Antiferromagnetic pinning of a phase-like mode below TN in NdMnO3

NÉSTOR E. MASSA ; LEIRE DEL CAMPO; DOMINGOS DE SOUSA MENESES ; PATRICK ECHEGUT. ; MARÍA JESÚS MARTÍNEZ-LOPE ; JOSÉ ANTONIO ALONSO

Boston, MA, USA

Congreso; American Physical Society March Meeting; 2012

American Physical Society

We report on reflectivity and emission far infrared spectra of NdMnO$_{3 }$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 $\sim $700 K a mid-infrared polaron band turns into a Drude tail suggesting hopping conductivity double exchange due to air heating oxidation Mn$^{3+}\to $ Mn$^{4+ }$+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 T$_{N}\sim $76 K we find strong phonon magnetostriction while the band turns asymmetric locking-in to the underlying magnetic order. Preliminary measurements of hexagonal TmMnO$_{3}$, show that asymmetry split due to lower symmetry and the triangular magnetic lattice two exchange integrals J$_{1}$ and J$_{2}$ 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.