Research Project: Optical properties of systems with the strongly correlated electrons: ferroelectricity in spiral magnets (multiferroics), spinels, hexa-manganites (multiferroics), high-temperature superconductors; electron-phonon and spin-(optical)phonon coupling in these materials.
TbMn2O5 is a representative of a group of multiferroic materials where ferroelectricity is induced by spiral magnetic ordering at low temperatures. These materials demonstrate several weak structural and magnetic transitions reflecting complex interplay between magnetic order and the lattice. Phonons serve as a probe of lattice changes and, via spin-phonon coupling, of magnetic ordering. We measured the reflectivity spectra in a- and b-polarizations of an orthorhombic single crystal. The Tb-dominating phonons are active in both polarizations but only b-polarization shows a magnetic shift below 24 K where Tb moments start to order. We made lattice dynamics calculations using popular GULP program to understand why a particular infrared phonon is strongly coupled to spin ordering.
Geometrically frustrated magnets can resist magnetic ordering and remain in a strongly correlated paramagnetic state well below the Curie-Weiss temperature. The spin-lattice coupling can play an important role in relieving the frustration in these systems. In ZnCr2O4, an excellent realization of the Heisenberg antiferromagnet on the pyrochlore network, a lattice distortion relieves the geometrical frustration through a spin-Peierls-like phase transition at Tc=12.5 K with a lowering of the symmetry from cubic to tetragonal. Conversely, spin correlations strongly influence the elastic properties of a frustrated magnet. By using infrared spectroscopy and published data on magnetic specific heat, we demonstrate that the frequency of the Cr optical phonon triplet in ZnCr2O4 tracks the nearest-neighbor spin correlations above Tc. Below Tc, the triplet splits into a singlet and a doublet, separated by 11 cm-1. This splitting gives a direct measurement of the spin-Peierls order parameter. From analysis of the ion displacements in the phonon modes we can conclude that direct Cr-Cr exchange dominates in ZnCr2O4. These experiments result in a clear understanding of spin-phonon coupling in ZnCr2O4 in contrast to other oxide magnets. Recent ab initio calculations confirm the magnetic origin of both the phonon splitting in ZnCr2O4 and the frequency shifts in the ferromagnetic insulating spinel CdCr2S4.
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 (TN ~ 90 K) with frustration. 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.
Other papers: here
MRSEC Materials Research Science and Engineering Center at the University of Maryland, University of Maryland
Group of Prof. Sang Cheong, Rutgers University
Group of Prof. Andrew Millis, Columbia University
Last Modified: January 17, 2006