Electric polarization induced by a proper helical magnetic ordering in a delafossite multiferroicCuFe1−xAlxO2

Abstract

Multiferroic CuFe1−xAlxO2 (x = 0.02) exhibits a ferroelectric ordering accompanied by a proper helical magnetic ordering below T = 7K under zero magnetic field. By polarized neutron diffraction and pyroelectric measurements, we have revealed a one-to-one correspondence between the spin helicity and the direction of the spontaneous electric polarization. This result indicates that the spin helicity of the proper helical magnetic ordering is essential for the ferroelectricity in CuFe1−xAlxO2. The induction of the electric polarization by the proper helical magnetic ordering is, however, cannot be explained by the Katsura-Nagaosa-Balatsky model, which successfully explains the ferroelectricity in the recently explored ferroelectric helimagnets, such as TbMnO3. We thus conclude that CuFe1−xAlxO2 is a new class of magnetic ferroelectrics.PACS numbers: 75.80.+q, 75.25.+z, 77.80.-e Novel types of couplings between dielectric property and magnetism, which produce colossal magnetoelectric (ME) effects, have been extensively investigated since a gigantic ME effect was discovered in RMnO 3 (R is a rare earth material) [1]. Among several types of couplings between spins and electric polarizations, a ferroelectricity induced by noncollinear spin arrangements has been most widely investigated experimentally and theoretically [2,3,4,5,6,7,8]. Katsura, Nagaosa and Baratsky (KNB) proposed that the local electric dipole moment p, which arises between neighboring two spins S i and S i+1 , can be described in the form of p ∝ e i,i+1 × (S i × S i+1 ), where e i,i+1 is a unit vector connecting two spins [2]. This formula successfully explains the ferroelectric property in cycloidal or conical magnetic orderings of some transition metal oxides, such as RMnO 3 (R=Tb, Tb 1−x Dy x ), Ni 3 V 2 O 8 , MnWO 4 and CoCr 2 O 4 [4,5,6,7,8]. Moreover, a recent polarized neutron diffraction study on TbMnO 3 demonstrated that the spin helicity, clockwise or counterclockwise, correlates with the direction of the electric polarization, as predicted in the formula [9]. It is, however, recently reported that ferroelectricity in a helical magnetic ordering of a delafossite multiferroic CuFe 1−x Al x O 2 cannot be explained by the above formula [10]. Therefore, CuFe 1−x Al x O 2 provides an opportunity to explore an another type of spin-polarization coupling.CuFeO 2 , which is one of model materials of a triangular lattice antiferromagnet, has been extensively investigated as a geometrically frustrated spin system for last fifteen years [11,12,13]. The ground state of CuFeO 2 is a collinear commensurate 4-sublattice (↑↑↓↓) state with the magnetic moments along the c axis, which is normal to the triangular lattice layers, in spite of the Heisenberg spin character expected from the electronic configuration of Fe. When a magnetic field is applied along the c axis at low temperature, CuFeO 2 exhibits a multi-step magnetization process consisting of several magnetization plateaus and slopes, which is accompanied by stepwise changes of lattice constants [14...

Magnetochiral dichroism resonant with electromagnons in a helimagnet

“…1b), which stacks along [001] direction, results in the complex magnetic phase diagram including the screw spin structure 24,25 . The slight doping of the non-magnetic ions, such as the Al 26,27 and Ga 28,29 , stabilizes the screw spin phase, whose magnetic modulation vector has an incommensurate wave number (q, q, 3/2) with q ¼ 0.202 below 7.4 K (see Fig. 1b,c).…”

Polarization memory in the nonpolar magnetic ground state of multiferroicCuFeO2

“…Among the large family of compounds now known to display this phenomenology, the CuFeO 2 delafossite is particularly interesting because ferroelectricity induced by an incommensurate proper-screw magnetic order, resulting in a handedness-dependent polarization, was first discovered in this material [1][2][3][4] . Neither of the multiferroic mechanisms known until then (spin-current/cycloidal and exchange striction) was found to be applicable to CuFeO 2 ; later, Arima proposed a new mechanism, known as spindependent d-p hybridization 5 to explain the multiferroic properties of this material.…”

Helical magnetic state in the distorted triangular lattice ofα-CaCr2O4Chapon1, Manuel2, Damay3 et al. 2011Phys. Rev. B3414530View full textAdd to dashboardCite

“…Most of the attention was initially focussed on cycloidal magnets such as TbMnO 3 1 for which the inverse Dzyaloshinskii-Moriya 2 and spincurrent model 3 predict the onset of a spontaneous polarization when the direction of the modulation (k-vector) is perpendicular to the spin-rotation axis (e) 4 . More recent work on incommensurate magnetic phases in triangular lattices [5][6][7][8][9][10] has established that improper ferroelectric order of similar magnitude also appears in spirals magnets, i.e systems that possess a true magnetic chirality (k parallel to e), however requiring another microscopic coupling mechanism such as hybridization effects proposed by Arima 11 . Those systems are quasi two-dimensional triangular lattices and display a nearly 120…”