Flop of Electric Polarization Driven by the Flop of the Mn Spin Cycloid in MultiferroicTbMnO3

Abstract

Using in-field single crystal neutron diffraction we have determined the magnetic structure of TbMnO3 in the high field P a phase. We unambiguously establish that the ferroelectric polarization arises from a cycloidal Mn spins ordering, with spins rotating in the ab plane. Our results demonstrate directly that the flop of the ferroelectric polarization in TbMnO3 with applied magnetic field is caused from the flop of the Mn cycloidal plane.PACS numbers: 61.12. Ld, 75.30.Kz, 75.47.Lx, 75.80.+q The antisymmetric Dzyaloshinski-Moriya (DM) interaction[1, 2] between two spins, S i , S i+1 separated by r i,i+1 , provides for a natural coupling between magnetism and ferroelectricity with the spontaneous ferroelectric polarization given by P s ∼ r i,i+1 × (S i × S i+1 ) [3,4,5]. This mechanism generates ferroelectricity in a wide variety of magnets such as RMnO 3 perovskites with R=Gd, Dy, and Tb, [6,7] [10,11]. In the RMnO 3 manganites the DM interaction results in cycloidal order of Mn-spins giving a spontaneous ferroelectric polarization along the c-axis (P c) ( Fig. 1(a)). The application of magnetic field results in the flop of the polarization from the c-to the a-axis and highlights a novel control of one ferroic property by an other [6]. It has been assumed that this change in the direction of the polarization reflects the flop of the Mn spin cycloid, implying that the antisymmetric DM interaction continues to be responsible for the polarization ( Fig. 1(b)) [3,12]. However, in this high field P a-phase, the commensurate magnetic wave vector for Mn is also compatible with other magneto-electric mechanisms such as exchange striction [13,14,15]. In this letter we present a determination of the magnetic structure of TbMnO 3 in a high magnetic field in the commensurate P a phase. We find that the magnetic structure of Mn-spins is characterized by an ab-cycloid that accurately describes the direction of the observed ferroelectric polarization via the DM interaction. Our finding validates the model that the polarization flops found in the perovskite manganites result from the flop of the Mn-spin cycloid.The manganite TbMnO 3 crystallizes in the orthorhombic perovskites structure Pbnm. On cooling, below T N =41K Mn spins order incommensurately point along the magnetic wave vector τ ∼ 0.275b * [5,6]. On further cooling below T S =28K, a c-axis component of the Mn moment orders with a phase shift of π/2 with respect to the b-component so as to form a cycloidal structure where Mn-spins rotate within the bc plane and around the aaxis as shown in Fig. 1(a)[5]. The axis of spin rotation defines the DM interaction, S i × S i+1 , while the distance r i,i+1 is parallel to the modulation vector τ . For this type of spin order, inversion symmetry is broken yielding for R=Tb and Dy, a ferroelectric polarization along the c-axis as indeed is observed (P s c ∼ a × b) [5,6].It is tempting to assume that the flop in the ferroelectric polarization arises from a flop in the Mn-spin spiral, but so far there is no experimental prove for ...

Ga substitution as an effective variation of Mn-Tb coupling in multiferroicTbMnO3

“…According to Ref. 30 , this transition is associated with the flop of Mn-cycloidal plane from bc to ab causing the electric polarization flop from P c to P a. 5,30 Both meta-magnetic transitions are observed in x = 4% sample.…”

Effects of Al substitution on the multiferroic properties of TbMnO3

“…12 This transition seems to be coupled to a metamagnetic transition of the Tb moments and it was accounted for by the change of the spiral plane of Mn moments from the bc plane to the ab plane. 13 Accordingly, the polarization at high fields is parallel to the a axis (P a). This type of ferroelectric phase is the one observed in compounds where the commensurate A-type magnetic ordering is preserved such as GdMnO 3 .…”

Infrared reflectivity of the phonon spectra in multiferroicTbMnO3

“…This cycloid of spins breaks the inversion symmetry of the crystal, 6 and induces a spontaneous electric polarization along the c-axis. 7 In addition to the fact that the cycloid magnetic and ferroelectric transitions occur at the same temperature, the magnetic origin of the ferroelectric order was evidenced by the fact that the electric polarization is tied to the cycloid plane 9 and chirality. 10 The electric polarization is given by P = e i,i+1 × (S i × S i+1 ) where S i and S i+1 are the spins on sites i and i + 1 and e i,i+1 is the unit vector directed from site i to site i + 1.…”