Super-exchange coupling in oxygen rich rare-earth based Sm₂MnRu O_6+δ double perovskite

A new material of rare-earth based Sm₂MnRu O_6+δ (SMRO) double perovskite was prepared using high temperature solid-state reaction. The structural, morphological, chemical, thermodynamic and magnetic properties were measured with X-ray diffraction (XRD), Energy dispersive spectroscopy (EDS), X-ray photoemission spectroscopy (XPS), and vibrating sample magnetometer (VSM), respectively. The XRD revealed a tetragonal structure belonging to the I4/mmm space group, number 139 with linear Mn–O–Ru bonds. Replacing the well-studied alkaline earth metal with a rare-earth element increased the Mn–O bond length difference between the shorter equatorial (Mn-O_ab) and the axial (Mn-O_c) bonds by approximately 6.3 %. The elemental composition showed an O-rich double perovskite with Ru deficit, which encourages the formation of Ru⁶⁺ (d²) state. XPS spectra of Sm-3d, Ru-3d, and Mn-2p revealed the coexistence of double oxidation state for each cation; Sm²⁺, Sm³⁺, Ru³⁺, Ru⁶⁺, Mn²⁺ and Mn³⁺, in varying proportions. Entropy studies showed drastic ordering of spins at low temperatures (up to 12.4 K), whilst increasing temperatures above this point resulted in a drastic increase of disorder of the spins (up to 43.3 K), beyond which a constant slope of entropy is observed. Magnetic measurements revealed two magnetic ground states at T_N = 12.4 K and T_C = 43.3 K ordering antiferromagnetically (AFM) and ferromagnetically (FM), respectively. Kneller fit further showed that the materials become completely paramagnetic at T_B = 88.1 K, (the blocking temperature). The existence of FM super-exchange coupling in this work originating from Mn³⁺ (t³_2ge¹_g)−O–Ru³⁺ (t⁵_2ge⁰_g) and Mn²⁺ (t³_2ge²_g)−O–Ru⁶⁺ (t²_2ge⁰_g) which plays an important role in suppressing the Mn/Ru–O–Mn/Ru AFM interactions.