Optical phonons, spin correlations, and spin-phonon coupling in the frustrated pyrochlore magnetsCdCr2O4andZnCr2

Abstract

We report on infrared, Raman, magnetic susceptibility, and specific-heat measurements on CdCr 2 O 4 and ZnCr 2 O 4 single crystals. We estimate the nearest-neighbor and next-nearest-neighbor exchange constants from the magnetic susceptibility and extract the spin-spin correlation functions obtained from the magnetic susceptibility and the magnetic contribution to the specific heat. By comparing with the frequency shift of the infrared optical phonons above T N , we derive estimates for the spin-phonon coupling constants in these systems. The observation of phonon modes which are both Raman and infrared active suggest the loss of inversion symmetry below the Néel temperature in CdCr 2 O 4 in agreement with theoretical predictions by Chern et al. ͓Phys. Rev. B 74, 060405͑R͒ ͑2006͔͒. In ZnCr 2 O 4 several new modes appear below T N but no phonon modes could be detected which are both Raman and infrared active.

Universal Exchange-Driven Phonon Splitting in Antiferromagnets

“…Although the exact lattice symmetry and the spin configuration of the magnetically ordered state are still subject of debate [8,15,37,38], the dominant struc- [15]. Details for sample preparation and characterization of the other samples have been given earlier [15,40]. The magnetic susceptibilities of our spinel samples [30] agree nicely to the ones of fully stoichiometric samples [41].…”

“…Fig. 3(g)] we encounter the opposite situation, with ∆ω = 11 cm −1 [11,15] and ∆ω = 14.3 cm −1 , respectively. The size of the splitting has previously been associated with the spin-phonon coupling effects due to the dominant direct nn exchange coupling J nn of the Cr 3+ ions residing on the frustrated pyrochlore lattice.…”

“…For transition-metal monoxides (TMMOs) a magnetisminduced anisotropy in the lattice response was predicted theoretically [6]. This approach has been extended to other material classes such as Cr based spinels, which are hallmark systems for highly frustrated magnets [7][8][9][10], where spin-phonon coupling leads to a splitting of characteristic phonon modes [11][12][13][14][15].TMMOs are both textbook examples for antiferromagnets governed by superexchange in a cubic rock-salt lattice and benchmark materials for the understanding of strongly correlated electronic systems [16,17]. The magnetic structure of the TMMOs consists of ferromagnetic planes coupled antiferromagnetically, e.g., along [1 1 1] as depicted in Fig.…”

“…For transition-metal monoxides (TMMOs) a magnetisminduced anisotropy in the lattice response was predicted theoretically [6]. This approach has been extended to other material classes such as Cr based spinels, which are hallmark systems for highly frustrated magnets [7][8][9][10], where spin-phonon coupling leads to a splitting of characteristic phonon modes [11][12][13][14][15].…”

Distinct quasiparticle relaxation dynamics in an electron-doped superconductor, BaFe1.9Ni0.1As2

“…The value θ D ∼ 140 K from a recent thermal conductivity measurement [34] was used in the calculation. Thus, we estimated the Coulomb pseudopotential µ * to be ∼−0.4.…”