Search for spin-lattice coupling mediated by itinerant electrons: Synchrotron x-ray diffraction and Raman scattering from GdAl₃

The coupling among the spin degree of freedom and the atomic displacements in intermetallic GdAl₃ was investigated by means of synchrotron x-ray diffraction and polarized Raman scattering. In this compound, the Gd 4f⁷ shell is spherical and the spin-lattice coupling provides a fingerprint of the exchange mechanism and degree of magnetic correlations. X-ray diffraction shows nonresonant symmetry-forbidden charge Bragg peaks below the long-range magnetic ordering temperature T_N=18 K, revealing a symmetry-lowering crystal lattice transition associated with Gd displacements, consistent with a Ruderman-Kittel-Kasuya-Yosida mechanism for the magnetic coupling. Raman scattering in fresh broken surfaces shows phonons with conventional frequency behavior, while naturally grown and polished surfaces present frequency anomalies below T^*∼50 K. Such anomalies are possibly due to a modulation of the magnetic energy by the lattice vibrations in a strongly spin-correlated paramagnetic phase. Such interpretation implies that the spin-phonon coupling in metals may depend on the surface conditions. A fully spin-correlated state immediately above T_N is inferred from our results in this frustrated system.