Thermodynamic and transport properties of single crystalline RCo$_{2}$Ge$_{2}$ (R = Y, La-Nd, Sm-Tm)

Abstract: Single crystals of RCo${2}$Ge${2}$ (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; YCo${2}$Ge${2}$ and LaCo${2}$Ge${2}$ are non-moment-bearing. Ce is trivalent in CeCo${2}$Ge${2}$ at high temperatures, and exhibits an enhanced electronic specific heat coefficient due to Kondo effect at low temperatures. In addition, CeCo${2}$Ge${2}$ 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 EuCo${2}$Ge${2}$ and GdCo${2}$Ge${2}$ manifest essentially isotropic paramagnetic properties consistent with J = S = 7/2. Clear magnetic anisotropy for rare-earth members with finite L was observed, with ErCo${2}$Ge${2}$ and TmCo${2}$Ge${2}$ manifesting planar anisotropy and the rest members manifesting axial anisotropy. The experimentally estimated crystal electric field (CEF) parameters B${2}^{0}$ were calculated from the anisotropic paramagnetic $\theta{ab}$ and $\theta_{c}$ values and follow a trend that agrees well with theoretical predictions. The ordering temperatures, T${N}$, as well as the polycrystalline averaged paramagnetic Curie-Weiss temperature, $\Theta{avg}$, for the heavy rare-earth members deviate from the de Gennes scaling, as the magnitude of both are the highest for Tb, which is sometimes seen for extremely axial systems. Except for SmCo${2}$Ge${2}$, metamagnetic transitions were observed at 1.8 K for all members that ordered antiferromagnetically.