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 magnetic long-range 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 the Jahn-Teller active Mn ³⁺ ions. X-ray diffraction and x-ray photoelectron spectroscopy of the sample yielded the composition (Zn0.82+Cu0.22+) _A [Fe0.42+Fe0.63+Mn3+] _B O _{4‒ δ} . Analysis of the temperature dependence of magnetization ( M ), ac magnetic susceptibilities ( χ' and χ'' ), dc susceptibility ( χ ), heat capacity C _p , and neutron diffraction (ND) measurements show complex temperature-dependent short-range order (SRO) but without LRO. The data of χ vs. T fits the Curie-Weiss law: χ = C /( T ‒ θ ) from T = 250 K to 400 K with θ ≃ 185 K signifying dominant ferromagnetic (FM) coupling with the FM exchange constant J / k _B = 17 K, and C = 3.29 emu K mol ^‒1 Oe ^‒1 yielding an effective magnetic moment µ _eff = 5.13 µ _B resulting from the high-spin states of Cu ²⁺ ( A site) and Fe ²⁺ ( B site), while the B site trivalent ions Mn ³⁺ and Fe ³⁺ are in their low-spin states. The extrapolated saturation magnetization obtained from the M vs. H data at T = 2 K is explained using the spin arrangement (Cu ²⁺ ↓) _A [Fe ²⁺ ↑, Fe ³⁺ ↓, Mn ³⁺ ↑] _B leading to FM clusters interact antiferromagnetically at low temperatures. Temperature dependence of d( χT)/ d T shows the onset of ferrimagnetism below ∼100 K and peaks near 47 K and 24 K. The relaxation time τ obtained from temperature and frequency dependence of χ″ when fit to the power law and Vogel-Fulcher laws confirm the cluster spin-glass (SG) state. The magnetic field dependence of the SG temperatureTSGHfollows the equation:TSGH=TSG01-AH2/ϕwith T _SG (0) = 46.6 K, A = 8.6 × 10 ^‒3 Oe ^‒0.593 andϕ= 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 becoming zero above 24 K, the T _SG ( H ) for H = 800 Oe. Variations of C _p vs. 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 weak peak typically of SRO becomes evident centered around 40 K. For T < 9 K, C _p varies as T ² ; a typical signature of spin-liquids (SLs). Comparison of the ND measurements at 1.7 K and 79.4 K shows absence of LRO. Time dependence of thermo-remanent magnetization M _TRM ( t ) 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 SG state at T _SG (0) = 46.6 K, followed by SL behavior below 9 K.
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