. Chapon
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  ABSTRACT: FeVO$_4$ has been studied by heat capacity, magnetic susceptibility, electric polarization and single crystal neutron diffraction experiments. The triclinic crystal structure is made of \emph{S}-shaped clusters of six Fe$^{3+}$ ions, linked by VO$_4^{3-}$ groups. Two long-range magnetic ordering transitions occur at T$_{N1}$=22K and T$_{N2}$=15K. Both magnetic structures are incommensurate. That stable below T$_{N1}$ is collinear with amplitude modulated moments whereas below T$_{N2}$ the arrangement is non-collinear with a helicoidal modulation. Below T$_{N2}$, \fevo becomes weakly ferroelectric coincidentally with the loss of the collinearity of the magnetic structure. We conclude that \fevo provides another example of frustrated spiral magnet similar to the classical TbMnO$_3$ compound. However, \fevo has quenched orbital moments and a particular structure clarifying the respective role of anisotropy and magnetic frustration in this type of multiferroic materials.
  01/2009;
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  Available from: Tapan Chatterji

  Article: Electric field switching of antiferromagnetic domains in YMn2O5: a probe of the multiferroic mechanism.

  P G Radaelli, L C Chapon, A Daoud-Aladine, C Vecchini, P J Brown, T Chatterji, S Park, S-W Cheong
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  ABSTRACT: We employ neutron spherical polarimetry to determine the nature and population of the coexisting antiferromagnetic domains in multiferroic YMn2O5. By applying an electric field, we prove that reversing the electrical polarization results in the population inversion of two types of in-plane domains, related to each other by inversion. Our results are completely consistent with the exchange-striction mechanism of ferroelectricity, and support a unified model where cycloidal ordering is induced by coupling to the main magnetic order parameter.
  Physical Review Letters 09/2008; 101(6):067205. · 7.37 Impact Factor
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  Available from: ArXiv

  Article: The incommensurate magnetic structure of YMn2O5: a stringent test of the multiferroic mechanism

  P G Radaelli, C Vecchini, L. C. Chapon, P J Brown, S Park, S-W Cheong
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  ABSTRACT: We have determined the magnetic structure of the low-temperature incommensurate phase of multiferroic YMn2O5 using single-crystal neutron diffraction. By employing corepresentation analysis, we have ensured full compliance with both symmetry and physical constraints, so that the electrical polarization must lie along the b axis, as observed. The evolution of the spin components and propagation through the commensurate-incommensurate phase boundary points unambiguously at the exchange-striction mechanism as the primary driving force for ferroelectricity.
  08/2008;
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  Available from: ArXiv

  Article: Orbital ordering promotes weakly-interacting S=1/2 dimers in the triangular lattice compound Sr3Cr2O8

  L. C. Chapon, C. Stock, P G Radaelli, C Martin
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  ABSTRACT: The weakly interacting S=1/2 dimers system Sr3Cr2O8 has been investigated by powder neutron diffraction and inelastic neutron scattering. Our data reveal a structural phase transition below room temperature corresponding to an antiferro-orbital ordering with nearly 90 degrees arrangement of the occupied 3z^2-r^2 d-orbital. This configuration leads to a drastic reduction of the inter-dimer exchange energies with respect to the high temperature orbital-disorder state, as shown by a spin-dimer analysis of the super-superexchange interactions performed using the Extended Huckel Tight Binding method. Inelastic neutron scattering reveals the presence of a quasi non-dispersive magnetic excitation at 5.4 meV, in agreement with the picture of weakly-interacting dimers.
  08/2008;
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  Available from: ArXiv

  Article: A Neutron diffraction study of multiferroics RMn2O5

  P G Radaelli, L. C. Chapon
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  ABSTRACT: The magnetic properties of RMn2O5 multiferrroics as obtained by unpolarized and polarized neutron diffraction experiments are reviewed. We discuss the qualitative features of the magnetic phase diagram both in zero magnetic field and in field and analyze the commensurate magnetic structure and its coupling to an applied electric field. The origin of ferrolectricity is discussed based on calculations of the ferroelectric polarization predicted by different microscopic coupling mechanisms (exchange striction and cycloidal spin-orbit models). A minimal model containing a small set of parameters is also presented in order to understand the propagation of the magnetic structure along the c-direction. Comment: IOP Review
  06/2008;
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  Available from: ArXiv

  Article: The commensurate phase of multiferroic HoMn2O5 studied by X-ray magnetic scattering

  G Beutier, A Bombardi, C Vecchini, P G Radaelli, S Park, S-W Cheong, L. C. Chapon
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  ABSTRACT: The commensurate phase of multiferroic HoMn2O5 was studied by X-ray magnetic scattering, both off resonance and in resonant conditions at the Ho-L3 edge. Below 40 K, magnetic ordering at the Ho sites is induced by the main Mn magnetic order parameter, and its temperature dependence is well accounted for by a simple Curie-Weiss susceptibility model. A lattice distortion of periodicity twice that of the magnetic order is also evidenced. Azimuthal scans confirm the model of the magnetic structure recently refined from neutron diffraction data for both Mn and Ho sites, indicating that the two sublattices interact via magnetic superexchange.
  03/2008;
• Article: Crystal and magnetic structure of (La0.70Ca0.30)(CryMn1-y)O3: a neutron powder diffraction study

  L. Capogna, A. Martinelli, M. G. Francesconi, P G Radaelli, J. Rodriguez Carvajal, O. Cabeza, M. Ferretti, C. Castellano, T. Corridoni, N. Pompeo
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  ABSTRACT: The crystal and magnetic structure of (La0.70Ca0.30)(CryMn1-y)O3 for y = 0.70, 0.50 and 0.15 has been investigated using neutron powder diffraction. The three samples crystallize in the Pnma space group at both 290 K and 5 K and exhibit different magnetic structures at low temperature. In (La0.70Ca0.30)(Cr0.70Mn0.30)O3, antiferromagnetic order with a propagation vector k = 0 sets in. The magnetic structure is Gx, i.e. of the G-type with spins parallel to the a-axis. On the basis of our Rietveld refinement and the available magnetisation data, we speculate that only Cr3+ spins order, whereas Mn4+ act as a random magnetic impurity. In (La0.70Ca0.30)(Cr0.50Mn0.50)O3 the spin order is still of type Gx, although the net magnetic moment is smaller. No evidence for magnetic order of the Mn ions is observed. Finally, in (La0.70Ca0.30)(Cr0.15Mn0.85)O3 a ferromagnetic ordering of the Mn spins takes place, whereas the Cr3+ ions act as random magnetic impurities with randomly oriented spins. Comment: 7 pages, 5 figures, 2 tables
  01/2008;
• Article: Electrical field switching of antiferromagnetic domains in YMn2O5: a probe of multiferroic mechanism

  P.G. Radaelli, L.C. Chapon, A. Daud-Aladine, C. Vecchini, P.J. Brown, T. Chatterji, S. Park, S.W. Cheong
  Physical Review Letters 01/2008; 101:067205. · 7.37 Impact Factor