Optical Magnons in Magnetostrictive CoFe2O4

The stability and tunability of spinel oxides, along with their favorable mechanical, electric and magnetic properties, have made them a consistent focus in the study of both fundamental condensed matter physics and device manufacture. CoFe2O4 exhibits the largest magnetostriction of all known transition metal spinel oxides with a high strain sensitivity and considerable chemical tunability which opens the possibility of a wide range of applications in actuators and high sensitivity sensors. It has long been suspected that the large magnetostriction originated from a strong uniaxial anisotropy associated with the Co2+ ions. However, despite the considerable interest in this compound, the magnetic properties of bulk CoFe2O4 have not yet been characterized.

In this talk, we will present detailed measurements of both the structure and dynamics of CoFe2O4. We show that despite the presence of orbitally degenerate Co2+, there is no evidence of a sizable crystallographic distortion. We present inelastic neutron scattering measurements which reveal two magnon branches of similar bandwidth, separated by a large gap. Surprisingly, we find that the lower branch has only a small gap. Using linear spin wave theory, we demonstrate that the upper optical magnon originates from the ferrimagnetic nature of the magnetic order in CoFe2O4. We suggest that the strong magnetostriction observed in CoFe2O4 originates from the interplay between anisotropy and the large net internal molecular field provided by the ferrimagnetic order.