Thermodynamic and transport properties of single crystalline RCo2Ge2 (R=Y, La–Nd, Sm–Tm)

Abstract

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 LaCo2Ge2 are non-moment-bearing. Ce is trivalent in CeCo2Ge2 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 1.8K. 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 θab and θc 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, Θavg, 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 1.8K for all members that ordered antiferromagnetically.