Fluctuation-induced first-order transition in Eu-based trillium lattices
AbstractAmong 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 Eu2+ moments ordering antiferromagnetically at TN=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 TN. The magnetic entropy reaches only roughly half of Rln8 at TN. These fluctuations are presumably the source for the pronounced first-order character of the transition at TN 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.