Coupling of magnetism and transport properties to the lattice degrees of freedom in NdBaCo2O5+δ(δ∼0.65).

We have studied the origin of zero volume expansion below the Curie temperature (Tc), variable range hopping (VRH) behavior using structural, magnetic, transport and thermal studies on the oxygen deficient double perovskite NdBaCo2O5+δ(δ∼0.65). The valence state of Co ions and the possible properties exhibited by such compound were studied using electronic structure calculations forδ= 0.75. Careful investigation of structure shows that the compound stabilizes in tetragonal structure (P4/mmm) having2ap×2ap×2ap(222) superstructure, where apis the cubic perovskite lattice parameter. The compound exhibits a minimum in resistivity, ferromagnetic (FM) and ferrimagnetic (FeM) transitions around 375 K, 120 K (Tc) and 60 K, respectively with signature of Griffiths phase aboveTc. Our detailed structural analysis suggests signature of the onset of the above magnetic transitions at temperatures well above its stabilization at long range level thereby leading to VRH behavior. The observed zero thermal expansion in volume belowTcappears to be due to competing magnetic interactions within and between the magnetic sublattices. Our electronic structure calculations in FM and FeM configurations show (a) Co ions stabilize in intermediate spin (IS) state, having oxidation state less than +3, (b) half metallicity, (c) the behavior of the density of states is in line with the resistivity results, and (d) unusually high orbital angular moment in Co ions with inclusion of spin orbit coupling (soc). Our results show the possibility of coupling between magnetism and ferroelectricity. We believe that our results especially on the valence state of the Co ion, zero thermal expansion in volume, short range magnetic orderings and the connection between different degrees of freedom will be helpful in clearing the ambiguities existing in literature on the nature of magnetism and thereby aiding in designing new functionalities by maneuvering the strength of soc.