Geometrical Frustration and Tunable Lattice Dynamics of Highly Anisotropic Pyrochlore Cuprospinels

Recently there has been a strong research interest in the field of frustrated magnetism in tertragonally distorted spinels (AB₂O₄) because they exhibit peculiar magnetic characteristics such as spin-liquid and reentrant spin-glass behavior. Among the various spinels, CuFe₂O₄ and ZnFe₂O₄ are unique systems in which the former exhibit Jahn-Teller distortion with high temperature ferrimagentic (FiM) ordering (743 K) while the later possesses pyrochlore structure and shows low-temperature antiferromagnetic (T_N ~ 10 K) ordering with high degree of magnetic frustration index (f ≥ 12). In this work, we report magnetic properties and lattice dynamics of few intermediate compositions (0 ≤ x ≤ 1) of the above two systems (Cu_1−xZn_xFe₂O₄) in the form of bulk polycrystals. Analysis of the magnetization data (χ = M/H) for x = 0.1 shows T_FiM ~ 700 K with two additional transitions at 65 K and 250 K, the latter is expected to result from the domain crossover dynamics. Based on the factor group analysis, 10 Raman active modes for tetragonal CuFe₂O₄ and 5 modes for cubic CuFe₂O₄ (A_1g + E_g + 3F_2g) are observed, whereas 18 modes for cubic ZnFe₂O₄ are evident. Our results across the morphotropic phase boundary (0.05 < x_P < 0.1) shows 6 vibrational modes. On the contrary, only one mode is noticed for x = 0.5 at ν = 500 cm⁻¹. A systematic comparative study between the parameters extracted from magnetic and spectroscopic data will be discussed in correlation to the changes occurring in the crystal structure.