Resumenes de las Charlas Invitadas
Temperature evolution of crystal field interactions across the Jahn-Teller transition in a
La7/8Sr1/8MnO3
single crystal
G. Alejandro1,
M. C. G. Passeggi2,
D. Vega3,
C. A. Ramos4, M. T. Causa4, M. Tovar4,
J. Fontcuberta5
1Centro Atómico Bariloche, San Carlos de Bariloche, Río Negro, Argentina
2Instituto de Desarrollo Tecnológico, CONICET-UNL and
Facultad de Bioquímica y Ciencias Biológicas-UNL Santa Fe, Argentina
3Centro Atómico Constituyentes, CNEA. Buenos Aires, Argentina
4Centro Atómico Bariloche, San Carlos de Bariloche, Río Negro, Argentina
5Institut de Ciencia de Materials de Barcelona, Catalunya, Spain.
Abstract
Electron spin resonance (ESR) measurements has been performed in a single crystal of
La7/8Sr1/8MnO3. The
experiments were carried out in the paramagnetic regime (TC ~ 180K),
from 220K to 570K and at a microwave frequency of 9.4GHz. Data of the ESR linewidth as a function
of temperature were taken for the magnetic field (H0) along the
crystallographic directions [100] and [001], referred to the orthorhombic (Pbnm) axes.
Detailed angular variation measurements at constant temperature for Ho
rotating in the (001), (010) and (-110) planes were also performed. The temperature dependence of
the linewidth for fixed directions clearly reflects the changes in the symmetry of the crystal. At
the Jahn Teller (JT) transition, a noticeable decrease of the linewidth is clearly observed.
The angular variation of the linewidth reveals an anisotropy that is also temperature dependent,
decreasing as T goes up and almost dissapears above TJT ~ 280K,
although a small remnant anisotropy can be observed up to 400K.
We showed that the temperature dependence of this anisotropy across the Jahn-Teller transition can
be related to the evolution of the crystal field interactions. The symmetry of the ESR spectra can
be explained in terms of a spin Hamiltonian that includes isotropic Heisenberg superexchange,
single ion-crystal fields, and Dzyaloshinsky-Moriya antisymmetic interactions. The experimental
angular variation of the ESR linewidth and its temperature dependence were fitted using the
calculated expressions for the linewidth as obtained from the Kubo-Tomita description of the ESR
absorption. We obtained a ratio E/D = -0.4 for the spin Hamiltonian coefficients associated
with the orthorhombic (E) and tetragonal (D) Jahn-Teller distortions of the
MnO6 octahedral units. The temperature dependence of the parameters of
the model are presented, and the correlation with the dilatometry results are discussed.
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