Hyperfine Interactions 136/137: 345–349, 2001. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 345 Hyperfine Interactions in CeT 2 Ge 2 (T = Mn, Co) Heavy Fermions Compounds Measured by TDPAC A. W. CARBONARI 1 , J. MESTNIK 1 , R. N. SAXENA 1 , R. DOGRA 1,⋆ and J. A. H. COAQUIRA 2 1 Instituto de Pesquisas Energéticas e Nucleares, São Paulo, SP, Brasil 2 Instituto de Física da Universidade de São Paulo, SP, Brasil Abstract. Time Differential Perturbed γ –γ Angular Correlation (TDPAC) technique was used to measure the magnetic hyperfine field at both Ge and Ce sites in CeMn 2 Ge 2 and CeCo 2 Ge 2 inter- metallic compounds. The 111 In ( 111 Cd) probe nuclei was used to investigate the hyperfine interaction at Ge sites, while the 140 La ( 140 Ce) nuclei was used to measure the magnetic hyperfine field at Ce site. The present measurements cover the temperature ranges from 10–460 K for CeMn 2 Ge 2 and 9–295 K for CeCo 2 Ge 2 , respectively. The result for 111 Cd probe showed two distinct electric quadrupole frequencies above magnetic transition temperatures, in both compounds and a combined interaction in the magnetic region. The temperature dependence of the magnetic hyperfine field at 111 Cd at Mn site for the CeMn 2 Ge 2 compound showed a transition from ferromagnetic to antiferro- magnetic phase around 320 K and from antiferromagnetic to paramagnetic phase at 420 K. While a small magnetic field was measured on 111 Cd at Co site, no magnetic field on 140 Ce site was observed in CeCo 2 Ge 2 . Key words: heavy fermions compound, magnetic hyperfine field, perturbed γ –γ angular correlation. 1. Introduction Highly correlated electron systems are usually called heavy Fermions and the term is generally used to designate rare earth and actinide metals, their alloys and in- termetallic compounds. The exotic properties associated with these systems are re- lated to a Fermi liquid behavior and, in a free electron model for a metal, refer to the conduction electron density of states at the Fermi level. CeMn 2 Ge 2 and CeCo 2 Ge 2 are ternary heavy Fermions compounds from the family RT 2 X 2 (R = rare earth or actinide, T = transition metal and X = Si, Ge) with the body-centered tetragonal ThCr 2 Si 2 -type crystalline structure. The CeMn 2 X 2 compounds are of particular interest because they are the only, in the whole family, which exhibit magnetic moments only at the Mn sites. In these compounds, the Mn atoms form a simple tetragonal lattice. It has been reported [1, 2] that the CeMn 2 Ge 2 has a collinear commensurate antiferromagnetic ordering with T N = 415 K. At 318 K, there is a transition to a incommensurate ferromagnetic component of the Mn moments in the c axis and a helical component in the ab plane. The CeCo 2 Ge 2 has no magnetic mo- ⋆ On leave from SBS College of Engg. and Tech. Ferozepur-152001, India.