Enhancement of Curie- and spin-spiral temperatures with doping Fe in multiferroic CoCr2O4 nanoparticles

Multiferroic, CoCr2O4 bulk material undergoes successive magnetic transitions such as paramagnetic to collinear ferrimagnetic state at Curie temperature (T-C) and non-collinear spin-spiral ordering temperature T-S followed by a lock-in-transition T-L. In this paper, the rich sequence of magnetic transitions in CoCr2O4 nanoparticles after doping with iron ions are investigated by varying the concentration from x = 10 to 40%. With increasing iron concentration from 10 to 40%, while T-C increases from 102 to 160 K higher than the T-C of pure CoCr2O4, T-S increases from 26 to 40 K. Later transition is clearly shown by sharp transition in specific heat measurement. While enhancement in T-C is attributed to increase in lattice expansion and the exchange interaction between tetrahedral (A) and octahedral (B) sites, spin-spiral ordering is dictated by B-B interaction. Further, we have examined the magnetic transitions through neutron scattering using polarized neutron beam along three orthogonal directions. We observe that the para to ferrimagnetic transition is found to be continuous and the spiral ordering is diffused in nature irrespective of composition.