Valence change and magnetic order in YbMn₆Ge_{6 −x}Sn_x

Abstract

The YbMn(6)Ge(6-x)Sn(x) compounds (0 < x < 6) have been investigated using x-ray diffraction, magnetic measurements, neutron diffraction and (170)Yb Mössbauer spectroscopy. The YbMn(6)Ge(6-x)Sn(x) system comprises three solid solutions: (i) 0 < x ≤ 1.1, (ii) 3.2 ≤ x ≤ 4.6 and (iii) 5.3 ≤ x < 6, all of which crystallize in the hexagonal (P6/mmm) HfFe(6)Ge(6)-type structure. The substitution of Sn for Ge yields changes in the type of magnetic order (antiferromagnetic, helimagnetic, ferromagnetic, conical and ferrimagnetic), in the easy magnetization direction (from easy axis to easy plane) as well as in the valence state of Yb (from trivalent to divalent). The Mn moments order at or above room temperature, while magnetic ordering of the Yb sublattice is observed at temperatures up to 110 K. While Yb is trivalent for x ≤ 1.1 and divalent for x ≥ 5.3, both magnetic and (170)Yb Mössbauer spectroscopy data suggest that there is a gradual reduction in the average ytterbium valence through the intermediate solid solution (3.2 ≤ x ≤ 4.6), and that intermediate valence Yb orders magnetically, a very unusual phenomenon. Analysis of the (170)Yb Mössbauer spectroscopy data suggests that the departure from trivalency starts as early as x = 3.2 and the loss of ytterbium moment is estimated to occur at an average valence of ∼2.5+.

Nonpareil Yb Behavior inYbMn6Ge6−xSnx

“…These measurements prove the intermediate valent character of Yb previously conjectured [7] and further evidence a temperature dependence of the 4f electron number which decreases upon increasing temperature, in contrast to the usual observation in intermediate valent Yb materials [2]. A scenario is proposed to explain this surprising and 6 Ge 6Àx Sn x samples with x ¼ 4:2 and 4.4 were prepared from high-purity elements in closed Mo crucibles to avoid losses of Yb during annealing, as described in Ref.…”

“…However, the situation of coexisting intermediate valence Yb and magnetically ordered transition metal sublattice has rarely been observed. Recently, unusual magnetic behaviors have been reported in the new YbMn 6 Ge 6Àx Sn x series [7]. The Yb ordering temperature was found to be $60 and 90 K for x ¼ 4:2 and 4.4, respectively (exceptionally high ordering temperatures in Yb compounds), while the Mn moments order near $340 K in both cases.…”

“…In the ground state, Yb is in a ferromagnetic Mn sublattice and carries a substantial magnetic moment ($ 1:5 B ) antiferromagnetically coupled with the Mn moments. Moreover, there is some indirect indication that Yb could be in an intermediate valent state [7]. These compounds exhibit unique physical properties with the interplay of Kondo interactions which reduce the Yb magnetic moment and the evidence of an astonishingly high magnetic ordering temperature of the Yb sublattice.…”

“…A scenario is proposed to explain this surprising and 6 Ge 6Àx Sn x samples with x ¼ 4:2 and 4.4 were prepared from high-purity elements in closed Mo crucibles to avoid losses of Yb during annealing, as described in Ref. [7]. The precise chemical composition of the two alloys was determined by microprobe measurements.…”

“…For simplicity, we choose a monotonic temperature dependence [insert of Fig. 2(c)] neglecting the complex changes in the Mn magnetic order with temperature [7]. The simple model yields a good qualitative description of the unusual experimental temperature dependence.…”