Abstract
We have investigated magnetic orders and excitations in a Y-type hexaferrite BaSrCo2Fe11AlO22 (BSCoFAO),
which was reported to exhibit spin-driven ferroelectricity at room temperature [S. Hirose, K. Haruki, A. Ando,
and T. Kimura, Appl. Phys. Lett. 104, 022907 (2014)]. By means of magnetization, electric polarization, and
neutron-diffraction measurements using single-crystal samples, we establish a H-T magnetic phase diagram for
magnetic field perpendicular to the c axis (H⊥c). This system exhibits an alternating longitudinal conical (ALC)
magnetic structure in the ground state, and it turns into a non-co-planar commensurate magnetic order with
spin-driven ferroelectricity under H⊥c. The field-induced ferroelectric phase remains as a metastable state after
removing magnetic field below ∼250 K. This metastability is the key to understanding of magnetic field reversal
of the spin-driven electric polarization in this system. Inelastic polarized neutron-scattering measurements in the
ALC phase reveal a magnetic excitation at around 7.5 meV, which is attributed to spin components oscillating
in a plane perpendicular to the cone axis. This phasonlike excitation is expected to be an electric-field active
magnon, i.e., electromagnon excitation, in terms of the magnetostriction mechanism.
