High-Field Magnetism of the S = 5/2 Triangular-Lattice Antiferromagnet CuFe1-xGaxO2 (x=0-0.028)

We have performed multifrequency electron spin resonance (ESR), electric polarization, and magnetization measurements on single crystals of the triangular-lattice antiferromagnet CuFe1-xGaxO2 (x = 0.000-0.028) in high magnetic fields. We compared ESR results with the other results (electric polarization and magnetization measurements) to clarify the origin of the expansion of a ferroelectric and incommensurate (FEIC) phase induced by the doping of nonmagnetic Ga3+ ions. We observed a rapid increase in the ESR linewidth and a large shift of the transition field from a four-sublattice collinear phase to the FEIC phase accompanied with a magnetization jump and a spontaneous electric polarization with slight doping of Ga3+ ions. We have found that the ESR linewidth is highly correlated with this shift. As a result, we have succeeded in explaining the origin of the expansion of the FEIC phase as well as the change of the transition field induced by broadening of the lowest excitation branch with an increase in Ga content.