Page 70 - ILLS Annual Report 2018

THEORY
The microscopic mechanism for the unusual spin-induced electric polarisation in the multiferroic compound GdMn O
Recently, a large magneto-electric effect (reversing of the polarisation under an applied magnetic field) has been found in two members of the RMn2O5 (R = rare earth) manganites, namely TbMn O [1] and
compound presents a large electric polarisation of ∼3600μC/m2 [2], a value nearly able
to compete with the so-called Bi manganite multiferroics. In this paper we explain the microscopic origin of this spin-induced ferroelectricity, and pinpoint the specificity of the Gd compound among other rare earths.
AUTHORS
M.-B. Lepetit (Institut Néel, CNRS, and ILL, Grenoble, France)
G. Yahia (Laboratoire de Physique des Solides, CNRS, Université Paris-Sud, Université Paris-Saclay, France and Laboratoire de Physique de la Matière Condensée, Université Tunis-El Manar, Tunisia)
F. Damay (Laboratoire Léon Brillouin, CEA-CNRS, France)
S. Chattopadhyay (Indian Institute of Technology, Kharagpur, India)
V. Bal´edent, W. Peng and P. Foury-Leylekian (Laboratoire de Physique des Solides, CNRS, Université Paris-Sud, Université Paris-Saclay, France)
S.W. Kim and M. Greenblatt (Dept. Chemistry and Chemical Biology, Rutgers, State University of New Jersey, USA)
ARTICLE FROM
Phys. Rev. B 97, 085128 (2018)
REFERENCES
[1] N. Hur, S. Park, P. Sharma, J.S. Ahn, S. Guha and S.-W. Cheong, Nature 429 (2004) 392
[2] N. Lee, C. Vecchini, Y.J. Choi, L.C. Chapon, A. Bombardi, P.G. Radaelli and S.-W. Cheong, Phys. Rev. Lett. 110 (2013) 137203
[3] P.G. Radaelli and L.C. Chapon, J. Phys. Condens. Matter 20 (2008) 434213
The RMn2O5 compounds crystallise in the Pm space group, but their structure is close to a Pbam symmetry (see figure 1). Along c, there are two relevant Mn4+ −Mn4+ exchange interactions: J1 (through the R3+ layers) and J2 (through
the Mn3+ layers) [3]. In the (a, b) plane there are three non-equivalent magnetic exchanges: J3 and J4 between
Mn3+ and Mn4+ spins, and J5 between two Mn3+ spins (figure 1). The main contribution to these exchanges is the antiferromagnetic (AFM) Mn−Mn super-exchange interaction through a shared oxygen. J4 and J5 are expected to be the dominant integrals, while J3 is frustrated. The influence of the rare earth is generally neglected, owing to the strong spatial localisation of their orbitals. However, at low temperature and in the particular case of Gd3+, the super-exchange interaction between Gd−Mn spins through a common oxygen (labelled J6s in the following and in figure 1) can become relevant.
Figure 1
Left) RMn2O5 structure and magnetic interactions.
Right) Magnetic structure of GdMn2O5 at 1.5 K. The blue (orange) ellipses show the Mn3+/Mn3+ (Gd3+/Mn3+) AF pairs. Stars identify the Gd/Mn3+ pair proposed by Lee et al. in their model [2]. The black dashed lines are guides to visualise the zig-zag chains running according to the dominant
magnetic interactions.
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GdMn O [2]. Furthermore, the GdMn O 25 25
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ANNUAL REPORT 2018