Magnetic structure of the ferromagnetic Kondo-lattice compounds YbPtGe and YbPdGe

YbPtGe and YbPdGe exhibit ferromagnetic ordering below Tc = 5.4 and 11.4 K with enhanced electronic specific heat coefficients of γ = 209 and 150 mJ K(-2) mol, respectively. In order to shed light on the origin of the coexistence of a ferromagnetic state and heavy-fermion behavior, we studied the powder neutron diffraction of YbPtGe and YbPdGe at low temperatures. Weak reflections due to magnetic ordering have been resolved. The data were analyzed using the Rietveld method together with group theory analysis. It has been found that YbPtGe exhibits a non-collinear ferromagnetic structure, with a spontaneous moment along the c-axis and a weak antiferromagnetic component along the a-axis. The presence of this antiferromagnetic component explains the origin of the observed metamagnetic-like behavior. In the case of YbPdGe, magnetization measurements confirmed the ferromagnetic moment along the b-axis and revealed a metamagnetic transition at 0.2 T for a field parallel to the c-axis. The neutron diffraction results indicate that the magnetic structure of YbPdGe is also of a non-collinear type, with ferromagnetic moments parallel to the b-axis and weak antiferromagnetic components along the c-axis, which is consistent with the magnetization data. A comparison of the results for YbPtGe and YbPdGe has been made. It is suggested that both the Kondo screening effect of ferromagnetic moments and the fluctuation of antiferromagnetic components can contribute to the enhanced mass in the ferromagnetic state.