Antiferromagnetic pinning of a phase-like mode below T$_{N}$ in NdMnO$_{3}$

NESTOR E. MASSA

Buenos Aires

Encuentro; 3¨¬ Reunión Materiales Ferroeléctricos; 2012

grupo ferroelectricos AFA

We report on reflectivity and emission far infrared spectra of NdMnO3 between 4K and its dissociation temperature. Phonon bands at 300K are in agreement with orthorhombic Pbnm space group assignments. In addition, a broad strong band. reminiscent to a phase-mode in quasi-one dimension metals, is found at very low frequencies that it is understood originating in charge fluctuations in d-orbitals. There is no distinctive behaviors between 1073 K and 1173 K, where orthorhombic O and O' coexist. Beyond ¡­ 700 K a mid-infrared polaron band turns into a Drude tail suggesting hopping conductivity double exchange due to air heating oxidation Mn3+¡æ Mn4+ +1e− . Below 300 K phonons are better defined and the low frequency giant dipole acquires strength. Few degrees above the antiferromagnetic transition it broadens as electrons loosing coherence. At TN¡­ 76 K we find strong phonon magnetostriction while the band turns asymmetric locking-in to the underlying magnetic order. Preliminary measurements of hexagonal TmMnO3 , show that asymmetry split due to lower symmetry and the triangular magnetic lattice two exchange integrals J1 and J2 in the a-b plane. Similar to a soft mode those two bands and a lower frequency resonance undergo strong hardening down to 4 K.