Raman spectroscopy of the magnetic coupling in Gd-i-MAX

We report the investigation of electronic collective modes in rare-earth-(RE) based magnets (Mo2/3RE1/3)2AlC (also known as RE-i-MAX phases), where RE=Gd, Yb, and Dy, using single crystal samples. In particular, a detailed investigation of the Raman spectra of Gd-i-MAX samples at low temperatures, with a focus on the phonon behavior in relation to the antiferromagnetic (AFM) phase transition at 26 K is presented. Significant shifts in the central frequencies of several low-frequency phonon modes were observed below 25 K, correlating with the Néel transition. Integrated Raman intensity measurements indicated a reduction in the electronic background below the AFM transition temperature, suggesting the opening of a magnetic gap. Our analysis showed no new phonon modes. Therefore, we do not see any indication of a Brillouin zone folding of phonon mode on cooling. However, the hardening of all phonon modes at low temperatures points to a strong spin-phonon coupling effect. Using a temperature-dependent model of phonon frequency, we determined the spin-phonon coupling constant λ to be less than 0.1 cm-1 for all frequencies, which is of the same order of magnitude as found in other antiferromagnetic materials such as MnF2 and FeF2 with TN=68 K and TN=78 K, respectively, but significantly lower than that of CuO with TN=213 K.