Interplay of lattice-spin-orbital coupling and Jahn-Teller effect in noncollinear spinel Ti_xMn_1-x(Fe_yCo_1-y)₂O₄: a...

Local magnetostructural changes and dynamical spin fluctuations in doubly diluted spinel Ti_xMn_1‒x(Fe_yCo_1‒y)₂O₄has been reported by means of neutron diffraction and magnetization studies. Two distinct sets of compositions (i)x(Ti) = 0.20 andy(Fe) = 0.18; (ii)x(Ti) = 0.40 andy(Fe) = 0.435 have been considered for this study. The first compound of equivalent stoichiometry Ti_0.20Mn_0.80Fe_0.36Co_1.64O₄exhibits enhanced tetragonal distortion across the ferrimagnetic transition temperatureT_C= 258 K in comparison to the end compound MnCo₂O₄(T_C∼ 180 K) with a characteristic ratioc_t/√2a_tof 0.99795(8) demonstrating robust lattice-spin-orbital coupling. However, in the second case Ti_0.40Mn_0.60Fe_0.87Co_1.13O₄with higherB-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 belowT_C. Such dispersive behaviour appears to be sensitive to the level of octahedral substitution. Further, the field dependence ofχ_ac(T) follows the weak anisotropic GT-line behaviour with crossover exponent Φ lies in the range 1.38-1.52 on theH-Tplane which is in contrast to theB-site Ti substituted MnCo₂O₄spinel 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.