Interplay of lattice-spin-orbital coupling and Jahn-Teller effect in noncollinear spinel Ti x Mn 1- x (Fe y Co 1- y ) 2 O 4 : a neutron diffraction study.

Local magnetostructural changes and dynamical spin fluctuations in doubly diluted spinel Ti x Mn 1‒ x (Fe y Co 1‒ y ) 2 O 4 has been reported by means of neutron diffraction and magnetization studies. Two distinct sets of compositions (i) x (Ti) = 0.20 and y (Fe) = 0.18; (ii) x (Ti) = 0.40 and y (Fe) = 0.435 have been considered for this study. The first compound of equivalent stoichiometry Ti 0.20 Mn 0.80 Fe 0.36 Co 1.64 O 4 exhibits enhanced tetragonal distortion across the ferrimagnetic transition temperature T C = 258 K in comparison to the end compound MnCo 2 O 4 ( T C ∼ 180 K) with a characteristic ratio c t /√2 a t of 0.99795(8) demonstrating robust lattice-spin-orbital coupling. However, in the second case Ti 0.40 Mn 0.60 Fe 0.87 Co 1.13 O 4 with higher B -site compositions, the presence of Jahn-Teller ions with distinct behavior appears to counterbalance the strong tetragonal distortion thereby ceasing the lattice-spin-orbital coupling. Both the investigated systems show the coexistence of noncollinear antiferromagnetic and ferrimagnetic components in cubic and tetragonal settings. On the other hand, the dynamical ac-susceptibility, χ ac ( T ) reveals a cluster spin-glass state with Gabay-Toulouse (GT) like mixed phases behaviour below T C . Such dispersive behaviour appears to be sensitive to the level of octahedral substitution. Further, the field dependence of χ ac ( T ) follows the weak anisotropic GT-line behaviour with crossover exponent Φ lies in the range 1.38-1.52 on the H - T plane which is in contrast to the B -site Ti substituted MnCo 2 O 4 spinel that appears to follow irreversible non-mean-field AT-line behaviour (Φ ∼ 3 + δ ). Finally, the Arrott plots analysis indicates the presence of a pseudo first-order like transition ( T < 20 K) which is in consonance with and zero crossover of the magnetic entropy change within the frozen spin-glass regime.