Spin-liquid state with precursor ferromagnetic clusters interacting antiferromagnetically in frustrated glassy tetragonal spinel Zn 0.8 Cu 0.2 FeMnO 4 .

Spinels (AB 2 O 4 ) with magnetic ions occupying only the octahedral B sites have inherent magnetic frustration which inhibits long-range magnetic order (LRO) but may lead to exotic states. Here we report on the magnetic properties of the tetragonal spinel Zn 0.8 Cu 0.2 FeMnO 4 , the tetragonality resulting from Jahn-Teller active Mn 3+ ions. Analysis of the temperature dependence of magnetization (M), heat capacity C p , and neutron diffraction measurements show complex temperature-dependent short-range ordering (SRO) but without LRO with spin arrangement (Cu 2+ ↓) A [Fe 2+ ↑, Fe 3+ ↓, Mn 3+ ↑] B leading to FM clusters interact antiferromagnetically (AFM) at low-T. The relaxation time τ obtained from the Power law and Vogel-Fulcher laws confirm the cluster spin-glass state. The field dependence of spin-glass temperature T SG (H) follows the equation: T SG (H)=T SG (0)[1-AH (2/ϕ) ] with T SG (0) = 46.6 K, A = 8.6 ×10 ‒3 Oe ‒0.593 and ϕ = 3.37. The temperature dependence of hysteresis loops yields coercivity H C ~ 3.8 kOe at 2 K without exchange-bias but H C decreases with increase in T and vanishes at 24 K, the T SG (H) for H = 800 Oe. Variations of C p -T from 2 K to 200 K in H = 0 and H = 90 kOe do not show any peak characteristic of LRO. However, after correcting for the lattice contribution, a broad transition typically of SRO becomes evident at 40 K. For T < 9 K, C p varies as T 2 ; a typical signature of spin-liquids. Comparison of the neutron diffraction measurements at 1.7 K and 79.4 K shows absence of LRO. Time dependence of thermoremanent magnetization studies below 9 K reveal weakening of the inter-cluster interaction with increase in temperature. A summary of these results is that in Zn 0.8 Cu 0.2 FeMnO 4 , ferromagnetic clusters interact antiferromagnetically without LRO but producing a cluster spin-glass state at T SG (0) = 46.6 K, followed by spin-liquid behavior below 9 K.