Spin-phonon coupling in multiferroic RCrO ₃ (R-Y, Lu, Gd, Eu, Sm): A Raman study

Abstract

-Raman study on a select few orthochromites, RCrO 3 (R = Y, Lu, Gd, Eu and Sm) shows that the phonon behavior at T N in compounds with magnetic R-ion (Gd and Sm) is remarkably different from that of non-magnetic R-ion (Y, Lu and Eu). While anomalies in most of the observed phonon frequencies in all these compounds may result from the distortion of CrO 6 octahedra due to size effect and magnetostriction arising from Cr-ordering, the anomalous behavior of their linewidths observed at T N for the compounds with only magnetic R-ion suggests spin-phonon coupling. The presence of spin-phonon coupling and the anomalies in the low frequency modes related to R-ion motion in orthochromites (R = Gd and Sm) support the suggestion that the coupling between 4f-3d moments play important role in inducing switchable electric polarization.Introduction.-Magnetoelectric multiferroic materials with their coupled ferroelectric and ferromagnetic order parameters are promising for developing a new generation of both electrically and magnetically controlled multifunctional devices [1][2][3][4][5]. Multiferroic materials are broadly classified into two types; in type I multiferroics, the ferroelectricity and magnetism occur at high temperatures but with different temperature scale. However, the coupling between the two order parameters are rather weak. Type II multiferroics are generally centrosymmetric and magnetic where the ferroelectricity is caused by certain type of magnetic ordering. In the well-known example of TbMnO 3 , the manganese moments order antiferromagnetically at T N = 41 K and at 25 K it undergoes another magnetic transition below which a cycloidal spin structure breaks the inversion symmetry and thus induces ferroelectricity [6]. In case of HoMnO 3 [7], a collinear magnetic ordering with E-type magnetic structure gives rise to ferroelectricity. It has been shown recently that canted antiferromagnetic ordering with two non-equivalent spin pairs in the orthoferrite, SmFeO 3 [8] induces ferroelectric polarization at the magnetic ordering temperature of iron. Rajeswaranet al. [9] have reported electric polar order at the magnetic ordering temperature of chromium in the isostructural orthochromites, RCrO 3 with magnetic R ion, where the interactions between R 3+ and Cr 3+ ions have been

Local inversion symmetry breaking and spin-phonon coupling in the perovskite GdCrO3

“…The dotted lines represent the fitted curves for anharmonic contributions to these modes according to Eq. 27 . This suggests the presence of spin-phonon coupling due to magnetic interaction between Gd 3+ and Cr 3+ moments which is mediated by the weak ferromagnetic coupling (canted) of Cr-sublattice.…”

“…Also, Raman studies show anomalous change of phonon frequency and a decrease in phonon life times across the mutiferroic transition temperature, both in the modes involving CrO 6 octahedra and magnetic R-ion 27 . This suggests that ferroelectricity in RCrO 3 is driven by magnetostriction mechanism caused by 3d-4f coupling, resulting in the displacement of the R-ion and the octahedral distortion via oxygen displacements.…”

Raman spectroscopy of rare-earth orthoferrites RFeO3 ( R =La, Sm, Eu, Gd, Tb, Dy)

“…More recently, it has been shown that RS is also an appropriate probe for the investigation of lattice distortions and slight changes in octahedra rotations [15][16][17][18]. Further to this, RS is an ideal probe for the investigation of spinphonon coupling phenomena [19][20][21][22][23]. Finally, RS is a now widely used technique for probing even subtle straininduced structural modifications in oxide thin films [24][25][26].…”

Spin-phonon and magnetostriction phenomena in CaMn7O12 helimagnet probed by Raman spectroscopy

“…In the first magnetic transition at 1~ 90 K the anomalies are due to the coupling directly between the magnetic ordering and lattice (phonon) since CMO does not show any anomaly in the lattice parameters in this magnetic transition 23 , what discards the possibility of the coupling be due to the magnetostriction effect 27 . However, the second spin-phonon coupling anomaly at 2~ 50 K could be due to magnetostriction effect, as once at this magnetic transition the lattice parameters of CMO change significantly.…”