Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinelCdCr2S4from first principles

Abstract

We have studied the dielectric properties of the ferromagnetic spinel CdCr2S4 from first principles. Zone-center phonons and Born effective charges were calculated by frozen-phonon and Berry phase techniques within LSDA+U. We find that all infrared-active phonons are quite stable within the cubic space group. The calculated static dielectric constant agrees well with previous measurements. These results suggest that the recently observed anomalous dielectric behavior in CdCr2S4 is not due to the softening of a polar mode. We suggest further experiments to clarify this point.PACS numbers: 77.80. Bh, 61.50.Ks, 63.20.Dj Multiferroics (MFs) displaying simultaneous ferroelectric (FE) and magnetic order are receiving considerable attention today. 1 Basic questions on the nature of ferroelectricity required to coexist with magnetism have been addressed, 2 facilitating the search for new materials. 3,4 Much effort has been directed towards finding MFs displaying the magnetoelectric effect, due to both the desire to understand this particular fundamental manifestation of spin-lattice coupling and to the potential technological applications of controlling the magnetization (polarization) by an applied electric (magnetic) field. 5,6 One avenue for finding new MEs is to revisit known ferromagnetic (FM) insulators to look for ferroelectricity. CdCr 2 S 4 is a member of a family of semiconducting FM chalcogenide ACr 2 X 4 spinels with A = Cd, Hg and X = S, Se. 7,8 The preponderance of evidence shows that CdCr 2 S 4 crystallizes in the normal cubic spinel Fd3m space group over a wide temperature range, from 4K (lowest temperature measured) to decomposition temperature. CdCr 2 S 4 is a simple Heisenberg ferromagnet with Cr 3+ spins (S=3/2) ordering at T c =84K (Θ CW =155K). 9 The valence band consists of a relatively narrow Cr t 2g peak hybridized with mostly sulfur 2p states, while Cr e g states make up the lowest unoccupied states, across a gap of 1.6-1.8eV, in the conduction band. 10,11 Early interest in these materials was due to the observed coupling of the electronic structure and the lattice to the magnetic subsystem. 12,13,14,15 While the intrinsic nature of many of these effects has subsequently been questioned, 16,17,18,19 spin ordering has clearly been shown to have a relatively strong effect on the infraredactive and selected Raman-active phonon modes, 15,18 making the chalcogenide spinels an attractive system to study spin-lattice effects from first principles. 20 Recently, CdCr 2 S 4 was revisited to look for ferroelectricity, leading to the suggestion that CdCr 2 S 4 is in fact a relaxor FE displaying a particularly large magnetocapacitive effect. 21,22 A broad, frequency dependent dielectric peak in the real part of the dielectric constant, of the type associated with relaxor behavior, was observed. The dielectric constant at 10 Hz was shown to change by 500% by application of a 5 T magnetic field, with the change decreasing rapidly with increasing excitation frequency.Finally, hysteresis loops indicating non...

Theoretical study of HgCr2Se3.5Te0.5: a doping-site-dependent semimetal

“…They are linear combinations of atomic orbitals (see the Methods Section for details). In addition, that CdCr 2 S 4 and CdCr 2 Se 4 can be well described by local density approximation (LDA) + U method has been reported by Fennie and Yaresko et al 2526. The role that the electronic correlation of the Cr-3 d electrons plays on the topological properties of HgCr 2 Se 3.5 Te 0.5 has also been checked.…”

Raman study of magnetic excitations and magnetoelastic coupling inα-SrCr2O4

“…This value has been previously shown to correctly reproduce the physics of different Cr compounds. 49,50 In order to show that the physical results obtained from our calculations are robust against variations of this parameter, we repeated the phonon calculations with different values of U , namely for U = 2 eV and U = 4 eV. In Table IX, the frequencies of the Raman active A g Γpoint phonons are listed for these values of U and the five different magnetic configurations considered.…”

First principles study of ferroelectric hexagonal compounds RInO3(R = Dy, Er, and Ho): electronic structure, optical and dielectric properties

“…Among the ABO 3 perovskite structures, RMnO 3 (R ¼ rare-earth element) has been widely studied due to its excellent ferroelectric and dielectric properties, [7][8][9][10][11][12][13] and hexagonal RInO 3 with the space group of P6 3 cm is isomorphic to the hexagonal RMnO 3 . 14 If we replace Mn 3+ ions with non-transition metal In 3+ ions, the dielectric and optical properties of the RInO 3 perovskite system are only contributed by the 4f-shell electrons of the rare-earth ions R 3+ , [15][16][17][18][19][20][21] which makes it much more straightforward to study the optical and dielectric properties of RInO 3 than in RMnO 3 .…”