Magneto-orbital helices as a route to coupling magnetism and ferroelectricity in multiferroic CaMn7O12

Abstract

Orbital physics drives a rich phenomenology in transition-metal oxides, providing the microscopic underpinning for effects such as Colossal Magnetoresistance. In particular, magnetic and lattice degrees of freedom are coupled through orbital ordering, and it has long been hoped that this coupling could be exploited to create high-temperature multiferroics with large values of the electrical polarization. Here we report an unprecedented magnetoorbital texture in multiferroic CaMn 7 O 12 , found to give rise to the largest magnetically induced ferroelectric polarization measured to date. X-ray diffraction characterization of the structural modulation in these 'magneto-orbital helices', and analysis of magnetic exchange shows that orbital order is crucial in stabilising a chiral magnetic structure, thus allowing for electric polarization. Additionally, the presence of a global structural rotation enables the coupling between this polarization and magnetic helicity required for multiferroicity. These novel principles open up the possibility of discovering new multiferroics with even larger polarization and higher transition temperatures.

Soft vibrational mode associated with incommensurate orbital order in multiferroicCaMn7O12

“…Since Raman scattering only detects phonons at the Brillouin zone center (Γ), and because A g is a one-dimensional representation of the high-temperature structural symmetry group, the systematic appearance of extra peaks below T o is most likely due to back-folding of phonon dispersions that gives rise to new optical modes at the Γ point. This result is consistent with X-ray diffraction observations of incommensurate structural distortion [17][18][19] that accompanies the orbital order. A closer inspection of the highly accurate data at 10 K (Figs.…”

“…1b) as one moves along the hexagonal c-axis, with an incommensurate propagation wave vector q o = (0, 0, 0.077) hex (Ref. 17). The order manifests itself in X-ray diffraction measurements as a modulation of the Mn2-O bond lengths along the local xand y-directions.…”

“…[13][14][15][16] In the latter case the Mn ions segregate themselves into 3+ and 4+ valence states, and because orbital order pertains only to the Mn 3+ sub-lattice, the ordering pattern develops on top of the charge order, usually in a commensurate fashion, even though the charge order itself may not be commensurate with the undistorted crystal lattice. [13][14][15][16] Recently, Perks et al 17 reported an unusual form of incommensurate orbital order in CaMn 7 O 12 . Through a careful analysis of single-crystal X-ray diffraction data that builds on earlier reports of incommensurate lattice distortions below ∼ 250 K, 18,19 the authors identified a continuous rotation of Mn 3+ e g orbital occupation between the local 3x 2 −r 2 and 3y 2 −r 2 states (Fig.…”

“…CaMn 7 O 12 exhibits multiferroic properties below T N1 = 90 K. An unprecedentedly large magnetically induced ferroelectric polarization [20][21][22] arises from a helical magnetic structure due to exchange striction, 22 and it is suggested that the orbital order at higher temperature sets the stage for the helical magnetic structure to be stabilized. 17 These novelties motivated us to study the mechanism for the orbital ordering in CaMn 7 O 12 using Raman spectroscopy, which is sensitive to symmetry breaking via the observation of collective excitations. The key finding of our work is a soft vibrational mode which can be regarded as amplitude excitation of the composite orbital order parameter below T o .…”

“…27), and (2) the orbital order appears to be responsible for the magnetic interactions that lead to the helical magnetic order below T N1 (Ref. 17). Details about the magnetic phases can be found in Refs.…”