Spin-lattice-coupling-mediated magnetoferroelectric phase transition induced by uniaxial pressure in multiferroic CuFe1−xMxO2 ( M=Ga , Al)

We have investigated magnetic and ferroelectric (dielectric) properties of multiferroic ${\mathrm{CuFe}}_0.982{\mathrm{Ga}}_0.018{\mathrm{O}}₂, {\mathrm{CuFe}}_0.965{\mathrm{Ga}}_0.035{\mathrm{O}}₂$, and ${\mathrm{CuFe}}_0.95{\mathrm{Al}}_0.05{\mathrm{O}}₂$ under applied uniaxial pressure $p$ up to 600 MPa. Unlike the results of the almost same experiments on ${\mathrm{CuFeO}}₂$ [Tamatsukuri et al., Phys. Rev. B 94, 174402 (2016)], we have found that the application of $p$ induces a new ferroelectric phase, which is different from the well-studied spin-driven ferroelectric phase with helical magnetic ordering, in all the doped samples investigated here. We have also constructed the temperature versus $p$ magnetoelectric phase diagrams of the three samples. The ferroelectric polarization in the $p$-induced ferroelectric phase lies along the [110] direction as in the helical magnetoferroelectric phase, and its value is comparable with or larger than that in the helical magnetoferroelectric phase. The magnetic structure in the $p$-induced ferroelectric phase seems to be of a collinear sinusoidal type. Although this magnetic structure itself does not break the inversion symmetry, it is considered to play an important role in the origin of ferroelectricity in the $p$-induced ferroelectric phase through the spin-lattice coupling in this system.