Tuning Magnetism of [MnSb₄]^9– Cluster in Yb₁₄MnSb₁₁ through Chemical Substitutions on Yb Sites: Appearance and Disappearance of Spin Reorientation

Single crystals of Yb_{14‑<i>x</i>}<i>RE</i>_<i>x</i>MnSb₁₁ (0 < <i>x</i> < 0.6, <i>RE</i> = Pr, Nd, Sm, and Gd) were synthesized by Sn flux. The compounds are iso-structural with Ca₁₄AlSb₁₁ (<i>I</i>4₁/<i>acd</i>), and their compositions were determined by wavelength-dispersive spectroscopy. Yb₁₄MnSb₁₁ is described as a partially screened d-metal Kondo system with the isolated [MnSb₄]^9– tetrahedral cluster having a d⁵ + hole configuration that results in four unpaired electrons measured in the ferromagnetically ordered phase. All of the Yb atoms in Yb₁₄MnSb₁₁ are present as Yb²⁺, and the additional <i>RE</i> in Yb_{14‑<i>x</i>}<i>RE</i>_<i>x</i>MnSb₁₁ is trivalent, contributing one additional electron to the structure and altering the magnetic properties. All compounds show ferromagnetic ordering in the range of 39–52 K attributed to the [MnSb₄]^9– magnetic moment. Temperature-dependent DC magnetization measurements of Yb_{14‑<i>x</i>}Pr_<i>x</i>MnSb₁₁ (0.44 ≤ <i>x</i> ≤ 0.56) show a sharp downturn right below the ferromagnetic transition temperature. Single-crystal neutron diffraction shows that this downturn is caused by a spin reorientation of the [MnSb₄]^9– magnetic moments from the <i>ab</i>-plane to <i>c</i>-axis. The spin reorientation behavior, caused by large anisotropy, is also observed for similar <i>x</i> values of <i>RE</i> = Nd but not for <i>RE</i> = Sm or Gd at any value of <i>x</i>. In Pr-, Nd-, and Sm-substituted crystals, the saturation moments are consistent with ∼4 unpaired electrons attributed to [MnSb₄]^9–, indicating that local moments of Pr, Nd, and Sm do not contribute to the ferromagnetic order. In the case of <i>RE</i> = Pr, this is confirmed by neutron diffraction. In contrast, the magnetic measurements of <i>RE</i> = Gd show that the moments of Gd ferromagnetically order with the moments of [MnSb₄]^9–, and reduced screening of moments on Mn²⁺ is evident. The sensitive variation of magnetic behavior is attributed to the various <i>RE</i> substitutions resulting in different interactions of the 4f-orbitals with the 3d-orbitals of Mn in the [MnSb₄]^9– cluster conducted through 5p-orbitals of Sb