At a tricritical point, where three lines of critical points intersect, the coexistence of three phases is allowed[1]. Besides the typical example of ³He-⁴He mixtures, the point is also observed in antiferromagnets showing the metamagnetic transition. In their temperature-field phase diagram, the nature of transition changes from second to first order at the point.

CePtGe₂ orders antiferromagnetically at 3.8 K and shows metamagnetic transition at 0.2 T with strong magnetic anisotropy[2]. The feature of the compound is having two crystallographically inequivalent Ce sites (4i and 4h) in its orthorhombic crystal structure. Recently, we revealed that Ce ions at 4i site host the antiferromagnetic ordering whereas ones at 4h contribute to a heavy-fermion state at low temperatures. Additionally, Y ions are found to replace Ce ions at 4h preferentially in (Ce_xY_1‒x)PtGe₂ system[3].

In this talk, we focus on metamagnetism and recently observed tricriticality in the H-T phase diagram of (Ce_0.5Y_0.5)PtGe₂ compound probed by magnetotransport. The suggested tricritical point will be discussed in comparison with the magnetic features of the parent CePtGe₂.

References:

[1] R. B. Griffiths, Phys. Rev. Lett. 24, 13 (1970).

[2] T. Nakano et. al., Phys. Rev. B 100, 035107 (2019).

[3] H. Muto et. al., J. Magn. Magn. Mater. 613, 172670 (2025).