## Polarizing an antiferromagnet by optical engineering of the crystal field

Nature Physics Nature Research (2020)

AS Disa, M Fechner, T Nova, B Liu, M Foerst, D Prabhakaran, P Radaelli, A Cavalleri

Strain engineering is widely used to manipulate the electronic and magnetic properties of complex materials. For example, the piezomagnetic effect provides an attractive route to control magnetism with strain. In this effect, the staggered spin structure of an antiferromagnet is decompensated by breaking the crystal field symmetry, which induces a ferrimagnetic polarization. Piezomagnetism is especially appealing because, unlike magnetostriction, it couples strain and magnetization at linear order, and allows for bi-directional control suitable for memory and spintronics applications. However, its use in functional devices has so far been hindered by the slow speed and large uniaxial strains required. Here we show that the essential features of piezomagnetism can be reproduced with optical phonons alone, which can be driven by light to large amplitudes without changing the volume and hence beyond the elastic limits of the material. We exploit nonlinear, three-phonon mixing to induce the desired crystal field distortions in the antiferromagnet CoF2. Through this effect, we generate a ferrimagnetic moment of 0.2 μB per unit cell, nearly three orders of magnitude larger than achieved with mechanical strain.

## Magneto-optical Kerr switching properties of (CrI3)2 and (CrBr3/CrI3) bilayers

ACS Applied Electronic Materials American Chemical Society (2020) 0c00154

K Yang, W Hu, H Wu, M-H Whangbo, P Radaelli, A Stroppa

We explore the magneto-optical Kerr effect (MOKE) for different spin configurations of (CrI3)2 bilayer and (CrBr3/CrI3) mixed bilayer, using symmetry arguments and first-principles electronic structure calculations. Starting from CrX3 (X = I, Br) monolayers, we considered collinear ferromagnetic (FM) and layered antiferromagnetic (AFM) states for (CrI3)2 and (CrBr3/CrI3) bilayers. The AFM (CrI3)2 bilayer does not show MOKE, consistent with the presence of a symmetry operator combining inversion (I) and time reversal (T) symmetries. The FM state preserves \textit{I} symmetry but breaks the T symmetry, thus allowing a non-zero Kerr angle, which is reversible by switching the FM spins. The (CrBr3/CrI3) bilayer breaks both the I and T symmetries and thus exhibits MOKE both in the FM and, remarkably, in AFM states. In both FM and AFM configurations, the Kerr angle switches by reversing the spins in both layers. Our study demonstrates that MOKE spectra can help characterize different magnetic configurations in these emerging two-dimensional (2D) materials due to a different stacking of the monolayers, even in the AFM case. Note that the present symmetry analyses and MOKE properties apply to more general 2D magnetic van der Waals heterostructures. Furthermore, we propose (CrBr3/CrI3) bilayer as a promising candidate for AFM spintronics, since the two time-reversed AFM states are associated with opposite Kerr rotation, i.e. they could be used as memory elements.

## Magnetic structure and spin-flop transition in the A -site columnar-ordered quadruple perovskite TmMn3O6

Physical Review B American Physical Society 99 (2019) 104424-

A Vibhakar, DD Khalyavin, P Manuel, L Zhang, K Yamaura, P Radaelli, AA Belik, R Johnson

We present the magnetic structure of $\mathrm{TmMn_3O_6}$, solved via neutron powder diffraction - the first such study of any $R\mathrm{Mn_3O_6}$ A-site columnar-ordered quadruple perovskite to be reported. We demonstrate that long range magnetic order develops below 74 K, and at 28 K a spin-flop transition occurs driven by $f$-$d$ exchange and rare earth single ion anisotropy. In both magnetic phases the magnetic structure may be described as a collinear ferrimagnet, contrary to conventional theories of magnetic order in the manganite perovskites. Instead, we show that these magnetic structures can be understood to arise due to ferro-orbital order, the A, A$'$ and A$''$ site point symmetry, $mm2$, and the dominance of A-B exchange over both A-A and B-B exchange, which together are unique to the $R\mathrm{Mn_3O_6}$ perovskites.

## Revealing the nature of photoluminescence emission in the metal-halide double perovskite Cs2AgBiBr6

Journal of Materials Chemistry C Royal Society of Chemistry 7 (2019) 8350-8356

SJ Zelewski, JM Urban, A Surrente, DK Maude, A Kuc, L Schade, R Johnson, M Dollmann, P Nayak, H Snaith, P Radaelli, R Kudrawiec, R Nicholas, P Plochocka, M Baranowski

<p>Double perovskite crystals such as Cs<small><sub>2</sub></small>AgBiBr<small><sub>6</sub></small> are expected to overcome the limitation of classic hybrid organic–inorganic perovskite crystals related to the presence of lead and the lack of structural stability. Perovskites are ionic crystals in which the carriers are expected to strongly couple to lattice vibrations. In this work we demonstrate that the photoluminescence (PL) emission in Cs<small><sub>2</sub></small>AgBiBr<small><sub>6</sub></small> is strongly influenced by the strong electron–phonon coupling. Combining photoluminescence excitation (PLE) and Raman spectroscopy we show that the PL emission is related to a color center rather than a band-to-band transition. The broadening and the Stokes shift of the PL emission from Cs<small><sub>2</sub></small>AgBiBr<small><sub>6</sub></small> is well explained using a Franck–Condon model with a Huang–Rhys factor of <em>S</em> = 11.7 indicating a strong electron–phonon interaction in this material.</p>

## Strain engineering a multiferroic monodomain in thin-film BiFeO3

Physical Review Applied American Physical Society 11 (2019) 024035

N Waterfield Price, A Vibhakar, R Johnson, J Schad, W Saenrang, A Bombardi, F Chmiel, CB Eom, P Radaelli

<p>The presence of domains in ferroic materials can negatively affect their macroscopic properties and hence their usefulness in device applications. From an experimental perspective, measuring materials comprising multiple domains can complicate the interpretation of material properties and their underlying mechanisms. In general, BiFeO<sub>3</sub> films tend to grow with multiple magnetic domains and often contain multiple ferroelectric and ferroelastic domain variants. By growing (111)-oriented BiFeO<sub>3</sub> films on an orthorhombic TbScO<sub>3</sub> substrate, we are able to overcome this, and, by exploiting the magnetoelastic coupling between the magnetic and crystal structures, bias the growth of a given magnetic-, ferroelectric-, and structural-domain film. We further demonstrate the coupling of the magnetic structure to the ferroelectric polarisation by showing the magnetic polarity in this domain is inverted upon 180° ferroelectric switching.</p>

## Structural and optical properties of Cs2AgBiBr6 double perovskite

ACS Energy Letters American Chemical Society 4 (2018) 299-305

L Schade, AD Wright, RD Johnson, M Dollmann, B Wenger, PK Nayak, D Prabhakaran, LM Herz, RJ Nicholas, HJ Snaith, PG Radaelli

We present a comprehensive study of the relationship between the crystal structure and optoelectronic properties of the double perovskite Cs2AgBiBr6, which has emerged as a promising candidate for photovoltaic devices. On the basis of single-crystal/powder X-ray diffraction and neutron powder diffraction, we have revealed the presence of a structural phase transition at Ts ≈ 122 K between the room-temperature cubic structure (space group Fm3̅m) and a new low-temperature tetragonal structure (I4/m). From reflectivity measurements we found that the peak exciton energy Eex ≈ 2.85 eV near the direct gap shifts proportionally to the tetragonal strain, which is consistent with the Eex being primarily controlled by a rotational degree of freedom of the crystal structure, thus by the angle Bi−Ag−Br. We observed the time-resolved photoluminescence kinetics and we found that, among the relaxation channels, a fast one is mainly present in the tetragonal phase, suggesting that its origin may lie in the formation of tetragonal twin domains.

## Magnetoelectric domains and their switching mechanism in a Y-type hexaferrite

PHYSICAL REVIEW B 100 (2019) ARTN 104411

FP Chmiel, D Prabhakaran, P Steadman, J Chen, R Fan, RD Johnson, PG Radaelli

## Spin-wave directional anisotropies in antiferromagnetic Ba3NbFe3Si2O14

Physical Review B American Physical Society 100 (2019) 134429

C Stock, RD Johnson, N Giles-Donovan, M Songvilay, JA Rodriguez-Rivera, N Lee, X Xu, P Radaelli, LC Chapon, A Bombardi, S Cochran, C Niedermayer, A Schneidewind, Z Husges, Z Lu, S Meng, S-W Cheong

Ba3NbFe3Si2O14 (langasite) is structurally and magnetically single-domain chiral with the magnetic helicity induced through competing symmetric exchange interactions. Using neutron scattering, we show that the spin waves in antiferromagnetic langasite display directional anisotropy. On applying a time-reversal symmetry breaking magnetic field along the c axis, the spin-wave energies differ when the sign is reversed for either the momentum transfer ±Q- or applied magnetic field ±μ0H. When the field is applied within the crystallographic ab plane, the spin-wave dispersion is directionally isotropic and symmetric in ±μ0H. However, a directional anisotropy is observed in the spin-wave intensity. We discuss this directional anisotropy in the dispersion in langasite in terms of a field-induced precession of the dynamic unit cell staggered magnetization resulting from a broken twofold symmetry. Directional anisotropy, often referred to as nonreciprocal responses, can occur in antiferromagnetic phases in the absence of the Dzyaloshinskii-Moriya interaction or other effects resulting from spin-orbit coupling.

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

Bulletin of the American Physical Society American Physical Society (2018)

F Chmiel, N Price, R Johnson, A Lamirand, J Schad, GVD Laan, DT Harris, J Irwin, C-B Eom, P Radaelli

Vortices are among the simplest topological structures, and occur whenever a flow field `whirls' around a one-dimensional core. They are ubiquitous to many branches of physics, from fluid dynamics to superconductivity and superfluidity, and are even predicted by some unified theories of particle interactions, where they might explain some of the largest-scale structures seen in today's Universe. 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 only observed when the effects of the dipole-dipole interaction is modified by confinement at the nanoscale, or when the parameter associated with the vorticity does not couple directly with strain. Here, we present the discovery of a novel form of vortices in antiferromagnetic (AFM) hematite ($\alpha$-Fe$_2$O$_3$) epitaxial films, in which the primary whirling parameter is the staggered magnetisation. Remarkably, ferromagnetic (FM) topological objects with the same vorticity and winding number of the $\alpha$-Fe$_2$O$_3$ 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 magnetisation), and indicate that the vortex/meron pairs can be manipulated by the application of an in-plane magnetic field, H$_{\parallel}$, giving rise to large-scale vortex-antivortex annihilation.

## 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)

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

## Evolution of magneto-orbital order upon B-site electron doping in Na1−xCaxMn7O12 quadruple perovskite manganites

Physical Review Letters American Physical Society 120 (2018) 257202-

R Johnson, F Mezzadri, P Manuel, DD Khalyavin, E Gilioli, PGR Radaelli

We present the discovery and refinement by neutron powder diffraction of a new magnetic phase in the Na1-xCaxMn7O12 quadruple perovskite phase diagram, which is the incommensurate analogue of the well-known pseudo-CE phase of the simple perovskite manganites. We demonstrate that incommensurate magnetic order arises in quadruple perovskites due to the exchange interactions between A and B sites. Furthermore, by constructing a simple mean field Heisenberg exchange model that generically describes both simple and quadruple perovskite systems, we show that this new magnetic phase unifies a picture of the interplay between charge, magnetic and orbital ordering across a wide range of compounds.

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

Nature Materials Nature Publishing Group 17 (2018) 581–585-

F Chmiel, N Waterfield Price, R Johnson, AD Lamirand, J Schad, G van der Laan, DT Harris, C-B Eom, P 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 nanoscale1,2,3, or when the parameter associated with the vorticity does not couple directly with strain4. 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 AMn7O12(A=Sr, Cd, and Pb)

Physical Review B American Physical Society 96 (2017) 054448-

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

<p>Through analysis of variable temperature neutron powder diffraction data, we present solutions for the magnetic structures of SrMn<sub>7</sub>O<sub>12</sub>, CdMn<sub>7</sub>O<sub>12</sub>, and PbMn<sub>7</sub>O<sub>12</sub> in all long-range ordered phases. The three compounds were found to have magnetic structures analogous to that reported for CaMn<sub>7</sub>O<sub>12</sub>. They all feature a higher temperature lock-in phase with <i>commensurate</i> magneto-orbital coupling, and a delocked, multi-<b>k</b> magnetic ground state where <i>incommensurate</i> magneto-orbital coupling gives rise to a constant-moment magnetic helix with modulated spin helicity. CdMn<sub>7</sub>O<sub>12</sub> represents a special case in which the orbital modulation is commensurate with the crystal lattice and involves stacking of fully and partially polarized orbital states. Our results provide a robust confirmation of the phenomenological model for magneto-orbital coupling previously presented for CaMn<sub>7</sub>O<sub>12</sub>. Furthermore, we show that the model is universal to the <i>A</i><sup<2+< sup=""> quadruple perovskite manganites synthesised to date, and that it is tunable by selection of the <i>A</i>-site ionic radius.</sup<2+<></p>

## Electrical switching of magnetic polarity in a multiferroic BiFeO3 device at room temperature

Physical Review Applied American Physical Society 8 (2017) 014033

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

<p>We have directly imaged reversible electrical switching of the cycloidal rotation direction (magnetic polarity) in a (111)<sub>pc</sub>-BiFeO3 epitaxial-film device at room temperature by non-resonant x-ray magnetic scattering. Consistent with previous reports, fully relaxed (111)<sub>pc</sub>-BiFeO3 epitaxial films consisting of a single ferroelectric domain were found to comprise a sub-micron-scale mosaic of magneto-elastic domains, all sharing a common direction of the magnetic polarity, which was found to switch reversibly upon reversal of the ferroelectric polarization without any measurable change of the magneto-elastic domain population. A real-space polarimetry map of our device clearly distinguished between regions of the sample electrically addressed into the two magnetic states with a resolution of a few tens of micron. Contrary to the general belief that the magneto-electric coupling in BiFeO3 is weak, we find that electrical switching has a dramatic effect on the magnetic structure, with the magnetic moments rotating on average by 90 degrees at every cycle.</p>

## 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.

## Coherent magnetoelastic domains in multiferroic films

Physical Review Letters American Physical Society 117 (2016) 177601-

N Waterfield Price, RD Johnson, W Saenrang, F Maccherozzi, SS Dhesi, A Bombardi, FP Chmiel, C-B Eom, P 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 non-resonant x-ray magnetic scattering, neutron diffraction and vector-mapped x-ray magnetic linear dichroism photoemission electron microscopy, we show that epitaxial (111)-BiFeO3 films support sub-micron antiferromagnetic domains, which are magneto-elastically coupled to a coherent crystallographic monoclinic twin structure. This unique texture, which is absent in bulk single crystals, should enable control of magnetism in BiFeO3 film devices via epitaxial strain.

## Polarization memory in the nonpolar magnetic ground state of multiferroic CuFeO2

Physical Review B American Physical Society (2016)

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

We investigate polarization memory effects in single-crystal CuFeO2, which has a magnetically induced ferroelectric phase at low temperatures and applied B fields between 7.5 and 13 T. Following electrical poling of the ferroelectric phase, we find that the nonpolar collinear antiferromagnetic ground state at B=0 T retains a strong memory of the polarization magnitude and direction, such that upon reentering the ferroelectric phase a net polarization of comparable magnitude to the initial polarization is recovered in the absence of external bias. This memory effect is very robust: in pulsed-magnetic-field measurements, several pulses into the ferroelectric phase with reverse bias are required to switch the polarization direction, with significant switching only seen after the system is driven out of the ferroelectric phase and ground state either magnetically (by application of B&gt;13 T) or thermally. The memory effect is also largely insensitive to the magnetoelastic domain composition, since no change in the memory effect is observed for a sample driven into a single-domain state by application of stress in the [110] direction. On the basis of Monte Carlo simulations of the ground-state spin configurations, we propose that the memory effect is due to the existence of helical domain walls within the nonpolar collinear antiferromagnetic ground state, which would retain the helicity of the polar phase for certain magnetothermal histories.

## Magnetostriction-driven ground-state stabilization in 2H perovskites

Physical Review B American Physical Society 94 (2016) 134404-

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

The magnetic ground state of Sr3ARuO6, with A=(Li,Na), is studied using neutron diffraction, resonant x-ray scattering, and laboratory characterization measurements of high-quality crystals. Combining these results allows us to observe the onset of long-range magnetic order and distinguish the symmetrically allowed magnetic models, identifying in-plane antiferromagnetic moments and a small ferromagnetic component along the c axis. While the existence of magnetic domains masks the particular in-plane direction of the moments, it has been possible to elucidate the ground state using symmetry considerations. We find that due to the lack of local anisotropy, antisymmetric exchange interactions control the magnetic order, first through structural distortions that couple to in-plane antiferromagnetic moments and second through a high-order magnetoelastic coupling that lifts the degeneracy of the in-plane moments. The symmetry considerations used to rationalize the magnetic ground state are very general and will apply to many systems in this family, such as Ca3ARuO6, with A=(Li,Na), and Ca3LiOsO6 whose magnetic ground states are still not completely understood.

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

Journal of Chemical Physics American Institute of Physics 145 (2016) 204501-

CG Salzmann, B Slater, P 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 computational 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.