Origin of weak ferromagnetism in YbxFe4Sb12, relationship between weak ferromagnetism and filling ratio x

The magnetization and specific heat of single-crystalline samples of filled-skutterudite compounds YbxFe4Sb12 were measured. Six single crystals with the filling ratio of Yb, x, from 0.875 to 0.910 were prepared. The weak ferromagnetism of YbxFe4Sb12 appearing at low temperatures was found to strongly correlate with x; with increasing x from 0.875 to 0.910, the transition temperature, T-C, decreases from 17 to 5 K and the weak-ferromagnetic moment, m(0), decreases from 0.066 to 0.005 mu(B)/Fe. From these trends, both T-C and m(0) were expected to reach zero at the critical concentration, x(c) = 0.93. In the temperature dependence of the specific heat, no anomaly at T-C was observed, reflecting that m(0) is extremely small compared with the effective Bohr magneton number. With the application of pressure, the T-C of the sample with x = 0.882 increases rapidly, with the ratio of dT(C)/dP = +5.27 K/GPa. This weak ferromagnetism of YbxFe4Sb12 originates from the itinerant 3d electrons of Fe in anions [Fe4Sb12].