Neutron scattering studies of multiferroic RMnO3 & BiFeO3
Je-Geun Park1,2
1 IBS, Center for Functional Interfaces of Correlated Electron System, Seoul National University, Seoul 151-742, Korea
2 Department of Physics & Astronomy, Seoul National University, Seoul 151-742, Korea
Multiferroic materials having a coexistence of otherwise seemingly incompatible phases of magnetic and ferroelectric ground states have been the focus of intensive materials researches recently. In particular, hexagonal manganites RMnO3 and BiFeO3 are arguably one of the most interesting examples.
Hexagonal manganites RMnO3 are an archetypal example of multiferroic systems having both ferroelectric transition around 900 K and antiferromagnetic transition below 100 K. There is several experimental evidence demonstrating direct coupling between the ferroelectric order parameter and the magnetic order parameter. The origin of such coupling has been suggested to lie in the unusually strong spin-lattice coupling [1]. Of particular interest from magnetism point of view is that Mn atoms form a natural two dimensional triangular lattice, thereby adding additional attractions to this already interesting material [2]. By exploring several experimental variables such as doping and external pressure, we have examined the full physical properties of several hexagonal manganites [3]. Furthermore, we have addressed the particular issue of spin-lattice coupling by using both neutron diffraction and inelastic neutron scattering.
On the other hand, BiFeO3 exhibits both magnetic and ferroelectric transitions above room temperature: TN=650 K and TC=1050 K. Moreover, it has an unusual incommensurate magnetic transition with an extremely long period of 650 Å. Despite the interesting and attractive physical properties as well as numerous studies carried out on this particular compound, some fundamental questions about the underlying mechanism of both transitions remain largely unanswered. Using single crystal neutron diffraction as well as synchrotron powder diffraction, we have demonstrated that there exists a strong, unanticipated spin-lattice coupling at TN [4]. Moreover, we showed that there are subtle magnetic field effects on the magnetic structure up to 25 Tesla [4]. Finally, by using 10 single crystals co-aligned within 3° of one another, we have recently measured the spin waves of the antiferromagnetic phase at two state-of-the-art inelastic neutron scattering instruments: one is AMATERA of J-PARC and another MERLIN of ISIS [5].
[1] Seongsu Lee et al., Phys. Rev. B 71, 180413(R) (2005); Nature 451, 805 (2008).
[2] Junghwan Park et al., Phys. Rev. B 68, 104426 (2003); Phys. Rev. B 82, 054428 (2010).
[3] P. A. Sharma et al., Phys. Rev. Lett. 93, 177202 (2004); M. Chandra Sekhar et al., Phys. Rev. B 72, 014402 (2005); D. P. Kozlenko et al., Phys. Rev. B 78, 054401 (2008).
[4] Junghwan Park et al., J. Phys. Soc. Jpn. 80, 114714 (2011): ibid 80, 125001 (2011).
[5] Jaehong Jeong et al., Phys. Rev. Lett. 108, 077202 (2012).