Orbital-spin-phonon coupling in Jahn-Teller-distortedLaMnO3: Softening of the 490 and610cm−1Raman-active m

Abstract

The spin-phonon coupling model that excludes a possible orbital-spin-phonon ͑OSP͒ coupling does not adequately explain the softening behavior of the two most intense Raman bands of LaMnO 3 , the parent compound of "colossal" magnetoresistivity. To quantitatively interpret the softening behavior of these two Raman bands at 490 and 610 cm −1 , we have derived theoretical expressions for the phonon softening ͑⌬ N ͒ that take account of a simultaneous OSP coupling. The present analysis successfully explains the softening behavior of these two Raman modes and indicates that a simultaneous OSP coupling is present below the Néel temperature, T N . The estimated effective force constant ͑k osp ͒ of the OSP coupling is approximately 70 dynes/ cm, which is an order of magnitude smaller than that of the spin-phonon coupling ͑k s-ph ͒. Divalentcation-modified lanthanum manganites ͑LaMnO 3 ; LMO͒ that display "colossal" magnetoresistivity have rekindled our interests in these strongly correlated electronic oxides. The subtle interplay between spin, charge, orbital, and lattice degrees of freedom is the most prominent feature in this class of perovskites and is believed to be the key to understanding their extraordinary transport and magnetic properties. [1][2][3][4] The study of the parent compound LMO is a useful starting point to a consistent understanding of this complex interplay between various degrees of freedom.In the perovskite orthorhombic LMO ͑space group Pnma͒, all of the Mn ions are trivalent with four electrons in 3d orbitals. Among these four electrons, one electron occupies one of the doubly degenerate e g orbitals, and thus the ion has both spin and orbital degrees of freedom. 5,6 Below 780 K, the orbital degree of freedom is spontaneously frozen by the real-space C-type ordering of the e g orbitals accompanied by a Jahn-Teller-type lattice distortion of the MnO 6 octahedra. 6,7 In addition, the spin ordering appears below T N Ϸ 140 K, where spins, due to the Hund's exchange coupling, 6 are aligned ferromagnetically in the basal plane and antiferromagnetically along the perpendicular direction ͑A-type antiferromagnetic ordering͒.Because the Raman scattering technique is highly sensitive to local symmetry change, this method has been applied to the study of the lattice modulation of spins, orbitals, and electrons ͑polarons͒ in manganites. 8,9 Indeed, Granado et al. 10,11 obtained an evidence of the spin-phonon coupling by analyzing the softening of the Raman active 610 cm −1 and 490 cm −1 modes of LMO below T N . Since the orbital ordering temperature ͑T oo ͒ of LMO is 780 K, a simultaneous spin-orbital-phonon coupling is expected below T N ͑140 K͒. In spite of this expectation, little progress has been made along this line. Here we report an interesting indication of a simultaneous coupling of orbital, spin, and lattice degrees of freedom by carefully examining the softening behavior of Raman active phonons.Orthorhombic LMO ͑Pnma͒ polycrystalline samples used in the present Raman and magnetization measurement...

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“…2), we could not fit the softening by conventional spin phonon couplings as those reported in the case of orthorhombic [17,18] or hexagonal [19] RMnO 3 manganites.…”

Structural, magnetic behaviors and temperature-dependent Raman scattering spectra of Y and Zr codoped BiFeO3 ceramics

“…This rapid mode-frequency softening can be explained by the model of spin-phonon coupling and structural instability due to modulation of Fe-O-Fe bond angle near T N . 23,24 As it can be seen in Fig. 5(a), for undoped BFO ceramics, the Raman modes E1, A1-1, A1-2 and E2 exhibit rapid softening at the temperature from 643 to 683 K, near T N , which is the manifestation of spin-phonon coupling and the BFO transition from a paramagnetic to antiferromagnetic.…”

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“…In Fe doped composition (xr0.1) in CaMn 1À x Fe x O 3À δ , oxygen non-stoichiometry is due to the presence of Fe þ 3 valence state in the lattice [14]. Raman spectroscopy has been efficiently applied for monitoring the structural changes and JahnTeller disorder in manganites at microscopic levels [15][16][17][18] . Fig.…”