Contrasting antiferromagnetic, spin-glass and magnetic glass states in the double perovskite Sr_2-xLa_xFeCoO₆

The magnetism of the double perovskite Sr_2-xLa_xFeCoO₆ (x = 0, 1, 2) strongly depends on the type of atom at the Sr-site and on the disorder at the Fe/Co sites. In this work, by employing a combination of magnetometry, neutron scattering and electron paramagnetic resonance spectroscopy, we demonstrate that as the content of La that replaces the Sr changes, the disorder and subsequently, the magnetism of Sr_2-xLa_xFeCoO₆ transforms from a canonical spin glass (for x = 0) to a magnetic glass (for x = 1) and further, to a long-range ordered antiferromagnet (for x = 2). The magnetism in Sr₂FeCoO₆ manifests through a spin freezing at T_g ≈ 75 K, with features resembling a canonical spin glass. In the case of SrLaFeCoO_6, strong evidence of diffuse magnetic scattering is present up to 300 K suggesting dominant short-range spin correlations at room temperature. Though two magnetic anomalies occur at T_a1 ≈ 75 K and T_a1 ≈ 250 K in this compound, no long-range magnetic order is established down to 2 K. In striking contrast to these two compounds, La₂FeCoO₆ has a long-range ordered magnetic state with a T_N ≈ 350 K. Protocols of cooling-and-heating-under-unequal-fields clearly map out the non-equilibrium magnetization dynamics.