Nicola A. Spaldin

Weak ferromagnetism and magnetoelectric coupling in bismuth ferrite

We analyze the coupling between the ferroelectric and magnetic order parameters in the magnetoelectric multiferroic

First-principles study of spontaneous polarization in multiferroic BiFeO 3

\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}

A Strain-Driven Morphotropic Phase Boundary in BiFeO3

using density functional theory within the local spin density approximation (LSDA) and the

Transition metal-doped TiO2 and ZnO—present status of the field

\mathrm{LSDA}+\mathrm{U}

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

method. We show that weak ferromagnetism of the Dzyaloshinskii-Moriya type occurs in this material, and we analyze the coupling between the resulting magnetization and the structural distortions. We explore the possibility of electric-field-induced magnetization reversal and show that, although it is unlikely to be realized in

Origin of the dielectric dead layer in nanoscale capacitors.

\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}

Influence of strain and oxygen vacancies on the magnetoelectric properties of multiferroic bismuth ferrite

, it is not in general impossible. Finally, we outline the conditions that must be fulfilled to achieve switching of the magnetization using an electric field.

Magnetic interactions in transition-metal-doped ZnO: An ab initio study

The ground-state structural and electronic properties of ferroelectric BiFeO 3 are calculated using density functional theory within the local spin-density approximation sLSDAd and the LSDA+U method. The crystal structure is computed to be rhombohedral with space group R3c, and the electronic structure is found to be insulating and antiferromagnetic, both in excellent agreement with available experiments. A large ferroelectric polarization of 90‐ 100 m C/c m 2 is predicted, consistent with the large atomic displacements in the ferroelectric phase and with recent experimental reports, but differing by an order of magnitude from early experiments. One possible explanation is that the latter may have suffered from large leakage currents. However, both past and contemporary measurements are shown to be consistent with the modern theory of polarization, suggesting that the range of reported polarizations may instead correspond to distinct switching paths in structural space. Modern measurements on well-characterized bulk samples are required to confirm this interpretation.

Anisotropic conductance at improper ferroelectric domain walls

Biffed into Shape BiFeO3 is known to have a very large ferroelectric polarization. Although the bulk ground state is rhombohedral (with the electrical polarization along the [111] direction), in thin films and under epitaxial strain, the material can be tetragonally distorted (polarization along [001]). Zeches et al. (p. 977) show that under compressive strain, these films are monoclinic, a phase that is highly stable because it comprises the same symmetry as the monoclinic phase which has been reported at the so-called morphotropic phase boundaries in technologically important ferroelectrics. This work offers the possibility of obtaining large piezoelectric responses in lead-free systems. Growth of epitaxial films of BiFeO3 on various substrates may provide a route toward making lead-free ferroelectric devices. Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFeO3) films. Electric field–dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications.

First principles study of the multiferroics BiFeO3, Bi2FeCrO6, and BiCrO3: Structure, polarization, and magnetic ordering temperature

There has been considerable recent interest in the design of diluted magnetic semiconductors, with a particular focus on the exploration of different semiconductor hosts. Among these, the oxide-based diluted magnetic semiconductors are attracting increasing attention, following reports of room temperature ferromagnetism in anatase TiO2 and wurtzite ZnO doped with a range of transition metal ions. In this review we summarize the current status of oxide-based diluted magnetic semiconductors, and discuss the influence of growth method, substrate choice, and temperature on the microstructure and subsequent magnetic properties of thin films. We outline the experimental conditions that promote large magnetization and high ferromagnetic Curie temperature. Finally, we review the proposed mechanisms for the experimentally observed ferromagnetism and compare the predictions to the range of available data.