Publications by Paolo Radaelli


Ab initio calculation of spin fluctuation spectra using time-dependent density functional perturbation theory, plane waves, and pseudopotentials

PHYSICAL REVIEW B 97 (2018) ARTN 024420

K Cao, H Lambert, PG Radaelli, F Giustino

We present an implementation of time-dependent density functional perturbation theory for spin fluctuations, based on plane waves and pseudopotentials. We compute the dynamic spin susceptibility self-consistently by solving the time-dependent Sternheimer equation, within the adiabatic local density approximation to the exchange and correlation kernel. We demonstrate our implementation by calculating the spin susceptibility of representative elemental transition metals, namely bcc Fe, fcc Ni, and bcc Cr. The calculated magnon dispersion relations of Fe and Ni are in agreement with previous work. The calculated spin susceptibility of Cr exhibits a soft-paramagnon instability, indicating the tendency of the Cr spins to condense in an incommensurate spin density wave phase, in agreement with experiment.


Breaking Symmetry with Light: Ultra-Fast Ferroelectricity and Magnetism from Three-Phonon Coupling

Physical review B: Condensed matter and materials physics American Physical Society (2018)

PG Radaelli

A theory describing how ferroic properties can emerge transiently in the ultra-fast regime by breaking symmetry with light through three-phonon coupling is presented. Particular emphasis is placed on the special case when two exactly degenerate mid-infra-red or THz phonons are resonantly pumped, since this situation can give rise to an exactly rectified ferroic response with damping envelopes of ~ 1 ps or less. Light-induced ferroelectricity and ferromagnetism are discussed in this context, and a number of candidate materials that could display these phenomena are proposed. The same analysis is also applied to the interpretation of previous femto-magnetism experiments, performed in different frequency ranges (visible and near-infrared), but sharing similar symmetry characteristics.


Ab initio calculation of spin fluctuation spectra using time dependent density functional perturbation theory, planewaves, and pseudopotentials

Physical review B: Condensed matter and materials physics American Physical Society (2018)

F Giustino, K Cao, P Radaelli


Observation of magnetic vortex pairs at room temperature in a planar α-Fe2O3/Co heterostructure.

Nature materials 17 (2018) 581-585

FP Chmiel, N Waterfield Price, RD Johnson, AD Lamirand, J Schad, G van der Laan, DT Harris, J Irwin, MS Rzchowski, C-B Eom, PG Radaelli

Vortices, occurring whenever a flow field 'whirls' around a one-dimensional core, are among the simplest topological structures, ubiquitous to many branches of physics. In the crystalline state, vortex formation is rare, since it is generally hampered by long-range interactions: in ferroic materials (ferromagnetic and ferroelectric), vortices are observed only when the effects of the dipole-dipole interaction are modified by confinement at the nanoscale1-3, or when the parameter associated with the vorticity does not couple directly with strain 4 . Here, we observe an unprecedented form of vortices in antiferromagnetic haematite (α-Fe2O3) epitaxial films, in which the primary whirling parameter is the staggered magnetization. Remarkably, ferromagnetic topological objects with the same vorticity and winding number as the α-Fe2O3 vortices are imprinted onto an ultra-thin Co ferromagnetic over-layer by interfacial exchange. Our data suggest that the ferromagnetic vortices may be merons (half-skyrmions, carrying an out-of plane core magnetization), and indicate that the vortex/meron pairs can be manipulated by the application of an in-plane magnetic field, giving rise to large-scale vortex-antivortex annihilation.


Magneto-orbital ordering in the divalent A-site quadruple perovskite manganites AMn(7)O(12) (A = Sr, Cd, and Pb)

PHYSICAL REVIEW B 96 (2017) ARTN 054448

RD Johnson, DD Khalyavin, P Manuel, PG Radaelli, IS Glazkova, N Terada, AA Belik


Electrical Switching of Magnetic Polarity in a Multiferroic BiFeO3 Device at Room Temperature

PHYSICAL REVIEW APPLIED 8 (2017) ARTN 014033

NW Price, RD Johnson, W Saenrang, A Bombardi, FP Chmiel, CB Eom, PG Radaelli


Deterministic and robust room-temperature exchange coupling in monodomain multiferroic BiFeO3 heterostructures.

Nature communications 8 (2017) 1583-1583

W Saenrang, BA Davidson, F Maccherozzi, JP Podkaminer, J Irwin, RD Johnson, JW Freeland, J Íñiguez, JL Schad, K Reierson, JC Frederick, CAF Vaz, L Howald, TH Kim, S Ryu, MV Veenendaal, PG Radaelli, SS Dhesi, MS Rzchowski, CB Eom

Exploiting multiferroic BiFeO3 thin films in spintronic devices requires deterministic and robust control of both internal magnetoelectric coupling in BiFeO3, as well as exchange coupling of its antiferromagnetic order to a ferromagnetic overlayer. Previous reports utilized approaches based on multi-step ferroelectric switching with multiple ferroelectric domains. Because domain walls can be responsible for fatigue, contain localized charges intrinsically or via defects, and present problems for device reproducibility and scaling, an alternative approach using a monodomain magnetoelectric state with single-step switching is desirable. Here we demonstrate room temperature, deterministic and robust, exchange coupling between monodomain BiFeO3 films and Co overlayer that is intrinsic (i.e., not dependent on domain walls). Direct coupling between BiFeO3 antiferromagnetic order and Co magnetization is observed, with ~ 90° in-plane Co moment rotation upon single-step switching that is reproducible for hundreds of cycles. This has important consequences for practical, low power non-volatile magnetoelectric devices utilizing BiFeO3.


Modulated spin helicity stabilized by incommensurate orbital density waves in a quadruple perovskite manganite

PHYSICAL REVIEW B 93 (2016) ARTN 180403

RD Johnson, DD Khalyavin, P Manuel, A Bombardi, C Martin, LC Chapon, PG Radaelli


Coherent Magnetoelastic Domains in Multiferroic BiFeO_{3} Films.

Physical review letters 117 (2016) 177601-

N Waterfield Price, RD Johnson, W Saenrang, F Maccherozzi, SS Dhesi, A Bombardi, FP Chmiel, C-B Eom, PG Radaelli

The physical properties of epitaxial films can fundamentally differ from those of bulk single crystals even above the critical thickness. By a combination of nonresonant x-ray magnetic scattering, neutron diffraction and vector-mapped x-ray magnetic linear dichroism photoemission electron microscopy, we show that epitaxial (111)-BiFeO_{3} films support submicron antiferromagnetic domains, which are magnetoelastically coupled to a coherent crystallographic monoclinic twin structure. This unique texture, which is absent in bulk single crystals, should enable control of magnetism in BiFeO_{3} film devices via epitaxial strain.


Polarization memory in the nonpolar magnetic ground state of multiferroic CuFeO2

PHYSICAL REVIEW B 94 (2016) ARTN 144411

J Beilsten-Edmands, SJ Magorrian, FR Foronda, D Prabhakaran, PG Radaelli, RD Johnson


Magnetostriction-driven ground-state stabilization in 2H perovskites

PHYSICAL REVIEW B 94 (2016) ARTN 134404

DG Porter, MS Senn, DD Khalyavin, A Cortese, N Waterfield-Price, PG Radaelli, P Manuel, H-C zur-Loye, C Mazzoli, A Bombardi


Ab initio cycloidal and chiral magnetoelectric responses in Cr2O3

PHYSICAL REVIEW B 94 (2016) ARTN 100405

N Tillack, JR Yates, PG Radaelli


Detailed crystallographic analysis of the ice VI to ice XV hydrogen ordering phase transition.

The Journal of chemical physics 145 (2016) 204501-

CG Salzmann, B Slater, PG Radaelli, JL Finney, JJ Shephard, M Rosillo-Lopez, J Hindley

The D2O ice VI to ice XV hydrogen ordering phase transition at ambient pressure is investigated in detail with neutron diffraction. The lattice constants are found to be sensitive indicators for hydrogen ordering. The a and b lattice constants contract whereas a pronounced expansion in c is found upon hydrogen ordering. Overall, the hydrogen ordering transition goes along with a small increase in volume, which explains why the phase transition is more difficult to observe upon cooling under pressure. Slow-cooling ice VI at 1.4 GPa gives essentially fully hydrogen-disordered ice VI. Consistent with earlier studies, the ice XV obtained after slow-cooling at ambient pressure is best described with P-1 space group symmetry. Using a new modelling approach, we achieve the atomistic reconstruction of a supercell structure that is consistent with the average partially ordered structure derived from Rietveld refinements. This shows that C-type networks are most prevalent in ice XV, but other structural motifs outside of the classifications of the fully hydrogen-ordered networks are identified as well. The recently proposed Pmmn structural model for ice XV is found to be incompatible with our diffraction data, and we argue that only structural models that are capable of describing full hydrogen order should be used.


First-principles study of structurally modulated multiferroic CaMn7O12

PHYSICAL REVIEW B 91 (2015) ARTN 064422

K Cao, RD Johnson, N Perks, F Giustino, PG Radaelli


Theory of Electromagnons in CuO.

Physical review letters 114 (2015) 197201-

K Cao, F Giustino, PG Radaelli

We develop a theory of electromagnons in CuO by combining a symmetry analysis based on irreducible corepresentations, ab initio calculations, and simulations of spin dynamics using a model Hamiltonian and the Landau-Lifshitz-Gilbert equation. We show that the electromagnon measured in [Jones et al., Nat. Commun. 5, 3787 (2014)] with the electric field along the [101] direction originates from a magnetoelectric coupling mediated by Dzyaloshinskii-Moriya interactions and consists of a rigid rotation of the Cu spins around the axis defined by the electric field. Furthermore we predict the existence of a second electromagnon originating from exchange striction and coupled to electric fields along the [010] direction in the AF2 phase.


The contribution of Diamond Light Source to the study of strongly correlated electron systems and complex magnetic structures.

Philosophical transactions. Series A, Mathematical, physical, and engineering sciences 373 (2015)

PG Radaelli, SS Dhesi

We review some of the significant contributions to the field of strongly correlated materials and complex magnets, arising from experiments performed at the Diamond Light Source (Harwell Science and Innovation Campus, Didcot, UK) during the first few years of operation (2007-2014). We provide a comprehensive overview of Diamond research on topological insulators, multiferroics, complex oxides and magnetic nanostructures. Several experiments on ultrafast dynamics, magnetic imaging, photoemission electron microscopy, soft X-ray holography and resonant magnetic hard and soft X-ray scattering are described.


Non-ferroelectric nature of the conductance hysteresis in CH3NH3PbI3 perovskite-based photovoltaic devices

APPLIED PHYSICS LETTERS 106 (2015) ARTN 173502

J Beilsten-Edmands, GE Eperon, RD Johnson, HJ Snaith, PG Radaelli


Policy: Crystallography needs a governing body.

Nature 505 (2014) 607-609

PG Radaelli


Crystallography needs a governing body

Nature 505 (2014) 607-609

PG Radaelli


CaBaCo4O7: A ferrimagnetic pyroelectric

PHYSICAL REVIEW B 90 (2014) ARTN 045129

RD Johnson, K Cao, F Giustino, PG Radaelli

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