Effect of structural and magnetic disorder on the 3d−5d exchange interactions in La2−xCax

Structural, electronic and magnetic properties of La$_{1.5}$Ca$_{0.5}$(Co$_{0.5}$Fe$_{0.5}$)IrO$_6$ double perovskite

“…The lattice parameters here observed are fairly close to those reported for LCCIO, for which a detailed investigation of the oxidation states by means of Xray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and Raman spectroscopy revealed mixed valence for Co, with ∼70% and 30% of Co 2+ and Co 3+ , respectively [12]. In case of the LCCFIO sample here investigated, Mössbauer indicated trivalent state for Fe, while Co and Ir are more likely in bivalent and tetravalent states, respectively, as will be discussed below.…”

“…[15] and slightly smaller than that found for La 1.5 Ca 0.5 CoIrO 6 (5.4 µ B /f.u.) [12]. For the FeIr-based series, Mössbauer spectroscopy has shown that Fe remains trivalent for all investigated samples [20], as for the LCCFIO sample here investigated.…”

“…In case of the LCCFIO sample here investigated, Mössbauer indicated trivalent state for Fe, while Co and Ir are more likely in bivalent and tetravalent states, respectively, as will be discussed below. This means that the introduction of Fe 3+ at Co site leads to a slight decrease of the average B-site ionic radius [19], which may be responsible for the subtle decrease of the unit cell volume (V) of LCCFIO (245.95 Å3 ) in comparison to LCCIO (247.09 Å3 ) [12]. It is established for DP compounds that an increase in the crystal distortion accompanies the volume shrinkage, generally ascribed to tilts in the oxygen octahedra [1], explaining the slightly smaller (Co/Fe)-O-Ir average bond angle of LCCIO with respect to LCCFIO.…”

“…For instance, electronic structure calculations have predicted a metallic ground state in La 2 CoIrO 6 . But this was not confirmed by experimental results, which showed a FM-like insulating state for this Co-and Ir-based DP [11,12]. Further studies of this material revealed magnetodielectric effect together with re-entrant spin-glass behavior at low-T [13], whereas Ca 2+ to La 3+ partial substitution leads to compensation temperatures and spontaneous exchange bias effect in La 1.5 Ca 0.5 CoIrO 6 (LCCIO) [14].…”

The spontaneous exchange bias effect in La2-xCaxCoMnO6 series

“…We can infer the Co/Mn electronic configurations for the x = 0 compound by comparing its µ ef f = 7.6 µ B /f.u. experimentally observed with the value obtained from the usual equation for systems with two or more different magnetic ions [38,39] µ = µ 1 2 + µ 2 2 + µ 3 2 + ... As aforementioned, one may expect mixed valences for the TM ions already for the x = 0 parent compound. Assuming the same proportion of Co 2+ /Mn 4+ and Co 3+ /Mn 3+ , considering that Co 3+ is in HS configuration and using the spin only (SO) approximation for these 3d ion moments (µ Co 2+ = µ Mn 4+ = 3.9 µ B ; µ HSCo 3+ = µ Mn 3+ = 4.9 µ B [40]) we get µ = 6.2 µ B /f.u., far below the experimental value.…”