Anomalous magnetic ground states of the cerium germanide system

The temperature and magnetic field dependence of the electrical resistivity, specific heat and magnetization has been carried out for the highly correlated CeGex system with 1.518 ≤ x ≤ 2.0. This system exhibits a variety of remarkable physical properties. We have found five distinct regions with the same type of crystal structure with different sets of lattice parameters across this series. Interestingly, each of these regions has a different magnetic ground state. Samples in region I with 1.713 ≤ x ≤ 2.0 crystallize in an orthorhombically distorted tetragonal structure and exhibit two magnetic transitions with an antiferromagnetic transition at TN = 7.0 K and subsequently a ferromagnetic transition at TC = 4.3 K. The remaining four regions are isomorphic and crystallize in a tetragonal structure. Samples in region II with 1.664 ≤ x ≤ 1.711 have a single antiferromagnetic transition at TN = 7.0 K. Samples in regions III with 1.627 ≤ x ≤ 1.664 are ferromagnetic with TC = 5.3 K. Samples in region IV with 1.59 ≤ x ≤ 1.627 are also ferromagnetic at a higher transition temperature TC = 7.0 K. Samples in region V with 1.518 ≤ x ≤ 1.59 return to a two magnetic transition scheme, similar to region I, with an antiferromagnetic transition at TN = 11.5 K and a ferromagnetic transition at TC = 7.0 K. The results of the high temperature susceptibility show that the cerium ions are trivalent throughout the series. Calculations of the entropy removal from the specific heat measurements indicate a doublet ground state for the 4f electrons. Moreover, the electronic specific heat coefficients deduced from temperatures above the magnetic phase transitions are enhanced compared to those of normal metals. The antiferromagnetic state in regions I, II and V undergoes a metamagnetic transition at a high magnetic field. This together with the change in magnetic ordered states across the CeGex series show that this is a magnetically frustrated system. It is believed that the change in Ce-Ce inter ion spacing leads to the change in the magnetic ground state across the series. The variation of antiferromagnetic and ferromagnetic ground states can best be described by the RKKY interaction between the localized 4f electrons mediated by the conduction electrons.