Magnetic excitations and anomalous spin-wave broadening in multiferroic FeV2O4

We report on the different roles of two orbital-active Fe 2+ at the A site and V 3+ at the B site in the magnetic exci-tations and on the anomalous spin-wave broadening in FeV 2 O 4. FeV 2 O 4 exhibits three structural transitions and successive paramagnetic (PM)–collinear ferrimagnetic (CFI)–noncollinear ferrimagnetic (NCFI)/ferroelectric transitions. The high-temperature tetragonal/PM–orthorhombic/CFI transition is accompanied by the appearance of a large energy gap in the magnetic excitations due to strong spin-orbit-coupling-induced anisotropy at the Fe 2+ site. While there is no measurable increase in the energy gap from the orbital ordering of V 3+ at the orthorhombic/CFI–tetragonal/NCFI transition, anomalous spin-wave broadening is observed in the orthorhombic/CFI state due to V 3+ spin fluctuations at the B site. The spin-wave broadening is also observed at the zone boundary without softening in the NCFI/ferroelectric phase, which is discussed in terms of magnon-phonon coupling. Our study also indicates that the Fe 2+ spins without the frustration at the A site may not play an important role in inducing ferroelectricity in the tetragonal/NCFI phase of FeV 2 O 4 .