Magnetoelectrics at the mesoscale

• Dr Neil Mathur, University of Cambridge, UK

Biography

Neil Mathur obtained his PhD degree in Physics from the University of Cambridge in 1996. He then joined the Cambridge department of Materials Science as a Postdoctoral Research Associate before being appointed to a Royal Society University Research Fellowship (1999), a University Lectureship (2005), and a University Readership (2008) which he currently holds. He has been a Fellow of Churchill College since 2000. In 2009 he was awarded the Rosenhain Medal & Prize by The Institute of Materials, Minerals and Mining (UK) for his work on device materials and his contribution to the understanding of magnetic and electronic materials.

• Professor James Scott FRS, University of Cambridge, UK

Biography

Professor James F Scott FRS is a Director of Research in the Department of Physics at the Cavendish Laboratory, Cambridge University. Prior to coming to Cambridge in 1999 he was Dean and Professor of Physics at universities in Australia for eight years (most recently UNSW in Sydney 1995-9) and Professor and Assistant Vice Chancellor for Research at the University of Colorado 1971-1992. His early career was at Bell Labs. Author of 800 publications, mostly on ferroelectrics, he has published five books, and his book "Ferroelectric Memories" (Springer 2000) has been translated into Japanese and Chinese. He won the MRS gold Medal in 2009 and has also received a Humboldt Prize from Germany, a Monkasho award from Japan, and election to the Slovenian Academy of Sciences. In 1986 he was a founder and first chairman of the board of directors of Symetrix Corp. A Fellow of the American Physical Society since 1974, he has had a number of visiting appointments, including the SONY Chair of Science (Atsugi, 1997).

• Professor Darrell G Schlom, Cornell University, USA
• Audio recording (mp3)

Biography

Darrell Schlom is the Hebert Fisk Johnson Professor in the Department of Materials Science and Engineering at Cornell University.  He received a BS degree from Caltech and MS and PhD degrees from Stanford University.  Following a post-doc at IBM Zurich he joined the faculty at Penn State University where he spent 16 years before moving to Cornell in 2008.  His research interests involve the replacement of SiO₂ as the gate dielectric in MOSFETs and the heteroepitaxial growth and characterization of oxide thin films, including their epitaxial integration with semiconductors.  His group synthesizes oxide heterostructures using molecular-beam epitaxy.  He has published over 400 papers and 8 patents resulting in an h-index of 56 and over 13,000 citations.  He received young investigator awards from ONR and NSF, an Alexander von Humboldt Research Fellowship, and the MRS Medal.  He is a Fellow of both the American Physical Society and the Materials Research Society. 

Abstract

When it comes to high temperature magnetoelectric multiferroics, LuFe₂O₄ stands out because it is reported to be simultaneously ferrimagnetic and ferroelectric at the highest temperature of any known material, 250 K.  The multiferroic status of LuFe₂O₄ has, however, recently come into question.  Nonetheless, we have found an adsorption-controlled regime in which single-phase epitaxial films of LuFe₂O₄ can be grown by molecular-beam epitaxy on (111) MgAl₂O₄, (111) MgO, and (0001) 6H-SiC substrates.  These LuFe₂O₄ films exhibit ferrimagnetism below 240 K.  A ferroelectric that is closely related to LuFe₂O₄ is the metastable hexagonal polymorph of LuFeO₃.  Although metastable, hexagonal LuFeO₃ has been grown in thin film form by epitaxial stabilization.  It is an improper structural ferroelectric, isostructural to YMnO₃, and polar at room temperature.  It should, therefore, display similar topologically protected cloverleaf domains.  Additionally,below about 120 K hexagonal LuFeO₃ orders antiferromagnetically in a pattern in which symmetry allows a slight canting of the spins giving rise to weak ferromagnetism.  Our preliminary results on intergrown LuFeO₃-LuFe₂O₄ samples suggest the tantalizing prospect of the existence of a room temperature ferrimagnetic ferroelectric in the LuFe₂O₄(LuFeO₃)_n homologous series.

Co-authors:
J A Mundy, C M Brooks, H Das, Q Mao, T Heeg, C J Fennie, Cornell University, USA
D A Muller, Cornell University and Kavli Institute at Cornell for Nanoscale Science, USA
R Misra, L A Zhang, V Gopalan, Z-K Liu and P Schiffer, Penn State University, USA
W Zander and J Schubert, Peter Gruenberg Institut (PGI-9), Germany
B S Holinsworth, K R O’Neal, and J L Musfeldt, University of Tennessee, USA

• Professor Lane Martin, University of Illinois Urbana-Champaign, USA
• Audio recording (mp3)

Biography

Professor Lane W Martin received his BS in Materials Science and Engineering from Carnegie Mellon University in Dec. 2003 and his MS and PhD in Materials Science and Engineering from the University of California, Berkeley in 2006 and 2008, respectively.  From 2008 to 2009, Lane served as a Postdoctoral Fellow in the Quantum Materials Program, Materials Science Division, Lawrence Berkeley National Laboratory.  Lane joined the Department of Materials Science and Engineering at the University of Illinois, Urbana-Champaign in August 2009.  Lane’s current research focuses on multiferroic and multifunctional materials and devices and oxide materials for energy conversion (eg, photovoltaics, photocatalysis, and waste heat energy conversion). Lane’s work as garnered a number of awards for his work including the National Science Foundation CAREER Award (2012), the Army Research Office Young Investigator Program Award (2010), a National Science Foundation IGERT Fellowship in Nanoscale Science and Engineering (2004-2007), an Intel Robert Noyce Fellowship in Microelectronics (2007-2008), the Graduate Excellence in Materials Science Award (2006), and the Materials Research Society’s Gold Medal Award for Graduate Students (2006).

Abstract

The parent structure of the multiferroic BiFeO₃ is a rhombohedrally distorted perovskite structure, but this material is known to exhibit a strain-induced structural phase transition under large compressive strains to a nearly tetragonally-distorted perovskite phase. At a critical compressive stain level of ~4.5% so-called mixed-phase structures have been observed in which both the tetragonal- and rhombohedral-like phases coexist. It is in these mixed-phase films that reversible electric field induced strains between 4-5% have been reported. Recent studies suggest that such films exhibit an exotic structural evolution accompanied by exciting properties including enhanced electromechanical responses. In this presentation, I will discuss a number of intriguing aspects of these complex materials including the structural evolution of these films as a function of strain, thickness, and temperature. The discussion will include details of new phases of BiFeO₃, the implications of these phases for the observed large electromechanical response, a model for the formation of these structures that builds upon the idea of a spinodal-modulated structure, and routes to stabilize these structures. We will examine the temperature- and thickness-dependent nanostructural evolution of the strain-induced phase boundaries in BiFeO₃/LaAlO₃ (001) heterostructures. It is observed that the fraction of the mixed-phase regions decreases with increasing temperature and that in 40 nm thick films, all evidence of the mixed-phase structure is removed by 300°C. Upon cooling the films, the mixed-phase structures are observed to return. We will discuss the possibility that in some films the mixed-phase structures form via a strain induced spinodal-instability and the resulting mixed-phase structures represent a strain-relaxation mechanism in these films. Details of the thermodynamic landscape and connections with other systems will be discussed. Furthermore, in films > 250 nm, a breakdown of this strain-stabilized metastable mixed-phase structure to non-epitaxial microcrystals of the parent rhombohedral structure of BiFeO₃ is observed. By a thickness of 300 nm, the entire film is observed to have experienced epitaxial breakdown. We will discuss a proposed mechanism for this breakdown and present a proposed phase stability map as a function of strain and film thickness at the growth temperature. We will also investigate chemical alloying routes to further stabilize the mixed-phase structures to greater thicknesses and implications of the the mixed-phase structure of magnetoelectric coupling near room temperature.

References:

1. A R Damodaran, C-W Liang, Q He, C-Y Peng, L Chang, Y-H Chu, L W Martin, Adv Mater 23, 3170 (2011).
2. A R Damodaran, S Lee, J Karthik, S MacLaren, L W Martin, Phys Rev B85, 024113 (2012).
3. A R Damodaran, E Breckenfeld, A Choquette, L W Martin, Appl Phys Lett100, 082904 (2012).

• Dr Xavier Moya, University of Cambridge, UK
• Audio recording (mp3)

Biography

Xavier Moya graduated in Physics in 2003 and obtained his PhD in Physics in 2008 at the University of Barcelona, Spain. In June 2008, he moved to Cambridge to continue his research at the Department of Materials Science and Metallurgy, where he is currently a Herchel Smith Research Fellow.

His research experience and interests span different aspects of condensed-matter physics and materials science. He is particularly interested in the multifunctional properties that arise in solid-state materials exhibiting strong coupling between their different degrees of freedom, eg structural, magnetic and electrical. Concretely, his research focuses on the study of magnetocaloric, electrocaloric and magnetoelectric effects.

Abstract

The local magnetic properties of ferromagnetic manganite films grown epitaxially on ferroelectric BaTiO₃ substrates can be controlled either electrically^1,2 via strain, or magnetically³ via strain‑mediated feedback. The resulting magnetoelectric and magnetocaloric effects depend dramatically on the strength of electron‑lattice coupling in the film, as revealed by 2D magnetic maps constructed from photoemission electron microscopy (PEEM) images with x-ray magnetic circular dichroism (XMCD) contrast. For La_0.67Sr_0.33MnO₃ films with relatively weak electron‑lattice coupling, strain due to thermally driven structural phase transitions in the substrate modifies the local magnetic anisotropy and forces sharp changes in the orientation of the local magnetization, permitting magnetoelectric effects. For films of La_0.7Ca_0.3MnO₃, stronger electron‑lattice coupling yields coexisting ferromagnetic and paramagnetic phases that may be interconverted by thermally driving phase transitions in the substrate. Magnetic cycling reveals that this interconversion may be driven via strain-mediated feedback to yield reversible changes in film entropy that are as large as the best magnetocaloric materials⁴.

[1] W Eerenstein, M Wiora, J L Prieto, J F Scott and N D Mathur, Nature Materials 6, 348 (2007)
[2] C Thiele, K Dörr, O Bilani, J Rödel and L Schultz, Phys Rev B 75, 054408 (2007)
[3] X Moya, L E Hueso, F Maccherozzi, A I Tovstolytkin, D I Podyalovskii, C Ducati, L Phillips, M Ghidini, O Hovorka, A Berger, M E Vickers, E Defaÿ, S S Dhesi and N D Mathur, in preparation
[4] K A Gschneidner Jr, V K Pecharsky and A O Tsokol, Rep Prog Phys68,1479 (2005)

Co-authors:
C Ducati, L C Phillips, W Yan, M Ghidini, M E Vickers, N D Mathur, University of Cambridge, UK
O Hovorka, A Berger, CIC nanoGUNE Consolider, Spain
L E Hueso, CIC nanoGUNE Consolider and Basque Foundation for Science, Spain
F Maccherozzi, S S Dhesi, Diamond Light Source, UK
A I Tovstolytkin, D I Podyalovskii, Institute of Magnetism, Ukraine
E Defaÿ, University of Cambridge, UK and CEA- LETI, France

• Professor Gopalan Srinivasan, Oakland University, Rochester, Michigan, USA
• Audio recording (mp3)

Biography

Gopalan Srinivasan is a Distinguished Professor of Physics at Oakland University in Michigan.  He graduated with a PhD from IIT-Bombay in 1980 and was a Research Associate at West Virginia University and a Research Professor at Colorado State University.  He joined Oakland University in 1988.  Gopalan’s research interests are magnetoelectric effects in composite multiferroics, smart sensors and high frequency phenomena and devices.  His research projects are supported by grants from the National Science Foundation, the Defense Advanced Research Project Agency, the Office of Naval Research and the Army Research Office.  He has 4 patents and more than 220 publications.

Abstract

Mechanical strain mediated magnetoelectric (ME) effects are studied in bilayers and trilayers  of magnetostrictive permendur (P) and piezoelectric single-crystal lanthanum gallium tantalate (LGT), lanthanum gallium silicate (LGS) or quartz.  It is shown that the ME voltage coefficient which is proportional to the ratio of the piezoelectric coupling coefficient to the permittivity, is higher in LGT, LGS or quart-based composites than for traditional ferroelectrics based ME composites. The piezoelectric LGT and LGS  are free of ferroelectric hysteresis, pyroelectric effects and phase transitions up to 1450 ⁰C and is of interest for ultrasensitive, high temperature magnetic sensors.

• Professor Yoshishige Suzuki, Osaka University and CREST, Japan
• Audio recording (mp3)

Biography

Yoshishige Suzuki is a Professor at Graduate school of Engineering Science, Osaka University. He had measured semiconductor physics in Tsukuba University Japan. Then, he had contributed to a development of the “metal-based spintronics” as a researcher in AIST Tsukuba. After he moved to Osaka in 2003, he is investigating high frequency dynamics and voltage effects in magnetic junctions. During this time, he was a visiting researcher in IEF, University Paris-Sud and IMEC in Belgium. He loves physics, cooking, and eating.

Abstract

Voltage control of high speed dynamics in all solid state devices will be introduced. We have first investigated voltage effects on an ultrathin Fe layer grown on Au surface and covered by MgO and find significant anisotropy change caused by a voltage application [1]. Then we tried to observe pulse- and rf-voltage responses in magnetic tunnel junctions with a few atomic FeCo (001) epitaxial layers adjacent to MgO barrier. By an application of voltage to the junction, the magnetic easy-axis in the FeCo ultrathin film changes from in-plane to out-of-plane. This change in anisotropy causes a precession of the magnetization. By determining an appropriate pulse length, the precession resulted in a two-way toggle switching of the magnetization [2]. On the other hand, an application of an rf-voltage causes an excitation of a magnetic resonance (FMR) in FeCo ultrathin films [3]. Since FMR associates with an oscillation in the resistance of the junction, the applied rf-signal is rectified and produces a dc voltage. From the spectrum of the dc voltage as a function of rf frequency, we could distinct voltage induced torque from spin-transfer torque and field like torque.

References:
[1] Maruyama, T et al, Nature Nanotechnology 4, 158-161 (2009)
[2] Shiota, Y et al, Nature Materials, 11, 39-43 (2012)
[3] Nozaki, T et al, Nature Physics, Online publication (2012)

• Professor Josep Fontcuberta, Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
• Audio recording (mp3)

Biography

Josep Fontcuberta was born in Barcelona (Spain) in 1953. He received the PhD degree in Physics at the University of Barcelona in 1982. After a post-doc stage at Laboratory of Inorganic Chemistry in Oxford, he became assistant professor at the University of Barcelona. In 1991 he moved to the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC). He is a Research Professor with interest on functional properties of materials, with focus on multiferroic oxides.

He has co-authored more than 350 peer-reviewed scientific papers and directed 15 PhD Thesis. He has delivered around 100 Invited Talks at Workshops and Conferences. He holds several patents. At the present, he is coordinating several research projects and has participated in variousl EU research projects (STREPS, Networks, Marie-Curie Tanning site, etc) and ESF and NATO grants. He is the director of the Laboratory of Magnetism and Magnetic Materials of ICMAB. Josep Fontcuberta, is currently at the Editorial Board of Solid State Communications.

Abstract

In some cycloidal antiferromagnets, electric dipoles are formed in a direction perpendicular to the propagation vector of the spin rotation. The resulting polarization is extremely sensitive to the magnetic order: it can be switched by 180° by reversing the sense of rotation of spins in the magnetic cycloid or by 90° by flopping the rotation plane of the cycloids by applying suitable magnetic fields. The intimate coupling of ferroelectric polarization and magnetic order implies that reversing the ferroelectric polarization by an appropriate field results in reversing the rotation sense of the cycloid.

Here we shall overview some recent progress on understanding these phenomena in a variety of cycloidal antiferromagnetic perovskite, both thin films and single crystals.

We will show that orthorhombic YMnO₃ thin films display the features described above. In these films, the  polarization vector direction can be controlled by applying electric and/or magnetic field, in an irreversible manner with memory effects. Therefore hysteresis in these antiferromagnetic materials occurs and, although they have a null magnetization, magnetic information can be encoded by the sense of spin rotation: “turning clockwise” or “anticlockwise”, or by the rotation plane of atomic spins across the lattice.

Data on related single crystals confirm a similar response and allow to show that electric and magnetic fields, however, have a dramatically different impact on the magnetoelectric response, namely the flopping ability control of the polarization, that appears to be rooted on the very nature of the multiferroic coupling mechanism.

Overall, these findings show that cycloidal magnets may offer an opportunity to build magnetic memories with antiferromagnetic.

• Dr Stephan Geprägs, Walther-Meißner-Institut, Germany
• Audio recording (mp3)

Biography

Stephan Geprägs graduated in Physics in 2004 and obtained his PhD degree in Physics in 2011 at the Technical University of Munich, Germany. Since 2012 he is a postdoctoral research assistant at the Walther-Meissner-Institute for low temperature research, which is part of the Bavarian Academy of Sciences and Humanities. His research focuses on oxide based multifunctional material systems. In particular, he is interested in the interaction of magnetic and dielectric degrees of freedom in intrinsic and extrinsic multiferroic thin films and composites.

Abstract

Extrinsic multiferroic composites, in which ferromagnetic and ferroelectric compounds are artificially assembled, allow for robust converse magnetoelectric effects at room temperature by exploiting the elastic coupling between the two constituents [1]. This indirect converse magnetoelectric coupling can be described phenomenologically by a product tensor property including both piezoelectric and magnetoelastic effects [2]. To predict the indirect magnetoelectric coupling in novel multiferroic structures, a detailed understanding of these effects and the elastic coupling across the interface is mandatory [3].

We here present a detailed study of the reversible and irreversible control of the magnetization by electric fields in ferromagnetic/ferroelectric hybrid structures using BaTiO₃ as the ferroelectric compound as well as Ni or Fe₃O₄ thin films as the ferromagnetic constituent [4]. In case of extrinsic multiferroic hybrid structures using BaTiO₃ crystals, the variation of the magnetization as a function of the applied electric field can be explained by the magnetic behaviour of those parts of the ferromagnetic thin film clamped to ferroelastic a-domains of the BaTiO₃ substrate, while the magnetization of regions of the ferromagnetic thin film located on top of c-domains stays nearly unaffected. We show that the experimentally obtained converse magnetoelectric effects at room temperature in multiferroic hybrids are well described within this approach. However, for ferromagnetic/BaTiO₃ heterostructures grown on SrTiO₃ substrates, the elastic coupling of the BaTiO₃ thin film to the rigid substrate does not allow for large magnetoelectric effects.

Financial support by the German Excellence Initiative via the Nanosystems Initiative Munich (NIM) is gratefully acknowledged.

[1] W Eerenstein et al, Nat Mater 6, 348 (2007).
[2] C-W Nan, Phys Rev B 50, 6082 (1994).
[3] S Geprägs et al, arXiv:1208.4738v1.
[4] S Geprägs et al, Appl Phys Lett 96, 142509 (2010).

• Dr Massimo Ghidini, University of Parma, Italy and University of Cambridge, UK
• Audio recording (mp3)

Biography

Massimo Ghidini holds a faculty position at the University of Parma  and is a visiting academic at Cambridge. After working at the CNRS Institut  Nèel in Grenoble, where he obtained his PhD, was a Marie Curie fellow at Oxford University and subsequently held appointments as visiting professor and scientist at the Institut National Polytechnique de Grenoble and Cornell University. His research focuses on the physics of magnetic and magnetoelectric  materials at the mesoscale. 

Abstract

Electrical control of in‑plane film magnetization has been previously achieved via changes in stress, exchange bias, carrier density and orbital occupation. I will describe changes of out‑of‑plane magnetization in polycrystalline films of magnetostrictive nickel, due to continuous and discontinuous electrically driven strains from polycrystalline and single‑crystal BaTiO₃, respectively.

Commercial multilayer capacitors display strain‑mediated magnetoelectric coupling between Ni electrodes and polycrystalline BaTiO₃-based dielectric layers. Magnetic Force Microscopy (MFM) reveals a perpendicularly magnetized feature that exhibits non-volatile electrically driven repeatable magnetization reversal with no applied magnetic field. This magnetization reversal is achieved in a dynamic process triggered by a temporary reduction of perpendicular anisotropy due to fast and reversible ferroelectric switching.

For nickel films on BaTiO₃ single-crystal substrates, I will report photoemission electron microscopy (with magnetic contrast from x-ray magnetic circular dichroism) and MFM studies. As expected, built-in strain yields an out‑of‑plane magnetization whose sign alternates every 125 nm along an in-plane direction that varies on a longer length scale. These stripe domains may be switched on and off by varying film strain, first by thermally cycling through structural phase transitions in BaTiO₃, then by electrically switching the ferroelectric domains. The electrical switching is non-volatile, and could inspire the design of electrically driven nanomagnets, eg. for electric-write magnetic-read processes in perpendicular recording media.

• Dr Yasujiro Taguchi, RIKEN, Japan
• Audio recording (mp3)

Biography

Dr Yasujiro Taguchi is the Team Leader of Exploratory Materials Team, Cross-correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG), RIKEN Advanced Science Institute. He obtained a PhD degree in engineering from the University of Tokyo under the supervision of Professor Y Tokura. He was an associate Professor in Institute of Materials Research, Tohoku University from 2002 to 2007, and worked on superconductivity induced by alkali-metal intercalation.

In 2007, he moved to RIKEN, and his current research interests involve multiferroic materials, thermoelectric materials, and strongly correlated materials mostly based on oxides.

Abstract

Two multiferroic materials showing strong coupling between polarization and magnetism are discussed: One is perovskite-type manganite (Sr,Ba)MnO₃ and the other is rare-earth ortho-ferrite RFeO₃ . In the manganite, magnetic Mn^4+ ions with S=3/2 are found to exhibit off-centering, independently of the magnetic ordering[1]. The ferroelectric transition in (Sr,Ba)MnO₃ is governed by a soft phonon mode, and its dynamics is revealed in detail by far-infrared reflectivity and momentum-resolved inelastic x-ray scattering measurements[2]. Strong coupling between spin and polarization is also found in the temperature-dependent crystal structure. In the rare-earth ferrite, polarization is induced by the symmetric exchange striction working between the rare-earth moment and iron spin. We succeed in reversing the weak-ferromagnetic moment associated with the iron spins that show G-type ordering, only with the use or electric field[3].

[1] H Sakai, J Fujioka, T Fukuda, D Okuyama, D Hashizume, F Kagawa, H Nakao, Y Murakami, T Arima, A Q R Baron, Y Taguchi, and Y Tokura, Phys Rev Lett 107, 137601 (2011).

[2] H Sakai, J Fujioka, T Fukuda, M S Bahramy, D Okuyama, R Arita, T Arima, A Q R Baron, Y Taguchi, and Y Tokura, Phys Rev B, in press.

[3] Y Tokunaga, Y Taguchi, T Arima, and Y Tokura, Nature Phys, in press.

• Professor Matt Rosseinsky FRS, University of Liverpool, UK
• Audio recording (mp3)

Biography

Matthew Rosseinsky obtained a degree in Chemistry from the University of Oxford and a D Phil under the supervision of Professor P Day FRS in 1990. He was a Postdoctoral Member of Technical Staff at AT&T Bell Laboratories in Murray Hill, New Jersey where his work with D W Murphy, A F Hebard and R C Haddon led to the discovery of superconductivity in alkali metal fullerides. In 1992, he was appointed University Lecturer at the Inorganic Chemistry Laboratory, University of Oxford, where he remained until 1999 when he moved to the University of Liverpool as Professor of Inorganic Chemistry. He was awarded the inaugural de Gennes Prize for Materials Chemistry (a lifetime award for achievement in this research area open to all scientists internationally) by the Royal Society of Chemistry in 2009 and the CNR Rao Award of the Chemical Research Society of India in 2010. He was elected a Fellow of the Royal Society in 2008, and was awarded the Hughes Medal of the Royal Society in 2011.  His work addresses the synthesis of new functional materials for energy and information storage applications, and has been characterised by extensive collaboration with many academic and industrial colleagues.

Abstract

The presentation will address the chemical synthesis and structural challenges to combine polarity⁽¹⁾ and magnetism⁽²⁾ in oxides, and will include the local ⁽³⁾ as well as conventional diffraction-based long-range average views of chemical structure.

(1)  M Dolgos, U Adem et al, Angewandte Chemie in press 2012
(2)  M Li, U Adem, S R C McMitchell et al, J Am Chem Soc 134, 3737, 2012
(3)  S Y Chong, R Szczencinski et al. J Am Chem Soc 134, 3737, 2012

• Professor Andrew Bell, University of Leeds, UK
• Audio recording (mp3)

Biography

Andrew Bell holds a degree in Physics from the University of Birmingham, UK (1978) and PhD in Ceramic Science and Engineering from the University of Leeds, UK (1984). He has spent almost 15 years in industrial research positions, with the Plessey Company, Cookson Group and Oxley Developments. He was a Senior Scientist in the Ceramics Laboratory of EPFL, Switzerland from1991 to 95 and has been Professor of Electronic Materials at the University of Leeds since March 2000 where he is currently Head of the School of Process, Environmental and Materials Engineering.

He has worked on a wide range of topics with a common theme of translating fundamental science into the development of applications-specific ferroelectric and dielectric materials. His approach has encompassed basic science, materials processing, structural and electrical characterization and device physics. His main achievements span the fields of pyroelectric materials and devices, microwave dielectrics and phenomenological modelling applied to BaTiO₃,relaxors, piezoelectric single crystals and PZT. More recently his work has focussed on BiFeO₃-PbTiO₃ based ceramics, single crystals and thin films for multiferroic and high temperature piezoelectric devices. His current research group comprises four postdoctoral researchers and 12 PhD students.

Abstract

Despite the large volume of relevant research, it seems that the occurrence of strong magnetoelectric coupling in multiferroic oxides is vanishingly rare. Given the inherent weakness of the direct magnetoelectric effect, the most likely mechanism by which electric field can significantly influence magnetisation is via the mutual dependence on lattice strain. In the perovskites, this is well understood in terms of traversing two sides of the multiferroic triangle. The premise of the current work is that the “indirect” magnetoelectric effect can be maximised at phase transitions in which there are large changes in lattice strain.

We have examined in detail the “tri-ferroic” phase boundary region between the rhombohedral and tetragonal distortions in the BiFeO₃-PbTiO₃ solid solution, which exhibits an exceptionally large differential strain of >15% along [001]. In situ neutron diffraction experiments shows that an isostatic pressure of 0.6 GPa is sufficient to transform the paramagnetic tetragonal phase to the antiferromagnetic rhombohedral phase, with a concomitant change in the ferroelectric polar axis. Experiments are in progress to demonstrate similar phase switching under applied electric field.

• Professor Paolo Radaelli, University of Oxford, UK
• Audio recording (mp3)

Biography

Paolo G Radaelli is the Dr Lee’s Professor of Experimental Philosophy at the Department of Physics, Oxford University. Following a Laurea degree at the Università degli Studia  di  Milano and a PhD at Illinois Institute of Technology, Professor Radaelli has held posts at the Argonne National Laboratory, CNRS Grenoble, the Institute Laue–Langevin and the ISIS Facility at the Rutherford Appleton Laboratory. His main interest is the study of transition metal oxides displaying novel physical phenomena, such as high-temperature superconductivity, “colossal” magneto-resistance or multiferroics behaviour, with the potential of device applications.

Abstract

The recent “renaissance” of research on multiferroics has led to the discovery of many materials where the appearance of a complex magnetic structure breaks inversion symmetry.  Electrical polarization then arises through an interaction between the magnetic and electronic structures and the lattice.  I will present examples of different types of magnetic structures and interactions, associated with distinct mechanisms of multiferroicity. In particular, I will show how direct coupling between the crystal and magnetic symmetry can be exploited to design new multiferroics and magnetoelectrics with improved properties.

• Professor Andrew Boothroyd, University of Oxford, UK
• Audio recording (mp3)

Biography

Andrew Boothroyd is a Professor of Physics at Oxford University, and also a Tutorial Fellow and currently Vice-Provost of Oriel College, Oxford. His research exploits neutron and x-ray scattering techniques to explore a range of emergent electronic states found in quantum materials, including novel magnetic states and unconventional superconductivity.

Andrew Boothroyd took undergraduate and graduate degrees at the University of Cambridge, completing his PhD in Physics at the Cavendish Laboratory in 1988. Immediately afterwards he joined the University of Warwick, first as a postdoctoral research associate and shortly afterwards as a University Lecturer. In 1991, he was appointed to a University Lectureship in Physics at Oxford University with a Tutorial Fellowship at Oriel College. He served as Head of Condensed Matter Physics from 2004 to 2008, and in 2006 was awarded the title of Professor of Physics. He was the winner of the Institute of Physics Superconductivity Group Prize in 2011.

Abstract

Cupric oxide (CuO) embodies many of the current themes in complex electronic matter: magnetic frustration, quantum (low-dimensional) magnetism, spin currents, and coupled magnetic and electric order parameters.  Recently, Kimura et al. [1] discovered a ferroelectric phase of CuO at relatively high temperatures (213 – 230 K) coincident with incommensurate magnetic order.  In this talk I shall present polarised neutron diffraction measurements [2] which demonstrate not only that helical magnetic order is coupled to ferroelectric order, but also that an electric field can be used to switch between chiral magnetic domains of opposite handedness. Possible mechanisms for the coupling will be discussed.

[1] T Kimura, Y Sekio, H Nakamura, T Siegrist, and A P Ramirez, Nat Mater 7, 291 (2008)

[2] P Babkevich, A Poole, R D Johnson, B Roessli, D Prabhakaran, and A T Boothroyd, Phys Rev B 85, 134428 (2012)

• Dr Markys Cain, National Physical Laboratory, UK
• Audio recording (mp3)

Biography

Markys Cain graduated with his PhD from Warwick University in 1990 and spent the next 2 years in the Materials Department of the University of California, Santa Barbara studying thin film epitaxial science. Subsequent research in ceramic composite materials technology in the UK utilised many of the principles learnt at Santa Barbara in the deployment of new interfacial fibre coatings for advanced high temperature ceramic matrix composites for gas turbine applications. Research with an Oxford based company led to the development of a prototype SEM based instrumented indentation system and he joined NPL in 1997 to lead the Functional Materials Research group. Research activity includes the development of measurement methods to elucidate materials behaviour in ferroelectric and piezoelectric ceramics and thin film materials, and more recently in multiferroic materials and materials metrology for Spintronics and Energy Harvesting. The focus of the research is materials metrology and he has published over 80 peer reviewed scientific papers in the field. He chairs the IOM3 Smart Materials and Systems Committee. He is Knowledge Leader for the Materials Division at NPL and Principal Research Scientist for the Multifunctional Materials technical area and also a member of the Institute of Physics.  In 2009 he was awarded the Institute of Materials’ Verulam medal for outstanding contributions to ceramic science, and is visiting Professor at Queen Mary, University of London.

Abstract

Multiferroics are a special class of materials exhibiting multiple cooperative phenomena that have recently attracted a lot of attention because of their potential for enhanced functionality for sensors and devices. They have been predicted since 1894 and pioneering work on multiferroic materials dates back to 1950s – 1960s.

By definition, multiferroics (also called magnetoelectrics) possess two or more switchable states such as electric polarization, magnetization or strain. Hence, they can change the electric polarization by a magnetic field or the magnetization by an electric field. The interplay between electric and magnetic states is realized via the magneto-electric (ME) effect, which is defined as the coupling between electric and magnetic fields in matter. Although this coupling can have non-linear components, the ME effect is usually described mathematically by the linear ME coupling coefficient (a), which is the dominant coupling term. In multiferroics, the internal magnetic / electric fields are enhanced by the presence of the multiple long-range ferroic ordering, which in turn produces large ME coupling effects.

In addition, the ME coupling can be stress mediated in samples exhibiting piezo-effects, especially when the two ferroic phases are clearly separated as in multiferroic laminates or composites. In this case, the ME coupling effect is described by a device effective ME coupling coefficient, which contains the linear effect and the stress mediated component. The exact knowledge of the effective ME coupling coefficient is essential for the design of the devices and sensors based on multiferroics. So far, a number of important applications based on the ME effect in multiferroic composite materials have been proposed or developed.

The overall functionality of these devices depends of the ME coupling strength and there is a growing demand for suitable metrologies to better define this parameter. The main aim of NPL’s work in this field is to develop an experimental capability for multiferroic materials and devices characterization (ie measurement of the ME coupling coefficient), in the framework of a National Measurement Institute.

There are two types of multiferroic materials: a) single phase, when the multiferroic state occurs within a single compound material, which is usually an oxide; b) multi-phase composite multiferroic, when the magnetic and electric components are represented by different compounds which are mechanically clamped to each other. The ME coupling in the case of composites is stress mediated and is often much larger than that of single-phase multiferroics. Moreover, the composite multiferroics can be easily engineered during fabrication so that the piezoelectric / ferroelectric and magnetostriction / ferromagnetic properties of the individual phases are maximized for any specific temperature or external field condition.

In this presentation we describe methods developed at NPL in which the coupling coefficient may be measured, we provide some examples of how the measurement may be dominated by artefacts and we outline a few other techniques which are currently under development.

• Professor Gustau Catalan, Institucio Catalana de Recerca i Estudis Avançats (ICREA) and Centre for Investigations in Nanoscience and Nanotechnology (CIN2), Spain
• Audio recording (mp3)

Biography

Professor Catalan graduated in Physics at the University of Barcelona (1997) and got his PhD, also in Physics, at the Queen’s University of Belfast (2001). He then held a postdoctoral position at the University of Groningen and a Research Fellowship at the University of Cambridge. Since 2009, he has been an ICREA research professor at the Centre for Investigations in Nanoscience and Nanotechnology (CIN2, Barcelona), where he leads the Oxide Nanoelectronics group.

His research interests are wide, having worked on various size effects in metal-insulator transitions, ferroelectrics, relaxors and multiferroics. Perhaps the best known contributions are the unveiling of a magnetorresistive mechanism that generates magnetocapacitance in non-magnetoelectric materials, the discovery of the flexoelectric contribution to the size effect in ferroelectric films, and the unification of the domain size theory for different ferroics. Currently, his work centres on two areas: the coupling between dielectric polarization and strain gradients (flexoelectricity), and the properties of “intrinsic” boundaries such as domain walls and surface layers. This last topic will be the basis of his talk.

Abstract

Kroemer’s idea that “the interface is the device” [1] has inspired much research on interface engineering whereby different materials are combined so as to achieve new interfacially-induced properties. But there are other interfaces that do not require growing different materials. One is a material’s interface with air –its “skin”. The other are domain walls, which are present in all ferroics. This talk will cover both. Starting with a thermodynamic explanation of why/how can domain walls present magnetoelectric properties different from those of the host material [2, 3], I will move on to experimental results on the discovery and characterization of the surface (“skin”) of multiferroic BiFeO₃ that suggest that it should be treated as a material different from the bulk, with its own structure and phase diagram [4,5].

References

[1]H Kroemer, Nobel Prize Acceptance Speech (Physics), 2000.
[2]M Daraktchiev, G Catalan, J F Scott, Physical Review B 81, 224118 (2010).
[3] G Catalan et al, Reviews of Modern Physics  84, 119 (2012).
[4]X Marti et al, Physical Review Letters 106, 236101 (2011).
[5]P Jarrier et al, Physical Review B 85, 184104 (2012).

• Professor James Scott FRS, University of Cambridge, UK
• Audio recording (mp3)

Biography

Professor James F Scott FRS is a Director of Research in the Department of Physics at the Cavendish Laboratory, Cambridge University. Prior to coming to Cambridge in 1999 he was Dean and Professor of Physics at universities in Australia for eight years (most recently UNSW in Sydney 1995-9) and Professor and Assistant Vice Chancellor for Research at the University of Colorado 1971-1992. His early career was at Bell Labs. Author of 800 publications, mostly on ferroelectrics, he has published five books, and his book "Ferroelectric Memories" (Springer 2000) has been translated into Japanese and Chinese. He won the MRS gold Medal in 2009 and has also received a Humboldt Prize from Germany, a Monkasho award from Japan, and election to the Slovenian Academy of Sciences. In 1986 he was a founder and first chairman of the board of directors of Symetrix Corp. A Fellow of the American Physical Society since 1974, he has had a number of visiting appointments, including the SONY Chair of Science (Atsugi, 1997).

Abstract

I will discuss recent work on multiferroic oxides that are single-phase compounds of PbFe(1/2)Ta(1/2)O3 and PbFe(2/3)W(1/3)O3 with PbZr(x)Ti(1-x)O3. The former material has been prepared both as ball-milled bulk ceramics and as thin films and exhibits a remarkably large magnetoelectric coefficient ca. 3 x 10e-7 (SI). The latter material has only biquadratic coupling, and an analysis of its dielectric properties under applied magnetic fields shows an inductive component probably due to charge injection at the electrode-dielectric interface. Some unpublished Asian work on room-temperature Aurivilius-phase Fe/Co oxides will be mentioned briefly.