Fluctuation-induced first-order transition in Eu-based trillium lattices

Among spin arrangements prone to geometric frustration, the so-called trillium lattice has not been very intensively investigated. A few theoretical works show that it is at the border between a degenerate, an only partially ordered, and a fully ordered ground state. However, only few compounds with this structure have been studied, and there is presently no good example of a trillium lattice with an antiferromagnetic ground state and clear evidence for frustration effects. We present magnetic and specific heat measurements on two realizations of a trillium lattice of local spins, EuPtSi and EuPtGe. Both compounds exhibit a similar magnetic behavior, with ${\mathrm{Eu}}^{2+}$ moments ordering antiferromagnetically at ${T}_{\mathrm{N}}$=4.1 K (EuPtSi) and 3.3 K (EuPtGe), albeit retaining a considerable amount of entropy in strong magnetic fluctuations extending to temperatures well above ${T}_{\mathrm{N}}$. The magnetic entropy reaches only roughly half of $Rln8$ at ${T}_{\mathrm{N}}$. These fluctuations are presumably the source for the pronounced first-order character of the transition at ${T}_{\mathrm{N}}$ and are likely due to magnetic frustration. Thus, EuPtSi and EuPtGe open a new door to experimental studies of frustration effects in the trillium lattice and provide a testing ground for theoretical predictions.