Multiferroics are materials in which ferroelectric and magnetic orders are closely related, opening the possibility for tuning the first one with the other and vice versa. This class of material has attracted recent interest for their potential applications in memory devices and other electronic components. In addition there is a drive from a more fundamental perspective - to understand the fundamental physics that give rise to these exotic properties. By combining x-ray diffraction studies at Diamond Light Source with neutron diffraction experiments at the nearby ISIS facility, scientists have gained an insight into how these materials function. This work has been published in the journal Physical Review B.
There has been very significant recent interest in magnetoelectric multiferroics, particularly magnetically frustrated transition-metal oxides. In this class of materials, the onset of ferroelectricity coincides with the transition to a magnetically ordered state characterized by a complex spin configuration. While the spontaneous electric polarization is much weaker than for proper ferroelectrics, the magnetoelectric coupling is strong, leading to giant effects in the physical properties upon application of an electric or a magnetic field.
Some key facts about these materials have been unambiguously established, but other aspects are much less clear. In compounds of the type RMn2O5 where R is a Rare-Earth ion, there is presently very little understanding of the role played by the magnetic rare earths. Rare earth magnetism not only leads to spectacular magnetoelectric effects at low temperatures, but can greatly increase the value of the electrical polarization even at high temperatures.
Neutron diffraction studies of the ferroelectric commensurate phase on powder and more recently single crystal specimens have shown a small moment on the magnetic rare-earth sites well above the main ordering temperature of the rare-earth sublattice, suggesting that this moment is induced by the complex Mn magnetic ordering. For HoMn2O5, the magnitudes and the directions of the Ho moments have been independently established by two groups, but there cannot be absolute confidence in these results since Ho’s overall contribution to the magnetic neutron scattering intensities is very small. More importantly, the nature of the interplay between Mn and Ho order—in particular, the induced nature of the Ho moments—is impossible to address with neutron diffraction, since the Ho and Mn contributions cannot be separated at temperatures close to the Neel temperature.
Azimuthal dependence of the magnetic Bragg peak intensities at 25 K.
This study examined the magnetic structure of HoMn2O5 in the commensurate/ferroelectric phase by magnetic x-ray scattering off resonance and at the Ho L3 resonance. The results clearly established that magnetic ordering of the Ho sublattice exists even at temperatures close to the Neel temperature and is induced by the magnetic ordering of the Mn sites. The azimuthal dependence of the magnetic scattering is in agreement with the overall magnetic structure derived by single crystal neutron diffraction and provides an independent confirmation of the magnetic ordering on the Ho sites at 25 K. The magnetic configuration on the Ho sites suggests that Ho and Mn ions interact through superexchange interactions rather than through magnetic dipolar field.
G. Beutier, A. Bombardi, C. Vecchini, P. G. Radaelli, S. Park, S-W. Cheong, and L. C. Chapon, "Commensurate phase of multiferroic HoMn2O5 studied by x-ray magnetic scattering" Physical Review B 77, 172408 (2008)
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