Unconventional magnetism and residual entropy in La ₇ Os ₃ O ₁₈

In this communication we describe the crystal structure and unusual magnetic properties of La₇Os₃O₁₈, a new layered osmate with structurally discrete (non-corner-sharing) octahedra containing Os⁵⁺ with and strong spin-orbit coupling. Investigations through powder X-ray diffraction, magnetometry and heat capacity measurements demonstrate that the material undergoes an antiferromagnetic ordering transition at <i>T</i>_N = 43 ± 1 K, with a further broad overturn in susceptibility at <i>T</i> ≈ 14 K indicating low dimensional short range magnetic ordering. Curie-Weiss fits give an effective moment of <i>µ</i>_eff = 3.26<i>µ</i>_B per Os⁵⁺ and Weiss constant of <i>θ</i>_W ≈ -92 K. Heat capacity data reveal that it is the low temperature feature with which the majority of the magnetic entropy is associated. However, between our lowest temperature of 2 K and well above <i>T</i>_N we find only approximately half of the expected magnetic entropy to be released. Magnetisation <i>versus</i> field measurements at low temperature reveal a hysteretic metamagnetic transition above 5 T, further indicating that the magnetism below <i>T</i>_N is non-trivial in nature.