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Research highlights

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Carrier-mediated magnetoelectricity in complex oxide heterostructures.

J. M. Rondinelli, M. Stengel and N. A. Spaldin, Nature Nanotechnology 3, 46 (2008).

The search for a general means to control the coupling between electricity and magnetism has intrigued scientists since Ørsted's discovery of electromagnetism in the early 19^th century. While tremendous success has been achieved in creating both single phase and composite magnetoelectric materials, the quintessential electric-field switching of magnetism remains a challenge. In this work we demonstrated a  linear magnetoelectric effect which arises from a carrier-mediated mechanism, and is a universal feature of the interface between a dielectric and a spin-polarized metal. Using first-principles density functional calculations, we illustrated this effect at the SrRuO₃/SrTiO₃ interface and describe its origin. To formally quantify the magnetic response of such an interface to an applied electric field, we introduced and defined the concept of spin capacitance. In addition to its magnetoelectric and spin capacitive behavior, the interface displays a spatial coexistence of magnetism and dielectric polarization suggesting a route to a new type of interfacial multiferroic.

Origin of the dielectric dead layer in nanoscale capacitors.

M. Stengel and N. A. Spaldin, Nature 443, 679-682 (2006).

Capacitors are omnipresent in electronic integrated circuits and devices, where they perform essential functions including storing electrical charge, and blocking direct current while allowing alternating currents to propagate. Since they are often the largest components in circuits, extensive efforts are directed at reducing their size through the use of high permittivity insulators such as perovskite-structure SrTiO₃, which should provide more capacitance per unit area of device. Unfortunately, however, most experiments on thin-film SrTiO₃ capacitors have yielded capacitance values that are orders of magnitude smaller than expected.

The microscopic origin of this reduced capacitance, which is often discussed in terms of a low permittivity interfacial "dead-layer", is not well understood. Whether such a dead layer exists at all, and if so whether it is an intrinsic property of an ideal metal-insulator interface or a result of processing issues such as defects and strains, are controversial questions of paramount importance, since the development of effective strategies for increasing the capacitance of nanoscale devices rests on knowing the origin of the capacitance suppression.

This project involved the first fully ab-initio calculations of the dielectric properties of realistic SrRuO₃/SrTiO₃/SrRuO₃ nanocapacitors, which showed that the observed dramatic capacitance reduction is indeed an intrinsic effect. We demonstrated the existence of a dielectric dead layer by calculating the dielectric profile across the interface and analyzed its origin by extracting the ionic and electronic contributions to the electrostatic screening.

Finally, we established a correspondence between the dead layer and the hardening of the collective SrTiO₃ zone-center polar modes, and determine the influence of the electrode by repeating our calculations for Pt/SrTiO₃/Pt capacitors. Our results provide practical guidelines for minimizing the deleterious effects of the dielectric dead layer in nanoscale devices.

Electrical control of antiferromagnetic domains in multiferroic BiFeO₃ films at room temperature.

T. Zhao, A. Scholl, F. Zavaliche, K. Lee, M. Barry, A. Doran, M. P. Cruz, Y. H. Chu, C. Ederer, N. A. Spaldin, R. R. Das, D. M. Kim, S. H. Baek, C. B. Eom and R. Ramesh, Nature Materials 5, 823 - 829 (2006).

Magnetoelectric coupling in multiferroic materials has attracted much attention because of the intriguing science underpinning this phenomenon and the exciting application potential in multiply controlled devices. BiFeO₃ is the only room temperature single-phase magnetoelectric multiferroic reported to date, with a ferroelectric Curie temperature of ~1100K and an antiferromagnetic Néel temperature of 640K.

The possibility of coupling between the ferroelectric polarization and weak ferromagnetism has been previously investigated using first principles density functional theory; in this work we presented a combined experimental and theoretical study of the coupling between ferroelectricity and antiferromagnetism. In addition to its fundamental interest, such a coupling offers the intriguing possibility of electric-field controllable ferromagnetism through exchange bias to an electrically controllable antiferromagnetic component. We imaged the antiferromagnetic domain structure of BiFeO₃ films and recorded the changes induced in the antiferromagnetic domains on switching of the ferroelectric polarization; we found a strong coupling between the two types of order. Antiferromagnetic domain switching induced by ferroelectric polarization switching was observed, in agreement with theoretical predictions.

Effect of epitaxial strain on the spontaneous polarization of thin film ferroelectrics.

C. Ederer and N. A. Spaldin Phys. Rev. Lett. 95, 257601 (2005).

The possible application of ferroelectric materials in microelectronic devices has led to strong interest in the properties of thin film ferroelectrics. One important question in this context is how epitaxial strain, which is incorporated in the ferroelectric material due to the lattice mismatch with the substrate, affects the ferroelectric characteristics of the thin film. It has been demonstrated that epitaxial strain can have drastic effects, such as inducing ferroelectricity at room temperature in otherwise paraelectric SrTiO₃, or increasing the ferroelectric Curie temperature of BaTiO₃ by nearly 500°C and the remanent polarization by 250~\% compared with the corresponding bulk values.

In this work, we investigated the variation of the spontaneous ferroelectric polarization with epitaxial strain for BaTiO₃, PbTiO₃, LiNbO₃, and BiFeO₃ using first principles calculations. We found that while the strain dependence of the polarization is very strong in the simple perovskite systems BaTiO₃ and PbTiO₃ it is only weak in LiNbO₃ and BiFeO₃. We showed that this different behavior can be understood purely in terms of the piezoelectric and elastic constants of the unstrained bulk material, and discussed several factors that determine the strain behavior of a certain material.

Schrødinger's mousetrap.

N. A. Spaldin, Nature 434, 25 (2005).

This one wasn't a research project but my first (and probably only) foray into fiction writing. As part of the World Year of Physics, Nature commissioned a detective story in ten instalments, each written by a different author. In the dramatic first episode a famous physicist dies by a mysterious accident while giving a demonstration at a Conference to inaugurate the World Year of Physics, each of the next eight episodes focusses on one of the main suspects. Go ahead and read the whole series, and see if you can figure out who did it!

The origin of ferroelectricity in magnetoelectric YMnO₃.

B. B. Van Aken, T. T. M. Palstra, A. Filippetti and N. A. Spaldin, Nature Materials 3 (3), 164 (2004).

In this project we identified the nature of the ferroelectric phase transition in the hexagonal manganite, YMnO₃, using a combination of single crystal X-ray diffraction, thorough structure analysis and first principles density functional calculations. We found that the ferroelectric phase is characterized by a buckling of the layered MnO₅ polyhedra, accompanied by displacements of the Y ions which lead to a net electric polarization.

Our calculations showed that the mechanism is driven entirely by electrostatic and size effects, rather than the usual changes in chemical bonding associated with ferroelectric phase transitions in perovskite oxides. As a result the usual indicators of structural instability, such as anomalies in Born effective charges on the active ions, do not hold. In contrast to the chemically-stabilized ferroelectrics, this mechanism for ferroelectricity permits the coexistence of magnetism and ferroelectricity and so suggests an avenue for designing novel magnetic ferroelectrics.

Epitaxial BiFeO₃ multiferroic thin film heterostructure.

J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, S. B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D. G. Schlom, U. V. Waghmare, N. A. Spaldin, K. M. Rabe, M. Wuttig and R. Ramesh, Science 299 (5613), 1719 (2003).

In this work we showed that thin films of BiFeO3 have a large room temperature ferroelectric polarization, as well as a small magnetization, suggesting that it is the first room temperature multiferroic to be identified. Prior to this study, results for bulk samples had shown only a small polarization (this is now known to be the result of incomplete switching) and no net magnetization.

The observed enhancement was corroborated by first-principles calculations and found to originate from large relative displacements of the Bi, Fe, and O sublattices. 5 years later, the origin of the magnetism is still unkown, although it is consistent with a canted antiferromagnetism.