Thermodynamic and transport properties of single crystalline RCo ₂Ge₂ (R=Y, La-Nd, Sm-Tm)

Single crystals of RCo₂Ge₂ (R=Y, La-Nd, Sm-Tm) were grown using a self-flux method and were characterized by room-temperature powder X-ray diffraction; anisotropic, temperature and field dependent magnetization; temperature and field dependent, in-plane resistivity; and specific heat measurements. In this series, the majority of the moment-bearing members order antiferromagnetically; YCo₂Ge₂ and LaCo₂Ge ₂ are non-moment-bearing. Ce is trivalent in CeCo₂Ge ₂ at high temperatures, and exhibits an enhanced electronic specific heat coefficient due to the Kondo effect at low temperatures. In addition, CeCo₂Ge₂ shows two low-temperature anomalies in temperature-dependent magnetization and specific heat measurements. Three members (R=Tb-Ho) have multiple phase transitions above 1.8 K. Eu appears to be divalent with total angular momentum L=0. Both EuCo₂Ge₂ and GdCo₂Ge₂ manifest essentially isotropic paramagnetic properties consistent with J=S=7/2. Clear magnetic anisotropy for rare-earth members with finite L was observed, with ErCo₂Ge₂ and TmCo₂Ge₂ manifesting planar anisotropy and the rest members manifesting axial anisotropy. The experimentally estimated crystal electric field (CEF) parameters B₂⁰ were calculated from the anisotropic paramagnetic θ_ab and θ_c values and follow a trend that agrees well with theoretical predictions. The ordering temperatures, ^TN, as well as the polycrystalline averaged paramagnetic Curie-Weiss temperature, Θ_avg, for the heavy rare-earth members deviate from the de Gennes scaling, as the magnitude of both is the highest for Tb, which is sometimes seen for extremely axial systems. Except for SmCo₂Ge₂, metamagnetic transitions were observed at 1.8 K for all members that ordered antiferromagnetically.