Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science

4C/2004G228,2005S2-003

Superlattice Modulation in Magnetoelectric Perovskite Manganites RMnO3
T. ARIMA1,2,*, T. GOTO3, Y. YAMASAKI3, S. MIYASAKA3, Y. TOKURA2,3
1
IMRAM, Tohoku University, Sendai 980-8577, Japan
2ERATO-SSS, Japan Science and Technology Agency, Tsukuba 305-8562, Japan3
Department of Applied Physics, University of Tokyo, Tokyo 113-, JapanIntroduction
Research on coupling between ferroelectric (FE) andmagnetic orders has revived after Kimura et al. reported
magnetic-field-induced flop of electric polarization P inTbMnO3.[1] Evidence has been increasing that thisunique ME effect is inherently connected to the Mn-spinordering structure. There are two possible mechanisms ofspin-order-driven ferroelectricity. It was suggested thatthe ferroelectricity in RMnO3 can be attributed to thesymmetric exchange striction at the earlier stage, due tothe locking of the antiferromagnetic (AFM) wavenumber
qmag at the FE Curie temperature TC. The superexchangeinteraction via oxygen ion between two Mn ions can be
modified with a Mn-O-Mn bond angle.[2] Conversely, aMn-O-Mn bond angle should be more or less affected by
the spin arrangement. In RMnO3, Mn-O-Mn bonds are notstraight due to rotation and tilt of the MnO6 octahedra.
Therefore, the Mn-O-Mn-O-… bonding along <110>forms a zigzag chain. Below the AFM transition
temperature TN, each Mn-O-Mn bond angle may befurther influenced by the alignment of Mn moments. Inthe original zigzag chain, the oxygen ions between up anddown spins move in the same direction when themagnetic modulation vector is represented as m/n, wherem and n are odd and even integers, respectively.
As an alternative mechanism of the spin-order-driven
ferroelectricity, it has recently been pointed out thattransverse spiral spin ordering can induce FE polarization
regardless of the commensurability between spin
50

Gd
Temperature [K]

40Tb

Ho
Dy (neutron)30
20
100GdTb
00.1

0.2

TbDy
0.3

0.4

0.5

qmag = qlattice/2

Fig. 1: Temperature dependence of antiferromagnetic
modulation wavenumber qmag in perovskite-type
RMnO3 compounds with various rare earth R. Arrows
indicate ferroelectric transition temperature.

modulation and the lattice.[3] This mechanism is closely
related to the antisymmetric superexchange or the socalled Dzialoshinski-Moriya (DM) interaction.[4] A
bending Mn-O-Mn bond lacking an inversion center
favors a canted spin arrangement. This antisymmetricsuperexchange term is linear to the spin-orbit coupling
term, and is expressed as D·(Si×Sj), where D is a constantvector. By considering the inverse effect, the stable
position of the oxygen ion between two Mn ions can be
modified using the vector product of the two magnetic
moments, Si×Sj. Since the vector product of any twoneighboring Mn moments is constant in case of transverse
spiral ordering, all the oxygen ions are displaced in thesame direction.
Results
Superlattice reflections with a modulation vector (02qmag 0) emerge in AFM phases of perovskite RMnO3
compounds with a spin modulation of (0 qmag 1). Figure1
shows the temperature dependence of superlattice
wavenumber 2qmag in various RMnO3 compounds. One
can note a general tendency that a compound with larger
rare earth element shows a modulation with smaller qmag.
Though the wavenumber qmag varies above TC in many
compounds, lock-in transition is absence in some (Tb,Dy)derivatives. We hence doubt that the FE transition could
be ascribed to the lock-in phenomenon of the AFM
modulation vector.
We could determine the x value in Tb1-xDyxMnO3 with
qmag=1/3 from the figure. The compound with qmag of 1/3
enabled us to perform a magnetic structure analysis
without assuming any model. We performed a neutron
diffraction measurement using FONDER at JRR-3 andproved that the transverse spiral AFM ordering causes theFE polarization.[5]References[1] T. Kimura et al., Nature 426, 55 (2003).
[2] J. B. Goodenough, Phys. Rev. 100, 564 (1955); J.
Kanamori, J. Phys. Chem. Solids 10, 87 (1959); P.W.
Anderson, Phys. Rev. 115, 2 (1959).
[3] H. Katsura et al., Phys. Rev. Lett. (2005).
[4] I. Dzialoshinski, J. Phys. Chem. Solids 4, 241 (1958);
T. Moriya, Phys. Rev. 120, 91 (1960).
[5] T. Arima et al., Phys. Rev. Lett. 96, 097202 (2006).
*arima@tagen.tohoku.ac.jp

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