Magnetostriction Reveals Orthorhombic Distortion in Tetrahedral Gd-compounds

We report detailed thermal expansion and magnetostriction experiments on GdCoIn$_5$ and GdRhIn$_5$ single crystal samples that show a sudden change in the dilation at a field B$^ast$ for temperatures below the Neel transition temperature TN. We present a first-principles model including crystal-field effects, dipolar and exchange interactions, and the dependence of the exchange couplings with lattice distortions in order to fully account for the magnetostriction and magnetic susceptibility data. The mean-field solution of the model shows that a transition between metastable states occurs at the field B$^ast$. It also indicates that two degenerate phases coexist in the sample at temperatures below TN. This allows to explain the lack of observation, in high resolution x-ray experiments, of an orthorhombic distortion at the Neeel transition even though the magnetic structure breaks the tetragonal symmetry and the magnetoelastic coupling is significant. These conclusions could be extended to other tetragonal Gd-based compounds that present the same phenomenology.

Landau theory for magnetic and structural transitions in CeCo$_{0.85}$Fe$_{0.15}$Si

We present a phenomenological analysis of the magnetoelastic properties of CeCo$_{0.85}$Fe$_{0.15}$Si at temperatures close to the Neel transition temperature $T_N$. Using a Landau functional we provide a qualitative description of the thermal expansion, magnetostriction, magnetization and specific heat data. We show that the available experimental results [Journal of Physics: Condensed Matter 28 346003 (2016)] are consistent with the presence of a structural transition at $T_sgtrsim T_N$ and a strong magnetoelastic coupling. The magnetoelastic coupling presents a Janus-faced effect: while the structural transition is shifted to higher temperatures as the magnetic field is increased, the resulting striction at low temperatures decreases. The strong magnetoelastic coupling and the proximity of the structural transition to the onset temperature for magnetic fluctuations, suggest that the transition could be an analogue of the tetragonal to orthorhombic observed in Fe-based pcnictides.

Evidence of a small crystal field anisotropy in GdCoIn$_5$

We investigate the effects of an applied magnetic field on the magnetic properties of the antiferromagnet GdCoIn$_5$. The prominent anisotropy observed in the susceptibility below $T_N$ is rapidly suppressed by a field of just a few Tesla. Further evidence of this low energy-scale is obtained from magnetoresistance and magnetostriction experiments. The lattice lenght, particulary, shows a sudden change below 2 Tesla when the magnetic field is applied perpendicular to the crystallographic $hat{c}$-axis. The experimental results as a whole can be attributed to a small but non negligible higher-order crystalline electric field.