Manifestation of geometric frustration on magnetic and thermodynamic properties of the pyrochlores Sm₂X₂O₇ (X=Ti,Zr)

Abstract

We present here magnetization, specific heat, and Raman studies on single-crystalline specimens of the first pyrochlore member Sm₂Ti₂O₇ of the rare-earth titanate series. Its analogous compound Sm₂Zr₂O₇ in the rare-earth zirconate series is also investigated in the polycrystalline form. The Sm spins in Sm₂Ti₂O₇ remain unordered down to at least T=0.5 K. The absence of magnetic ordering is attributed to very small values of exchange (θ_cw~-0.26 K) and dipolar interaction (μ_eff~0.15 μ_B) between the Sm³⁺ spins in this pyrochlore. In contrast, the pyrochlore Sm₂Zr₂O₇ is characterized by a relatively large value of Sm-Sm spin exchange (θ_cw~-10 K); however, long-range ordering of the Sm³⁺ spins is not established at least down to T=0.67 K due to frustration of the Sm³⁺ spins on the pyrochlore lattice. The ground state of Sm³⁺ ions in both pyrochlores is a well-isolated Kramer's doublet. The higher-lying crystal field excitations are observed in the low-frequency region of the Raman spectra of the two compounds recorded at T=10 K. At higher temperatures, the magnetic susceptibility of Sm₂Ti₂O₇ shows a broad maximum at T=140 K, while that of Sm₂Zr₂O₇ changes monotonically. Whereas Sm₂Ti₂O₇ is a promising candidate for investigating spin fluctuations on a frustrated lattice, as indicated by our data, the properties of Sm₂Zr₂O₇ seem to conform to a conventional scenario where geometrical frustration of the spin excludes their long-range ordering.