Investigation of magnetoelastic coupling in multiferroic geometrically frustrated magnets: CuFeO₂, CuCrO₂, and CuCrS₂

Aktas, Oktay (2012) Investigation of magnetoelastic coupling in multiferroic geometrically frustrated magnets: CuFeO₂, CuCrO₂, and CuCrS₂. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Elastic and structural properties of triangular lattice antiferromagnets CuCr0₂ , CuFe0₂ , and CuCrS₂ were investigated to elucidate the role played by spin-phonon coupling in the magnetic and multiferroic properties of a large class of triangular latt ice antiferromagnets. Using Brillouin scattering, five of six elastic constants of CuCr0₂ are determined at room temperature. Low temperature elastic properties of CuCr0₂ are extensively investigated with the ultrasonic pulse echo method. According to these measurements, the elastic constants C₁₁, C₄₄, and C₆₆ show softening as the temperature is reduced down to the antiferromagnetic transition temperature Tn₁ = 24.3 K. The Landau analysis of the ult rasonic data indicates a first order pseudoproper ferro elastic transition at T n₁ , where magnetic moments can act as a secondary order parameter. The transition corresponds to a structural change from the tetragonal point group 3m to the monoclinic point group 2/m. In addition, the symmetry lowering at Tn₁ seems to aid the spin-driven ferroelectricity below Tn₂ = 23.8 K, at which the crystal symmetry should change from 2/m to 2. The existence of Tn₂ is confirmed by simultaneous measurements of the dielectric constant E[uo) and acoustic modes. Unlike CuCr0₂ , isostructural CuFe0₂ seems to show a second order 3m -'- 2/m ferroelastic transition coincident with the antiferromagnetic transition at Tn₁ = 14 K [1] . In order to confirm if the t ransition is second order, Brillouin scattering measurements were performed on CuFe0₂ . Due to the opacity of CuFe0₂ , Brillouin spectra show only surface acoustic modes for waves propagating in the xy and xz planes. The velocity of the modes depends on the elastic constants c₄₄ and c₃₃ . Raman measurements were performed to possibly determine if the ferroelastic transitions at Tn₁ in CuFe0₂ and CuCr0₂ and the R3m ---> Cm structural and antiferromagnetic transition at TN = 38 K in another geometrically frustrated magnet, CuCrS₂, are driven by a soft optic mode. Based on these measurements, the temperature dependencies of all modes in CuCr0₂, CuFe0₂ and CuCrS₂ are attributed to anharmonic phonon-phonon interactions. Therefore, Raman modes in CuCr0₂ and CuFe0₂ cannot account for the ferroelastic transit ions observed at Tn₁ , leaving the driving mechanism of the ferroelastic transit ions uncertain. Similarly, measurements on CuCrS₂ does not reveal any soft optic modes. Finally, simultaneous measurements of the dielectric constant and acoustic velocities of CuCr0₂ were performed to determine the magnetic phase diagram of CuCr0₂ for magnetic fields along the [110] and [l10] directions (hexagonal setting). For magnetic fields parallel to the [110] direction the dielectric constant and acoustic modes show an anomaly at Hflop ~5 T between 2 K and 23.7 K, which correspond to a 90° flop in the spin-spiral plane and electric polarization. The anomaly observed in the longitudinal acoustic mode propagating in the basal plane is attributed to the field dependence of magnetic susceptibility. Measurements performed with magnetic fields parallel to the [110] direction suggest a reorientation in the spin-spiral plane. The ferroelastic transition, coincident with the antiferromagnetic transition at Tn₁ in CuCr0₂ as well as acoustic anomalies at the spin flop transition clearly indicate that magnetoela. stic coupling has a. strong impact on the magnetic and multiferroic proper ties of CuCr0₂. For a complete understanding of the role of magnetoelastic coupling on these properties in CuCr0₂ and a. large class of triangular lattice antiferromagnets, results on the ultrasonic velocity measurements on CuCr0₂ have to be analyzed using a Landau model that includes magnetoela.stic coupling terms as well as the coupling between the order parameter and magnetic moments.

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