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  Jena, S.K.; Seehra, M.S.; Sarkar, T.; Reehuis, M.; Hoser, A.; Weise, B.; Thota, S.: Spin-liquid state with precursor ferromagnetic clusters interacting antiferromagnetically in frustrated glassy tetragonal spinel Zn_0.8Cu_0.2FeMnO₄. Journal of Physics: Condensed Matter 35 (2023), p. 375802/1-18

10.1088/1361-648x/acdbfa

Abstract:
Spinels (AB2O4) 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 Zn0.8Cu0.2FeMnO4, the tetragonality resulting from the Jahn–Teller active Mn3+ ions. X-ray diffraction and x-ray photoelectron spectroscopy of the sample yielded the composition (Zn2+0.8Cu2+0.2 )A[Fe2+0.4Fe3+0.6 Mn3+]BO4-d. Analysis of the temperature dependence of magnetization (M), ac magnetic susceptibilities (chi' and chi''), dc susceptibility (chi), heat capacity Cp, and neutron diffraction (ND) measurements show complex temperature-dependent short-range order (SRO) but without LRO. The data of chi; vs. T fits the Curie-Weiss law: chi = C/(T - theta) from T = 250 K to 400 K with theta = 185 K signifying dominant ferromagnetic (FM) coupling with the FM exchange constant J/kB = 17 K, and C = 3.29 emu K mol^-1Oe-1 yielding an effective magnetic moment mueff = 5.13 mB resulting from the high-spin states of Cu2+ (A site) and Fe2+ (B site), while the B site trivalent ions Mn3+ and Fe3+ 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 [Cu2+(down)]A[Fe2+(up)Fe3+(down)Mn3+(up)]B leading to FM clusters interact antiferromagnetically at low temperatures. Temperature dependence of d(chiT)/dT shows the onset of ferrimagnetism below ~100 K and peaks near 47 K and 24 K. The relaxation time tau; obtained from temperature and frequency dependence of chi'' when fit to the power law and Vogel–Fulcher laws confirm the cluster spin-glass (SG) state. The magnetic field dependence of the SG temperature TSG (H) follows the equation: TSG (H) = TSG (0) [1-AH^2/phi;] with TSG(0) = 46.6 K, A = 8.6 × 10^-3 Oe^-0.593 and phi = 3.37. The temperature dependence ofhysteresis loops yields coercivity HC ~ 3.8 kOe at 2 K without exchange-bias, but HC decreases with increase in T becoming zero above 24 K, the TSG(H) for H = 800 Oe. Variations of Cp 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, Cp varies as T2; 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 MTRM(t) studies below 9 K reveal weakening of the inter-cluster interaction with increase in temperature. A summary of these results is that in Zn0.8Cu0.2FeMnO4, ferromagnetic clusters interact antiferromagnetically without LRO but producing a cluster SG state at TSG(0) = 46.6 K, followed by SL behavior below 9 K.