Magnetic interactions in the multiferroic phase of CuFe _1-xGa _xO ₂ (x=0.035) refined by inelastic neutron scattering with uniaxial-pressure control of domain structure

We have performed inelastic neutron scattering measurements in the ferroelectric noncollinear-magnetic phase of CuFe _1-xGa _xO ₂ (CFGO) with x=0.035 under applied uniaxial pressure. This system has three types of magnetic domains with three different orientations reflecting the trigonal symmetry of the crystal structure. To identify the magnetic excitation spectrum corresponding to a magnetic domain, we have produced a nearly "single-domain" multiferroic phase by applying a uniaxial pressure of 10 MPa onto the [11̄0] surfaces of a single-crystal CFGO sample. As a result, we have successfully observed the single-domain spectrum in the multiferroic phase. Using the Hamiltonian employed in the previous inelastic neutron scattering study on the "multi-domain" multiferroic phase of CFGO (x=0.035), we have refined the Hamiltonian parameters so as to simultaneously reproduce both of the observed single-domain and multidomaim spectra. Comparing between the Hamiltonian parameters in the multiferroic phase of CFGO and in the collinear four-sublattice magnetic ground state of undoped CuFeO ₂, we suggest that the nonmagnetic substitution weakens the spin-lattice coupling, which often favors a collinear magnetic ordering, as a consequence of the partial release of the spin frustration.