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Magnetoelectrics at the mesoscale

Event

Starts:

September
262012

09:00

Ends:

September
272012

17:00

Location

Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ

Overview

Ferroelectric domain pattern of hexagonal YMnO3, courtesy of Professor Sang-Wook Cheong.

This meeting will focus on the mesoscale properties of multiferroic and magnetoelectric materials for future device applications that exploit magnetic and electrical order parameters that are coupled either directly or via strain. A theoretical and experimental treatment of surfaces and interfaces, including domain walls, will provide insight into current perspectives and future trends.

Biographies of the organisers and speakers are available below. Recorded audio of the presentations will be available on this page after the event.

Attending this event

This is a residential conference, which allows for increased discussion and networking.  It is free to attend, however participants need to cover their accommodation and catering costs if required.

Participants are also encouraged to attend the related Discussion meeting Magnetoelectric phenomena and devices which immediately precedes this event.

Enquiries: Contact the events team.

Event organisers

Select an organiser for more information

Schedule of talks

Session 1

4 talks Show detail Hide detail

Epitaxial engineering of magnetoelectric multiferroics

Professor Darrell G Schlom, Cornell University, USA

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

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Unraveling the complex phase evolution in highly-strained BiFeO3 thin films: thickness, temperature, and chemical-alloying evolution

Professor Lane Martin, University of Illinois Urbana-Champaign, USA

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:

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

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Strain-control of local magnetism in manganite films on barium titanate substrates

Dr Xavier Moya, University of Cambridge, UK

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Magnetoelectric effects in composites of ferromagnetic alloys and piezoelectric langatate, langasite or quartz

Professor Gopalan Srinivasan, Oakland University, Rochester, Michigan, USA

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.

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Session 2

4 talks Show detail Hide detail

Dynamic switching of a magnetization using voltage pulses

Professor Yoshishige Suzuki, Osaka University and CREST, Japan

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)

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Chiral memory in magnetic ferroelectrics

Professor Josep Fontcuberta, Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain

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.

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Converse magnetoelectric interactions in extrinsic multiferroic composites

Dr Stephan Geprägs, Walther-Meißner-Institut, Germany

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 BaTiOas 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 SrTiO3substrates, 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).

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Thermal and electrical control of perpendicular magnetization

Dr Massimo Ghidini, University of Parma, Italy and University of Cambridge, UK

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.

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Session 3

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Strong coupling between polarization and magnetism in new multiferroic manganite and ferrite

Dr Yasujiro Taguchi, RIKEN, Japan

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.

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New polar and polar-magnetic oxides

Professor Matt Rosseinsky FRS, University of Liverpool, UK

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

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Multiferroic properties of morphotropic phase boundary perovskites

Professor Andrew Bell, University of Leeds, UK

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.

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More than one twist: materials and mechanisms of multiferroicity

Professor Paolo Radaelli, University of Oxford, UK

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.

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Session 4

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Electric-field control of chiral magnetic domains in the high temperature multiferroic CuO

Professor Andrew Boothroyd, University of Oxford, UK

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)

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Metrological development for multiferroic materials characterisation

Dr Markys Cain, National Physical Laboratory, UK

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.

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Magnetoelectric boundaries

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

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).

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Quaternary oxides: new room-temperature multiferroics

Professor James Scott FRS, University of Cambridge, UK

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.

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Magnetoelectrics at the mesoscale Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ