Griffiths-like phase, large magnetocaloric effect, and unconventional critical behavior in the NdSrCoFeO6 disordered double perovskite

A B-site disordered double perovskite NdSrCoFeO6 was successfully synthesized by the conventional sol-gel method. Detailed experimental analyses revealed that NdSrCoFeO6 crystallizes in the orthorhombic Pnma space group, in which Co2+/3+ and Fe3+/4+ ions are randomly distributed at the BB' sites, and Sr2+ and Nd3+ ions are respectively ordered at the A and A' sites in an alternating arrangement along the c direction. NdSrCoFeO6 has a semimetallic-to-semiconducting transition nature, and a paramagnetic-ferromagnetic (FM) second-order phase transition originating from the complex hybridization between Co 3d and O 2p states is also found to occur at T-C approximate to 150 K. Then the spin coupling between Fe4+ <-> Co3+ and Fe3+ <-> Co2+ randomly distributed on the B and B' sites leads to a FM cluster spin-glass behavior with characteristic parameters of k = 0.01, T-SG = 82.7 K, zv = 1.89, and tau(0) = 0.46 x 10(-4) s. Additionally, Griffiths-like phase behavior was observed in the region T-C < T < T-GP, with T-GP = 245 K, consistent with the power law exponent of lambda = 0.74. The maximum isothermal magnetic entropy change - Delta S-M(max) approximate to 1.84 J kg(-1) K-1 and relative cooling power approximate to 43.8 J kg(-1) under a field of 40 kOe also indicate a magnetocaloric coupling wherein fitted critical exponents beta = 1.384, gamma = 0.621, and delta = 1.421 are far from any conventional universality class. Density functional theory calculations demonstrated spin short- and long-range ordering competitions for Fe/Co at BB' sites, which arise predominantly from the stronger negatively charged ligand interaction with Co 3d orbitals and the weakest Fe 3d orbitals. This unconventional behavior is expected to be the main reason for the experimentally observed magnetic exchange distance decreasing with J(r) approximate to r(-4.7).

Automated summary

In this study, a B-site disordered double perovskite material was synthesized and found to exhibit unique magnetic properties and magnetocaloric coupling. Experimental results demonstrated the significant influence of the distribution of iron and cobalt ions on the material's magnetism.