Manifestation of geometric frustration on magnetic and thermodynamic properties of pyrochlores $S{m}_2{X}_2{O}_7$ (X=Ti, Zr)

Abstract

We present here magnetization, specific heat and Raman studies on single-crystalline specimens of the first pyrochlore member $S{m}_{2}T{i}_2{O}_7$ of the rare-earth titanate series. Its analogous compound $S{m}_{2}Z{r}_2{O}_7$ in the rare-earth zirconate series is also investigated in the polycrystalline form. The Sm spins in $S{m}_{2}T{i}_2{O}_7$ remain unordered down to at least T = 0.5 K. The absence of magnetic ordering is attributed to very small values of exchange (${\theta }_{cw}-0.26K$) and dipolar interaction (${\mu }_{eff}0.15{\mu }_B$) between the $S{m}^{3+}$ spins in this pyrochlore. In contrast, the pyrochlore $S{m}_{2}Z{r}_2{O}_7$ is characterized by a relatively large value of Sm-Sm spin exchange (${\theta }_{cw}-10K$); however, long-range ordering of the $S{m}^{3+}$ spins is not established at least down to T = 0.67 K, due to frustration of the $S{m}^{3+}$ spins on the pyrochlore lattice. The ground state of $S{m}^{3+}$ 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 $S{m}_{2}T{i}_2{O}_7$ shows a broad maximum at T = 140 K while that of $S{m}_{2}Z{r}_2{O}_7$ changes monotonically. Whereas $S{m}_{2}T{i}_2{O}_7$ is a promising candidate for investigating spin-fluctuations on a frustrated lattice as indicated by our data, the properties of $S{m}_{2}Z{r}_2{O}_7$ seem to conform to a conventional scenario where geometrical frustration of the spin exclude their long-range ordering.

arXiv comment24 pages, 6 figures, Accepted for publication in Phys. Rev. B

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