# Publications by Roger Johnson

## 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 11 (2019) ARTN 024035

NW Price, AM Vibhakar, RD Johnson, J Schad, W Saenrang, A Bombardi, FP Chmiel, CB Eom, PG Radaelli

## Spin Jahn-Teller antiferromagnetism in CoTi$_2$O$_5$

Physical Review B American Physical Society 99 (2019) 064403-

F Kirschner, R Johnson, F Lang, DD Khalyavin, P Manuel, T Lancaster, D Prabhakaran, S Blundell

We have used neutron powder diffraction to solve the magnetic structure of orthorhombic CoTi$_2$O$_5$, showing that the long-range ordered state below 26 K identified in our muon-spin rotation experiments is antiferromagnetic with propagation vector ${\bf k}=(\pm \frac{1}{2}, \frac{1}{2}, 0)$ and moment of 2.72(1)$\mu_{\rm B}$ per Co$^{2+}$ ion. This long range magnetic order is incompatible with the experimentally determined crystal structure because the imposed symmetry completely frustrates the exchange coupling. We conclude that the magnetic transition must therefore be associated with a spin Jahn-Teller effect which lowers the structural symmetry and thereby relieves the frustration. These results show that CoTi$_2$O$_5$ is a highly unusual low symmetry material exhibiting a purely spin-driven lattice distortion critical to the establishment of an ordered magnetic ground state.

## 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.

## Unconventional Field-Induced Spin Gap in an S=1/2 Chiral Staggered Chain

PHYSICAL REVIEW LETTERS 122 (2019) ARTN 057207

J Liu, S Kittaka, RD Johnson, T Lancaster, J Singleton, T Sakakibara, Y Kohama, J van Tol, A Ardavan, BH Williams, SJ Blundell, ZE Manson, JL Manson, PA Goddard

## The magnetic structures of rare-earth quadruple perovskite manganites RMn7O12

Physical review B: Condensed matter and materials physics American Physical Society 98 (2018) 104423-

R Johnson, D Khalyavin, P Manuel, L Zhang, K Yamaura, AA Belik

We report a neutron powder diffraction study of RMn7O12 quadruple perovskite manganites with R = La, Ce, Nd, Sm, and Eu. We show that in all measured compounds concomitant magnetic ordering of the A and B manganese sublattices occurs on cooling below the N´eel temperature. The respective magnetic structures are collinear, with one uncompensated Mn3+ moment per formula unit as observed in bulk magnetisation measurements. We show that both LaMn7O12 and NdMn7O12 undergo a second magnetic phase transition at low temperature, which introduces a canting of the B site sublattice moments that is commensurate in LaMn7O12 and incommensurate in NdMn7O12. This spin canting is consistent with a magnetic instability originating in the B site orbital order. Furthermore, NdMn7O12 displays a third magnetic phase transition at which long range ordering of the Nd sublattice modifies the periodicity of the incommensurate spin canting. Our results demonstrate a rich interplay between transition metal magnetism, orbital order, and the crystal lattice, which may be fine tuned by cation substitution and rare earth magnetism.

## Magneto-orbital texture in the perovskite modification of Mn2O3

PHYSICAL REVIEW B 98 (2018) ARTN 014426

DD Khalyavin, RD Johnson, P Manuel, AA Tsirlin, AM Abakumov, DP Kozlenko, Y Sun, L Dubrovinsky, SV Ovsyannikov

## Helical magnetism in Sr-doped CaMn7O12 films

PHYSICAL REVIEW B 98 (2018) ARTN 224419

A Huon, AM Vibhakar, AJ Grutter, JA Borchers, S Disseler, Y Liu, W Tian, F Orlandi, P Manuel, DD Khalyavin, Y Sharma, A Herklotz, HN Lee, MR Fitzsimmons, RD Johnson, SJ May

## High-Pressure Synthesis, Structures, and Properties of Trivalent A-Site-Ordered Quadruple Perovskites RMn7O12 (R = Sm, Eu, Gd, and Tb).

Inorganic chemistry 57 (2018) 5987-5998

L Zhang, N Terada, RD Johnson, DD Khalyavin, P Manuel, Y Katsuya, M Tanaka, Y Matsushita, K Yamaura, AA Belik

A-site-ordered quadruple perovskites RMn7O12 with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and ∼1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group I2/ m at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic Im3̅ structure at ∼664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at TN1 ≈ 82-87 K at zero magnetic field, but additional magnetic transitions below TN2 ≈ 12 K were observed in SmMn7O12 and EuMn7O12 at high magnetic fields. Very weak kinklike dielectric anomalies were observed at TN1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18-35 K)-these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn7O12 shows signs of nonstoichiometry expressed as (Tb1- xMn x)Mn7O12, and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn7O12 and EuMn7O12 were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn7O12 and (Tb0.88Mn0.12)Mn7O12 were studied by synchrotron X-ray powder diffraction at 295 K.

## 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.

## Spin-induced multiferroicity in the binary perovskite manganite Mn2O3

Nature Communications Nature Publishing Group 9 (2018) 2996-

J Cong, K Zhai, Y Chai, D Shang, DD Khalyavin, R Johnson, DP Kozlenko, Kichanov, AM Abakumov, AA Tsirlin, L Dubrovinsky, X Xu, Z Sheng, SV Ovsyannikov, Y Sun

The ABO3 perovskite oxides exhibit a wide range of interesting physical phenomena remaining in the focus of extensive scientific investigations and various industrial applications. In order to form a perovskite structure, the cations occupying the A and B positions in the lattice, as a rule, should be different. Nevertheless, the unique binary perovskite manganite Mn2O3 containing the same element in both A and B positions can be synthesized under high-pressure high-temperature conditions. Here, we show that this material exhibits magnetically driven ferroelectricity and a pronounced magnetoelectric effect at low temperatures. Neutron powder diffraction revealed two intricate antiferromagnetic structures below 100 K, driven by a strong interplay between spin, charge, and orbital degrees of freedom. The peculiar multiferroicity in the Mn2O3 perovskite is ascribed to a combined effect involving several mechanisms. Our work demonstrates the potential of binary perovskite oxides for creating materials with highly promising electric and magnetic properties.

## 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.

## Intrinsic Triple Order in A-site Columnar-Ordered Quadruple Perovskites: Proof of Concept.

Chemphyschem : a European journal of chemical physics and physical chemistry (2018)

AA Belik, DD Khalyavin, L Zhang, Y Matsushita, Y Katsuya, M Tanaka, RD Johnson, K Yamaura

There is an emerging topic in the science of perovskite materials: A-site columnar-ordered A2 A'A''B4 O12 quadruple perovskites, which have an intrinsic triple order at the A sites. However, in many examples reported so far, A' and A'' cations are the same, and the intrinsic triple order is hidden. Here, we investigate structural properties of Dy2 CuMnMn4 O12 (1) and Ho2 MnGaMn4 O12 (2) by neutron and X-ray powder diffraction and prove the triple order at the A sites. The cation distributions determined are [Ho2 ]A [Mn]A' [Ga0.66 Mn0.34 ]A'' [Mn3.66 Ga0.34 ]B O12 and [Dy2 ]A [Cu0.73 Mn0.27 ]A' [Mn0.80 Dy0.20 ]A'' [Mn1.89 Cu0.11 ]B1 [Mn2 ]B2 O12 . There are clear signatures of Jahn-Teller distortions in 1 and 2, and the orbital pattern is combined with an original type of charge ordering in 1. Columnar-ordered quadruple perovskites represent a new playground to study complex interactions between different electronic degrees of freedom. No long-range magnetic order was found in 2 by neutron diffraction, and its magnetic properties in low fields are dominated by an impurity with negative magnetization or magnetization reversal. On the other hand, 1 shows three magnetic transitions at 21, 125, and 160 K.

## 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>

## Temperature-induced phase transition from cycloidal to collinear antiferromagnetism in multiferroic Bi0.9Sm0.1FeO3 driven by f-d induced magnetic anisotropy

Physical Review B - Condensed Matter and Materials Physics American Physical Society 95 (2017) 054420-

R Johnson, PA McClarty, DD Khalyavin, P Manuel, P Svedlindh, CS Knee

In multiferroic BiFeO3 a cycloidal antiferromagnetic structure is coupled to a large electric polarization at room temperature, giving rise to magnetoelectric functionality that may be exploited in novel multiferroic-based devices. In this paper, we demonstrate that substituting samarium for 10% of the bismuth ions increases the periodicity of the room-temperature cycloid, and upon cooling to below ∼15 K the magnetic structure tends towards a simple G-type antiferromagnet, which is fully established at 1.5 K. We show that this transition results from f-d exchange coupling, which induces a local anisotropy on the iron magnetic moments that destroys the cycloidal order - a result of general significance regarding the stability of noncollinear magnetic structures in the presence of multiple magnetic sublattices.

## 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 Springer Nature 8 (2017) 1583-

W Saenrang, BA Davidson, F Maccherozzi, JP Podkaminer, J Irwin, R 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, 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.

## Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S=1 quantum antiferromagnet

PHYSICAL REVIEW B 95 (2017) ARTN 134435

J Brambleby, JL Manson, PA Goddard, MB Stone, RD Johnson, P Manuel, JA Villa, CM Brown, H Lu, S Chikara, V Zapf, SH Lapidus, R Scatena, P Macchi, Y-S Chen, L-C Wu, J Singleton

## Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3

ADVANCED MATERIALS 29 (2017) UNSP 1602327

A Agbelele, D Sando, C Toulouse, C Paillard, RD Johnson, R Ruffer, AF Popkov, C Carretero, P Rovillain, J-M Le Breton, B Dkhil, M Cazayous, Y Gallais, M-A Measson, A Sacuto, P Manuel, AK Zvezdin, A Barthelemy, J Juraszek, M Bibes