Condensed Matter Physics Seminar

2 p.m., Thursday, September 11, 2003
Room 1201, Physics Building

Exchange effects in the optics of the multiferroic manganites

A. B. Souchkov

(University of Maryland)

Abstract: The colossal magnetoresistance compounds based on doped pseudo-cubic LaMnO₃ have excited much attention because of their interesting physical properties and potential applications. YMnO₃ and LuMnO₃ are members of another series of RMnO₃ materials with smaller radius R³⁺ ions that crystallize in a hexagonal lattice. The hexagonal manganites are interesting as examples of multiferroics — they are both ferroelectric (Tc ~ 900K) and antiferromagnetic (T_N ~ 90 K) with frustration [1]. The coupling between ferroelectric and magnetic order parameters in multiferroics opens the possibility to manipulate electric properties through magnetic fields and vice versa which, in turn, gives these materials potential for applications in spintronics and as read/write heads. These materials are also interesting because of their non-linear optical properties [2].

We have studied optical and Raman spectroscopy of single crystal LuMnO₃ at temperatures between 4 and 300 K. A symmetry allowed on-site Mn d-d transition near 1.7 eV is observed to blue shift (~0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the IR active TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in temperature dependence of the quasi-static dielectric constant ε₀ below the T_N ~ 90 K is overwhelmingly dominated by the phonon contributions [3].

All Raman phonons are resonant to an excitation energy just below the optical feature. The intensity of the low-energy modes, including the Lu (Y) phonon, correlate strongly with T_N. Photoluminescence measurements show a spectral feature at ~1.4 eV, which corresponds to the radiative recombination to a Jahn-Teller split level in the optically excited state [4]. Intensities of the narrow Raman modes and of a broad scattering anti-correlate with temperature around T_N, and the photoluminescence intensity drops dramatically above T_N. Both these observations reveal an important role of spin fluctuations in relaxation process in this frustrated magnetic system.

We have shown that optical measurements give important information about the exchange interactions, the spin-lattice interactions and the interaction between the ferroelectricity (Lu or Y ion) and magnetism (Mn ion) in hexagonal manganites.

1. G. Smolenskii and I. Chupis, Usp. Fiz. Nauk 136-138,415 (1982 [Sov. Phys. Usp. 25, 475 (1982)]).

2. M. Fiebig, Th. Lottermoser, D. Fröhlich, A.V. Goltsev, and R.V. Pisarev, Nature 419, 818 (2002).

3. A.B. Souchkov, J.R. Simpson, M. Quijada, H.D. Drew, H. Ishibashi, N. Hur, J.S. Ahn, S.W. Cheong, and A. J. Millis, PRL 91, 027203 (2003).

4. A. B. Souchkov, D.B. Romero, H. D. Drew, H. Ishibashi, N. Hur, J.S. Ahn, and S.W. Cheong, in preparation.

Host: Drew
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