Magnetoelectronics, developing next generation device technology

06-08-2013

More efficient data storage may come from the study of magneto-electric materials. An international team of researchers at the beamline BM28 (XMaS CRG) is investigating the highly prized properties of magneto-electric materials that could underpin the next generation of computers. For the first time, X-ray analysis has explained a unique property of these materials which allows for control of the magnetic state with an electric field, a finding that could help improve the energy efficiency of computer memory and push computational power past the limitations of binary code.

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Magneto-electric materials have the unusual characteristic of coupling electric and magnetic parameters. This unique property has been the subject of intense research over the past decade due to the potential to revolutionise the electronics industry in two areas:

  1. Memory – typically written using magnetism as on the hard disk in your computer, it is an extremely robust way to store information, lasting for 1000s of years but requires significant amounts of energy to write, erase and rewrite information. Electrically driven memory is the alternative approach, which consumes far less energy but is not as stable and has a lower storage density. A combination of the best parts of these two capabilities through magneto-electric materials could significantly reduce costs without compromising the lifetime.
  1. Computational processing codes – currently everything in computing comes down to a binary coding system formed from 1s and 0s. By combining magnetic and electric states with magneto-electric materials, future computers could take advantage of a new multistate numerical system to increase computational power.

Europium titanate (EuTiO3) is one such magneto-electric material. Previous experiments have demonstrated that when sliced into nanometre-thick layers and stretched at temperatures close to absolute zero, this typically paraelectric - antiferromagnetic material becomes a ferroelectric - ferromagnet. As a result theorists suggested that if strained in exactly the right way europium titanate could display a balance between the magnetic states, with only the addition of an electric field required to ‘switch’ between the two.

To test this theory, a team from Argonne National Laboratory utilised XMaS’ unique sample environment to create the combination of a high magnetic field, a high electric field and the very low temperature required to draw out the dual personalities of the compound.

Resonant magnetic scattering at the Eu LII edge through the antiferromagnetic reflection, with increasing temperature

Figure 1. a) Resonant magnetic scattering at the Eu LII edge through the antiferromagnetic reflection, with increasing temperature; above the Néel Temparture TN the magnetic signal disperses. The inset shows the europium resonant response at 1.5 K. This peak clearly indicates the G-type magnetic structure of the EuTiO3 film grown on the LSAT substrate. b) Temperature dependence of the magnetic order of the three differently strained EuTiO3 films. The black data set shows a linear onset, indicating magnetic competition within the film, induced by the strain of the substrate. The inset shows the scattering geometry adopted during the experiment: vertical scattering geometry with s- to p- polarisation selection analysis used to suppress charge and optimise the magnetic/charge scattering ratio.

With the switch from antiferromagnet to ferromagnet order induced by an electric field, the team studied the underlying phenomenon driving this transition using X-ray resonance magnetic scattering (Figure 1). In conjunction with advanced first principles calculations, the analysis helped provide information on the relationship between europium spins and explained for the first time how both magnetic states coexist and how, through the magnetoelectric degree of freedom, one can switch from one state to the other.

Experiment setup applying a voltage to the EuTiO3 film grown on LSAT substrate and series of data scans showing the magnetic reflection with increasing voltage

Figure 2. a) Illustrates the experiment setup applying a voltage to the EuTiO3 film grown on LSAT substrate. b) A series of data scans showing the magnetic reflection with increasing voltage. c) As the electric field pushes the central titanium atom, shown in purple, the magnetic interaction through this atom weakens. d) The response of the magnetic strength is reversible with increasing and decreasing field strength; the peak intensity follows inversely. e) Calculated difference in strength of magnetic orders as the field increases and the titanium atom moves; the system responds from being preferably antiferromagnetic to ferromagnetic.

Previous research suggested that the relationship between the europium atoms were consistently ferromagnetic, making it difficult to explain the compound’s strong antiferromagnetic structure. However using XMaS the team was able to show for the first time that whilst this was true for the first and second neighbouring europium atoms, the third neighbouring atoms were dominated by an antiferromagnetic relationship. By stretching the material, scientists could bring the influence of these two different interactions closer into equilibrium, once at this balanced state the system becomes susceptible to control using the electric field (Figure 2).

The researchers also showed that the introduction of the electric field moves the titanium atoms towards the centre of the structure (Figure 2). They believe it is this phenomenon that affects the molecules overall magneto-electric balance, tipping the magnetic ordering one way or the other.

Europium titanate only expresses its highly prized split personality at very low temperatures, making its use in commercial applications unlikely. However, understanding the coupled nature between both the electric and magnetic parameters will help in the development of new materials with the same properties that can operate at higher temperatures.

Principal publication and authors
Reversible control of magnetic interactions by electric field in a single phase material, P.J. Ryan (a), J.-W. Kim (a), T. Birol (b), P. Thompson (c,d), J.-H. Lee (a), X. Ke (e), P.S. Normile (f), E. Karapetrova (a), P. Schiffer (g), S.D. Brown (c,d), C.J. Fennie (b), and D.G. Schlom (h), Nature Communications 4, 1334 (2013).
(a) X-ray Science Division, Argonne National Laboratory, Illinois (USA)
(b) School of Applied Engineering Physics, Cornell University, New York (USA)
(c) Department of Physics, University of Liverpool (UK)
(d) XMaS, the UK-CRG, ESRF
(e) Quantum Condensed Matter Division, Oak Ridge National Laboratory, Tennessee (USA)
(f) Instituto Regional de Investigación Científica Aplicada (IRICA) and Departamento de Física Aplicada, Universidad de Castilla-La Mancha, Ciudad Real (Spain)
(g) Department of Physics and Materials Research Institute, Pennsylvania State University (USA)
(h) Department of Materials Science and Engineering, Cornell University, New York (USA)

Top image: This image presents a model of the simple perovskite structure. The red, white, purple, balls represent the oxygen, titanium, and europium atoms. The central Ti atom is displaced from its central position. This effect determines the resulting magnetic spin ordering of the Eu indicated by arrows illustrating the antiferromagnetic to ferromagnetic order. Courtesy of Renee Carlson, Argonne National Laboratory.