Impurity-doping-induced ferroelectricity in the frustrated antiferromagnetCuFeO2

Abstract

Dielectric responses have been investigated on the triangular-lattice antiferromagnet CuFeO2 and its site-diluted analogs CuFe1−xAlxO2 (x=0.01 and 0.02) with and without application of magnetic field. We have found a ferroelectric behavior at zero magnetic field for x=0.02. At any doping level, the onset field of the ferroelectricity always coincides with that of the noncollinear magnetic structure while the transition field dramatically decreases to zero field with Al doping. The results imply the further possibility of producing the ferroelectricity by modifying the frustrated spin structure in terms of site-doping and external magnetic field. In all these materials, the ferroelectric behavior is observed in the noncollinear magnetic phase. Katsura et al. proposed the spin current theory for the origin of ferroelectricity in the noncollinear magnetic phase [9]. In the theory, the electric dipole is induced by the spin current, which is expected to flow between noncollinear magnetic moments in analogy to the magnetic dipole induced by electric current. To produce such a ferroelectricity of magnetic origin, modification or partial lifting of spin state degeneracy may be prerequisite with use of the frustrated spin systems.Delafossite CuFeO 2 (See Fig. 1(d) for the structure) is one of candidates for such magnetic ferroelectrics. The crystal structure is characterized by the space group R3m, with two-dimensional triangular lattice layers stacked rhombohedrally along the caxis ( Fig. 1(d)). The magnetic structure of this material has been studied extensively by neutron diffraction measurements [10,11,12,13,14,15]. At zero field, a 4-sublattice collinear structure (CM-4) is observed at the lowest temperature. When the field is increased up to 7 T, a noncollinear spin structure (NC) emerges [16]. Above 12 T, a 5-sublattice collinear structure (CM-5) is realized [11]. Recently, Kimura et al. reported that the spontaneous polarization is observed in the noncollinear magnetic phase in between the 4-sublattice and 5-sublattice phases [18]. Similarly to the magnetic field, Al doping is known to easily modify the magnetic structure [13]. The 2 % substitution of Fe with Al induces the transition from CM-4 to NC at zero field and low temperature [12]. To clarify the relation between the magnetic structure and dielectric properties, the search for electric polarization is desirable also for the Al-doped crystals. In this work, we report the finding of ferroelectricity in the noncollinear magnetic phase of the Al doped crystal even in the absence of external magnetic field as well as the systematic evolution of the magneto-electric phase with Al-doping. This ensures the close relation between the noncollinear magnetic structure and ferroelectricity.Single crystals of CuFe 1−x Al x O 2 (x=0.00, 0.01, 0.02) were prepared by a floating zone method [19]. For the measurements of pyroelectric current and dielectric constant, the crystals were cut into thin planes with the widest faces parallel to (110) plane. As the electrodes, we ...

Effect of Spin Dilution on the Magnetic State of Delafossite CuFeO2 with an S = 5/2 Antiferromagnetic Triangular Sublattice

“…The existence of electric polarization in all samples related to magnetic transition at T N2 implies the existence of intrinsic magnetoelectric coupling in the present polycrystalline samples. This is in accordance with a previous paper reporting on Al dopinginduced electric polarization in monocrystalline sample of CuFeO 2 [4].…”

“…Geometric frustration is a broad phenomenon that results from an intrinsic incompatibility between some fundamental interactions and the underlying lattice geometry usually based on triangles or tetrahedra. The delafossite CuFeO 2 is of particular interest because of the discovery of multiferroic phenomena with either application of a magnetic field or the substitution of Fe 3+ with nonmagnetic Al 3+ ions [3,4]. As a model material of triangular lattice antiferromagnet (TLA), CuFeO 2 forms an Ising-like 4-sublattice antiferromagnetic order at low temperature, with spin moment pointing along the c-axis [5].…”

Robust ferroelectric state in multiferroicMn1−xZnxWO4

“…Magnetic fields may stabilize or destroy the multiferroic state or result in the rotation of the ferroelectric polarization by 90 • or even by 180 • . 7, [13][14][15] Similarly, the application of external pressure [16][17][18][19] as well as chemical substitutions [20][21][22][23] are a viable tool to change and tune the multiferroic properties of different compounds.…”

Spin-wave dynamics for the high-magnetic-field phases of the frustrated CuFeO2antiferromagnet: Predictions for inelastic neutron scattering

“…The spin-wave gap that appears in the CL-5i phase will increase linearly with field. Magnetization and diffraction measurements 13,14 strongly suggest that the NC-5i phase appears above the CNC phase in doped samples: the wavevector k = (4π/5,0) of the 5-SL phase remains constant while the magnetization linearly increases with field. INS measurements can be used to confirm the appearance of the NC-5i phase in those materials.…”

“…Above 7 T, it exhibits multiferroic behavior and is characterized by a complex non-collinear (CNC) state 12 . The CNC phase is also produced by Al or Ga doping, which decreases the easy-axis anisotropy 13,14 perpendicular to the hexagonal planes. Characterization of the CNC phase in Ga-doped CuFeO 2 shows that the magnetic structure is a distorted spiral with alternating small and large turn angles fluctuating between (19 • -25 • ) and (130 • -140 • ) 15,16 .…”