New concepts in multiferroics and magnetroelectrics

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

Multiferroics and magnetoelectrics are materials that develop a ferroelectric polarization in a magnetic state, either spontaneously or in a magnetic field. Because they can in principle convert electric into magnetic signals, it has been proposed that they could be used as key components in a new generation of information storage and processing devices, alternative and better than the familiar magnetic (e.g., hard disks) and ferroelectric (e.g., smart-card chips) storage media. A true renaissance in the field was triggered by the discovery of a new class of multiferroics, in which magnetism and ferroelectricity are tightly coupled. However, after almost a decade of research, no material has yet emerged as a viable candidate for applications, since the observed effects are weak and generally restricted to low temperatures. Here, we propose to explore at the fundamental level a number of novel concepts, which depart in a radical way from the thoroughly-explored `cycloidal magnetism' paradigm. In particular, we will attempt to unlock the potential of the strongest of the mageto-electric interactions, the so-called `exchange striction' effect. In contrast to the weaker effects mostly considered so far, obtaining electrical polarisation from exchange striction requires an exquisite control of the crystal symmetry and of the magnetic interactions at the atomic level. We propose to employ an innovative research methodology, which combines conventional measurements of electrical and magnetic properties, `imaging' of the spins and electric dipoles at different length-scales, from atomic to macroscopic, and state-of-the-art ab-initio theoretical calculations of the static and dynamic properties of these systems, both at low temperatures and at room temperature. The breakthrough we seek is a new microscopic "working principle" that can be deployed to perfect practical multiferroics and magnetoelectrics materials. Our new approach, which strongly emphasizes the interface between theory and experiments, will also pave the way for similar studies on related classes of materials, with applications in information storage, energy conversion and storage and many others.

Planned Impact

This proposal represents the first, basic-research step in of a "pathway" that may eventually lead to building commercial devices based on novel operating principles. The ultimate beneficiaries of our work would then be the electronic and IT industries, since practical multiferroic and magnetoelectric materials would transform the information processing and storage technologies. Once our underpinning `new concepts' are validated and tested on model systems, the subsequent steps in the process will be to identify materials with "practical" temperature and field operating parameters, to grow the materials in appropriate form for integration with existing technologies (e.g., CMOS) and to demonstrate a prototype device "package", incorporating all the relevant technologies. Although these activities do not correspond to specific deliverables in our proposal, we will establish a close exchange of communication with key partners with the right combination of scientific and technological skills, who will be in the best position to exploit any breakthrough from our research. They include solid state chemists (design and synthesis of high-temperature materials), experts in the growth (e.g., Molecular Beam Epitaxy and Pulsed Laser Deposition) and characterisation (Transmission Electron and Scanning Probe Microscopies) of oxide thin films, device fabrication experts and commercial companies. The exchange of information will have a focal points during at the end of Y2, in the form of a workshop entitled "Applications of new concepts in multiferroics and magnetoelectrics". Likewise, we plan to organize an hands-on tutorial on "Calculations in multiferroics and magnetoelectrics" at the end of Y3. The tutorial will be structured so that both experimentalists (with little or no knowledge of DFT) and theorists (with little knowledge of characterization techniques) could attend.
Furthermore, all the methodological developments will be open-sourced and a large part of academic and industrial users of quantum-Espresso can benefit from them, in other areas of materials research, such as: magnetic oxides, high-k dielectrics, heavy fermion and strongly correlated systems.

Publications

10 25 50
 
Description Key Findings

Multiferroics and magnetoelectrics are materials that develop a ferroelectric polarization in a magnetic state, either spontaneously or in a magnetic field. Because they can in principle convert electric into magnetic signals, it has been proposed that they could be used as key components in a new generation of information storage and processing devices, alternative and better than the familiar magnetic (e.g., hard disks) and ferroelectric (e.g., smart-card chips) storage media. The main focus of this grant was to explore at the fundamental level a number of novel concepts, which depart in a radical way from the existing paradigms. The Key Findings associated with this awards are:

We discovered a novel multiferroic mechanism, which we called "ferroaxial mechanism", which can result in a spontaneous electrical polarization upon helical magnetic ordering in certain materials.

We discovered several ferroaxial multiferroics, including Cu3Nb2O8 - a previously unknown compound - and CaMn7O12, which displays the largest magnetically-induced electrical polarisation ever measured, appearing at a rather high temperature (~90K).

In CaMn7O12, we discovered a novel form of electronic (orbital) ordering, which is coupled to the helical magnetic ordering. The interplay between charge, magnetism and the crystal lattice in transition metal oxides is one of the foundational topics in condensed matter physics, and has defined entire fields of research, including, most notably, high-temperature superconductivity. The way this occurs in CaMn7O12 is entirely unprecedented, and represents an important milestone in the wider context of this research.

We have developed a series of atomistic methods to calculate the static properties (e.g., the electrical polarization) and the dynamic properties (e.g., the response to electromagnetic radiation) of multiferroics from first principles. Moreover, we have applied these methods to the novel systems we discovered, which guided the interpretation of our results and enabled us to make predictions for new experiments.

Very recently, we modelled the prototypical magneto electric material Cr2O3 ab initio using density functional methods, and determined quantitatively all the components of the Cr2O3 magneto-electric tensor both in the zero-field phase (where they were found to agree with experimental results) and in the high-field "Spin Flop" phase, for which no measurements exist. We found that different components of the tensor arise from different mechanisms.

We have developed novel experimental methodologies to image multiferroic domains and determine their properties quantitatively, down to lengthscales of a few hundred nm. We have applied these techniques both to bulk crystals and to films and devices of the technologically relevant material BiFeO3. Very recently we managed to achieve electrical control of the spins in BiFeO3, and to image the electrical switching process in situ in a device configuration, paving the way for the development of oxide electronics based on multiferroics.
Exploitation Route The most immediate application of our findings will be in the area of multiferroic devices. In fact, as a consequence of the results associated with this award, we have undertaken collaborative work with Prof. Chang-Beom Eom at University of Wisconsin - Madison, who is one of the leading experts in the area of multiferroic devices. For example, we have already been able to image the switching behavior of multiferroic "bits" in situ on a chip, and to determine the exact mechanism of magnetic switching upon application of a voltage to the devices. Many other groups will benefit from the atomistic methodologies we have developed, which are of general applicability beyond the field of multiferroic research.
Sectors Electronics

 
Description Johnson RS-URL Fellowship
Amount £505,068 (GBP)
Organisation The Royal Society 
Sector Academic/University
Country United Kingdom
Start 01/2016 
End 12/2020
 
Description Oxford Qantum Materials Platform Grant
Amount £1,736,109 (GBP)
Funding ID EP/M020517/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 04/2015 
End 03/2020