John M. Attah-Baah, Pascal Manuel, D. D. Khalyavin, Nilson S. Ferreira, Roger D. Johnson
Through detailed analysis of neutron powder diffraction data, we confirm that the double perovskite Nd 2 NiMnO 6 adopts a monoclinic P 2 1 / n structure with nearly complete B -site ordering of Ni 2 + and Mn 4 + cations. Below T 1 = 198 K , magnetic susceptibility measurements and further analysis of neutron diffraction data reveal that the Ni 2 + and Mn 4 + sublattices undergo ferromagnetic ordering, as expected of strong 3 d − 3 d exchange interactions. Upon cooling through T 2 = 22 K , a secondary magnetic transition is observed, below which we have discovered an additional noncollinear symmetry-breaking order of Nd 3 + moments. We argue that the canting of the rare-earth moments naturally arises through the competition of f − d and f − f Heisenberg exchange interactions, which may be finely balanced in the double perovskite framework. Furthermore, the symmetry of the ground state magnetic structure implies significant Nd 3 + easy plane anisotropy, and an effective decoupling of the antiferromagnetic spin canting from the transition metal sublattice. Between T 1 and T 2 , Nd 2 NiMnO 6 shows anomalous behavior in the frequency-dependent ac magnetic susceptibility that is characteristic of reentrant spin-glass-like properties attributed to antisite disorder and competing interactions. Finally, analysis of isothermal magnetization yields magnetic entropy changes that suggest Nd 2 NiMnO 6 and related compounds have potential for magnetic refrigeration, showing a peak in the magnetic entropy change of 2.25 J kg − 1 K − 1 at T 1 under a 7 T field. The scaling behavior of the magnetic entropy, paired with analysis of other critical exponents, shows that the ferromagnetic transition at <bb:math xmlns