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

Spectroscopy Methods for Molecular Nanomagnets


ML Baker, SJ Blundell, N Domingo, S Hill

Local Structure and Bonding of Transition Metal Dopants in Bi2Se3 Topological Insulator Thin Films

JOURNAL OF PHYSICAL CHEMISTRY C 119 (2015) 17344-17351

AI Figueroa, G van der Laan, LJ Collins-McIntyre, G Cibin, AJ Dent, T Hesjedal

Engineering coherent interactions in molecular nanomagnet dimers


A Ardavan, AM Bowen, A Fernandez, AJ Fielding, D Kaminski, F Moro, CA Muryn, MD Wise, A Ruggi, EJL McInnes, K Severin, GA Timco, CR Timmel, F Tuna, GFS Whitehead, REP Winpenny

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.

An ultra-compact, high-throughput molecular beam epitaxy growth system.

The Review of scientific instruments 86 (2015) 043901-

AA Baker, W Braun, G Gassler, S Rembold, A Fischer, T Hesjedal

We present a miniaturized molecular beam epitaxy (miniMBE) system with an outer diameter of 206 mm, optimized for flexible and high-throughput operation. The three-chamber system, used here for oxide growth, consists of a sample loading chamber, a storage chamber, and a growth chamber. The growth chamber is equipped with eight identical effusion cell ports with linear shutters, one larger port for either a multi-pocket electron beam evaporator or an oxygen plasma source, an integrated cryoshroud, retractable beam-flux monitor or quartz-crystal microbalance, reflection high energy electron diffraction, substrate manipulator, main shutter, and quadrupole mass spectrometer. The system can be combined with ultrahigh vacuum (UHV) end stations on synchrotron and neutron beamlines, or equivalently with other complex surface analysis systems, including low-temperature scanning probe microscopy systems. Substrate handling is compatible with most UHV surface characterization systems, as the miniMBE can accommodate standard surface science sample holders. We introduce the design of the system, and its specific capabilities and operational parameters, and we demonstrate the epitaxial thin film growth of magnetoelectric Cr2O3 on c-plane sapphire and ferrimagnetic Fe3O4 on MgO (001).

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.

Evidence for magnetic clusters in Ni1-xVx close to the quantum critical concentration


R Wang, S Ubaid-Kassis, A Schroeder, PJ Baker, FL Pratt, SJ Blundell, T Lancaster, I Franke, JS Moeller, T Vojta

Anisotropic local modification of crystal field levels in Pr-based pyrochlores: a muon-induced effect modeled using density functional theory.

Physical review letters 114 (2015) 017602-

FR Foronda, F Lang, JS Möller, T Lancaster, AT Boothroyd, FL Pratt, SR Giblin, D Prabhakaran, SJ Blundell

Although muon spin relaxation is commonly used to probe local magnetic order, spin freezing, and spin dynamics, we identify an experimental situation in which the measured response is dominated by an effect resulting from the muon-induced local distortion rather than the intrinsic behavior of the host compound. We demonstrate this effect in some quantum spin ice candidate materials Pr(2)B(2)O(7) (B=Sn, Zr, Hf), where we detect a static distribution of magnetic moments that appears to grow on cooling. Using density functional theory we show how this effect can be explained via a hyperfine enhancement arising from a splitting of the non-Kramers doublet ground states on Pr ions close to the muon, which itself causes a highly anisotropic distortion field. We provide a quantitative relationship between this effect and the measured temperature dependence of the muon relaxation and discuss the relevance of these observations to muon experiments in other magnetic materials.

Soft chemical control of superconductivity in lithium iron selenide hydroxides Li(1-x)Fe(x)(OH)Fe(1-y)Se.

Inorganic chemistry 54 (2015) 1958-1964

H Sun, DN Woodruff, SJ Cassidy, GM Allcroft, SJ Sedlmaier, AL Thompson, PA Bingham, SD Forder, S Cartenet, N Mary, S Ramos, FR Foronda, BH Williams, X Li, SJ Blundell, SJ Clarke

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li(1-x)Fe(x)(OH)Fe(1-y)Se (x ∼ 0.2; 0.02 < y < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction, and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesized samples when the iron vacancy concentration is low (y < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasized by the demonstration that reductive postsynthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li(1-x)Fe(x)(OH) reservoir layer to fill vacancies in the selenide layer.

Fermi surface of IrTe2 in the valence-bond state as determined by quantum oscillations

PHYSICAL REVIEW B 91 (2015) ARTN 121105

SF Blake, MD Watson, A McCollam, S Kasahara, RD Johnson, A Narayanan, GL Pascut, K Haule, V Kiryukhin, T Yamashita, D Watanabe, T Shibauchi, Y Matsuda, AI Coldea

Linear magnetoresistance caused by mobility fluctuations in n-doped Cd(3)As(2).

Physical review letters 114 (2015) 117201-

A Narayanan, MD Watson, SF Blake, N Bruyant, L Drigo, YL Chen, D Prabhakaran, B Yan, C Felser, T Kong, PC Canfield, AI Coldea

Cd(3)As(2) is a candidate three-dimensional Dirac semimetal which has exceedingly high mobility and nonsaturating linear magnetoresistance that may be relevant for future practical applications. We report magnetotransport and tunnel diode oscillation measurements on Cd(3)As(2), in magnetic fields up to 65 T and temperatures between 1.5 and 300 K. We find that the nonsaturating linear magnetoresistance persists up to 65 T and it is likely caused by disorder effects, as it scales with the high mobility rather than directly linked to Fermi surface changes even when approaching the quantum limit. From the observed quantum oscillations, we determine the bulk three-dimensional Fermi surface having signatures of Dirac behavior with a nontrivial Berry phase shift, very light effective quasiparticle masses, and clear deviations from the band-structure predictions. In very high fields we also detect signatures of large Zeeman spin splitting (g∼16).

Inverse order-disorder transition of charge stripes

PHYSICAL REVIEW B 92 (2015) ARTN 205114

S-H Lee, Y-C Lai, C-H Du, AF Siegenfeld, Y-J Kao, PD Hatton, D Prabhakaran, Y Su, D-J Huang

Local electronic and structural environment of transition metal doped Bi2Se3 topological insulator thin films

Journal of Physical Chemistry C (2015)

AI Figuorea, G van der Laan, Collins-McIntyre, G Cibin, AJ Dent, T Hesjedal

Transition metal (TM) doped topological insulators have been the focus of many recent studies since they exhibit exotic quantum and magneto-electric effects, and offer the prospect of potential applications in spintronic devices. Here we report a systematic study of the local electronic and structural environment using x-ray absorption fine structure (XAFS) in TM (= Cr, Mn, and Fe) doped Bi2Se3 thin films grown by molecular beam epitaxy. Analysis of the TM K-edge XAFS reveals a divalent character for Cr, Mn, and Fe when substituting Bi in the films, despite the trivalent character of the Bi. All dopants occupy octahedral sites in the Bi2Se3 lattice, which agrees with substitutional incorporation onto the Bi sites. With the incorporation of TM dopants a local structural relaxation of the Bi2Se3 lattice is observed, which strengthens the covalent character of the TM–Se bond. The presence of additional phases and interstitial incorporation for the Mn and Fe dopants is also observed, even at low concentrations.

Tailoring of magnetic properties of ultrathin epitaxial Fe films by Dy doping

AIP ADVANCES 5 (2015) ARTN 077117

AA Baker, AI Figueroa, G van der Laan, T Hesjedal

Study of Dy-doped Bi₂Te₃: thin film growth and magnetic properties.

Journal of physics. Condensed matter : an Institute of Physics journal 27 (2015) 245602-

SE Harrison, LJ Collins-McIntyre, S-L Zhang, AA Baker, AI Figueroa, AJ Kellock, A Pushp, SSP Parkin, JS Harris, G van der Laan, T Hesjedal

Breaking the time-reversal symmetry (TRS) in topological insulators (TIs) through ferromagnetic doping is an essential prerequisite for unlocking novel physical phenomena and exploring potential device applications. Here, we report the successful growth of high-quality (Dy(x)Bi(1-x))2Te3 thin films with Dy concentrations up to x = 0.355 by molecular beam epitaxy. Bulk-sensitive magnetisation studies using superconducting quantum interference device magnetometry find paramagnetic behaviour down to 2 K for the entire doping series. The effective magnetic moment, μeff, is strongly doping concentration-dependent and reduces from ∼12.6 μ(B) Dy(-1) for x = 0.023 to ∼4.3 μ(B) Dy(-1) for x = 0.355. X-ray absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Dy M4,5 edge are employed to provide a deeper insight into the magnetic nature of the Dy(3+)-doped films. XMCD, measured in surface-sensitive total-electron-yield detection, gives μ(eff )= 4.2 μ(B) Dy(-1). The large measured moments make Dy-doped films interesting TI systems in which the TRS may be broken via the proximity effect due to an adjacent ferromagnetic insulator.

Universal Magnetic Hall Circuit Based on Paired Spin Heterostructures


S Zhang, AA Baker, J-Y Zhang, G Yu, S Wang, T Hesjedal

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


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

Magnetostructural relationship in the tetrahedral spin-chain oxide CsCoO2

PHYSICAL REVIEW B 91 (2015) ARTN 024419

NZ Ali, RC Williams, F Xiao, SJ Clark, T Lancaster, SJ Blundell, DV Sheptyakov, M Jansen

Transverse field muon-spin rotation signature of the skyrmion-lattice phase in Cu2OSeO3

PHYSICAL REVIEW B 91 (2015) ARTN 224408

T Lancaster, RC Williams, IO Thomas, F Xiao, FL Pratt, SJ Blundell, JC Loudon, T Hesjedal, SJ Clark, PD Hatton, MC Hatnean, DS Keeble, G Balakrishnan