Unconventional magnetism and residual entropy in La₇Os₃O₁₈

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 S = 3/2 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 T_N = 43 ± 1 K, with a further broad overturn in susceptibility at T ≈ 14 K indicating low dimensional short range magnetic ordering. Curie–Weiss fits give an effective moment of µ_eff = 3.26µ_B per Os⁵⁺ and Weiss constant of θ_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 T_N we find only approximately half of the expected magnetic entropy to be released. Magnetisation versus field measurements at low temperature reveal a hysteretic metamagnetic transition above 5 T, further indicating that the magnetism below T_N is non-trivial in nature.