Magnetic Phase Transition in the Quasi-One-Dimensional Spin Chain System Fe0.75Cu0.25NbO4.

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 Fe0.75Cu0.25NbO4. 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.