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Instruments / Technical Area	Publication Details	Published
I21-Resonant Inelastic X-ray Scattering (RIXS)	Quadrupolar magnetic excitations in an isotropic spin-1 antiferromagnet A. Nag , A. Nocera , S. Agrestini , M. Garcia-Fernandez , A. C. Walters , S.-W. Cheong , S. Johnston , K.-J. Zhou Nature Communications 13 DOI: 10.1038/s41467-022-30065-5 Diamond Proposal Number(s): [24593] Open Access Show Abstract ▼ Abstract: The microscopic origins of emergent behaviours in condensed matter systems are encoded in their excitations. In ordinary magnetic materials, single spin-flips give rise to collective dipolar magnetic excitations called magnons. Likewise, multiple spin-flips can give rise to multipolar magnetic excitations in magnetic materials with spin S ≥ 1. Unfortunately, since most experimental probes are governed by dipolar selection rules, collective multipolar excitations have generally remained elusive. For instance, only dipolar magnetic excitations have been observed in isotropic S = 1 Haldane spin systems. Here, we unveil a hidden quadrupolar constituent of the spin dynamics in antiferromagnetic S = 1 Haldane chain material Y2BaNiO5 using Ni L3-edge resonant inelastic x-ray scattering. Our results demonstrate that pure quadrupolar magnetic excitations can be probed without direct interactions with dipolar excitations or anisotropic perturbations. Originating from on-site double spin-flip processes, the quadrupolar magnetic excitations in Y2BaNiO5 show a remarkable dual nature of collective dispersion. While one component propagates as non-interacting entities, the other behaves as a bound quadrupolar magnetic wave. This result highlights the rich and largely unexplored physics of higher-order magnetic excitations.	Apr 2022
I16-Materials and Magnetism	Absolute crystal and magnetic chiralities in the langasite compound Ba3NbFe3Si2O14 determined by polarized neutron and x-ray scattering N. Qureshi , A. Bombardi , S. Picozzi , P. Barone , E. Lelièvre-Berna , X. Xu , C. Stock , D. F. Mcmorrow , A. Hearmon , F. Fabrizi , P. G. Radaelli , S.-W. Cheong , L. C. Chapon Physical Review B 102 DOI: 10.1103/PhysRevB.102.054417 Diamond Proposal Number(s): [4073, 1803, 17569] Show Abstract ▼ Abstract: We present a combined polarized neutron and x-ray scattering study on two enantiopure langasite single crystals aimed at the determination of their absolute structural and magnetic chiralities and the coupling between them. Our respective data sets unambiguously reveal two samples of opposite structural chirality, where the magnetic handedness is pinned by the structural one. Simple energy considerations of the magnetic exchange and single-ion anisotropy parameters reveal that it is not the Dzyaloshinskii-Moriya interaction but the local single-ion anisotropy on a triangular plaquette which plays a key role in stabilizing one of the two magnetic helices.	Aug 2020
I11-High Resolution Powder Diffraction	From first- to second-order phase transitions in hybrid improper ferroelectrics through entropy stabilization Fernando Pomiro , Chris Ablitt , Nicholas C. Bristowe , Arash A. Mostofi , Choongjae Won , Sang-Wook Cheong , Mark S. Senn Physical Review B 102 DOI: 10.1103/PhysRevB.102.014101 Diamond Proposal Number(s): [25166, 18786] Open Access Show Abstract ▼ Abstract: Hybrid improper ferroelectrics (HIFs) have been studied intensively over the past few years to gain an understanding of their temperature-induced phase transitions and ferroelectric switching pathways. Here we report a switching from a first- to a second-order phase transition pathway for HIFs Ca 3 − x Sr x Ti 2 O 7 , which is driven by the differing entropies of the phases that we identify as being associated with the dynamic motion of octahedral tilts and rotations. A greater understanding of the transition pathways in this class of layered perovskites, which host many physical properties that are coupled to specific symmetries and octahedral rotation and tilt distortions—such as superconductivity, negative thermal expansion, fast ion conductivity, ferroelectricity, among others—is a crucial step in creating novel functional materials by design.	Jun 2020
I11-High Resolution Powder Diffraction	Mapping the structural transitions controlled by the trilinear coupling in Ca3-xSrxTi2O7 Marie Kratochvilova , Fei-Ting Huang , Maria-Teresa Fernandez Diaz , Milan Klicpera , Sarah Day , Stephen P. Thompson , Yoon-Seok Oh , Bin Gao , Sang-Wook Cheong , Je-Geun Park Journal Of Applied Physics 125 DOI: 10.1063/1.5089723 Diamond Proposal Number(s): [16074] Show Abstract ▼ Abstract: We present the results of the high-temperature neutron and x-ray diffraction experiments on the Ca3–xSrxTi2O7 (x = 0.5, 0.8, 0.85, 0.9) compounds. The ferro- to paraelectric transition in these hybrid improper ferroelectric materials arises from the so-called trilinear coupling. Depending on the strontium content, various structures and phase transitions, different from theoretical predictions, emerge. The in situ x-ray powder diffraction indicates a direct ferro- to paraelectric transition between the orthorhombic A21am and the tetragonal undistorted I4/mmm phase for x ≤ 0.6. We identified a reduction in the trilinear coupling robustness by increasing the Sr-doping level to lead to the emergence of the intermediate tetragonal P42/mnm phase and the gradual suppression of the orthorhombic phase. The observed character of the structure transitions and the Ca3–xSrxTi2O7 phase diagram are discussed in the framework of theoretical models of other related hybrid improper ferroelectric systems.	Jun 2019
I11-High Resolution Powder Diffraction	Symmetry switching of negative thermal expansion by chemical control Mark Senn , Claire Murray , Xuan Luo , Lihai Wang , Fei-Ting Huang , Sang-Wook Cheong , Alessandro Bombardi , Chris Ablitt , Arash A. Mostofi , Nicholas C. Bristowe Journal Of The American Chemical Society DOI: 10.1021/jacs.5b13192 Diamond Proposal Number(s): [9677] Show Abstract ▼ Abstract: The layered perovskite Ca3–xSrxMn2O7 is shown to exhibit a switching from a material exhibiting uniaxial negative to positive thermal expansion as a function of x. The switching is shown to be related to two closely competing phases with different symmetries. The negative thermal expansion (NTE) effect is maximized when the solid solution is tuned closest to this region of phase space but is switched off suddenly on passing though the transition. Our results show for the first time that, by understanding the symmetry of the competing phases alone, one may achieve unprecedented chemical control of this unusual property.	Mar 2016
I11-High Resolution Powder Diffraction	Negative thermal expansion in hybrid improper ferroelectric Ruddlesden-Popper perovskites by symmetry trapping M. S. Senn , A. Bombardi , C. A. Murray , C. Vecchini , A. Scherillo , X. Luo , S. W. Cheong Physical Review Letters 114 (3) DOI: 10.1103/PhysRevLett.114.035701 PMID: 25659007 Diamond Proposal Number(s): [9677] Show Abstract ▼ Abstract: We present new results on the microscopic nature of the ferroelectricity mechanisms in Ca3Mn2O7 and Ca3Ti2O7. To the first approximation, we confirm the hybrid improper ferroelectric mechanism recently proposed by Benedek and Fennie for these Ruddlesden-Popper compounds. However, in Ca3Mn2O7 we find that there is a complex competition between lattice modes of different symmetry which leads to a phase coexistence over a large temperature range and the 'symmetry trapping' of a soft mode. This trapping of the soft mode leads to a large uniaxial negative thermal expansion (NTE) reaching a maximum between 250 and 350 K (3.6 x 10 -6 K-1) representing the only sizable NTE reported for these and related perovskite materials to date. Our results suggest a systematic strategy for designing and searching for ceramics with large NTE coefficients.	Jan 2015
I16-Materials and Magnetism	Magnetic phase diagram and ordered ground state of GdMn2O5 multiferroic studied by x-ray magnetic scattering C. Vecchini , A. Bombardi , L. C. Chapon , N. Lee , P. G. Radaelli , S.-W. Cheong Journal Of Physics: Conference Series 519, 3796-3802 REXS 2013 — Workshop on Resonant Elastic X-ray Scattering in Condensed Matter DOI: 10.1088/1742-6596/519/1/012004 Open Access Show Abstract ▼ Abstract: The magnetic structure of multiferroic GdMn2O5 is studied by x-ray magnetic scattering, both off resonance and in resonant conditions at the Gd LIII edge. Temperature dependence of magnetic Bragg reflections shows an initial incommensurate (ICM) ordering appearing at T~40K with magnetic propagation vector kICM ~(0.49 0 0.18) which condenses at T~33K in a commensurate (CM) wave vector kCM=(½ 0 0). In the CM phase Gd3+ ions appear to order spontaneously at the same temperature as Mn ions. Azimuthal scans allowed for the precise determination, from x-ray magnetic scattering only, of the orientation and magnitude of the Gd3+ magnetic moments. The Gd3+ magnetic isotropy allows for the Gd ions to mimic the Mn4+ magnetic sublattice via Gd-Mn-Gd superexchange interactions.	May 2014
I16-Materials and Magnetism	Magnetically induced femtoscale strain modulations in HoMn2O5 C. Vecchini , A. Bombardi , L. C. Chapon , G. Beutier , P. G. Radaelli , S. Park , S.-W. Cheong Physical Review B 89 (12) DOI: 10.1103/PhysRevB.89.125114 Diamond Proposal Number(s): [6066] Show Abstract ▼ Abstract: X-ray scattering was used to investigate the magnetically induced ionic displacements in the low-temperature commensurate ferroelectric/antiferromagnetic phase of the multiferroic HoMn2O5. The structural modulation signal appearing at twice the magnetic wave vector km = ( 1 2 0 1 4 ) has been used, combined with symmetry analysis, to determine a model for the ionic displacements up to a precision of 10−3 A° . The symmetry-breaking operations that are associated to the active irreducible representation have been experimentally determined from the analysis of the modulation modes. They reveal a lowering of symmetry to the polar point group m2m. Calculations based on the determined model show that the magnetic structure along the c direction is stabilized via nearest-neighbor and next-nearest-neighbor interactions through the rare-earth layer. We also show that by knowing both the magnetic and the magnetically induced strain waves patterns, it is possible to remove any phase ambiguity between the crystal and magnetic structures.	Mar 2014
I06-Nanoscience (XPEEM) I10-Beamline for Advanced Dichroism - scattering	Magnetic fan structures in Ba0.5Sr1.5Zn2Fe12O22 hexaferrite revealed by resonant soft x-ray diffraction A. Hearmon , R. D. Johnson , T. Beale , S. S. Dhesi , X. Luo , S.-W. Cheong , P. Steadman , P. Radaelli Physical Review B 88 (17) DOI: 10.1103/PhysRevB.88.174413 Show Abstract ▼ Abstract: The hexaferrites are known to exhibit a wide range of magnetic structures, some of which are connected to important technological applications and display magnetoelectric properties.We present data on the lowmagnetic field structures stabilized in a Y-type hexaferrite as observed by resonant soft x-ray diffraction. The helical spin block arrangement that is present in zero applied magnetic field becomes fanlike as a field is applied in plane. The propagation vectors associated with each fan structure are studied as a function of magnetic field, and a new magnetic phase is reported. Mean field calculations indicate this phase should stabilize close to the boundary of the previously reported phases.	Nov 2013
I16-Materials and Magnetism	X-ray imaging and multiferroic coupling of cycloidal magnetic domains in ferroelectric monodomain BiFeO3 R. D. Johnson , P. Barone , A. Bombardi , R. J. Bean , S. Picozzi , P. G. Radaelli , Y. S. Oh , S.-W. Cheong , L. C. Chapon Physical Review Letters 110 (21) DOI: 10.1103/PhysRevLett.110.217206 PMID: 23745922 Show Abstract ▼ Abstract: Magnetic domains at the surface of a ferroelectric monodomain BiFeO3 single crystal have been imaged by hard x-ray magnetic scattering. Magnetic domains up to several hundred microns in size have been observed, corresponding to cycloidal modulations of the magnetization along the wave vector k ¼ ð	May 2013

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