Unified understanding of the first-order nature of the transition in TbCo2

The determination of the nature of phase transitions, especially those that involve two symmetry-breaking order parameters, is a fundamental issue in condensed matter physics. For the Laves-phase rare-earth-transition-metal intermetallic compounds, their phase transitions involve both magnetic ordering and structural ordering. As a typical material of the Laves-phase intermetallics, TbCo2 has been studied extensively for its transition around 230 K. However, the understanding on the nature of this transition has remained controversial (first/second order) for decades. Here, in this paper, based on the criteria that determine first-order and second-order transitions for magnetic materials, which are (1) latent heat, (2) thermal hysteresis, (3) coexistence of phases, and (4) the Banerjee criterion, we show direct evidence to reveal the first-order nature of the transition in TbCo2, which is further interpreted by a Landau theory based phenomenological approach. Our work reconciles the lasting arguments on the transition of TbCo2 and may pave the way for deepening the understanding on the transitions of magnetic materials that involve both magnetic and structural transitions.