Magnetic frustration on a Kagome lattice in R3Ga5SiO14 langasites with R = Nd, Pr

In the R3Ga5SiO14 compounds, the network R of rare earth cations forms well separated planes of corner sharing triangles topologically equivalent to a Kagome lattice. Powder samples and single crystals with R = Nd and Pr were prepared and magnetostatic measurements were performed under magnetic field up to 10 T in the temperature range from 1.6 to 400 K. Analysis of the magnetic susceptibility at the high temperatures where only the quadrupolar term of the crystal electric field prevails suggests that the Nd and Pr magnetic moments can be modelled as coplanar elliptic rotators perpendicular to the threefold axis of the crystal structure that interact antiferromagnetically. Nonetheless, a disordered phase that can be ascribed to geometric frustration persists down to the lowest temperature, which is about 25 times smaller than the energy scale for the exchange interactions.