Magnetic Phase Transition in the Quasi-One-Dimensional Spin Chain System Fe_0.75Cu_0.25NbO₄

In the present work, we conducted a comprehensive study using multiple experimental techniques to elucidate the structural, charge-state, vibrational, and magnetic properties of the quasi-one-dimensional spin chain system Fe_0.75Cu_0.25NbO₄. We found that substituting Fe³⁺ (3d⁵, s = 5/2) with Cu²⁺ (3d⁹, s = 1/2) induces significant changes in the lattice parameters, mixed valence states of Cu (Cu²⁺ and Cu¹⁺), and oxygen vacancies, as well as a ferrimagnetic (FiM) ordered state at room temperature. The FiM state is supported by double-exchange interactions between Fe³⁺ and Cu²⁺, mediated by oxygen, which highlight the magnetization mechanism and help describe the complex magnetic interactions among the Fe ions in this compound. We examined the temperature dependence of the magnetic order and, notably, observed a transition from ferrimagnetic to antiferromagnetic (FiM to AFM) at T = 38.4 K. ⁵⁷Fe Mössbauer spectroscopy indicates two Fe species, both in the trivalent state: one arising from Fe ions in a distorted octahedral configuration that interact with octahedrally substituted Cu polyhedra, favoring a FiM state among Fe ions with an ordering temperature above 300 K, and a second species associated with conventional Fe-Fe interactions that lead to the ordinary AFM state at 38 K. The magnetic transition was confirmed by the M vs T and ΔS _M vs T curves. Furthermore, Raman spectroscopy captures this magnetic transition and reveals clear spin-phonon coupling.