Spin-Wave Spectrum in “Single-Domain” Magnetic Ground State of Triangular Lattice Antiferromagnet CuFeO₂

Abstract

By means of neutron scattering measurements, we have investigated spin-wave excitation in a collinear four-sublattice (4SL) magnetic ground state of a triangular lattice antiferromagnet CuFeO2, which has been of recent interest as a strongly frustrated magnet, a spin-lattice coupled system and a multiferroic. To avoid mixing of spin-wave spectrum from magnetic domains having three different orientations reflecting trigonal symmetry of the crystal structure, we have applied uniaxial pressure on [110] direction of a single crystal CuFeO2. By elastic neutron scattering measurements, we have found that only 10 MPa of the uniaxial pressure results in almost 'single domain' state in the 4SL phase. We have thus performed inelastic neutron scattering measurements using the single domain sample, and have identified two distinct spinwave branches. The dispersion relation of the upper spin-wave branch cannot be explained by the previous theoretical model [R. S. Fishman: J. Appl. Phys. 103 (2008) 07B109]. This implies the importance of the lattice degree of freedom in the spin-wave excitation in this system, because the previous calculation neglected the effect of the spin-driven lattice distortion in the 4SL phase. We have also discussed relationship between the present results and the recently discovered "electromagnon" excitation.

Multifrequency ESR measurements of the triangular lattice antiferromagnet CuFeO2in high magnetic fields

“…The theoretical curves deviate slightly from the experimental ESR modes. In fact, the excitation energy, 2.3 meV, of ω H at (1/4 1/4 3/2) observed by neutron scattering measurements 26,27 is slightly lower than the zero-field gaps of the ω b± modes. Furthermore, the splitting between ω b+ and ω b− at zero field is not reproduced.…”

“…The magnetic excitation at zero field was studied by neutron scattering measurements of several groups. [25][26][27] Dispersive behaviors of the observed magnetic excitation indicate that CuFeO 2 has a Heisenberg-type character and thus elementary excitation in this compound can be described by a spin wave. The dispersion curves reported for the (h h 3/2) scan are rather complicated because several kinds of excitation modes are superimposed.…”

“…To reproduce the experimental valuehω H,0 ≈ 2.3 meV, 19,26 inequities in the nearest-neighbor interactions are required, i.e., J 1 , J 2 , and J 3 are evaluated to take different values. Our analysis suggests that the lattice distortion strengthens the exchange interactions J 1 and J 2 between the spins with an antiparallel alignment and weakens the interaction J 3 with parallel alignment to stabilize the 4-sublattice structure.…”

“…In fact, zero-field frequencies of the ω a± mode agree with the excitation energy of the lower frequency spinwave modehω L,0 at (1/4 1/4 3/2) observed in neutron scattering measurements. 26,27 Small splitting between ω a+ and ω a− at a zero magnetic field can be explained by taking the rhombic anisotropy E term into account. As shown in Fig.…”

Spin-driven bond order in a15-magnetization plateau phase in the triangular lattice antiferromagnet CuFeO2

“…This is because CFO has three magnetic domains in the magnetically ordered phases owing to the threefold rotational symmetry about the c axis, and volume fractions of the three domains can be controlled by a small uniaxial pressure applied in the triangular lattice plane. 27 The sample with the uniaxial-pressure clamp was mounted in the cryomagnet so that the c axis is parallel to the magnetic field. By measuring elastic magnetic Bragg reflections in the 5SL phase, we have found that the magnetic domain having the magnetic modulation wave vector parallel to the [110] direction has the volume fraction of 64 %, and dominates over the other two domains (14% and 22%).…”