Multiferroic phase of doped delafossite ${\text{CuFeO}}_2$ identified using inelastic neutron scattering

We report inelastic neutron scattering measurements that provide a distinct dynamical “fingerprint” for the multiferroic ground state of 3.5% Ga-doped ${\text{CuFeO}}_2$. The complex ground state is stabilized by the displacement of the oxygen atoms, which contribute to the multiferroic coupling predicted in the “spin-driven” model. By comparing the observed and calculated spectrum of spin excitations, we conclude that the magnetic ground state is a distorted screw-type spin configuration with a distribution of turn angles.

Figure 1

(Color online) (a) The three-dimensional crystal structure of ${\text{CuFeO}}_2$. (b) The two-dimensional hexagonal lattice structure of the ${\text{Fe}}^{3+}$ layers and the intralayer interactions. The orange (gray) and blue (gray/white) colored atoms illustrate the displacements of oxygen atoms lying above and below the hexagonal plane. The magnetic structure and $yz$ projections of the [(c) and (d)] proper and [(e) and (f)] CNC spiral or screw-type spin configurations are also shown.

Figure 2

(Color online) Energy versus momentum transfer taken from (a) the triple-axis spectrometer at HFIR and (b) the time-of-flight CNCS at the SNS. (c) Predicted simulation of the INS data using intralayer and interlayer interactions along with a lattice distortion and anisotropy. (d) Wave vector scans with different energy transfers for the CNCS data.

Figure 3

(Color online) (a) Energy versus momentum transfer taken from the CNCS clearly showing the SW modes along the $\left[0,0,L\right]$ direction with $H=0.2$. The dispersion with $L$ indicates the importance of interlayer interactions. (b) Predicted INS spectra using intralayer and interlayer interactions along with a lattice distortion. (c) Intensity versus momentum transfer for various energies. (d) A three-dimensional view of the ${\text{Fe}}^{3+}$ lattice structure illustrating the interlayer interactions.

Figure 4

(Color online) (a) Intensity versus energy for the [0.20,0.20,1.5] (blue/gray squares) and [0.30,0.30,1.5] (red/gray circles) modes along the $\left[H,H,0\right]$ direction with $L=1.5$. The ratio of mode intensities (open triangles) shows that the main [0.20,0.20,1.5] mode is three times as intense as the [0.30,0.30,1.5] mode. The simulated intensity is consistent with this intensity ratio. (b) SW velocities for the two modes along the $\left[H,H,0\right]$ direction with $L=1.5$ together with the predicted values (black circles).